Soil Water Repellency: Occurrence, Consequences, and Amelioration

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SOIL WATER REPELLENCY Occurrence, consequences, and amelioration

ThisPageIntentionallyLeftBlank .

SOIL WATER REPELLENCY Occurrence, consequences, and amelioration

Edited by

C.J. Ritsema and L.W. Dekker Wageningen, The Netherlands

2003

ELSEVIER Amsterdam – Boston – London – New York – Oxford – Paris San Diego – San Francisco – Singapore – Sydney – Tokyo

ELSEVIER SCIENCE B.V. Sara Burgerhartstraat 25 P.O. Box 211, 1000 AE Amsterdam, The Netherlands q 2003 Elsevier Science B.V. All rights reserved. This work is protected under copyright by Elsevier Science, and the following terms and conditions apply to its use: Photocopying Single photocopies of single chapters may be made for personal use as allowed by national copyright laws. Permission of the Publisher and payment of a fee is required for all other photocopying, including multiple or systematic copying, copying for advertising or promotional purposes, resale, and all forms of document delivery. Special rates are available for educational institutions that wish to make photocopies for non-profit educational classroom use. Permissions may be sought directly from Elsevier Science via their homepage (http://www.elsevier.com) by selecting ‘Customer support’ and then ‘Permissions’. Alternatively you can send an e-mail to: [email protected], or fax to: (+44) 1865 853333. In the USA, users may clear permissions and make payments through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA; phone: (+1) (978) 7508400, fax: (+1) (978) 7504744, and in the UK through the Copyright Licensing Agency Rapid Clearance Service (CLARCS), 90 Tottenham Court Road, London W1P 0LP, UK; phone: (+44) 207 631 5555; fax: (+44) 207 631 5500. Other countries may have a local reprographic rights agency for payments. Derivative Works Tables of contents may be reproduced for internal circulation, but permission of Elsevier Science is required for external resale or distribution of such material. Permission of the Publisher is required for all other derivative works, including compilations and translations. Electronic Storage or Usage Permission of the Publisher is required to store or use electronically any material contained in this work, including any chapter or part of a chapter. Except as outlined above, no part of this work may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without prior written permission of the Publisher. Address permissions requests to: Elsevier Science Global Rights Department, at the fax and e-mail addresses noted above. Notice No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be made.

First edition 2003 Extended version of the Journal of Hydrology, Volumes 231–232 Library of Congress Cataloging in Publication Data A catalog record from the Library of Congress has been applied for. British Library Cataloguing in Publication Data A catalogue record from the British Library has been applied for.

ISBN: 0-444-51269-1

1 W

The paper used in this publication meets the requirements of ANSI/NISO Z39.48-1992 (Permanence of Paper). Printed in The Netherlands.

Contents Chapter 1 Introduction C.J. Ritsema and L.W. Dekker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 2 Historical overview of soil water repellency L.F. DeBano . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ORIGIN OF SOIL WATER REPELLENCY Chapter 3 Hydrophobic compounds in sands from New Zealand D.J. Horne and J.C. McIntosh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 4 Chemical characterisation of water repellent materials in Australian sands C.M.M. Franco, P.J. Clarke, M.E. Tate and J.M. Oades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ASSESSMENT OF SOIL WATER REPELLENCY Chapter 5 Characterizing the degree of repellency J. Letey, M.L.K. Carrillo and X.P. Pang . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 6 Sessile drop contact angle method J. Bachmann, A. Ellies and K.H. Hartge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 7 Water-entry value as an alternative indicator Z. Wang, L. Wu and Q.J. Wu . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . OCCURRENCE AND HYDROLOGICAL IMPLICATIONS Chapter 8 Soil wettability in forested catchments in South Africa D.F. Scott . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 9 Soil water repellency in arid and humid climates D.F. Jaramillo, L.W. Dekker, C.J. Ritsema and J.M.H. Hendrickx . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 10 Water repellency in dunes along the Dutch coast L.W. Dekker, C.J. Ritsema and K. Oostindie . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 11 Water repellent soils on UK golf greens C.A. York and P.M. Canaway . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 12 Soil water repellency in the Natural Park of Donana, southern Spain F.J. Moral Garcia, L.W. Dekker, K. Oostindie and C.J. Ritsema . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 13 Soil water repellency in northeastern Greece A.K. Ziogas, L.W. Dekker, K. Oostindie and C.J. Ritsema . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 14 Soil moisture: a controlling factor in water repellency? S.H. Doerr and A.D. Thomas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 15 Wetting patterns in water repellent Dutch soils L.W. Dekker and C.J. Ritsema . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 16 The impact of water-repellency on overland flow and runoff in Portugal A.J.D. Ferreira, C.O.A. Coelho, R.P.D. Walsh, R.A. Shakesby, A. Ceballos and S.H. Doerr . . . Chapter 17 The erosional impact of soil water repellency: an evaluation R.A. Shakesby, S.H. Doerr and R.P.D. Walsh . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1 3

25 37

51 57 67

77 93 99 113 121 127 137 151 167 179

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EFFECT OF FIRE ON WATER REPELLENCY Chapter 18 The role of fire and soil heating on water repellency L.F. DeBano . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 Chapter 19 Infiltration rates after prescribed fire in Northern Rocky Mountain forests P.R. Robichaud . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 203 PHYSICS AND MODELING ON WATER REPELLENT SOILS Chapter 20 Physics of hydrophobic soils T.W.J. Bauters, T.S. Steenhuis, D.A. DiCarlo, J.L. Nieber, L.W. Dekker, C.J. Ritsema, J.-Y. Parlange and R. Haverkamp . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 21 Solute transport through a hydrophobic soil B.E. Clothier, I. Vogeler and G.N. Magesan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 22 Effects of water repellency on infiltration rate and flow instability Z. Wang, Q.J. Wu, L. Wu, C.J. Ritsema, L.W. Dekker and J. Feyen . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 23 Non-equilibrium model for gravity-driven fingering in water repellent soils: Formulation and 2D simulations J. Nieber, A. Sheshukov, A. Egorov and R. Dautov . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Chapter 24 Modeling implications of preferential flow in water repellent sandy soils C.J. Ritsema and L.W. Dekker . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

215 225 235

245 259

AMELIORATION TECHNIQUES AND FARMING STRATEGIES ON WATER REPELLENT SOILS Chapter 25 Clay spreading on water repellent sands M.A. Cann . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 273 Chapter 26 Treating water repellent surface layer with surfactant L.W. Dekker, K. Oostindie, S.J. Kostka and C.J. Ritsema . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281 Chapter 27 Management of water repellency in Australia P.S. Blackwell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 291 Chapter 28 Water repellency: a whole farm bio-economic perspective A.K. Abadi Ghadim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 303 BIBLIOGRAPHY AND REFERENCES Chapter 29 More than one thousand references related to soil water repellency L.W. Dekker, L.F. DeBano, K. Oostindie and E. van den Elsen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 313 Chapter 30 References not listed in chapter 29 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347

Chapter 1

Introduction Coen J. Ritsemap and Louis W. Dekker Alterra, Land Use and Soil Processes Team, Droevendaalsesteeg 3, Building 101, 6708 PB Wageningen, The Netherlands

Recently, it has become clear that soil water repellency is much more wide-spread than formerly thought. Water repellency has been reported in most continents of the world for varying land uses and climatic conditions. Soil water repellency often leads to severe runoff and erosion, rapid leaching of surface-applied agrichemicals, and losses of water and nutrient availability for crops. At present, no optimum management strategies exist for water repellent soils, focusing on minimizing environmental risks while maintaining crop production. One of the reasons is that knowledge on water repellent soils is scattered among researchers of different disciplines working at different places throughout the world. To promote cross-disciplinary discussion and to obtain an integral view on many aspects related to soil water repellency, a three-days international workshop had been organized at the DLO Winand Staring Centre for Integrated Land, Soil and Water Research (now Alterra) on September 2 –4, 1998. The workshop was sponsored by several institutions and organizations, and was attended by around 150 participants originating from 14 countries. The book starts with a historial overview of water repellency research by DeBano in chapter 2, followed by seven thematic sections covering 26 research chapters. In the first section dealing with the the orgin of soil water repellency, two chapters are presented. In chapter 3, Horne and McIntosh discuss extraction p

Corresponding author. E-mail addresses: [email protected] (C.J. Ritsema), [email protected] (L.W. Dekker)

q 2003 Elsevier Science B.V. All rights reserved.

techniques, characterisation methods and proposed mechanisms for water repellency expression. Franco et al. present results on the properties and chemical characterisation of natural water repellent materials in Australian sands in chapter 4. The second section covers chapters dealing with the assessment of soil water repellency. In chapter 5, Letey et al. present an overview of approaches usable for characterizing the degree of water repellency. Bachmann et al. introduce and apply a new sessile drop contact angle method to assess the degree of water repellency in chapter 6, and Wang et al. describe the water-entry value as an alternative indicator of soil water repellency and wettability in chapter 7. Occurrence and hydrological implications of soil water repellency are discussed in the third section of this book. Occurence of water repellency is reported in South Africa by Scott (chapter 8), USA and Colombia by Jaramillo et al. (chapter 9), the Netherlands by Dekker et al., and Dekker and Ritsema (chapters 10 and 15), United Kingdom by York and Canaway (chapter 11), Spain by Moral Garcia et al. (chapter 12), Greece by Ziogas et al. (chapter 13), and Portugal by Doerr and Thomas, and Ferreira et al. (chapters 14 and 16). Effects of the presence of water repellent substances on soil charactistics, vegetation, infiltration, and moisture distribution within the soil are illustrated by most of these authors. Shakesby et al. discuss the erosional impact of soil hydrophobicity, and future research directions in chapter 17. The fourth section of this book is devoted to the effect of fire on water repellency. DeBano presents a

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C.J. Ritsema and L.W. Dekker

review on past and current work on the role of fire and soil heating on the inducement of water repellency in wildland environments (chapter 18). The effect of prescribed fire on actual infiltration rates in the field is highlighted by Robichaud in chapter 19. Section five deals with the physics and modeling of flow and transport in water repellent soils. Bauters et al. present a general overview of the underlying physics of water repellent soils in chapter 20. In an experimental study, Clothier et al. illustrate the effect of the breakdown of water repellency on infiltration rates and solute transport in a soil from New Zealand (chapter 21). Wang et al. highlight the effects of soil water repellency on water infiltration rates and the generation of flow instability in the soil profile in chapter 22. Nieber et al. present extensive results of experimental gravity-driven unstable flow in water repellent soils using a two-dimensional numerical simulation model in chapter 23. In chapter 24, Ritsema and Dekker show field evidence and twodimensional modeling results of finger formation and finger recurrence in a water repellent sandy soil, and, additionally, postulate an alternative approach to incorporate these processes in one-dimensional water flow and transport models. Amelioration techniques and farming strategies to combat soil water repellency are presented in the sixth section. Cann focuses on promoting sustainable agriculture in South Australia by clay spreading on water repellent sands (chapter 25). Dekker et al. discuss the effects of systematic surfactant applications on the amelioration of soil water repellency and soil moisture distributions (chapter 26).

Blackwell summarizes management strategies for water repellent soils in Australia in chapter 27, and discusses associated risks of preferential flow, pesticide concentrations and leaching. Abadi Ghadim introduces a whole farm bio-economic perspective for agricultural use of water repellent soils in chapter 28. Dekker et al. conclude the special issue with an extensive bibliography on soil water repellency research, containing over more than 1000 references (chapter 29). Additional non water repellent references have been listed in Chapter 30. We would like to express our sincere hope that this first book ever on soil water repellency will act as a stimulus for initiating further research on a broad range of topics related to water repellent soil systems world-wide.

Acknowledgements This publication has been made possible through financial support of the following organizations and institutions: † The European Union, Grant ENV4-CT97-6129 † The Royal Netherlands Academy of Arts and Sciences, RC 241.612.2697 † The Netherlands Integrated Soil Research Programme, Subsidy 185 † The Australian Department of Industry, Science and Tourism, Grant 97/686 † Alterra

Chapter 2

Historical overview of soil water repellency L.F. DeBano Watershed Management, School of Renewable Natural Resources, University of Arizona, Tucson, AZ 85721, USA

Abstract The purpose of this paper is to document some of the more important highlights of the research and historical aspects concerning soil water-repellency. This effort traces the evolution of interests and concerns in water repellency from basic studies in the nineteenth century to the earlier part of the 20th century and up to our current-day understanding of this subject. The interactions among different scientific disciplines, various manager-scientists efforts, and specific scientific and management concerns are presented chronologically. This growing interest in water repellency generated an earlier conference in 1968 which was devoted exclusively to water repellency and has since initiated productive discussions and debate on water repellency during several peripherally related national and international conferences. The 1968 conference held in Riverside, California (USA), mainly involved scientists from the United States and Australia. Since this early conference, a large body of information has been published in a wide range of scientific disciplines throughout the world. This worldwide attention has produced many recent research findings, which have improved the understanding of water-repellent soils, particularly of the dynamics of the water movement and redistribution in these unique systems. Intermingled with the effort in water repellency is a related, although somewhat separate, body of information dealing with soil aggregation and water harvesting, which are important for improving the productivity of fragile arid ecosystems. A summary is presented of the literature on water repellency, showing changes in subject areas and national interests over time.

1. Introduction Water repellency has been a concern of both scientists and land managers for well over a century. During this time, the interest in water repellency has evolved from an isolated scientific curiosity to an established field of science that is recognized worldwide. The wide range of topics discussed on this issue exemplify the range of interest in water repellency. The purpose of this paper is to present a detailed overview of the research and institutional history of the field of water repellency. This effort traces the evolution of knowledge and concerns about water repellency from the beginning of this century to our current-day understanding of this subject. The interactions among different scientific disciplines, various q 2003 Elsevier Science B.V. All rights reserved.

manager-scientists efforts, and specific scientific and management concerns are presented both chronologically and by subject area content.

2. Information base The information used as the basis for this paper consisted of: (1) an extensive bibliography of over 500 published papers reporting on various aspects of water repellency; (2) a bibliography of over 200 published papers which contributed information directly related to the understanding of some of the basic physical, biological, and chemical processes— the kind of knowledge essential to the present level of understanding of water repellency phenomena;

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L.F. DeBano

and (3) the personal knowledge gained by this author in over 30 years of interest and research in the field of water repellency. The bibliography of related literature was derived mainly from important citations in published papers on water repellency. This list cannot be claimed to be complete, but reflects the author’s evaluation of the importance of individual citations and their contributions to the field. The papers cited in this paper represent only a sample of the entire bibliography used and the cited references were selected at the discretion of the author. A comprehensive bibliography is being published separately for those interested in a more complete list of citations. The bibliography described above was first examined chronologically in order to identify changes in emphasis over time and to track the rise and fall of interest in different scientific and applied aspects of water repellency. This review of literature was also used to identify the evolution of regional emphases on different research topics pertaining to water repellency. The final section of this paper summarizes the chronological development of the knowledge and the

regional centers that were involved in water repellency research at different times. 3. A global perspective As during the evolution of many sciences, the earlier years produced only a few publications and the numbers remained low for several decades until interest and fundamental understanding accumulated, after which the numbers of publications mushroomed. Fig. 1 illustrates the number of papers published during different time periods on water repellency per se and in areas that contributed directly to the understanding of water repellency. The total numbers themselves are not too informative, but when examined in detail they reveal some noteworthy landmarks and stages of development. The database provided the basis for identifying the overall flow of information, the emphasis of different topic areas, the reasons for the increased number of publications, and the overall evolution of the science of water repellency. A more detailed review of the chronology of these publications and examples of important

Fig. 1. The number of publications concerning soil water-repellency and related fields during the 20th century.

Historical overview of soil water repellency

publications are presented below within the framework of different decades.

4. Chronological highlights The disciplines that provide the scientific and utilitarian basis for our current understanding of water repellency are: firstly, the study of the important role of organic matter in agricultural systems, and secondly, the knowledge of soil – water – plant relationships, particularly the physics of soil water movement. The accumulation of knowledge in these two areas evolved both from academic curiosity and from the importance of organic matter in the productivity of agricultural systems. These two areas of scientific inquiry continued to be keystone sciences underlying the study of water repellency phenomena over the years and also appear as an integral part of many papers included in this issue. 4.1. The roots (pre-20th century) Interest in water repellency phenomena began well before the 20th century, although it was not identified as such. It is not the purpose of this paper to establish an irrefutable beginning point for the study of water repellency, but instead to identify selected references found in the literature before 1900, which increased the awareness of organic matter (humus) and provided a basis for later studies of water repellency that began in the early 20th century. An examination of the literature before 1900 indicates that water repellency was mostly associated with observations on organic matter and its decomposition, particularly where fungi were involved. Humic substances were first investigated during the later part of the 18th century when Achard attempted to isolate humic substances in 1786 (Stevenson, 1994). DeSaussure introduced the word “humus” in 1804 and humic acids were designated by Dobereiner in 1822. The first comprehensive reports on the chemical nature of humic substances were written between 1826 and 1862 (Stevenson, 1994). In the second half of the nineteenth century, most of the reports were concerned with classifying products produced during the decomposition of organic substances (Kononova, 1961). By the end of the nine-

5

teenth century, it was well established that humus was a complex mixture of organic substances that were mostly colloidal and had weakly acidic properties (Stevenson, 1994). Studies on the fungal decomposition of organic matter were first reported by Waring in 1837 (as reviewed by Bayliss, 1911) and these reports were probably the first publications that discussed the effect of mycelium growth on the rate of absorption of water by soil. These studies described a phenomenon known as “fairy rings”. The term “fairy ring” was used by early investigators to describe the arrangement of plants (usually grass or crop plants) in an approximately circular form, where plant growth on the inside of the circle was stimulated. Circles of bare ground or concentric zones of withered plants surrounded this inner circle of healthy plants. These concentric rings were attributed to various natural and supernatural sources such as the paths created by dancing fairies, thunder, lightning, whirlwinds, ants, moles, haystacks, urine of animals, and so on. In many cases the fairy ring phenomena was so abundant locally that it materially affected the yield of crops. Almost a half century later, quantitative data were reported at Rothamsted indicating that more soil moisture was present in the healthy ring of plants than either outside or inside it (Lawes et al., 1883). Although none of these pre-20th century publications used the term “water repellency”, it was obvious that many of these earlier scientists were observing the phenomenon of water repellency as we know it today. Another building block, which would contribute to the understanding of water repellency, was the discipline of soil physics, which was just starting to appear at the end of the nineteenth century. Two papers written by German scientists during the last part of the nineteenth century described the physics of air and water relationships in soils (Puchner, 1896) and the relationships between rainfall and soil –plant systems (Wollny, 1890). Physical relationships describing the cohesive properties of water had been published much earlier (Young, 1805). 4.2. Decades of awareness (from 1900 to 1919) Interest in organic matter, particularly humic substances, continued into the earlier part of the 20th

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century. Starting in 1908, Schreiner and Shorey (1910) initiated a series of studies to identify organic chemicals contained in a California soil. During their investigations of humic substances, they reported studying a soil that “could not be wetted, either by man, by rain, irrigation or movement of water from the subsoil” (Schreiner and Shorey, 1910, p. 9). During the earlier part of the 20th century the interest in “fairy ring” phenomena continued. Bayliss (1911) reported on the fairy ring phenomena and cited measurements reported earlier by Molliard (1910) that showed that the soil proliferated with fungal mycelium was comparatively dry. The area occupied by the mycelium contained only 5 –7% moisture, compared to 21% in the areas inside and outside the ring, which were not occupied by mycelium. Bayliss (1911) validated that soils containing mycelium were difficult to wet and cited an example where rain did not penetrate the soil in mycelia-infested areas but penetrated to a depth of 10 cm in the adjacent nonmycelial areas. Measurements of soil water content, associated with rings formed by a fleshy fungus (Agaricus tabularis ) that had infected grasslands in eastern Colorado (Schantz and Piemeisel, 1917), indicated that during the spring there were no differences in moisture content between the mycelial and non-mycelial zones. After the soil had dried out in later summer, however, the mycelia-infested soil rings did not permit penetration of water. As a result, large differences in soil water contents were found between the bare areas and the inner and outer vegetated rings, particularly in the upper foot and early in the growing season. During these first two decades of the 20th century, soil physics and water use by plants began emerging as important sciences. A review of the few studies on soil water movement was published (Buckingham, 1907), as was a report on the importance of transpiration in crop production (Kiesselbach, 1916). 4.3. Decades of contemplation (from 1920 to 1939) The period between 1920 and 1939 witnessed the development of scientific knowledge in peripheral disciplines that later provided the basis for better describing soil water movement and the physical – chemical nature of wetting. Soil physicists (Zunker,

1930; Richards, 1931) began quantifying the concept of water movement and the importance of capillary forces on water in the soils. Interest was also developing in the methods of quantifying aggregate stability for erosion control studies (Middleton, 1930). One of the earlier methods of quantifying erosion potential was based on the stability of soil aggregates to slaking when exposed to excess water (Yoder, 1936). The interest in the stability of aggregates to wetting continues today, although more sophisticated procedures are available for assessing this characteristic. More detailed studies on the stability of soil aggregates to wetting were also reported during these two decades, particularly as related to organic matter and microbial processes (Kanivetz and Korneva, 1937; Waksman, 1938). During these early decades of the 20th century, the physical –chemical nature and the wetting of low surface tension solids (e.g. talcs, waxes and resins) were being investigated from an industrial engineering perspective (Bartell and Zuidema, 1936; Wenzel, 1936). Only two publications between 1920 and 1939 were found that discussed water repellency. These were: a report of resistance to wetting in sands (Albert and Ko¨hn, 1926), and a second report describing the creation of “ironclad” or artificial catchments (Kenyon, 1929). 4.4. Decades of recognition ( from 1940 to 1959) Between 1940 and 1959, published papers reporting observations on water-repellent soils began appearing in several scientific journals. Studies by Jamison (1946) showed that resistance to wetting was affecting the productivity of citrus orchards in Florida, USA. Elsewhere in the world, Van’t Woudt (1959) reported that organic particle coatings were affecting the wettability of soils in New Zealand. The results of an investigation on difficult-to-wet soils was also reported in the Netherlands (Domingo, 1950). Finally, in 1959, detailed microscopic examinations of the aggregating effect of microbiological filaments on the aggregation of sand grains were reported in Australia (Bond, 1959). Although water repellency was not specifically mentioned as being a factor in aggregate stability, this initial publication was the beginning of a


series of fruitful research reports about water repellency that was published later by a group of Australian scientists (Bond, Emerson and others) during the 1960s and 1970s. During these two decades, interest in soil aggregation increased (Robinson and Page, 1950; Martin et al., 1955). This interest included increasing the stability of clay soils (Childs, 1942) using synthetic polyelectrolytes to improve aggregation (Hedrick and Mowry, 1952), and improving aggregation to decrease wind erosion (Chepil, 1958). The role of microorganisms in enhancing aggregation was gaining interest (Martin and Waksman, 1940; Swaby, 1949). Molds and algae were found to be particularly effective agents for soil crusting (Fletcher and Martin, 1948) and aggregation (Gilmour et al., 1948). An interest in characterizing contact angles also began emerging (Bikerman, 1941) along with a continuing interest in the physical –chemical process of wetting from an chemical engineering perspective (Barr et al., 1948). A book on surface-active agents and detergents was published near the end of these two decades (Schwartz et al., 1958). Both the theoretical and applied dimensions of water movement in soils were gaining closer attention. Theoretical concepts being developed in soils included: describing capillarity (Miller and Miller, 1956); recognition of contact angles’ role during infiltration (Fletcher, 1949); soil water energetics during infiltration (Bodman and Colman, 1943); and the numerical solution of concentration dependent diffusion equations (Philip, 1957). During this same period, viscous flow was described in porous media (Chouke et al., 1959) and in the Hele –Shaw cell (Saffman and Taylor, 1958). These theoretical developments served as the basis for a more comprehensive approach to describing water movement in hard-towet soil systems during the following decades. Applied research was done on the effect of plants on interception, stemflow, and ground rainfall (Specht, 1957) and the effect of profile characteristics (Hursh and Hoover, 1941) and roots (Gaiser, 1952) on hydrologic processes in forest soils. Infiltration into soils found in wildland environments was also attracting attention, particularly in soils that had been exposed to wildfires (Scott and Burgy, 1956). Water repellency was also starting to be utilized for beneficial uses, including its use for water harvesting

7

where paved drainage basins provided a source of water for livestock or game (Humphrey and Shaw, 1957), its application as moisture, thermal and electric insulator during highway construction (Kolyasev and Holodov, 1958), and its potential for decreasing soil water evaporation (Lemon, 1956). 4.5. Decade of renewed interest (from 1960 to 1969) The decade of the 1960s witnessed a flurry of interest in soil water-repellency and in related fields. As a result, several milestone publications appeared during this decade, in addition to a substantial increase in the knowledge about water repellency in soils and related fields. 4.5.1. Significant milestones The first milestone was represented by a surge in the number of scientific papers, primarily by scientists in Australia and the United States, on a wide range of topics concerning water repellency. Between 1960 and 1970, over 90 publications dealing with various aspects of water repellency were published (Fig. 1), with about one-third of these publications appearing in the proceedings of the first international conference at Riverside, CA in 1968 (DeBano and Letey, 1969). An addition of 31 publications reporting scientific findings related to water repellency were also published during this decade. A second significant milestone during this decade was the development of physical methods for characterizing soil water-repellency using contact angle methodology. In 1962, Letey and coworkers at the University of California, Los Angeles published two significant papers, one describing the measurement of liquid – solid contact angles in soil and sand (Letey et al., 1962a), and a second describing the influence of water –solid contact angles on water movement in soil (Letey et al., 1962b). These publications were closely followed in 1963 by a publication by Emerson and Bond (1963), working in Australia, who described a technique of using the rate of water entry into dry sand to calculate the advancing contact angle. A third milestone was the summary and synthesis of all available knowledge of water repellency conducted during the 1960s along with earlier findings. This formed the basis for discussion among interested

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scientists at the first international conference on water repellency held in May 1968 at the University of California, Riverside, USA (DeBano and Letey, 1969). Thirty-one presentations covering a wide range of topics concerning water repellency were discussed at this conference. Specific presentations included: physics of water movement through soil, distribution of water repellency in different ecosystems, theoretical and practical implications of surface-active agents (particularly wetting agents), factors responsible for water repellency (microorganisms and wildland fires), water harvesting, methods of measuring water repellency, and soil erosion processes. 4.5.2. Other advances in water repellency The accumulation of knowledge about water repellency and its treatment had accumulated to such an extent during the 1960s that synthesis papers were beginning to be published. Important summary papers included a state-of-the art publication on soil wettability and wetting agents (DeBano et al., 1967), and a separate review describing the chemistry of surface-active agents (Black, 1969). In addition to the synthesis papers, significant papers appeared describing: use of wetting agents to ameliorate water repellency; identification of fire-induced water repellency as a contributor to postfire erosion; interrelationships among organic matter, soil microorganisms and water repellency; and better definition of the role of liquid – solid contact angles in water movement. The use of wetting agents to increase infiltration and enhance water movement in water-repellent soils attracted considerable attention during the 1960s (Letey et al., 1961; Watson et al., 1969). Particularly noteworthy was a group of scientists and cooperators at the University of Riverside who studied the usefulness of wetting agents for irrigating waterrepellent soils (Letey et al., 1962c), established guidelines and techniques for using nonionic wetting agents (Letey et al., 1963), and evaluated the longevity of wetting agents (Osborn et al., 1969a). This interest in wetting agents expanded to their use for reducing postfire erosion (Osborn et al., 1964b) and enhancing turfgrass growth (Morgan et al., 1967). Also, several studies were published about the effects of surfactants on plant growth (Parr and Norman, 1965) and seed germination (Osborn et al., 1967).

The increased use of surfactants also prompted an evaluation of their effect on soil aggregation (Mustafa and Letey, 1969). Large increases in water erosion following wildfires had been a long-standing concern in southern California, USA, particularly in the Los Angles Basin. Research showed that water repellency on these erosive watersheds was intensified by the soil heating occurring during a fire (DeBano and Krammes, 1966). The decrease in infiltration due to water repellency had been overlooked previously by these watershed investigators (Krammes and DeBano, 1965), because it was assumed that the decreased infiltration after fire resulted primarily from the loss of a protective plant cover and the plugging of soil pores with ashy residue remaining on the soil surface. Awareness of fireinduced water repellency in other wildland environments in the United States was quickly reported by other investigators: in forested environments of the Sierra Nevada of Nevada and California (Hussain et al., 1969) and in many vegetation types throughout the western United States (DeBano, 1969a). The relationships among soil fungi, soil heating, and water repellency were also demonstrated (Savage et al., 1969b). A keen interest in the relationships among organic matter, soil microorganisms, and water repellency also developed during the 1960s. In Australia, research inquiries into the effect of microbial filaments on soil properties were gaining momentum (Bond, 1962). Field studies on water repellent sandy soils (Bond, 1964) revealed that filamentous algae and fungi were responsible for the water repellency (Bond and Harris, 1964). In the United States, the production of water repellency by fungi was also confirmed (Savage et al., 1969b), and the roles of humic acids and polysaccharides were evaluated (Savage et al., 1969a). The use of hydrophobic materials for harvesting rainfall (water harvesting) attracted substantial interest, particularly in arid regions of the western United States. Water harvesting interests were summarized in reviews, including a description of waterproofing soil to collect precipitation (Myers and Frasier, 1969) and a comprehensive book on waterproofing and water repellency (Moilliet, 1963). A better understanding was also evolving of the chemistry of


9

a variety of synthetic substances that could make soils hydrophobic and of their effect on soil physical properties (Bozer et al., 1969). Characterizing water repellency and the effect of hydrophobic substances on water movement was the focus of several studies. In addition to the pioneering publications on contact angle methodology described above, fundamental relationships between contact angles of water and saturated hydrocarbons and exchangeable cations were reported (Cervenka et al., 1968). Additional techniques for characterizing water repellency in soils were also reported, including measurements of liquid – solid contact angles (Emerson and Bond, 1963; Yuan and Hammond, 1968). Studies on soil water movement in hydrophobic soils consisted of determining the influence of wetting on the liquid water movement in sand (Vladychenskiy and Rybina, 1965), the role of capillary movement in soils (Wladitchensky, 1966; Rybina, 1967), and the processes of water movement through water-repellent soils (DeBano, 1969b), including layered systems (Mansell, 1969).

Fundamental information on water movement in layered systems was being acquired (Miller and Gardner, 1962). There was also continued interest in the more theoretical aspects of growth by fingers in Hele –Shaw cells (Wooding, 1969). Other important theoretical topics studied during this decade included: development of stability theory for miscible liquid – liquid displacements (Elrick and French, 1966); a better understanding of capillary flow (Waldron et al., 1961); contact angle hysteresis (Johnson and Dettre, 1964) and equilibria (Zisman, 1964); and the linkage between infiltration in sand and ground water recharge (Smith, 1967). The use of surface-active materials (nonionic surfactants, fatty alcohols, hexadeconal) was found to suppress evaporation (Law, 1964) and to alter soil water diffusivity (Gardner, 1969). Two comprehensive reviews were published during this decade, one on soil water theory (Childs, 1967) and a second on contact angle wettability and adhesion (Gould, 1964).

4.5.3. Related scientific inquiry Numerous publications also appeared during this decade that contributed fundamental knowledge in fields related to water repellency. A comprehensive synthesis of information on the dynamics of aggregation was published (Harris et al., 1966) in addition to a classification of aggregates based on their coherence in water (Emerson, 1967). The role of microorganisms and their byproducts on soil structure stabilization was the focus of some papers (Bond and Harris, 1964; Harris et al., 1964). Polysaccharides were found to play an important role in stabilizing natural aggregates (Acton et al., 1963). Synthetic substances such as 4-tert-butylpyrocatechol were also evaluated for their effectiveness in enhancing soil structural stability (Hemwall and Bozer, 1964). Synthetic soil conditioners were also used to enhance infiltration and reduce erosion (Kijne, 1967). Basic information that later contributed to understanding water movement in soils began emerging during the 1960s, and provided the basis for describing better the water movement in water repellent systems during the following decades. The theoretical basis for describing infiltration that was developed during the 1950s was summarized (Philip, 1969).

During the 1970s, over 130 papers were published on various aspects of water repellency and an additional 55 publications on closely related subject matter (Fig. 1). Many of the publications of this decade clearly reflected the spinoffs arising from the information reported at the 1968 conference.

4.6. Decade of spinoffs (from 1970 to 1979)

4.6.1. Understanding water repellency The interest in water repellency and its management implications began to attract worldwide attention. In the United States water-repellent soils were reported in: desert shrub communities in the American Southwest (Adams et al., 1970); granitic forest soils in the Sierra Mountains of the western United States (Meeuwig, 1971a); pinyon – juniper woodlands (Scholl, 1971), chaparral (Scholl, 1975), and ponderosa pine forests (Zwolinski, 1971; Campbell et al., 1977) in Arizona; mixed conifer forests in California (Agee, 1979); sagebrush-grass communities in the western United States (Salih et al., 1973); the high Cascade Mountains in the northwestern United States (Dyrness, 1976); coal mine spoils of New Mexico (Miyamoto et al., 1977); and forest soils in upper Michigan (Reeder and Jurgensen, 1979) and in Wisconsin (Richardson and Hole, 1978). Elsewhere

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in the world, water repellency was reported in: Australia (Roberts and Carbon, 1971), Egypt (Bishay and Bakhati, 1976), India (Das and Das, 1972), Japan (Nakaya et al., 1977a), Nepal (Chakrabarti, 1971), Mali (Rietveld, 1978), and New Zealand (John, 1978). The management concerns focused primarily on the effect water repellency had on plant growth, including: the “fairy ring” phenomenon (Stone and Thorp, 1971), impacts on the production of barley (Bond, 1972a), and non-wettable spots on golf greens (Miller and Wilkinson, 1977). Other interests in water repellency during the 1970s were similar to those during the 1960s and additional research continued to be conducted on: using wetting agents for remedial treatments, fireinduced water repellency, water harvesting, characterizing water repellency, and soil water movement. 4.6.2. Remedial treatments The use of wetting agents and other remedial techniques continued to capture the interest of scientists studying techniques for ameliorating water repellency during this decade. The addition of cores containing a loam soil to water repellent sands was found to increase the overall infiltration rates into sandy soils in Australia (Bond, 1978). Chemical remedial treatments, however, continued to receive most of the attention in treating water-repellent soils. The understanding of wetting agents and/or surfactants challenged both scientists and managers. Noteworthy publications describing the basic functioning of wetting agents in soils included the following topics: factors responsible for increasing the effectiveness of wetting agents (Mustafa and Letey, 1970), quantifying their effect on penetrability and diffusivity relationships in soils (Mustafa and Letey, 1971), evaluating their movement and leaching through wettable and water-repellent soils (Miller et al., 1975), and assessing their effect on pesticide mobility in the soil (Huggenberger et al., 1973). A major concern was the effect of surfactants on plant physiology which led to studies that addressed their effects on: seed germination (Burridge and Jorgensen, 1971); plant cells (Haapala, 1970); growth, porosity, and uptake by barley roots (Valoras et al., 1974a); and the germination and shoot growth of grasses (Miyamoto and Bird, 1978).

Concurrent with the basic studies on wetting agents described above were several reports which emphasized their overall application (Letey, 1975) and their specialized uses in forestry (DeBano and Rice, 1973), including erosion reduction (Valoras et al., 1974b). The success of using wetting agents to remedy water repellency encountered in field situations was variable. In one study, the use of wetting agents improved infiltration into water-repellent coal mine spoils to a limited degree (Miyamoto, 1978). An effort to use operational-level wetting agent treatments to reduce soil erosion on burned watersheds was not successful, however (Rice and Osborn, 1970). Two important synthesis publications on surfactants were also published during this decade: a stateof-the-art review of soil water-repellency and the use of nonionic wetting agents (Letey et al., 1975) and a book describing the fundamental relationships of surfactants to interfacial phenomena (Rosen, 1978). 4.6.3. Fire-induced water repellency A better understanding of fire-induced water repellency (Savage, 1974; DeBano et al., 1976) and its importance in postfire erosion on watersheds (Megahan and Molitor, 1975) were subjects of active research during the 1970s. Fire-induced water repellency was also reported in different situations, including: under campfires (Fenn et al., 1976); under piles of burned logs (DeByle, 1973), and in the upper soil layers during prescribed fires in mixed conifer forests (Agee, 1979). The author and others present a more detailed discussion of fire-induced water repellency and its erosional consequences elsewhere in this issue. 4.6.4. Water harvesting The use of water repellency principles provided the basis for the rapid expansion of water harvesting technology during the 1970s. Over a dozen published papers dealt with different aspects of water harvesting, including: developing the technology of bonding water repellent films to soil particles (Frasier and Meyers, 1972), assessing the resistance of organo-film-coated soils to infiltration (Fink, 1970), utilizing wax-treated soils for water harvesting (Fink, 1977), developing laboratory evaluation techniques (Fink, 1976), assessing freeze-thaw effects on soils treated for water repellency (Fink and Mitchell, 1975), establishing


water harvesting efficiencies for different surface treatments (Rauzi et al., 1973), utilizing water harvesting as a reforestation tool (Mehdizadeh et al., 1978). Two major efforts to synthesize information on water harvesting occurred during the 1970s. First, a state-of-the-art synthesis on water harvesting was published (Cooley et al., 1975). The second effort was the convening of an international water harvesting conference held in Phoenix, Arizona in 1974 (Frasier, 1975). This conference produced numerous papers on all aspects of water harvesting. 4.6.5. Characterizing water repellency Major advances in characterizing both the physical and chemical nature of water repellency occurred during the 1970s. Investigation of physical effects was concerned with assessing those factors affecting wetting phenomena, while the studies involving chemical characterization of water repellency focused on humic acids and their interactions with various substances, including soils. Detailed studies were reported on proposed techniques for physically characterizing water repellency in terms of: wetting coefficients (Bahrani et al., 1970), surface roughness (Bond and Hammond, 1970), and liquid-surface tension and liquid – solid contact angles during liquid entry into porous media (Watson et al., 1971). Indices for characterizing water repellency were developed using contact angle – surface tension relationships (Watson and Letey, 1970) and solid – air surface tensions of porous media (Miyamoto and Letey, 1971). Concerns about the limitations of scaling when using contact angles also emerged (Philip, 1971; Parlange, 1974). A better understanding of the chemistry of hydrophobic substances responsible for water repellency and their interactions with other substances was gained during this decade. A comprehensive book on the chemistry of natural waxes appeared (Kolattukady, 1976). Detailed studies reported on the adsorption of water by soil humic substances (Chen and Schnitzer, 1976) and on the surface tensions of aqueous solutions of soil humic substances (Chen and Schnitzer, 1978). A method was developed to quantify hydrophobic sites on soil minerals using aliphatic alcohols (Tschapek and Wasowki, 1976b). The wettability of different natural substances was being

11

characterized, including that of humic acid and its salts (Tschapek et al., 1973) and of zeolites (Chen, 1976). 4.6.6. Water movement During this decade more was being learned about the effect of hydrophobic substances on soil water movement during infiltration and evaporation (DeBano, 1975). The beneficial use of water repellency to save water (Hillel and Berliner, 1974), particularly by reducing the capillary rise of water to the soil surface where it was evaporated (Hergenhan, 1972), and to reduce fertilizer leaching (Snyder and Ozaki, 1974) also gained attention during the 1970s. 4.6.7. Related research Interest in the interrelationships among aggregation, soil structure, and water repellency was galvanized during the 1970s, when a conference was held in Las Vegas, Nevada, USA, under the sponsorship of the Soil Science Society of America, dealing specifically with “Experimental Methods and Uses of Soil Conditioners” (Stewart et al., 1975). This conference considered soil stabilization to control wind and water erosion, structural improvement of sodic and clay soils, water harvesting, soil conditioning with bentonite, water repellency (water movement, fire-induced water repellency), the role of organic matter and other natural mulches (e.g. bark) to improve soil structure, and the use of synthetic material such as bitumen emulsion and polyacrylamide for soil stabilization. This conference did much to link strongly an independent line of investigations on soil structure and conditioners to those being conducted on water repellency phenomena. Other important publications on aggregation also were published during the 1970s. Fundamental work on cementing substances of iron and aluminum on soil aggregates was being published in Italy by Giovannini and Sequi (1976b) and interest continued about the role of microorganisms in stabilizing aggregates (Aspiras et al., 1971b). A book on modification of soil structure, including chapters on aggregate formation and stabilization, was also published (Emerson et al., 1978). Important theoretical efforts by soil physicists began identifying more realistic models for describing soil water movement in water repellent systems.

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Foundations were established for describing the hydrodynamic instability of miscible fluids in porous media (Bachmat and Elrick, 1970), solving flow equations for unsaturated soils (Parlange, 1975), and determining the extent of equilibrium vapor adsorption by water-repellent soils (Miyamoto et al., 1972). The complications arising from wetting front instability and unstable flow in layered soils were beginning to be more fully recognized (Hillel, 1972; Hill and Parlange, 1972) and a theoretical framework for analyzing wetting front instability was proposed (Parlange and Hill, 1976). Experimental studies were also conducted on the effect of sudden changes in pressure gradients on wetting front instability in heterogeneous media (White et al., 1977); spatial variability in field-measured water properties (Nielsen et al., 1973); and unstable flow during infiltration (Raats, 1973; Philip, 1975b). Preferential flow through large pores was also receiving attention (Ehlers, 1975; Scotter, 1978). A conceptual model for hysteresis was described (Mualem, 1974). 4.7. Decade of enrichment (from 1980 to 1989) The decade starting in 1980 was characterized not only by continuing strong interest in water repellency per se, but also saw the development of knowledge in related areas which would be used during the 1990s as the basis for describing water movement in hard-to-wet soils. Over 150 publications on water repellency appeared during this decade; in addition, more than 55 papers describing related theory were published (Fig. 1). The decade began with a state-of-the-art publication, which synthesized much of the published information on water-repellent soils up through the mid-1970s (DeBano, 1981). The management concerns enlarged from a limited focus on the effects of water-repellent soils on plant productivity during the 1960s and 1970s, to more complicated and broader environmental issues in the 1980s. These emerging management concerns prompted the initiation of a wide spectrum of new research. 4.7.1. Understanding water repellency Water repellency continued to be reported as a problem when managing sandy and heavy textured

soils in: Australia (McGhie and Posner, 1980; Ma’shum and Farmer, 1985), Japan (Nakaya, 1982), Poland (Prusinkiewicz and Kosakowski, 1986), and the United States (Hubbell, 1988). Numerous publications continued to address the concerns with the wettability of golf greens as related to thatch and dry patch in: Australia (Charters, 1980), Great Britain (Shiels, 1982), New Zealand (Wallis et al., 1989), and the United States (Taylor and Blake, 1982). The effect of plant shoot material on the development of water repellency was also examined (McGhie and Posner, 1981). The wettability of humus and peat materials was found to be an important factor in managing forested ecosystems. Humus wettability had to be considered when managing forests in Poland (Grelewicz and Plichta, 1985b). Interest in the management of peat bogs was highlighted by a book on peat and water (Fuchsman, 1986). Of special interest were studies on the effect of mineralization on the hydrophilic properties of peat soils when managing peat bogs (Lishtvan and Zuyev, 1983). Several areas of water repellency studied during the 1960s and 1970s continued to attract interest during the 1980s. These include: remedial treatments, fire-induced water repellency, characterizing water repellency, soil structure and aggregation, and the effect of water repellency on soil water movement. 4.7.2. Remedial treatments Remedial treatments captured the interest of several authors. Application of surfactants was by far the most popular treatment (Rieke, 1981; Sawada et al., 1989). The use of dispersible clays, however, was emerging as a promising technique for reducing water repellency in sandy soils in Australia (Ma’shum et al., 1989). Physical methods of ameliorating water repellency other than claying were to emerge later during the 1990s when they would become efficient and widespread methods for treating water repellency, particularly in Australia and New Zealand. 4.7.3. Fire-induced water repellency The interest in the effect of fire-induced water repellency continued and in the 1980s it was reported in: California (Wells, 1987), Canada (Henderson and Golding, 1983), southern Chile (Ellies, 1983); Calluna heathlands in England (Mallik and Rahman, 1985),


Italy (Giovannini and Lucchesi, 1983), Oregon (McNabb et al., 1989), the Pacific Northwest of the USA (Poff, 1989), South Africa (Scott, 1988), Spain (Almendros et al., 1988), and Turkey (Sengonul, 1984). The effect of water-repellent soils on erosion from burned watersheds continued to capture the interest of several investigators. A conceptual model relating hillside rill erosion to the formation of a waterrepellent soil layer during fire was developed for chaparral areas in southern California (Wells, 1987). The Universal Soil Loss Equation (USLE) was evaluated on burned forest areas in northwestern Spain (Diaz-Fierros et al., 1987). 4.7.4. Characterizing water repellency Assessing and quantifying water repellency also remained a continuing interest. Publications during this decade included those on the methods for measuring severity of water repellency (King, 1981), field techniques for quantifying water repellency using soil survey information (Richardson, 1984), the use of effective contact angle and water drop penetration time to classify water repellency (Wessell, 1988), and measurement of water repellency using intrinsic sorptivity measurements (Tillman et al., 1989). The effect of humidity on water repellency was also assessed (Jex et al., 1985). 4.7.5. Soil aggregation and structure Soil aggregation and its stability remained a popular subject of many studies. An energy-based index was developed for assessing the stability of soil structure (Skidmore and Powers, 1982). The importance and overall role of organic matter in the structural stability of soil aggregates was discussed by some authors (Chaney and Swift, 1984; Oades, 1984). Still other authors focused specifically on the effect of humic substances on the stability of soil aggregates (Chaney and Swift, 1986; Piccolo and Mbagwu, 1989). Significant research results continued to be published on water repellency and aggregation stability (Giovannini and Lucchesi, 1983) and on the identification of substances responsible for hydrophobicity in soils (Giovannini and Lucchesi, 1984; Ma’shum et al., 1988). Lastly, a comprehensive review was

13

published in a book describing the interactions between microorganisms and soil minerals (Huang and Schintzer, 1986). 4.7.6. Water movement in soils During the decade starting in 1980, substantial progress was made in establishing the theoretical basis for describing water flow through water repellent systems. This theoretical framework provided the basis for making substantial progress in describing water movement in hydrophobic soils later during the 1990s. During the 1980s, the theoretical developments in the field of fluid dynamics which described fingering phenomena in the Hele – Shaw cells were published (Saffman, 1986). These theoretical developments provided the basis for extending the concept of fingering to describe water movement in soils. Hillel and Baker (1988) presented a descriptive theory of fingering during infiltration in layered soils, which was expanded to describe fingering phenomena in two-dimensional, homogeneous, unsaturated porous media (Tamai et al., 1987). Preferential flow patterns were being reported regularly in agricultural soils (Van Ommen et al., 1988). Concurrent with the interest in fingering phenomena described above were reports on the stability of water movement through unsaturated porous media (Diment and Watson, 1985), particularly in the vadose zone (Glass et al., 1988), which led to a suggested mechanism for finger persistence in homogeneous, unsaturated porous media (Glass et al., 1989a). Fundamental studies were also conducted to assess penetration coefficients in porous media (Malik et al., 1981). Characterization of sorptivity and soil water diffusivity was being expanded to describe these processes under field conditions (Clothier and White, 1981). Subsurface flow processes were also being evaluated above fragipan horizons (Parlange et al., 1989) and in forest soils (Mosely, 1982). A successful climax to the research efforts on soil movement in unsaturated soil was the convening of an international conference in New Mexico that was devoted entirely to examining flow and transport models used to characterize water flow in unsaturated zones (Wieringa and Bachelet, 1998). One of the papers at this conference extended recent soil water theory to describe water and solute movement

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L.F. DeBano

through water repellent sands (Hendrickx et al., 1988b). 4.7.7. Miscellaneous research In addition to the areas of interest discussed above, some miscellaneous information on water repellency was also published during the 1980s. One such study described relationship of vegetation age to soil water-repellency in California chaparral ecosystems (Teramura, 1980). Water repellency was also reported to be an important factor in the reclamation of soils containing degraded lignite (Richardson and Wollenhaupt, 1983). Water repellency was reported in coastal sand dunes of the Netherlands where its presence affected the erosion processes on sand dune landscapes (Jungerius and Van Der Meulen, 1988). Finally, the role of hydrophobic substances was evaluated in the adaptation of leaves to periodic submersion by tidal water in a mangrove ecosystem (Misra et al., 1984). 4.7.8. Related research The increased environmental awareness during the 1980s began to capture the interest of several soil scientists. Of particular concern was the rapid movement of contaminants into the groundwater via preferential flow patterns. Although most of the early efforts were concentrated on developing new theoretical approaches to infiltration and water movement through soils, this effort evolved into the theoretical and operational frameworks necessary for quantifying the effect of water repellency on groundwater contamination later during the 1990’s. This approach, therefore, provided an initial stimulus for attacking the complex process of ultimately merging surface hydrologic theory with models describing integrated soil – water systems. 4.7.8.1. Surface hydrology. Surface hydrology and watershed performance began emerging as major interests during the 1980s (Beven, 1989); some of this work had a direct bearing on the effect of water repellency on catchment responses. Theoretical work on surface hydrology focused on lateral flow through layered soils on sloping topography was beginning to appear in the literature (Zaslavsky and Sinai, 1981; Selim, 1987; Miyazaki, 1988). Detailed evaluations were also made of unsaturated and saturated flow

through a thin porous layer on hillslopes (Hurley and Pantelis, 1985). Concurrent with this interest in surface hydrology was the interest in extending the principles of water repellency to erosion and hydrologic performance on a watershed level (Topalidis, 1984; Burch et al., 1989). 4.7.8.2. Integrated soil water systems. Concerns intensified during the 1980s about the rapid transport of pollutants from the soil surface through the soil into the underlying ground water. Stemflow was quickly recognized as an important mechanism capable of delivering rainfall rapidly to the soil surface (Van Elewijck, 1989). Once the water had been delivered to the soil surface, macropores and other discontinuities in the soil provided pathways that quickly moved water, containing dissolved and suspended matter along with adsorbed contaminants, through the soil into the underlying water table (Beven and Germann, 1982; Bouma, 1982). Specific examples of such processes included nitrogen leaching during sprinkler irrigation (Dekker and Bouma, 1984); chloride movement through a layered field soil (Starr et al., 1986); and the movement of water and solute pollutants through unsaturated zones (Raats, 1984). The rapidly growing interest in the storage and movement of materials in soils, particularly pollutants, resulted in a special publication describing the reaction and movement of organic chemicals in soils (Sawhney and Brown, 1989). Fundamental studies were also conducted on developing models to describe adsorption and transport of hydrophobic organic chemicals in aqueous and mixed solvent systems (Rao et al., 1985) and in natural sediments and soils (Karickhoff, 1981). 4.8. Decade of maturity (from 1990 to 1998) Between 1990 and 1998, a record breaking 150, or more, papers on water repellency and over 60 additional papers on related subjects were published (Fig. 1). Substantial progress was made in all aspects of water repellency, although the increased understanding of water movement through these hard-to-wet systems was particularly noteworthy. The publications concerned with water movement reflected a close cooperative effort between scientists


working on the cutting edge of soil physics and scientists concerned with water repellency phenomena. 4.8.1. Occurrence and amelioration of water repellency The 1990s also witnessed a more profound recognition of the implications of water-repellent soils on the productivity of both cultivated and natural ecosystems, as well as recognition of their role in emerging environmental issues. This increased awareness led to a concerted effort to manage and/or ameliorate the adverse effects of water repellency. 4.8.1.1. Occurrence. During the 1990s, water repellency continued to be reported to occur in a wide range of natural and agricultural environments and as a result was fast becoming an important component in the management and productivity of soils worldwide during the 1990s. The most frequent reports described problems on agricultural lands. The effect of water repellent sands on crop and pasture production was particularly acute on thousands of hectares in Australia and New Zealand, where the importance of this problem stimulated intensive research on the problem and intensified a search for economic methods of ameliorating this soil condition. (Blackwell, 1993; Carter and Howes, 1994). As a result, studies on the physiochemical and biological mechanisms responsible for water repellency in sands were initiated (Franco et al., 1994), as were studies on the role of particulate organic matter (Franco et al., 1995). In the Netherlands, special concerns arose about ground water contamination with fertilizers and pesticides that had been transport rapidly downward through wettable fingers in an otherwise waterrepellent soil (Van Dam et al., 1990). Other, less widespread, interest on water repellency was also reported. Localized areas of highly water-repellent soils were created by oil spills, thus requiring intensive remedial efforts (Roy and McGill, 1998). Furthermore, water repellency was also becoming recognized as a key mechanism responsible for the self-cleaning features of plant surfaces (Neinhuis and Barthlott, 1997). The diminished aesthetics and playing qualities of golf greens that

15

were afflicted by the age-old problem of “dry patch” still concerned golf green supervisors (Tucker et al., 1990; York and Baldwin, 1992b; Hudson et al., 1994). Water repellency was also reported in wildland soils (i.e. uncultivated soils supporting natural stands of trees, shrubs, and grass), both in fire and non-fire environments. Water-repellent soils were reported in several wildland environments, including: dry sclerophyll eucalyptus in Australia (Crockford et al., 1991), eucalyptus and pine forests in Portugal (Doerr et al., 1996), eucalyptus forests in South Africa (Scott, 1991), and under windbreaks in Taiwan (Lin et al., 1996). The fire-induced water repellency described above continued to have its primary impact on wildland ecosystems. 4.8.1.2. Amelioration. Coping with water-repellent soils continued to present a challenge to managers of agricultural and pasture lands worldwide (Abadi, 1994; Capriel, 1997). Interest in the usefulness of wetting agents as a remedial treatment for water repellency continued (Effron et al., 1990), although it did not capture as much interest as during the 1960s and 1970s. Wetting agents were also used to enhance irrigation (Wallis et al., 1990b) and drainage (Zartman and Bartsch, 1990). Their effects were studied on aggregation and colloidal stability in tropical soils (Mbagwu et al., 1993). Remedial treatments other than wetting agents were beginning to be tested and used more extensively, particularly in Australia and New Zealand (Carter and Howes, 1994). These treatments included: direct drilling (Chan, 1992), wide furrow sowing (Blackwell et al., 1994f; Blackwell and Morrow, 1997), and the use of microorganisms (Roper, 1994) and fertilizers to stimulate microbial breakdown of water repellency (Michelson and Franco, 1994). Soil claying, a treatment involving mixing large amounts of clay in the upper water repellent layer, received widespread use in Australia (Blackwell et al., 1994d; Carter and Hetherington, 1994; Dellar et al., 1994). Intensive examinations were made of the effect of clay mineralogy and exchangeable cations on waterrepellent soils that had been amended with clay (Ward and Oades, 1993). High pH soil treatments offered some alleviation of the hydrophobic condition found on golf greens (Karnok et al., 1993).

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4.8.2. Water movement in soils Water movement into and through water-repellent soils was a major focus of research during the 1990s. This effort involved the creation of a forum for exchanging and assessing relevant information among scientists (e.g. workshops, conferences), utilizing cutting-edge soil water theory for describing water movement in water-repellent soils, and identifying the role of water repellency in present-day environmental issues. 4.8.3. Information exchange A significant landmark for this decade was the bringing together of the theoretical and practical dimensions of distribution flow and fingering phenomena, as studied during the early 1990s, the 1980s, and earlier, during a one-day workshop held at the Winand Staring Centre in Wageningen in April 1994 (Steenhuis et al., 1996). During this workshop, 14 papers were presented on various aspects of water repellency and supporting theoretical sciences. This collection of papers included several on characterizing field moisture patterns in water-repellent soils, on the three-dimensional portrayal of preferential flow patterns, and on the use of ground-penetrating radar for identifying structural layers that affect finger flow phenomena. Theoretical papers reviewed the instability of fingered flow and the heterogeneity of these fingers. Another paper described the field validation of laboratory-based equations for determining finger dimensions. The use of laboratory theory to predict the risk of ground water contamination was also presented. A final set of papers presented various models describing: instability-driven fingers, finger formation based on laboratory findings, and heterogeneity-driven fingers. These presentations were combined into one document and were published in 1996, vol. 70, no. 2– 4, of Geoderma. Numerous additional papers were published on the theoretical and applied aspects of water movement per se. The application of such theoretical analyses to describing water movement in hard-to-wet soil systems both in laboratory and field environments was particularly useful. Preferential flow was of such importance that a national symposium was devoted to this subject at Chicago, Illinois in 1991 (Gish and Shirmohammadi, 1991).

4.8.4. Theoretical development In addition to the papers presented at the Wageningen and Chicago conferences, numerous other papers appeared which described theoretical studies done on the transport of water and associated solutes through the soil. These reports described the instability of wetting fronts during infiltration into unsaturated porous media (Glass et al., 1990; Selker et al., 1992a), infiltration into layered soils (Baker and Hillel, 1990; Steenhuis et al., 1991), preferential and lateral flow (Kung, 1990a,b; Heijs et al., 1996), fingered flow (Selker et al., 1992b,c; Glass and Nicholl, 1996; Nieber, 1996), and the complications arising from hysteresis in soil – water systems (McCord et al., 1991). Other studies focused on the effect of different moisture contents on the formation and persistence of fingered flow in coarse-grained soils (Liu et al., 1994a) and outlined closed-form solutions for predicting finger width development in these soils (Liu et al., 1994b). A better understanding of the theoretical basis of distribution flow, unstable moisture movement, and fingering phenomena was quickly extended to describe water movement through water repellent systems. Detailed field measurements had established that uneven moisture patterns developed as a result of water repellent sites throughout the soil profile (Dekker and Ritsema, 1994b). The theoretical models describing fingering and instability of wetting fronts began to be applied intensively to describe the water movement in the water-repellent soils, particularly by scientists in the Netherlands or their cooperators (Hendrickx et al., 1993; Dekker and Ritsema, 1995, 1996b; Ritsema et al., 1998), the dynamics of finger (or preferential) flow in water repellent systems (Ritsema et al., 1993, 1996, 1997a,b; Ritsema and Dekker, 1994b; Dekker and Ritsema, 1996; Bauters et al., 1998), the contribution of finger flow to solute movement through the soil (Van Dam et al., 1990), and water movement through macropores in soils (Mallants et al., 1996). Distribution flow was demonstrated to be an important process in the top layer of water-repellent soils (Ritsema and Dekker, 1995) that directed surface-applied water to preferrential flow paths within the soil profile (Ritsema and Dekker, 1996b). Studies were designed to predict the occurrence and diameters of fingers occurring in field soils (Ritsema et al., 1996) and to examine the recurrence


of fingered flow pathways through water repellent field soils (Ritsema and Dekker, 1998). The effect of cover type on preferential flow and resulting moisture patterns in water-repellent soils was studied under field conditions (Dekker and Ritsema, 1995, 1996d, 1997). The information gained by the above studies served as the basis for modeling finger formation and recurrence (Ritsema et al., 1998a), and three-dimensional fingered flow patterns (Ritsema et al., 1997a; Ritsema and Dekker, 1998). The role of hysteresis when describing unsaturated flow in water-repellent soils was reported (Van Dam et al., 1996). A numerical model was developed for describing the movement of heat and water in water repellent sands that were furrowed for remedial purposes (Yang et al., 1996). 4.8.5. Environmental implications The awareness of environmental issues emerging during the previous two decades lead to several studies during the 1990s that examined the role of water repellency within the context of environmental management. The most apparent need was for developing models describing the transport of fieldapplied chemicals through the soil (Flu¨hler et al., 1996). Estimates of pollutant loading via fingered flow were reported (Selker et al., 1991). Information on water and chemical transport was used as the basis for developing a soil classification system that reflected these processes (Quisenberry et al., 1993). The role of solute leaching was beginning to be modeled for water-repellent soils (De Rooij and De Vries, 1996). The movement of contaminants via finger flow (Selker et al., 1991) was particularly important in water repellent sandy soils (Van Dam et al., 1990). Also, adsorption of hydrophobic substances on soil and sediments was receiving attention for at least two reasons. First, the adsorption of hydrophobic substances was necessary for inducing permanent water repellency for water harvesting purposes (Blackwell et al., 1994b), which had received considerable attention in the 1970s. Secondly, the adsorption and transport of hydrophobic contaminants by soils and sediments was emerging as a sensitive environmental concern (Ghosh and Keinath, 1994; Huang et al., 1998; Weber et al., 1998).

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4.8.6. Surface hydrology and watershed responses During the 1990s, initial efforts were made to extend the soil water theory described above to surface hydrology of hillslopes and even further to describe entire watershed responses. 4.8.6.1. Surface hydrology. Surface hydrology drew the interest of some investigators. Models were developed to describe infiltration, soil moisture, and unsaturated flow (Beven, 1991a) and were expanded into a conceptual approach for predicting runoff (Beven, 1991b). The concepts of hysteresis and state-dependent anisotrophy were being incorporated into the modeling of unsaturated hillslope hydrologic processes (McCord et al., 1991). Important hydrologic relationships in unsaturated air systems and their role in contaminant transport were also receiving attention (Scanlon et al., 1997). 4.8.6.2. Watershed responses. Hydrologic responses of watersheds, particularly after fire, gained special attention during the 1990s. Theoretical work on predicting the relationships among infiltration, unsaturated flow, and runoff was reported for unburned watersheds (Beven, 1991a,b). There was particular interest in quantifying the hydrologic processes involved in subsurface transport from an upper subcatchment during storm events (Wilson et al., 1991), including hillslope infiltration and lateral downslope unsaturated flow (Jackson, 1992). Recent advances in modeling of hydrologic systems also served as the theme of a book (Bowles and O’Connell, 1991). Watershed behavior and hydrologic responses to wildfire and changes in soil wettability received considerable attention by investigators in Africa (Scott, 1997), Portugal (Shakesby et al., 1993; Walsh et al., 1994), Spain (Imeson et al., 1992) and the United States (Robichaud, 1996). Water repellency was recognized as playing an increasingly important role in erosional processes. In non-fire environments, water repellency induced erosion in dunes along the coast of the Netherlands (Jungerius and Dekker, 1990; Witter et al., 1991). Raindrop splash was recognized as an important process during the erosion of both hydrophobic and wettable soils (Terry and Shakesby, 1993). Special attention was also directed toward modeling the spatial

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L.F. DeBano

variability in hillslope erosion following timber harvesting and prescribed burning (Robichaud, 1996). Comparisons of experimental results with those predicted by the Water Erosion Prediction Program (WEPP) models showed reasonable accuracy, although the WEPP model showed a consistent tendency to underestimate runoff and erosion (Soto and Diaz-Fierros, 1998). 4.8.7. Fire-induced water repellency There was a continuing interest in determining the heat-induced changes that occur in soils during fire in natural ecosystems (Soto et al., 1991; Giovannini, 1994; Sala and Rubio, 1994) and in the associated creation of water repellency (bibliography by Kalendovsky and Cannon, 1997). Changes in soil waterrepellency in response to soil heating during fire were reported for several ecosystems, including: pinyon– juniper woodlands in the United States (Everett et al., 1995), forests in Spain (Almendros et al., 1990), eucalyptus forests in Portugal (Walsh et al., 1994; Doerr et al., 1998), and chaparral in the United Sates (DeBano et al., 1998). Fire effects studies on closely related changes included studies on the effect of fire intensity on soil changes (Giovannini and Lucchesi, 1997), the overall changes in soil quality produced by fire (Giovannini, 1994), and fire-induced changes in aggregate stability (Molina et al., 1991). 4.8.8. Characterizing water repellency A wide range of approaches were used to characterize water repellency, including: evaluating traditional methodologies; utilizing new analytic techniques; and designing statistical sampling designs to better describe and assess overall water repellency under field-scale conditions. 4.8.8.1. Traditional and new methodologies. Specific techniques were evaluated, included using intrinsic sorptivity as an index for assessing water repellency (Wallis et al., 1991) and standardizing the “water drop penetration time” and the “molarity of an ethanol droplet” techniques to classify soil hydrophobicity (Doerr, 1998). A method of characterizing disaggregated nonwettable surface soils found at old crude oil spill sites was also reported (Roy and McGill, 1998). A detailed examination was made of the relationship

between water repellency, measured in the sieve fractions of sandy soils containing organic matter, and soil structure (Bisdom et al., 1993). A need to better designate the differences between “potential” and “actual” water repellency and to establish a “critical soil water content” when assessing water repellency was identified (Dekker and Ritsema, 1994b). The water repellency of an ovendried sample is designated as “potential”, in contrast to the water repellency of a field moist sample which is referred to as “actual”. The need to distinguish between the two arises because water repellency is a time-dependent soil property and wetting resistance decreases with time, particularly when the soil is exposed to high humidity or water. These changes make static measurements of water repellency inadequate (Dekker and Ritsema, 1994b). Several new analytical and visualization techniques were reported which could potentially improve the assessment of water repellency and its effect on soil water movement. One such technique is the use of computed tomographs as a tool for non-destructive analysis of flow patterns in macroporous clay (Heijs et al., 1995). Time Domain Reflectometry (TDR) employing standard three-rod probes was also used to measure volumetric water contents at different times and positions in the soil profile (Ritsema et al., 1997a). A visualization techniques, useful for more vividly portraying three-dimensional finger flow patterns, was generated using modular visualization software and associated computer hardware (Heijs et al., 1996; Ritsema et al., 1997a). Another promising technique was the use of NMR (nuclear magnetic resonance) to quantify and study organic matter in whole soils (Preston, 1996). Furthermore, a more detailed chemical analysis of organic matter using reflectance Fourier transform infrared spectroscopy (DRIFT) was tested as a potential tool for characterizing water repellency by providing aliphatic C –H-toorganic C ratios (Capriel et al., 1995). A higher ratio indicated greater water repellency. 4.8.8.2. Sampling and landscape characterization. The importance of sampling, characterizing, and portraying the spatial distribution of water repellency under field conditions received the attention of several investigators. Spatial variability of soil hydrophobicity was studied in fire-prone eucalyptus and pine


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forests in Portugal (Doerr et al., 1996, 1998). A detailed study was conducted on the influence of sampling strategy on detecting preferential flow paths in water-repellent sand (Ritsema and Dekker, 1996a). It was found that as the physical size of the soil sample increase, the detection of detailed preferential flow patterns diminished until they were eventually unobservable. Therefore, because preferential flow patterns vary in space and time, the optimal number of samples to detect these paths varied, indicating that sampling strategies needed to be flexible in design. There is an emerging interest in characterizing water repellency on a landscape basis. Such a relationship would involve first relating specific soil properties to water repellency (McKissock et al., 1997), and then using the spatial distribution of these diagnostic soil parameters to predict the occurrence of water repellency over large landscapes (Harper and Gilkes, 1994).

sowing techniques to improve infiltration, farmer experiences when managing water-repellent soils, amelioration of water repellency with soil additives (e.g. claying), economic impact of water repellency on farm management, and establishment of perennial pastures on water repellent sands (Carter and Howes, 1994). The Barcelona conference was concerned with fire effects, particularly on erosion and soil degradation. Many of the papers discussed fire-induced water repellency and erosion processes following the fire. The important topics that were discussed were the action of forest fires on: vegetative cover and soil erosion, soil quality, physical and chemical properties of the soil, changes in aggregate stability, and overall post-fire management and erosion response.

4.8.9. Summary publications The 1990s also were highlighted by several summary publications on water repellency, or closely related subjects. These included a state-of-the-art publication on water repellency (Wallis and Horne, 1992). The proceedings for two conferences in Australia also appeared, one held in 1990 in Adelaide (Oades and Blackwell, 1990) and the Second National Water Repellency Workshop held at Perth in 1994 (Carter and Howes, 1994). In addition, an international conference on soil erosion and degradation as a consequence of forest fires was held in Barcelona, Spain in 1991, with the proceedings published in 1994 (Sala and Rubio, 1994). The two regional conferences in Australia brought together an excellent review of the ongoing research and application of knowledge being conducted there. Topics discussed at the 1990 conference included a review of the historical problems created by water repellency; the physics of water-repellent soils; the chemistry of water repellency; the effect of waterrepellent soils on wind erosion of sand soils; and the use of a furrow sowing press wheel, wetting agents, additions of clays, and plants to ameliorate water repellency. The second conference in 1994 expanded on the earlier conference and the topics discussed were: causes and extent of water repellency, biological control of water repellency, the use of furrow

The body of knowledge concerning water repellency has evolved from a little known academic curiosity arising during vegetation studies in the early 1900s to a highly complex field of study during the 1990s. Early in the 1900s, scientists began reporting hard-to-wet soils, especially those associated with the “fairy ring” or “dry patch” phenomenon. Restricted water infiltration and redistribution in water-repellent soils and their effect on the productivity of horticultural crops, citrus trees, pasture production, and turf management were of concern to managers, starting in the 1940s and continuing to the present. Concerns over runoff and erosion from wildlands, particularly following fires, attracted concentrated interest in the 1960s and 1970s. This stimulated interest in studying the entire soil – water system and led to methods of characterizing water repellency in terms of water penetration, liquid-solid contact angles, and eventually the dynamics of both laboratory and field soil water systems. The concern with restricted water movement in soil led to a concurrent effort to develop mitigating techniques that would improve plant productivity and reduce runoff and erosion. The past 30 years have witnessed an everincreasing interest in water repellency. This evolving interest has shifted the centers of research and the areas of interest concerning water repellency in soil.

5. Summary

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During the 1940s and 1950s, the primary pragmatic interest was in citrus production in Florida, USA, although “bare patch” disease was reported in clover pastures in south Australia. In the 1960s, waterrepellent soils became a high profile concern both in southern California, USA and in Australia. In the United States, a substantial research effort on water repellency by faculty and graduate students at the University of California, Riverside (UCR) focused on developing contact angle theory to characterize water repellency and also on applying remedial wetting agent treatments to increase infiltration rates into water-repellent soils. Working closely with the UCR group was a small group of USDA Forest Service scientists working on fire-induced water repellency and post-fire erosion on the San Dimas Experiment Forest in southern California. Simultaneously, during the 1960s, a concerted effort was being conducted on the other side of the world by Australian scientists, who were concentrating on understanding microbial effects on soil physical properties, including water repellency in sandy soils and is effect on pasture productivity. The worldwide interest at this point in time was reviewed and synthesized at the first international conference on water repellency at Riverside, CA in June of 1968. The 1970s witnessed an awakening to the worldwide occurrence of water repellency and as such was titled the decade of “spinoffs” Water repellency was reported in a wide range of agricultural and natural environments. During this decade there was a concentrated interest in the use of wetting agents as a remedial treatment for water-repellent soils. The research and technology necessary to implement water harvesting techniques were well established during this decade and their application continues to occur worldwide up to the present. Another significant occurrences during the 1970s was the bridging of interests of scientists working on aggregation and on water-repellent soils during a special conference on soil conditioners. Significant advances in soil physics relationships describing the instability of flow in homogeneous and layered systems provided the background for implementing these concepts in the following decades. The study of hysteresis and preferential flow through macropores was in its

infancy and just beginning to attract the serious attention of scientists interested in water movement in soils. The decade of the 1980s experienced a combination of the old and the new. The age-old concerns with “dry patch” and fire-induced water repellency remained. However, some new aspects of water repellency began emerging, including an interest in the wetting of peat soils and organic forest soils. Most noteworthy were the advances that were being made in understanding surface hydrology and water movement through soils. A great deal of theoretical work on lateral flow in sloping and layered soils was beginning to make its appearance. These developments had important implications in describing catchment responses. New approaches for describing soil water movement in non-uniform systems were rapidly being developed. The first reports on the concept of water movement through porous media by fingering were beginning to appear. The concept of preferential flow was to become the “center” of interest in describing soil water movement in the following decade. Another important concept that emerged during the 1980s was the role of the integrated soil – water system in the rapid transport of pollutants from the surface into the underlying groundwater. These interests in integrated systems and preferential water flow were to blossom and provide the basis for describing water and pollutant movement through hydrophobic soil systems in the 1990s. Thus, it seems appropriate to designate this decade as one of “enrichment” in that it was setting the stage for immense progress in understanding water-repellent soil systems in the following decade. The 1990s started at full acceleration with a workshop, at Wageningen in the Netherlands, on the theoretical and practical dimensions of distribution flow and fingering phenomena. Many of the papers presented at this workshop represented a strong cooperative effort by scientists from the Netherlands and the United States. This workshop set the stage for a full frontal attack on the process of water movement into and through hydrophobic soils by lateral flow on the surface and finger flow through the soil profile. The practical dimension of this effort was the environmental concern with the rapid movement of chemicals and other pollutants through the soils


into the underlying groundwater aquifers. Meanwhile, on the other side of the world, in Australia and New Zealand, a strong interest in understanding and ameliorating water repellency was maturing. Two regional conferences in Australia, one in 1990 and a second in 1994, permitted significant discussions of new and innovative techniques for ameliorating water-repellent soils. The treatments to improve water-repellent soils that were discussed at these two conferences include: claying, wide furrowing, use of

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press wheels, direct seeding, promoting microbial decomposition of hydrophobic substances, and high pH soil treatments. The remainder of this decade promises to continue to be highly productive, as is indicated by the papers included in this issue. The information presented here is truly a rewarding and significant platform for future endeavors in understanding the role of water repellency in the management of agricultural and wildland environments.


ORIGIN OF SOIL WATER REPELLENCY


Chapter 3 Hydrophobic compounds in sands from New Zealand D.J. Hornea,* and J.C. McIntoshb b

a Institute of Natural Resources, Massey University, Private Bag 11 222, Palmerston North, New Zealand Institute of Food, Nutrition and Human Health, Massey University, Private Bag 11 222, Palmerston North, New Zealand

Abstract Organic compounds were extracted from samples of water repellent sandy soils from two development sequences found on the south-west coast of the North Island, New Zealand. A number of extraction procedures and solvent mixtures are effective at reducing the repellency of these soil samples as measured by the Molarity of Ethanol Droplet (MED) test. The extracted compounds were partitioned into lipid and water-soluble fractions. The lipid fraction includes neutral lipids (predominantly alkane hydrocarbons and triglycerides), acidic lipids (mainly long-chain fatty acids), and polar lipids. The water-soluble fraction exhibits amphipathic behaviour, and shares some similarities with the hymetamelonic fraction of the humic acid pool. When added to soil, this material binds very successfully and its effect on MED depends on the type of solvent in which it is added. There is no good correlation between repellency (MED value) and total carbon content, the quantity of water-soluble material, the quantities of total lipid extract or any of the lipid fractions. The repellency of soil samples can be modified in vitro by the addition of soil lipid extract (increases MED values), or by the addition of soil water-soluble extract (decreases MED values): this is true, regardless of whether the lipid and water-soluble extract have been sourced from repellent or wettable soil. When a sample of repellent Himatangi sand is serially extracted by fresh isopropanol:ammonia (7:3 v:v) mixtures interspersed with extraction by fresh solutions of ammonia, the MED value fluctuates markedly. The former solvent mixture removes lipid compounds and lowers the MED value; ammonia removes polar and amphipathic material and increases the MED value. The results of this study are best explained by the hypothesis that water repellency is determined by the composition or nature of the outermost layer of organic material, particularly amphipathic compounds, rather than the characteristics of the bulk of the organic matter. Four mechanisms, which are consistent with the results reported here, are proposed to explain the expression of repellency in both laboratory studies and in the field.

1. Introduction Many of the young soils of the ‘brown soils’ (Hewitt, 1992) development sequence of the southwest coast of the lower North Island of New Zealand are frequently water repellent (Wallis et al., 1993). The onset of repellency results in very low water * Corresponding author. Fax: þ 64-6-350-5632. E-mail address: [email protected] (D.J. Horne). q 2003 Elsevier Science B.V. All rights reserved.

infiltration rates, poor plant growth, and erosion where the vegetative cover is lost. Some characteristics of the repellent soils in one of these sequences have been reported by Wallis et al. (1993). They found that severe repellency (MED values ranging from 2 to 4) developed in less than 130 years in soils with limited amounts of organic carbon (20 – 40 kg C m23 in the surface 50 mm of soil). In addition, the dune phase soils were more repellent than low lying soils of equal age. Soil topographic position was found to affect

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D.J. Horne and J.C. McIntosh

surface soil moisture contents and repellency expression. Compared with the number of studies of the effects of water repellency on soil water relations and plant performance, the biochemistry of the hydrophobic phenomena in soils has received little attention in the literature. A few Australian workers have successfully removed repellency by extracting sandy soil samples with organic solvents. Ma’shum et al. (1988) identified long-chain fatty acids as the primary cause of repellency. More recently, Franco et al. (1995) have proposed a mechanism for the transfer of lipids from particulate organic matter onto soil particles. To date, no one in New Zealand has identified the specific compounds that cause repellency or the processes by which repellency is expressed. The objectives of this study were to isolate the compounds that play an important role in the development and expression of repellency, to characterise them, and to elucidate some aspects of the mechanisms by which repellency is expressed.

2. Materials and methods 2.1. The soils Soil samples were taken from two development sequences; one near the townships of Waitarere and Himatangi (including: Waitarere sand, Himatangi sand, and Foxton black sand) and the other near the township of Waverley (Castlecliff sand, Mosstown sand, and Patea black sand). The sand dunes (Waitarere and Castlecliff sands) were covered with dune grass and lupins while the other samples were removed from under dairy pasture. The soils of the Waitarere development sequence have been described in detail elsewhere (Claridge, 1961; Cowie et al., 1967; Gibbs, 1980), but in brief they are derived from a parent material of predominantly quartz and feldspar minerals with small amounts of mica. Most sand grains are 0.15 – 0.3 mm in diameter, and the proportion of silt and clay increases from approximately 1% in the youngest sands to 10% silt and 20% clay in the oldest soils. The Waverley area has very similar climate and vegetation to the Waitarere region. The major

difference between the soils from the two regions is in their mineralogy: the Waverley soils contain 90% titanomagnetite (Fleming, 1953; Wright, 1964). At each site, the soil was sampled to a depth of 50 mm from 5 –6 areas selected at random, and pooled. The samples were air-dried and passed through a 2-mm sieve before further analysis. 2.2. Water repellency measurements The repellency of soil samples was measured by the Molarity of Ethanol Droplet test (MED) as developed by King (1981). Ordinarily, MED values are measured on air-dried soils. To study the effect of pre-heating soil samples prior to the measurement of MED, soil samples were re-wetted and dried by heating at 708C for seven days or 1058C for 48 h. Samples were cooled in a desiccator before the MED test was performed. 2.3. Extraction procedures Shaking technique: 40 g of air-dried Himatangi sand was mixed with 120 ml of solvent and allowed to stand for 1 h at room temperature with brief shaking at 10-min intervals. The following solvent mixtures were used: chloroform:methanol:water (9:1:1 v:v), chloroform:methanol:1 M hydrochloric acid (9:9:1 v:v), chloroform:methanol:ammonia (10:10:1 v:v), and toluene:ethanol (2:1 v:v). The extract was decanted off (without prior centrifugation) and the process was repeated twice more with fresh solvent. The MED value was measured after the soil had airdried. Column technique: 100 g of air-dried Himatangi sand was packed into a glass column (in diameter 2.5 cm). One litre of solvent was allowed to slowly percolate through the soil. Chloroform:methanol:ammonia (10:10:1 v:v), and isopropanol:ammonia (7:3 v:v) were the solvent mixtures used. The MED value was measured after the soil had air-dried. Material extracted by the isopropanol:ammonia solvent mixture (7:3 v:v) was collected and weighed after the removal of the solvent. The solid extract was partitioned into chloroform- and water-soluble fractions, and the fractions quantified gravimetrically. Soxhlet extraction: 40 g of Himatangi sand was placed in a thimble and extracted in Soxhlet apparatus

Hydrophobic compounds in sands from New Zealand

for 6 h. The soil was wetted with the solvent for 15 min before refluxing. The procedure was repeated twice more with fresh solvent. This is a modification of the technique of Ma’shum et al. (1988). The solvents mixtures used were isopropanol:ammonia (7:3 v:v) and chloroform:methanol:ammonia (10:10:1 v:v). The MED value was measured after the soil had air-dried. Material extracted by an isopropanol:ammonia solvent mixture (7:3 v:v) was collected and weighed after the removal of the solvent. The solid extract was partitioned into chloroform- and water-soluble fractions, and the fractions quantified gravimetrically. 2.4. Separation and analysis of soil extracts The MED values of a range of soils were measured (Waitarere sand, Himatangi sand, Foxton black sand, Castlecliff sand, Mosstown sand, and Patea black sand) before they were placed in a Soxhlet apparatus and refluxed using an isopropanol:ammonia (7:3 v:v) solvent mixture as described above. The extracts were evaporated to dryness under reduced pressure on a rotary evaporator at 40 – 508C. The drying was completed on a lyophiliser. Following acidification with a drop of formic acid to ensure that the fatty acids were ionised and so able to be extracted into chloroform, the dried extract was dissolved in a mixture of chloroform and water. The phases were separated and shaken with the other solvent. The aqueous phase contained amphipathic and polar material, and the chloroform contained the lipid compounds. The aqueous phase was evaporated to dryness in a rotary evaporator. This material was then dissolved in methanol. A small portion did not dissolve in the methanol: this residual material is probably highly polar in nature. For simplicity, the methanol-soluble part of the water-soluble fraction is denoted, hereinafter, as the water-soluble fraction. The dried aqueous phase was also dissolved in ethyl acetate (for use in an addition experiment as described below), again with a small amount of residual material left. The lipid in the chloroform phase was separated by column chromatography with Florisil as the absorbent. The hydrocarbon fraction was eluted with hexane, neutral lipids with diethyl ether, and acidic lipids with ether and 2% formic acid. Chloroform was

27

then used to elute polar lipids, followed by methanol to elute the very polar lipids. In many of the experiments reported here, these last two fractions were combined. Some lipid material cannot be eluted from the Florisil column. The eluted fractions were dried over NaOH/P2O5 in an evacuated desiccator before being weighed. The very polar lipid fraction was re-dissolved in chloroform to remove any Florisil that had dissolved in the methanol. The partitioning techniques and solvents used here follow standard procedures (Kates, 1986). The constituents of the extracted fractions were identified using standard methods and instrumentation including thin layer chromatography (TLC), gas liquid chromatography (GLC), ultraviolet and infrared spectroscopy. TLC was carried out on silica gel plates (0.25 mm) using sequential development, first in hexane, then toluene and finally in a hexane:diethylether:acetic acid (80:20:1 v:v) mixture (Colton et al., 1986). The lipid material was detected by spraying with a 0.1% solution of 2,7-dichlorofluorescein in methanol and viewed under UV light. Other organic compounds were detected by the copper acetate– phosphoric acid charring procedure of Fewster et al. (1969). The fatty acids were converted to their methyl esters by the method of Van Wijngaarden (1967). GLC was performed with a Shimadzu GC-8A Instrument. The esters were separated on 20% FFAP on Anakoom Q (60/70 mesh) in a glass column (1.8 m in length £ 3.2 mm in diameter). The hydrocarbons were separated on 3% Dexsil 300 on Chromosorb W-AW (80/100 mesh) in the same size glass column (Tulloch, 1972). The abilities of the water-soluble fraction as described above, the standard humic acid fraction (Stevenson, 1994), and the hymetamelonic fraction of humic acid as defined by Hoppe Seyler (1845), to ameliorate repellency were compared. These three fractions were sourced from Himatangi sand and added to Himatangi sand. Humic acids were extracted from a sample of Himatangi sand (100 g) using standard procedures (Stevenson, 1994). Further extraction in methanol gave the hymetamelonic fraction (Hoppe Seyler, 1845). These three fractions were added to air-dried Himatangi sand at equivalent rates (see below for details of addition procedure). Soil samples were then air-dried and the MED values

28


determined. A further comparison was made by observing whether the water-soluble fraction and the hymetamelonic fraction of humic acid were retained in a dialysis sack. The material was dialysed overnight at 48C against a ten fold volume of water. Samples of soil (approximately 0.5 g) were placed in a Leco induction furnace to determine the total carbon content (Nelson and Sommers, 1982). 2.5. Sequential extractions of Himatangi sand Forty grams of air-dried Himatangi sand were extracted in a sequential manner. Firstly, with an isopropanol:ammonia mixture (7:3 v:v) in a Soxhlet apparatus as described above. The MED value was measured after the soil had air-dried. Secondly, this same soil sample was shaken in aqueous ammonia (2.4 M) at room temperature for 1 h before the extract was decanted off (without prior centrifugation). The MED value was measured after the sample had airdried. This sample was then Soxhlet-extracted with an isopropanol:ammonia mixture (7:3 v:v) for a second time, and then shaken with aqueous ammonia again. The MED values were measured after each extraction. This procedure was repeated a third and fourth time. 2.6. Addition experiments Lipid and water-soluble fractions obtained from Himatangi sand by Soxhlet extraction with isopropanol:ammonia (7:3 v:v) were added to airdried samples of untreated, and Soxhlet-extracted Himatangi sand. Addition rates were adjusted to equal the concentration of individual water-soluble and lipid fractions originally extracted from the source Himatangi sand. The extract removed from the Soxhlet apparatus containing 160 g of soil was partitioned as described above. The lipid fraction, dissolved in chloroform, was added to the sand at the rate of approximately 1.8 mg g21, and the water-soluble fraction, dissolved in water was added to the sand at the rate of approximately 3.4 mg g21. The solvent volume used to transfer the extract solution was set at 5 ml. The stoppered tubes (15 ml, glass screw cap with liner) containing the sand and the extract solution were shaken occasionally over a period of 8 h. Surplus liquid was then removed by decantation (without prior centrifugation) and the samples allowed to air-dry

before soil water repellency was assessed. The excess solvent (and the material it contained) was decanted, rather than being left to evaporate, in order to gauge the nature of the interaction between the added material and soil sample. More specifically, excess solvent was decanted off so as to determine if it is possible to add extract compounds to soil samples simply by bringing them into contact (compared with the risk of ‘forcing’ or depositing material back onto the soil matrix by evaporating the solvent off)? The abilities of lipid and water-soluble compounds extracted from a repellent soil (Himatangi sand) to modify repellency were compared with those of lipid and water-soluble material extracted from a wettable soil (Manawatu fine sandy loam). The lipid materials were added to air-dried samples of Waitarere sand, Himatangi sand, Foxton black sand, Castlecliff sand, and Manawatu fine sandy loam. The water-soluble extracts were added to air-dried samples of Himatangi sand and Waitarere sand. In order to distinguish between the effect on the repellency of the added fraction, and the solvent in which it was added, chloroform was added to Manawatu fine sandy loam. The procedures for extraction, partitioning, addition of material, and measurement of MED were the same as those described above. Water-soluble material extracted from Himatangi sand was dissolved in water and added to air-dried Himatangi sand. The soil was air-dried and the MED value was then measured. This was compared with the effect of adding the water-soluble fraction to Himatangi sand using ethyl acetate as the solvent. All the extraction, sequential extraction and addition experiments were replicated twice, therefore, in nearly all cases the values given in the tables are the means of two measurements.

3. Results 3.1. Variation in MED with different drying temperatures Heating soil samples prior to MED measurement does not consistently increase MED values above those measured on air-dried soils (Table 1). The greatest difference in the MED values at different drying temperatures is for Castlecliff sand where the MED

29

Hydrophobic compounds in sands from New Zealand Table 1 The effect of drying soils to different temperatures on MED values Soil

Himatangi sand Waitarere sand Foxton black sand Castlecliff sand Mosstown sand Patea black sand

Table 2 A comparison of extraction procedures and solvents for lowering the repellency of Himatangi sand (initial MED ¼ 2:8Þ

MED after drying at 208C (air-dried)

708C

1058C

2.7 1.2 1.2 2.1 0.8 0.8

2.8 1.3 0.8 1.4 1.4 1.1

3.0 1.4 1.3 1.4 0.8 0.9

value for the air-dried sample is 0.7 molar units greater than the values measured after heating to either 70 or 1058C. As there is no advantage to be gained from heating these sandy soils prior to the measurement of the MED value, all subsequent MED measurements reported here were made on air-dried samples. Replicated (12) measurements of the MED value of Himatangi sand have a standard deviation of 0.3. Some Australian workers (Ward and Oades, 1993; Franco et al., 1995) have reported that the temperature to which soil samples are dried prior to the performance of the MED test has a marked effect on the magnitude and consistency of the MED measurements. In contrast to the consistent MED values for New Zealand coastal sands across a range of premeasurement heating treatments, the MED values of non-wettable Australian sands are typically greatest and most consistent after heating at elevated temperatures. 3.2. Extraction of soil samples A number of solvent mixtures and extraction techniques successfully reduce the water repellency of Himatangi sand (Table 2). The solvent mixture requires a base (ammonia) and an alcohol (either isopropanol or methanol) to remove, effectively and consistently, the compounds responsible for repellency. Better lipid solvents like toluene or chloroform do not increase the efficiency of the extraction process. Provided that the solvent mixture comprises an alcohol and a base, the reduction in the MED value is not sensitive to the extraction technique used. Generally, pouring the solvent through columns of soil is as effective at lowering repellency as shaking soil samples in the solvent mixture or

Technique and solventa

MED

(a) Shaking Chloroform:methanol:water (9:1:1) Chloroform:methanol:1M hydrochloric acid (9:9:1) Chloroform:methanol:ammonia (10:10:1) Toluene:ethanol (2:1)

2.0 2.2 0.8 1.6

(b) Column Chloroform:methanol:ammonia (10:10:1) Isopropanol:ammonia (7:3)

0.2 0.4

(c) Soxhlet reflux Isopropanol:ammonia (7:3) Chloroform:methanol:ammonia (10:10:1)

0 1.6

a

Solvent composition is given in v:v.

Soxhlet-refluxing using the solvent mixture (Table 2). Despite the presence of both an alcohol and a base, the MED value measured following reflux with the chloroform:methanol:ammonia mixture ðMED ¼ 1:6Þ is greater than that measured following extraction with the isopropanol:ammonia mixture ðMED ¼ 0Þ: This may be due to the fact that the latter extraction was carried out at a warmer temperature (208C greater) or there may have been some interaction between the chloroform (non-polar) and hydrophobic compounds on, or near, the soil surface in the former extraction (McGhie and Posner, 1980). Although extractions using the column and Soxhlet techniques produce similar reductions in the MED values, the Soxhlet apparatus removes considerably more material (Table 3). The column technique Table 3 Quantities of material extracted from Himatangi sand using the column and Soxhlet techniques and an isopropanol:ammonia (7:3 v:v) mixture Extract fraction

Technique Column

Soxhlet

Total (mg g21) Lipida (mg g21) Polarb (mg g21)

2.4 0.8 1.9

5.0 1.8 3.4

MED at conclusion of extraction

0.4

0

a b

Chlorofrom-soluble. Water-soluble.

30


extracts only 48, 44 and 55% of the quantities of total, lipid and water-soluble materials removed by Soxhlet extraction, respectively. However, the ratio of total: lipid:water-soluble material extracted is approximately the same for both techniques. Only 4 h of refluxing with an isopropanol:ammonia (7:3 v:v) solvent mixture is necessary to eliminate water repellency ðMED ¼ 0Þ: However, 28% more material (total) is removed when soil samples are refluxed a second time with fresh solvent mixture (for no further change in repellency). A fourth reflux extracted only 10% more material (w:w); hence, the use of the three-step process described above for the routine Soxhlet extraction of samples. In contrast to the results reported here, some Australian nonwetting sands require 16 h of refluxing to remove hydrophobicity (Ma’shum et al., 1988). As the extraction of soil samples in the Soxhlet apparatus with the isopropanol:ammonia (7:3 v:v) solvent mixture is a convenient and reliable method, and yields the greatest quantity of extracted material, all routine extractions for subsequent experiments reported here were performed using Soxhlet extraction. 3.3. Identifying compounds Two types of compounds, lipids and water-soluble substances, were extracted from soil samples: both have a role in repellency. The lipid fraction includes neutral, acidic and polar lipids. Examination by TLC revealed two prominent lipid bands: a major one located at the solvent front Rf 0.9 –1.0 and a second of much slower mobility at Rf 0.2 –0.3. These major and minor bands were made up of hydrocarbons/alkanes and fatty acids, respectively. There were numerous other organic compounds as shown by charring. The neutral lipids are predominantly alkane hydrocarbons (carbon chain length from 23 to 33 with maximum at 29) and triglycerides. The evidence for the presence of hydrocarbons in the lipid fraction is as follows. Firstly, on chromatographic separation, these compounds moved with the solvent front, showing no inclination to bind to the silica gel even when an extremely non-polar solvent like hexane was used. Secondly, the infrared spectra showed only bands due to C –H bonds. There was no evidence of a carbonyl group which would be seen if esters were

present. Thirdly, the absence of staining after exposure of the TLC plate to osmium tetroxide suggests there were no double bonds. In contrast, staining was observed in the fatty acid region of the chromatogram. Fourthly, GLC at high temperature (200 –4008C), when the hydrocarbons are volatile, gives peaks corresponding to straight chains with carbon numbers from 23 to 33 with a maximum at 29. This was confirmed by running standards purchased from Sigma Chemical Company. The acidic lipids are mainly long-chain fatty acids. The fatty acids contained in the lipid extract are the same as those commonly found in animals and plants. The major fatty acids identified were: C16:0 palmitic, C16:1 palmitoleic, C18:1 oleic and C18:2 linoleic. The presence of fatty acids was indicated by the following observations. The fatty acids present were not produced from triglycerides (saponification) in the extraction process as coconut oil subjected to the same procedure as soil samples showed only trace amounts of fatty acids. This lipid fraction had the same chromatographic mobility as palmitic acid—a representative fatty acid. Methylation of this fraction gave compounds with the same chromatographic mobility as the methyl ester of palmitic acid. A GLC examination of these methyl esters gave peaks that correspond to the common esters of fatty acids. The polar lipids are mostly humic acids as they have a high molecular weight, are non-dialysable, and their solubility is dependent on pH. They are chloroform-soluble in acidic conditions, but water-soluble in alkaline conditions. The water-soluble compounds share some similarities with the hymetamelonic fraction of humic acids described by Hoppe Seyler (1845). On addition to Himatangi sand, both fractions reduce the MED value to 0. In contrast, addition of the humic acid fraction has very little impact on repellency ðMED ¼ 2:4Þ: Both the water-soluble fraction and the hymetamelonic fraction of humic acid are non-dialysable. The water-soluble fraction is also soluble in ethyl acetate. Therefore, this fraction might be thought of as being comprised of very large, amphipathic compounds, presumably with phenolic hydroxyl and carboxylic acid groups as the major polar functional groups (Stevenson, 1994).

31

Hydrophobic compounds in sands from New Zealand Table 4 Carbon content and quantities of fractions extracted from a range of soils Units

MED Total carbon Soxhlet-extractable Water-soluble fraction Lipid material Neutral lipids Acidic lipids Polar lipidsb Not eluted Vegetation a b

% mg g21 mg g21 mg g21 mg g21 mg g21 mg g21 mg g21

(%)a (%) (%) (%)

Castlecliff sand

Mosstown sand

Patea black sand

Waitarere sand

Foxton black sand

Himatangi sand

2.0 0.8 1.7 0.9

0.5 2.1 1.5 0.8

1.1 8.6 5.3 2.9

1.5 0.5 2.5 2.0

1.2 3.5 5.8 4.8

2.7 4.9 5.0 3.4

0.6 0.10 (17) 0.12 (20) 0.15 (25) 0.23 (39)

0.9 0.15 (17) 0.18 (20) 0.16 (18) 0.41 (45)

3.0 0.43 (18) 0.78 (26) 0.87 (29) 0.82 (27)

0.6 0.20 (33) 0.12 (20) 0.08 (13) 0.20 (33)

1.7 0.20 (12) 1.30 (76) 0.25 (14) 0 (0)

1.8 0.3 (17) 0.8 (45) 0.4 (22) 0.3 (17)

Lupins & Dune grass

Pasture

Pasture

Lupins & Dune grass

Pasture

Pasture

% of total lipid. Polar lipids includes polar and very polar fractions.

3.4. Quantities of extract fractions and relationship to MED There is no apparent relationship between the MED value and the total soil carbon content or between the MED value and either the total lipid content or the quantity of any of the individual lipid fractions (Table 4). The MED value does not seem to be related to the quantity of the water-soluble fraction. Neither is there a strong correlation between the total soil carbon content and the quantity of material extracted using the Soxhlet apparatus (Table 4). Indeed, the Soxhlet extraction removes only a small amount of the organic matter (commonly less than 10%). However, there is a linear relationship between the total lipid fraction and total soil carbon content, which suggests that Soxhlet refluxing may be more efficient at extracting this fraction. 3.5. Sequential extraction of Himatangi sand When a sample of Himatangi sand, that has been Soxhlet-extracted in isopropanol:ammonia (7:3 v:v), is shaken with aqueous ammonia at room temperature, an additional quantity of material is extracted (Table 5). Surprisingly, the removal of this additional material is accompanied by a marked increase in repellency (from MED ¼ 0 to MED ¼ 4:2Þ: When this extremely repellent soil is Soxhlet-extracted for

a second time with isopropanol:ammonia (7:3 v:v), repellency is again reduced to 0 MED. This sequence can be repeated three times before the increase in MED on shaking with aqueous ammonia becomes small. 3.6. Addition experiments An addition of the total lipid fraction extracted from Himatangi sand to untreated Himatangi sand increases repellency, and correspondingly, an addition of the water-soluble fraction reduces the MED value (Table 6). A similar result is observed Table 5 Effect of sequential extraction of Himatangi sand (initial MED value ¼ 2.8) on MED values and the quantity of material removed Procedure

MED value

Quantity extracted (mg g21)

1st IPAa 1st ammoniab 2nd IPA 2nd ammonia 3rd IPA 3rd ammonia 4th IPA 4th ammonia

0 4.2 0 3.2 0 1.2 0 0.2

1.5 8.2 0.5 2.1 0.4 1.8 0.1 1.2

a Soxhlet extraction using isopropanol:ammonia (7:3 v:v) solvent mixture. The pH of this solution was approximately 13. b Soil extracted at room temperature with 2.4 M ammonia.

32


Table 6 Effect of adding soil extract fractions (lipids and water-soluble compounds) to samples of untreated Himatangi sand and Himatangi sand that has been Soxhlet-refluxed

Table 8 Effect of adding water-soluble extract from repellent and wettable soil on repellency Soil

Untreated

MED

Soxhlet-extracted

MED

Initial soil Plus lipids only Plus water-soluble only

2.8 4.6 1.4

Initial soil Plus lipids only Plus water-soluble only

0.2 1.6 0.2 Waitarere sand Himatangi sand a

when the lipid fraction is added to soil previously extracted in the Soxhlet apparatus. The lipid compounds extracted from a wettable soil, Manawatu fine sandy loam, are equally effective at increasing MED values when added to soil as the lipid fraction removed from the repellent Himatangi sand (Table 7). Addition of either of these lipid extracts increases the MED value of the soils by approximately 1 – 1.5 MED units. In a similar fashion, the water-soluble material extracted from Manawatu fine sandy loam is equally effective at lowering the MED values when added to repellent soils as the water-soluble fraction removed from Himatangi sand (Table 8). The data presented in Tables 6 – 8 should be interpreted with some caution. As excess solvent was decanted from the soil sample rather than being allowed to evaporate, it might be argued that little can be said about the true or final addition rate. An early attempt to quantify the lipid and water-soluble

Table 7 Effect of adding lipid extract from repellent and wettable soil on repellency Soil

Waitarere sand Himatangi sand Foxton black sand Castlecliff sand Manawatu fine sandy loam a b

MED values Initial value

Plus lipid from repellent soila

Plus lipid from wettable soilb

1.5 2.7 1.2 2.1 0

2.5 3.3 2.2 3.0 1.0

3.4 3.6 2.9 3.0 1.0

Extracted from Himatangi sand. Extracted from Manawatu fine sandy loam.

b

MED values Initial value

Plus water-soluble fraction from repellent soila

Plus water-soluble fraction from wettable soilb

1.5 2.7

1.0 1.5

1.0 1.0

Extracted from Himatangi sand. Extracted from Manawatu fine sandy loam.

fractions decanted off in the solvents following the addition of fractions extracted from Himatangi sand to Soxhlet-refluxed Himatangi sand, revealed that 3 mg of water-soluble material was bound to a gram of sand and 0.4 mg of lipid material was retained per gram of sand. In other words, approximately 22 and 88% of the added lipid and water-soluble compounds, respectively, were retained by the Himatangi soil sample. Again, the very small quantity of lipid material required to impart hydrophobicity is noteworthy. When the water-soluble fraction is dissolved in water and added to repellent Himatangi sand ðMED ¼ 2:7Þ; the soil is rendered wettable ðMED ¼ 0Þ: When this same fraction is added to Himatangi sand using ethyl acetate as the solvent, hydrophobicity is ameliorated to a much smaller degree ðMED ¼ 1:6Þ: This is further evidence for the amphipathic character of this fraction. As the water-soluble compounds are readily retained by soil samples, this fraction may play a key role in repellency expression. The addition of the chloroform solvent alone did not alter the MED value of the Manawatu fine sandy loam i.e. it remained at 0. While the solvents used in these addition studies may have some direct effect on the repellency of the soil samples, it is the added material that has the major impact on the MED value. This notwithstanding, the solvent may affect MED values indirectly through its effect on the behaviour of the fraction being added, as demonstrated by the addition of the water-soluble fraction in different solvents.


4. Discussion No obvious, single cause of repellency formation or expression emerges from the consideration of the results presented above. The lack of correlation between the MED value of the soils and their total carbon content, the quantity of lipid or of any lipid fraction extracted suggests that the severity of repellency can not be accounted for by the amount of lipid or of any lipid fraction in the ‘bulk’ soil. Peculiarly, repellency of a soil sample can be made to fluctuate, in a serial manner, over a wide range of MED values depending on the sequence of solvents it is extracted in. In terms of generating repellency, there is nothing unique about the compounds extracted from hydrophobic soils i.e. lipid compounds extracted from repellent soils seem no more efficient at imparting hydrophobicity than similar compounds removed from wettable soils. It would appear that a subtle mechanism is required to explain repellency expression in these soils. Repellency seems to be a phenomenon best explained by reference to the nature of the surface that water encounters when attempting to infiltrate. More specifically, repellency is determined by the properties of the outer surface of the organic coating. Amphipathic compounds are key constituents of this outer layer. Given that differences in the MED values of these coastal sands cannot be accounted for by concomitant variations in the quantity of bulk humus material, four possible mechanisms for repellency development and expression are proposed. Firstly, the amphipathic compounds may change orientation. In the wettable context, these compounds are likely to have their polar end pointing outwards. If, for some reason, such as dehydration, there is re-configuration or re-orientation, then these substances may well present a water repellent end at the surface. Secondly, repellency may vary according to the ionisation status of carboxylic groups in amphipathic compounds. If protonated, this functional group will be hydrophobic in character: upon ionisation, the resultant charged carboxylate group will be mostly hydrophilic. The ionisation form of the carboxylic groups will be dependent on moisture content: in moist conditions, the carboxylate anion will be more common. The ionisation status will also be affected by soil pH. The third mechanism relates to the extent to which

33

the hydrophobic compounds are screened or covered. When the soil is wettable, the hydrophobic material is effectively screened, especially if the surface is well hydrated. Repellency may develop when the hydrophobic compounds are more exposed. For instance, this may be due to the contraction of carboxylate and other polar groups associated with dehydration described in mechanisms one and two above. Fourthly, as was clearly demonstrated in the present study, extraction and/or addition of compounds, be they water-soluble or lipid, may change repellency. The mechanisms proposed here are likely to interact. Indeed, the first three mechanisms might be thought of as, stages in, or parts of, a larger process. Other workers, most notably Ma’shum and Farmer (1985) and Swift (1989), have proposed mechanisms for repellency expression similar to those given here. Ma’shum and Farmer (1985) claim that when soils are wet, polar groups interact with water molecules, but as the soil dries and water is lost, polar groups interact with each other and organic matter presents largely non-polar groupings on its surface. Building on a model of flexible macromolecules, Swift (1989) suggests that, as soils dry, the last water to be removed is entrained within molecules. At this stage, hydrophilic and other charged sites will attempt to accumulate and/or orientate towards the centre of the molecule leaving hydrophobic sites to accumulate and/or orientate towards the outside, drier part, of the molecule. On re-wetting polar sites are re-solvated. The mechanisms for changes in hydrophobicity proposed above are consistent with the compounds identified in the present study and supported by the observations of the other experiments reported here. For example, they explain the ability of the relatively ‘gentle’ column and shaking techniques to remove repellency, and the fact that their success does not seem to be directly related to the quantity of lipid material extracted. To ameliorate repellency, it was not necessary that these procedures extract large quantities of lipid material, only that they facilitate some of the changes in surface chemistry discussed above. Presumably, in the most successful extractions ðMED < 0 at their completion), the alcohol was removing lipid material from near the surface (mechanism four), but this material was more effectively accessed if amphipathic compounds covering it were removed by the base (mechanism three).

34


Mechanisms one, three and four can be invoked to make sense of the sequential extraction procedure. In this sequence, layers of compounds were removed from the surface, and this was probably accompanied by re-orientation or re-configuration of amphipathic molecules at the outer layer. It is not known what can be deduced from the fact that this sequence had four cycles i.e. whether it is possible to speculate about compounds responsible for repellency expression in Himatangi sand being approximately four layers deep. Most of the changes in repellency reported in the addition experiments are probably best explained by reference, in the first instance, to the fourth mechanism listed above i.e. the fraction added simply becomes, or defines the nature of, the new outer layer. Further support is lent to this mechanism by the finding that, in terms of repellency expression, there is nothing unique about either the water-soluble or lipid material present in these repellent sandy soils i.e. the water-soluble and lipid material from repellent and wettable soils are equally effective at modifying the chemistry of the outer layer of the humus –soil matrix when added to soils. It is acknowledged that little evidence from the present study can be offered for the suggestion that the ionisation status of carboxylate groups plays a key role in repellency expression (mechanism two). As alluded to above, perhaps this mechanism should be combined with the first mechanism to give a larger picture of how functional groups behave according to the degree of hydration. Unfortunately, because the MED values were measured on air-dried soils, little can be said directly from this study about the effect of the degree of hydration on the expression of repellency. A cautionary note should be made about the translatability of the results reported here to repellency as experienced in very different contexts. For example, it would appear that different processes for repellency formation and expression might be operative in these New Zealand sandy soils compared with Australian non-wetting sands. The hydrophobic behaviour of the coastal sands found on the southwest coast of New Zealand does not appear to be as recalcitrant as it is in other soils, and it is unlikely that repellency in these soils (formed in a relatively temperate and humid climate) will develop in a manner similar to the heat-induced process described

by Franco et al. (1995). The differences in the severity of repellency, and mechanisms by which it develops and is expressed, both within the soils studied here, and between these and other soils, probably relate, principally, to differences in texture, climate, position in the landscape, and to a lesser extent, vegetation, and the composition of hydrophobic, amphipathic and polar, compounds. The central role given to repellency as a surface phenomenon in this study suggests that soil texture is an especially important factor. Some comments on the limitations of some of the experimental procedures used in this study are warranted. There may also be limitations associated with the categorisation schema adopted here. For example, there may be no clear demarcation between the categories of polar lipids and water-soluble material but rather a continuum. A more useful delineation may have only two categories; very hydrophobic compounds (e.g. the hydrocarbons and possibly the fatty acids) and substances that exhibit some polar or amphipathic behaviour (e.g. acidic, polar and very polar lipids, and polar material), albeit to varying degrees. This probably merits further work. For example, given that there is some indication that the very polar lipids may exhibit some amphipathic characteristics, it would be interesting to compare their impact on repellency with that of the watersoluble fraction as reported above. While the amelioration of repellency by the extraction of hydrophobic and hydrophilic humus compounds from soils, and modifications of repellency through addition of these compounds, have been demonstrated, the applicability of these results to the development and expression of repellency in the field context may not be straightforward. For example, humus fractions will, clearly, not be present in soils as solutes of chloroform or methanol solutions. What of the field situation, where there are no discrete fractions as such, and everything is altogether more complex than the laboratory-based study reported here might imply? For example, consider the fact that these coastal sands are wettable in winter, and very water repellent only in summer. Presumably the quantities of hydrophobic and/or polar compounds do not change in the span of a few months. In other words, the fourth mechanism given above has little utility in describing the seasonal nature of repellency expression in the field. How is it then that repellency


expresses itself in such a marked seasonal fashion? The first three mechanisms proposed above are suggestive for the seasonal nature of repellency expression in the field. In winter, the outer layer will be well hydrated through a combination of factors: amphipathic compounds will be orientated in such a fashion as to have their polar end outwards, the carboxylate anion will predominate, and hydrophobic groups will be effectively screened by more hydrophilic groups. During drying, a more hydrophobic outer surface is the norm as the amphipathic substances change in orientation, the carboxylate groups are protonated, and due to contraction of the ‘surface screen’, hydrophobic substances are exposed.

5. Conclusions Organic compounds were extracted from a range of coastal sandy soils using a number of extraction procedures and solvent mixtures. The extracted

35

compounds were partitioned into operationally defined lipid and water-soluble fractions, and the constituent compounds identified. In the neutral lipid fraction, there are mostly alkane hydrocarbons and triglycerides; the acidic lipids are comprised of mainly long-chain fatty acids. The water-soluble fraction exhibits amphipathic behaviour, and plays an important role in repellency expression. This fraction bears some similarities with the hymetamelonic fraction of the humic acid pool. There is no clear relationship between repellency (MED value) and either total carbon content, that portion of the extract that is water-soluble, total extracted lipid or any lipid fraction. The repellency of soil samples can be modified in vitro by the addition of soil lipid extract or water-soluble extract. This is so, regardless of whether the lipid and water-soluble extracts have been extracted from repellent or non-repellent soil. Water repellency is a surface phenomenon. Four mechanisms for repellency expression both in laboratory studies and in the field are proposed for further consideration.


Chapter 4

Chemical characterisation of water repellent materials in Australian sands C.M.M. Francoa,*, P.J. Clarkeb, M.E. Tateb and J.M. Oadesb a

Biotechnology, School of Medicine, Flinders University of South Australia, GPO Box 2100, Adelaide, SA 5001, Australia b Department of Soil Science, University of Adelaide, P.M.B.1, Glen Osmond, Adelaide, SA 5064, Australia

Abstract Water-repellency in non-wetting sands is due to hydrophobic waxes present on the surface of sand grains and contained in particulate organic matter present in these sands. This study investigates the physico-chemical characteristics of these natural waxes and compares them to waxes extracted from potential original source materials. Non-polar and polar hydrophobic wax extracts were obtained from whole non-wetting sand, and its individual constituents, and associated organic matter. These included the sand fraction, the intrinsic particulate organic matter, tree litter, eucalyptus leaves, bark, lucerne and lupin plants, and fungi and actinomycetes isolated from these sands. Waxes were characterised for their hydrophobic properties and composition of their chemical constituents. The hydrophobicities of the waxes were assessed by measuring the water-repellency induced after treating acid washed sand with wax extracts. Non-polar and polar wax extracts of the tree litter displayed hydrophobic properties that were similar to the corresponding waxes isolated from non-wetting sand and intrinsic particulate organic matter. Unlike these plant-derived waxes, the microbial wax extracts possessed different hydrophobic properties. Characterisation of the components of the extracted waxes by gas chromatography-mass spectroscopy (GC-MS) analysis revealed a strong similarity in the composition of waxes isolated from non-wetting sand, tree litter and other plant material. The major components found were unbranched and branched C16 to C36 fatty acids and their esters, alkanes, phytanols, phytanes, and sterols. Some of these components were not detected in the microbial waxes. Unextracted samples, as well as wax extracts of non-wetting sand, intrinsic particulate organic matter, tree litter and fresh plant material were further analysed by solution and solid state Nuclear Magnetic Resonance spectroscopy which revealed the relative content of the different chemical species present.

1. Introduction A research effort focussed on the amelioration of the problem of water repellency (Ma’shum et al., 1989; Ward and Oades, 1993; Franco et al., 1994, 1995) requires the detailed characterisation of the * Corresponding author. Fax: þ 61-8-8277-0085. E-mail address: [email protected] (C.M.M. Franco). q 2003 Elsevier Science B.V. All rights reserved.

components that cause water repellency in nonwetting sand (NWS). A recent study of the role of intrinsic particulate organic matter (IPOM) in these sands described the contribution of these waxcontaining particles, and that of a wax-coated sand surface, to the development of water repellency (Franco et al., 1995). In the current paper, we describe the physical and chemical properties of the hydrophobic waxes extracted from NWS

38

C.M.M. Franco et al.

and compare them to other materials that may be implicated in the development of water repellency in the field. These materials include the organic coating on sand particles (Roberts and Carbon, 1972), IPOM (Franco et al., 1995), waxes from microorganisms including basidiomycete fungi (Bond and Harris, 1964), fungal growth (Chan, 1992), plant materials (De Bano et al., 1970; McGhie and Posner, 1981) and eucalyptus tree litter (McGhie and Posner, 1980). Ma’shum et al. (1988) studied the components of the hydrophobic extracts of non-wetting sands and reported the presence of both free and esterified long-chain, C16 to C32 fatty acids. Simulation of water repellency using these straight chain fatty acids and esters showed that although the addition of these compounds imparts hydrophobicity to wettable sand surfaces, the nature of the repellency is different to that found with the natural polar extract from NWS (Ward and Oades, 1993). Consequently, the hydrophobic waxes must also contain other chemical constituents which either have different hydrophobic properties or which influence the behaviour of the straight chain fatty acids. Our investigation involves the detailed characterisation of the waxes from NWS and correlates the findings with analysis of other related materials so as to establish the origin of these waxes. The characterisation of organic material in complex mixtures found in soil and sediments have been the subjects of detailed analyses. The GC/MS technique requires the relatively volatile, extractable organic matter to be separated from the mineral component in sufficient quantity in order to identify the major components extracted from the soil and related samples. In addition to conventional MS techniques (e.g. GC/MS and MS/MS) that aid in analysis of the more volatile compounds, it is necessary to characterise the significant proportion of the organic matter that consists of high molecular weight non-volatile material and the more polar organic matter. This was accomplished using Solid State Cross Polarisation/Magic Angle Spinning-13C Nuclear Magnetic Resonance (CP/MAS-NMR) Spectroscopy to analyse the extracts and particulate organic matter.

These studies form an integral part of a broader strategy, which aims to ameliorate the problem of water repellency by biological remediation (Franco et al., 2000b).

2. Materials and methods 2.1. Soils and associated materials Non-wetting sand (NWS) samples were collected from sites located at Western Flat and Coombe in the south –east of South Australia. Samples were taken from the top 15 cm of a homogenous siliceous sand horizon. The sand was air dried, sieved , 2 mm and stored at room temperature. Samples were also collected from the area beneath the clumps of eucalyptus trees adjacent to the NWS sites that were rich in tree litter. These litter samples typically contain 55 – 65% organic matter (Franco et al., 1995) which is predominantly freshly decomposing plant material, the rest being sand with a wax coating. MED values and size distribution of the sands and litter from both sites have been presented earlier (Franco et al., 1995). Eucalyptus wood, bark and leaves were sampled from the trees in the area. Ten week-old lupin and lucerne plants were obtained from adjoining fields. Wettable sand used for these studies was either acid washed sand (AWS) (BDH chemicals) or NWS fired in a muffle furnace to burn off all the organic matter. 2.2. Measurement of water repellency The Molarity of an Ethanol Droplet (MED) test was used to measure water repellency (King, 1981). The MED values reported are of samples heated to 1058C before measurement, unless otherwise stated, as this procedure gave the “potential” repellency value (Dekker and Ritsema, 1994b). However, drying at high temperatures may lead to higher water repellency than may be achieved in the field (Dekker et al., 1998). Samples were allowed to cool in a desiccator over silica gel and the test carried out in a constant temperature room at 258C.

Chemical characterisation of water repellent materials in Australian sands

2.3. Isolation of intrinsic particulate organic matter and washed (IPOM-free) sand IPOM was isolated from water repellent sand by differential sedimentation to obtain size fractions of , 53, 53 to150, 150 to 250 and . 250 mm and IPOMfree sand as described previously (Franco et al., 1995). 2.4. Isolation and culture of actinomycetes and fungi from non-wetting sand Actinomycete cultures were isolated by the dilution plate method using starch – casein agar containing cycloheximide (25 mg ml21), streptomycin – ampicillin (each 15 mg ml21) and sodium azide (5 mg ml21). Fungi were isolated by one of two methods: (a) by the dilution plate method using 1/6 strength Czapek – Dox agar and potato-dextrose agar containing streptomycin – ampicillin (each 15 mg ml21) and sodium azide (5 mg ml21); and (b) by the Warcup method (Warcup, 1955) used to isolate basidiomycetes, using 1/6 strength Czapek –Dox agar. Both agar media contained rifampicin (5 mg ml21) and sodium azide (5 mg ml21). Twenty five morphologically distinct actinomycete isolates were cultured for 96 h in a liquid medium containing (w/v) 1% glucose, 1% starch, 1% defatted soybean meal, 0.5% yeast extract, 0.5% CaCO3, pH 7.2. Fifteen fungal cultures were grown for 96 h in a liquid medium comprising (w/v) 2% glucose, 2% glycerol, 1.5% water extract of NWS, 0.3% yeast extract, 0.3% soyapeptone, 1% KNO3, 0.5% NaCl, 0.3% K2HPO4, 0.3% MgSO4.7H20, pH 6.5. Growth was in Erlenmeyer flasks incubated at 278C in a rotary shaker. Harvested mycelium was washed three times with distilled water, dried in aluminium trays at 608C, and ground in a mortar and pestle with liquid nitrogen. The ground mycelium was stored under nitrogen in a desiccator at 48C. 2.5. Extraction and fractionation of water repellent waxes Soils and related organic samples were extracted using a 500 ml Soxhlet apparatus. With the exception of NWS, all other samples were ground to , 250 mm. All samples for extraction were weighed and placed into the filter paper thimble. Chloroform was used first

39

to extract the “non-polar wax” fraction. Samples were then extracted with an isopropanol:15.7 M ammonia (7:3, v/v) solution to yield the “polar wax” fraction, and finally distilled water was used for the “water extract”. Extraction was done on two scales: 30 –50 g or 1.5 –1.8 kg amounts, with reflux times for each solvent extraction step set at 16 and 60 h, respectively. After extraction, solvents were removed in vacuo at 458C. The dried isopropanol – ammonia extract was further fractionated, first with chloroform, with the soluble portion pooled with the non-polar wax extract, and subsequently with hot methanol to yield the “methanol-soluble subfraction”. Finally, the watersoluble portion was combined with the water extract. All fractions were evaporated to dryness before use. 2.6. Assessment of hydrophobic behaviour of extracts The extracts of NWS, plant litter, related plant materials and microbial cultures were added as solutions to dry, AWS at rates of 400, 800, 1200 and 1600 mg £ 10 g21 : The non-polar wax fractions were dissolved in chloroform and the polar wax fraction in warm methanol. After addition of the extracts, a 1:1 mixture of methanol:chloroform was added to the sand to wet it completely and to allow for ease of mixing with a spatula. This solvent mixture was used to introduce uniformity to both the polar and non-polar wax fractions. The vials were capped with an aluminium foil seal and mixed for 2 h in an endover-end shaker, then opened and dried at 1058C. Samples were cooled in a desiccator over silica gel before MED values were recorded. 2.7. Interactions between the extracted wax subfractions The interactions between the water-extract and either the non-polar (chloroform-soluble) or the polar (methanol-soluble) extract fractions were assessed in two ways: (1) the non-polar or polar extract fractions were added at different concentrations alone, and compared with the same concentrations to which 400 mg £ 10 g21 water-extract were added; (2) to a fixed amount of non-polar

40


extract (200 and 400 mg £ 10 g21 ) and polar extract (400 and 800 mg £ 10 g21 ) fractions increasing amounts of water-extract was added. 2.8. Gas chromatography – mass spectroscopy analysis Samples of dried non-polar and polar wax extracts were each dissolved in chloroform or methanol respectively and filtered through tightly packed glass wool in order to remove insoluble particles. All GC –MS analyses were carried out on a Varian 3400 GC –MS instrument using a splitless injector and a 30 m £ 0:25 mm fused silica column with a bonded DB-5 phase (J and W Scientific). The temperature gradient was 608C, 1 min, to 3008C, 20 min, increasing at 108C min21 with helium as the carrier gas. The GC was connected to a Finnigan MAT TSQ 70 Triple Quadrupole mass spectrometer linked to a DEC PDP11 68000 Minicomputer system. The mass spectrometer was operated in either electron impact (EI) or chemical ionisation (CI) mode, with a reagent gas pressure of 6500 mTorr. The ion source was maintained at 1508C and the electron energy at 70 eV with an emission current of 300 mA. High purity Argon at 1.8 mTorr was used as the collision gas in all collision activation experiments. Identification of compounds was based on interpretation of the mass spectroscopy fragmentation, comparison with spectra published in the literature and those available in the computerised database of the instrument, and relative GC-retention times. 2.9. Solid state

13

C CP/MAS NMR analysis

Solid state high resolution 13C nuclear magnetic resonance with CP/MAS was used to obtain spectra at 50.309 MHz on a Varian Unity 200 spectrometer with a 4.7 T wide-bore Oxford superconducting magnet. Samples were spun at 5 kHz in 7 mm diameter zirconia rotors with Kel-F or Vespel caps in a Doty Scientific MAS probe. All spectra were attained with a 1 ms contact time and between 300 and 1000 ms recycle time. The number of transients required for acceptable S/N ranged from 1000 to 1,000,000. Using the standard Varian polar pulse sequence, the free

induction decays were acquired with a sweep width of 40 kHz, acquisition time of 20 ms in a 1600 point database. Spectra were obtained with 32 k zero filling, 50 Hz lorentzian line broadening and 0.010 s gaussian broadening. Chemical shift assignments were externally referenced to the methyl resonance of hexamethyl benzene at 17.36 ppm. Extracts were applied as solutions to an AWS matrix and dried at 708C prior to analysis. Samples of NWS, size fractions of IPOM and tree litter were ground to a , 150 mm particle size prior to analysis.

3. Results and discussion

3.1. Yield of the extracted waxes Average extraction yields and percentage composition after fractionation are shown in Table 1. Nonwetting sands have a lower amount of total extractable material, as well as a lower content of non-polar material (5.75 – 6.6% of total waxes) compared to fresh tree litter, eucalyptus plant materials and microorganisms (11.95 –20.85%). The polar wax fraction constituted between 41.7 –51.75% of all the extracts analysed.

Table 1 Amount and distribution of extracted material from water repellent sands and associated plants and microorganisms. Results shown are the mean values of three analyses Extracted from

mg kg21 dry wt.

NWS-Coombe 1670 NWS-Western Flat 2310 TreeLitter-Coombe 14,310 TreeLitter-Western Flat 18,916 Eucalyptus bark 62,068 Eucalyptus leaves 50,078 Lupin plant 160,325 Lucerne plant 131,644 Fungi 88,105 Actinomycetes 69,089

% Non-polar % Polar-wax wax (CHCl3- (MeOHsoluble) soluble) 6.6 5.75 20.7 20.85 20.35 15.5 14.4 15.2 11.95 19.65

46.9 46.65 41.7 30.75 43.1 43.3 50.75 51.75 46.6 49.85


3.2. Water repellent properties of the extracts The hydrophobic properties of AWS treated with the extracts (Fig. 1A) and extract subfractions (Fig. 1B) of NWS were measured by MED determination. The chloroform extract and the non-polar wax (chloroform-soluble) fractions both imparted a high degree of hydrophobicity to the AWS. It should be noted that the MED values for chloroform-soluble samples heated to 1058C are about 1.0 unit lower than for similar samples heated to 708C only. The water extract treated AWS had a very low hydrophobicity with MED values less than 0.6 at the highest rate of added extract (Fig. 1A). Treatment with the methanol-soluble fraction (Fig. 1B) resulted in a higher level of hydrophobicity than the polar extract, with maximum hydrophobicity achieved at a treatment rate of 1200 mg £ 10 g21 : The higher hydrophobicity can be explained by the selective exclusion of very polar materials that are insoluble in methanol. This indicates that these very polar (hydrophilic) materials have a tempering effect on repellency. When heated to 1058C an increase in MED values was noted with both the isopropanol:ammonia extract and its methanol-soluble fraction. The hydrophobic properties observed with the polar waxes mirrors that obtained with an untreated

41

NWS, i.e. low hydrophobicity when wet, MED values not exceeding 4.0 when dry, and an increase with heat treatment. As the polar fraction constitutes approximately half of all the waxes extracted, it is a key component in attempting to understand the process of water repellency development. The hydrophobic properties of AWS treated with the chloroform extracts of all the samples isolated in this study are shown in Fig. 2A and B. The extracts of the intermediate size fractions were more hydrophobic than either the smallest (, 53 mm) or the largest (. 250 mm) fractions, and comparable to the whole NWS. The non-polar extracts of fresh litter, lupin plants and eucalyptus bark extracts had hydrophobicities comparable to that of the non-polar extract of the original NWS, whereas the hydrophobic properties of the extracts of eucalyptus leaves and lucerne plants were lower. AWS treated with the non-polar wax extract of the fungi gave MED values between 4– 5 when added at 200 – 400 mg £ 10 g21 sand, whereas at higher rates of addition a decrease in hydrophobicity was observed. Similar patterns were observed with polar wax extracts of tree litter, fresh eucalyptus bark and leaves and the IPOM (Fig. 2C and D). Polar extracts from other fresh plant materials such as lucerne and lupins, appeared not to induce any significant water

Fig. 1. The hydrophobic behaviour of (A) the non-polar (CHCl3), polar (I/N) and water soluble extracts of NWS from Western Flat. (B) the chloroform (CHCl3)-soluble and methanol (MeOH)-soluble fractions of the polar wax component. Extracts were applied to an AWS and heated to 70 or 1058C prior to MED analysis.

42


Fig. 2. The hydrophobic behaviour of the non-polar (chloroform) and polar (isopropanol:ammonia) extracts obtained from the IPOM fractions from NWS and the associated plants and microorganisms, respectively.

repellency. Polar waxes derived from the microbial sources and the water extracts isolated from all sources tested (not shown) imparted no measurable hydrophobicity to the AWS.

3.3. Interactions between the extracted wax subfractions The water extract of NWS is able to lower the water repellent effect of the polar wax fraction (Fig. 3A) of NWS as well as the non-polar wax fraction (Fig. 3B). The decrease in the water-repellency of the nonpolar wax was less severe in comparison to the decrease with the polar wax fraction. Nevertheless, this result confirms that the interaction between the different components has an effect on hydrophobicity.

3.4. GC –MS analysis of the non-polar and polar extracts A typical GC chromatogram of the polar wax fraction from the NWS is presented in Fig. 4. The numbered peaks have been identified and are listed in Table 2. The results from the GC – MS analysis of both the non-polar and polar wax extracts for each sample were analysed and the constituents identified were combined for each fraction and then tabulated (Table 3). The rigorous extraction procedure used is expected to provide a representative sample of the materials tested and could be compared to the extract obtained using a supercritical extraction procedure (Schnitzer et al., 1986). Constituents present in all of the plant derived materials, including tree litter and whole and size-


43

Fig. 3. The effect of the water-extract on the hydrophobic behaviour of the non-polar and polar extracts from NWS. (A) Increasing amounts of wax extracts added to 400 mg of water extract on AWS. (B) Increasing amounts of water extract added to fixed amounts of wax extracts on AWS.

fractions of the IPOM from NWS, showed very similar trends of chemically related or homologous series of constituents present in each fraction, albeit in different concentrations. Whereas the GC – MS analysis is biased towards volatile constituents such as the straight chain fatty acids, we are still able to detect other components due to the higher temperature used. The predominant compounds detected in these samples were straight and branched chain fatty acids

and fatty acid esters, alkanes, phytols, phytanols and sterols. The fatty acids and esters are likely to have hydrophobic properties characteristic of the non-polar components, whereas the branched chain fatty acids and the other chemical constituents containing hydroxyl groups such as the phytols, phytanols and sterols would make up the polar waxes. Further characterisation of the non-volatile components of the polar waxes and water extract require detailed

Fig. 4. GC –MS analysis of a typical methanol-soluble fraction of the polar wax extract of a water repellent sand. The identification of the peaks is shown in Table 2.

44


Table 2 Identification of GC peaks shown in Fig. 4

Table 2 (continued)

Peak no.

Compound

Peak no.

Compound

1 2 3 4 5 6 7 7a 8 9 10 11 12, 12a 13 14 14a 15 16 16a 17 18 19 20 20a 21 22 23 24 25 25a 26a 27 28 29 30 31 32 33 34 35 36 37 38,39 40 41 42 43 44 45 46 47

Octanoic acid 2,3-dihydrobenzofuran 4-ethyl-2-methoxyphenol Dodecanoic acid Benzoic acid, 4-hydroxy,3-methoxy Tetradecanoic acid Pentadecanoic acid Tetradecanoic acid, methylester Tetradecanoic acid, 12-methyl 6,10,14 trimethyl pentadecanone C14/C16 acid/alcohol ester Pentadecanoic acid, 13-methyl, methylester Hexadecanoic acid Hexadecanoic acid, methylethylester Heptadecanoic acid Octadecadienoic acid methylester Oleic acid, eicosylester 2-Hexadecanol, 3,7,11,15-tetramethyl Eicosadienoic acid methylester C14/C18 acid/alcohol ester Octadecanoic acid Octadecanamide Indacenedione, tetrahydrotetramethyl Hexadecanol 3,7,11,15-tetramethyl Eicosane Benzapyranone, 5-methoxy-6-methyl-2phenyl Eicosanoic acid methylester Eicosane cyclohexane Anthracenedione 2-hydroxy-2phenyl 9-Octadecenamide Octadecanamide 26 Eicosanoic acid Docosanol Octadecanoic acid 5,9,13,17-tetramethyl Benzopyranol derivative Benzenedicarboxylic acid/C16 ester Docosanoic acid Docosanol Tetracosanoic acid methylester 2-propeneone drivative Estatrienone 2-hydroxy-3methoxy Tetracosahexane hexamethylester Hexacosane Benzopyranol-phenyl derivative Stigmatrienol Hexatriacontaine Cholesterol Ergostadienol Ergosterol Stigmastadienol Stigmasterol Friedoolean-enol (continued)

48 49 50

Ursenol Ursenoic acid methylester Taraxasterol

chemical analysis which is vitiated by the number of components present as well as a lack of pure standards for comparison. GC –MS analysis of the actinomycete and fungal extracts show a very different chemical composition profile to the other samples, for example, the complete absence of any phytols. The actinomycete extracts, unlike the fungal extracts, were also free of any detectable sterols.

3.5. CP/MAS NMR analysis of the extracts and selected whole samples As GC-MS analysis is not optimal for non-volatile components, NMR techniques were employed. Solution NMR of the non-polar, chloroform soluble materials (spectra not shown) indicated the presence of strong methylenic signals characteristic of long chain fatty acids (Ma’shum et al., 1988). Solution state NMR could not be carried out with the polar wax fractions due to problems of solubility, hence solid state CP/MAS-NMR was carried out on the whole IPOM and IPOM size fractions (Fig. 5), and the polar extracts of IPOM (Fig. 6) and related plants and microorganisms (Fig. 7). The whole soil and particulate matter contained O-alkyl carbon (72 and 105 ppm) due to carbohydrates, and alkyl carbon (30 –32 ppm) due to polymethylene type structures found in fatty acids and waxes, as the predominant signals. Aromatic carbon signals (130 ppm) and phenolics (150 ppm) are also observed. The larger particles contained a higher composition of carbohydrates whereas the finer particles are enriched in alkyl carbon. This trend towards a build up of polymethylenic moieties is characteristic of the decomposition sequence of plant materials in soils (Baldock et al., 1992). In particular, sandy soils

Table 3 Composition of waxes extracted from NWS and associated organic matter Peak no.

NWS

Washd NWS

,53 mm

532150 mm

150–250 mm

.250 mm

Tree Litter

Fungi

Actinomycete

Leaves

Lupin

Lucerne

C8–C15 acids & methylesters C16 –C20 acids & methylesters C16 –C20 acids phytolesters Acid/alc. Esters C14, C18, C20 C20 –C30 alkanes Eicosane cylcohaexane C22 –C36 acids & alcohols Estatrione, Pregnandione Ergosterol, dienol Stigmast-erol, di-& trienol Cholesterol, adienol Ursenol, Taraxasterol, etc.

1, 4, 6, 7, 8

þ

þ

þ

þ

þ

þ

þ

þ

þ

þ

þ

þ

11–14, 18, 19, 26, 27 9, 16, 20, 28, 36

þ

þ

þ

þ

þ

þ

þ

þ

þ

þ

þ

þ

þ

þ

þ

þ

þ

þ

þ

W

W

þ

þ

þ

10, 15.17

þ

þ

þ

þ

þ

þ

þ

þ

W

þ

þ

W

21, 23 24

þ þ

þ þ

þ þ

þ þ

þ þ

þ þ

þ W

W W

þ W

þ þ

þ W

þ W

31–33, 37, 41

þ

þ

þ

þ

þ

þ

þ

W

W

þ

W

þ

35

þ

W

þ

þ

þ

þ

þ

W

W

þ

W

W

43, 44 40, 45, 46

þ þ

þ þ

þ þ

þ þ

þ þ

þ þ

þ þ

þ þ

W W

þ þ

W þ

W W

42

þ

þ

þ

þ

þ

þ

þ

þ

W

W

W

W

47–50

þ

W

þ

þ

þ

þ

þ

W

W

W

W

W


Compound class

45

46


Fig. 5. CP/MAS 13C NMR of IPOM from NWS—whole IPOM and different size fractions.

have been found to contain relatively higher ratios of alkyl C (Capriel et al., 1995) which is expected to contribute to water repellency. The polar extracts of the whole IPOM and the IPOM size fractions (Fig. 6) are comprised of the polymethylenic components with low amounts of aromatic compounds. This correlates with the data from the GC – MS analysis. The whole IPOM also contains larger amounts of short chain compounds (25 ppm) which are reduced in the smaller particles. Extracts of fungi (Fig. 7) presented spectra that were similar to the extracts of IPOM fractions and eucalyptus bark, even though the GC – MS profile

and hydrophobic behaviour is different from these materials. This discrepancy is not uncommon as the CP/MAS-NMR analysis indicates the presence of chemical groups, but does not provide detailed information on how these are linked to each other or their location. The power to discriminate between waxes or fatty acids with different polarities is also not high. Nevertheless, the spectra obtained for eucalyptus leaves shows the presence of polymethylenic material of shorter chain length (18 – 25 ppm) and proteinaceous material (45– 60 ppm). Protein and carbohydrate rich extracts were also observed in the extracts of actinomycetes and lupins. The lupin extract also showed the presence of aromatic compounds


47

Fig. 6. CP/MAS 13C NMR of the polar extracts (methanol-soluble fraction) of IPOM from NWS—whole IPOM and the different size fractions.

characteristic of lignins and tannins. The presence of these polar components correlates with the hydrophilic behaviour of the lupin and actinomycete extracts.

4. Conclusions The physico-chemical analysis of water repellent sandy soils from the southeast of South Australia indicates that the major component that contributes to hydrophobicity is a polar wax. The behaviour of this polar wax, which is present on the surface of the sand

grains and in the particulate organic matter (Franco et al., 1995), is mediated by the presence of low levels of a non-polar wax, that is highly hydrophobic, and the relatively high amounts of water soluble, hydrophilic components. The waxes appear to be similar, in chemical composition and hydrophobic properties, to those found in plant materials, such as eucalyptus trees found in the region. This correlates with the findings of Tegelaar et al. (1989) for the origins of aliphatic moieties in humic substances, though algal/ bacterial components are absent (Vella and Holzer, 1992). The direct contribution of microorganisms to

48


Fig. 7. CP/MAS 13C NMR of the polar extracts (methanol-soluble fraction) of plants and microorganisms associated with NWS—eucalyptus bark and leaves, lupins, actinomycetes and fungi.

the wax components appears to be low, though microbial activity is essential to the development and appearance of the polar wax materials. For example, the content of the highly non-polar waxes, found in fresh eucalyptus tree litter in this region, is reduced considerably in the particulate matter found

in the sands. This reduction is due to the activity of microorganisms that can degrade these fatty acids (data not shown). It is also expected that during the microbial degradation of plant constituents such as lignins and suberins, new polar organic materials are formed that contribute to the polar waxes.

ASSESSMENT OF SOIL WATER REPELLENCY

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Chapter 5 Characterizing the degree of repellency J. Letey*, M.L.K. Carrillo and X.P. Pang University of California, 900 University Avenue, Riverside, CA 92521, USA

Abstract Measurement techniques that quantify the degree of soil water repellency are important for research and for the communication of research findings. The water drop penetration time (WDPT) is a commonly used measurement. If a water drop does not enter the soil spontaneously, the soil – water contact angle is greater than 908 and the soil is considered to be water repellent. The time for the drop to enter the soil (WDPT) provides an indication of the stability of the repellency. The liquid– air surface tension of an aqueous ethanol concentration series that enters the soil in approximately 5 s is identified as the ninety degree (ND) surface tension, gND, of the soil. The gND number can be used to calculate the solid – air surface tension, gs, by gs ¼ gND =4: The water – soil contact angle can also be calculated from the gs value by the relationship cos u ¼ ½ðgND =gw Þ1=2 2 1; where u is the contact angle and gw the water – air surface tension. The water entry pressure, hp, which is a function of both the soil water repellency and pore size, is an important parameter for predicting infiltration and the stability of water flow in the field. Measurements of WDPT, gND, and hp provide a complete characterization of the degree of water repellency.

1. Introduction

2. Contact angle (0 –908)

Soil water repellency has become increasingly recognized as an important consideration in hydrology as evidenced by the attendance of participants from several countries at the International Workshop on Water Repellency held at Wageningen, Netherlands, September 1– 3, 1998. Measurement techniques that quantify the degree of soil water repellency are important for research and for the communication of research findings. Tschapek (1984) reviewed criteria for determining the hydrophilicity– hydrophobicity of soils. This paper reviews various approaches to characterizing soil water repellency.

An index of water repellency of plane surfaces is the contact angle between the liquid and solid. Soils do not provide planar surfaces that allow the geometric measurement of a contact angle. Thus, an alternative to geometric measurement is required. Soils have pores and occasionally have been represented as being composed of a bundle of capillary tubes. A capillary tube is a vast oversimplification of the complex geometric arrangement of soil pores. Nevertheless, helpful insight can be achieved by assuming that capillary tube model for soils. The capillary rise equation is

* Corresponding author. Fax: þ 1-909-787-5295. E-mail address: [email protected] (J. Letey). q 2003 Elsevier Science B.V. All rights reserved.

h ¼ 2gl cos u=r rg

ð1Þ

where h is the height of rise, gl the liquid –air surface tension, u the liquid – solid contact angle, r the capillary radius, r the liquid density and g

52

J. Letey et al.

the gravitational constant. According to Eq. (1), water will not spontaneously enter the soil if cos u is zero or a negative number (u equal to, or greater than, 908). A soil is commonly classified as being water repellent if a drop of water placed on the soil does not spontaneously enter the soil. By this convention, a water repellent soil is one which has a water – solid contact angle equal to, or greater than, 908. Soils classified as being wettable by this approach may have differing contact angels between 0 and 908 which can affect soil – water relationships such as infiltration rates. Letey et al. (1962a) reported a technique for measuring the water – solid contact angle for soils using a capillary rise approach. Assume that ethanol wets all soil with u equal to zero. According to Eq. (1) and using the subscript e to represent ethanol r ¼ 2ge =he gre :

ð2Þ

This value of r can be substituted into Eq. (1) when the height of rise is measured with water in the same soil. This leads to the equation for calculation of uw, where the subscript w refers to water cos uw ¼ hw ge rw =gw he re :

ð3Þ

The assumption that ethanol wets all soil materials at a contact angle equal to zero was used by Letey et al. (1962a) and Tillman et al. (1989) to measure the water repellency of materials that had an initial contact angle less than 908. Tillman et al. (1989) measured sorptivity of water and ethanol in soil columns and used the ratio of ethanol to water sorptivity as a repellency index. Letey et al. (1962a) measured infiltration rates of ethanol and water into soil columns to measure the water – solid contact angles. These indirect procedures circumvent the need for a geometric measurement of u, which is impossible in soils. Since water repellent soils are usually identified as those having a u value greater than 908, the remainder of this paper will be devoted to procedures for characterizing the level of repellency for these soils.

3. Water drop penetration time (WDPT) This procedure involves placing a drop of water on the soil and measuring the time for it to penetrate. Because of its simplicity, this procedure is almost always used, even if other procedures are also invoked. This procedure separates soils which are classified as being water repellent from those which are not. Since water penetrates the soil if u is less than 908, WDPT is a measure of the time required for u to change from its original value, which was greater than 908, to a value approaching 908. Therefore, it is a measurement of the stability of the repellency and not necessarily an index of u. Marmur (1988) reported that a water drop could penetrate a capillary tube that had a u value greater than 908 if the radius of the drop was small relative to the capillary radius. For example, a water drop to capillary tube radius ratio of 20 could penetrate a capillary tube that had a u value equal to 938. However, even though the drop could penetrate the tube, a portion of the drop remained outside the tube if u was greater than 908.

4. Ninety degree surface tension The liquid surface tension which wets the soil material with a 908 contact angle was proposed as an index of water repellency by Watson and Letey (1970). This procedure employs the concept that a liquid can only completely enter the soil if u is less than 908. A series of aqueous ethanol solutions producing various surface tensions is prepared for this measurement. Drops of these solutions are placed on the soil. The higher surface tension solutions will set on the surface and the lower surface tension solutions will spontaneously penetrate the soil. The 908 surface tension (gND) is the surface tension of the solution where there is transition from penetration to setting on the surface. Five seconds was arbitrarily chosen as the reference time. In other words, the solution surface tension which penetrates in 5 s is assumed to be the solution surface tension which wets the soil at 908. King (1981) proposed a similar procedure as the 908 surface tension approach except he recommended measuring the molarity (rather than surface tension)

Characterizing the degree of repellency

of ethanol in a droplet of water required for soil infiltration within 10 s. This procedure has been referred to as the MED test. King proposed a classification where soils with a MED index < 1 are not significantly water repellent and soils with a MED index > 2.2 are severely water repellent. In other cases, for example Dekker and Ritsema (1994b), the results of the ethanol drop test are reported in terms of the volumetric ethanol percentage. The 908 surface tension, MED index and volumetric ethanol percentage procedures are identical, only the reported numbers differ. One value can be converted to another, using the relationships depicted in Fig. 1. Butler and Wightman (1932) reported the experimental relationship between mole fraction of ethanol in water and the surface tension at 258C. These data were used to construct the results presented in Fig. 1. Since the ethanol purchased for laboratory research is usually 95% ethanol, the vol% numbers in the figure are for the 95% ethanol material. Therefore no correction is required if 95% ethanol is used in the laboratory. The molarity was computed on the basis that the molecular weight of ethanol is 50 g mol21 and its density at 258C is 0.79 g cm23.

53

As will be detailed later, gND is directly related to the solid –air surface tension. Therefore, it characterizes a fundamental physical – chemical property of the solid rather than merely providing an index. The relationship between gND and MED index or volumetric ethanol percentages is not linear. This factor becomes important when statistical procedures are used to determine significant differences in water repellency between soils. The results of statistical analyses using MED index values can produce misleading conclusions.

5. Solid– air surface tension The solid-air surface tension, gs, is a fundamental physical –chemical property of a solid which affects its wetting properties. Therefore, characterizing the magnitude of water repellency by measuring the solid – air surface tension would be valuable. Miyamoto and Letey (1971) derived an equation whereby gs could be determined by measuring the height of rise of liquids with various surface tensions. This procedure is cumbersome and requires the

Fig. 1. Relationships between liquid–air surface tension and the vol% of 95% ethanol in an ethanol–water solution and the molarity of an ethanol–water solution.

54

J. Letey et al.

liquids to be in contact with the soil for some time to reach the equilibrium height of rise. This time of contact between the liquid and solid could modify the wetting behavior just as placing a water drop on the surface does during the WDPT. Carrillo et al. (1999) used an alternative procedure for measuring gs based on combining theoretical relationships in a manner to obtain the relationship between gs and a measurable parameter. Assuming the solid –vacuum surface tension is the same as the solid – air surface tension, gs, and the liquid – vacuum surface tension is the same as the liquid –air surface tension, gl; then the solid – liquid surface tension, gsl, is (Good and Girifalco, 1960)

gsl ¼ gs þ gl 2 2Fðgs gl Þ1=2

ð4Þ

where F is a function of molecular properties of the solid and liquid. For a water – hydrocarbon system, F is approximately unity. Young’s (1805) equation is given by

gl cos u ¼ ðgs 2 gsl Þ:

ð5Þ

Combining Eqs. (4) and (5) leads to cos u ¼ 2ðgs =gl Þ1=2 2 1:

The capillary rise technique previously described is not a good procedure for measuring u when u is greater than 908. Water will not enter the column so capillary rise will not occur. This shortcoming could be overcome by immersing the soil column in water such that the hydraulic pressure would “force” water into the column. The height of capillary rise could be measured relative to the water elevation outside the column (a negative value). The contact time with the water would likely modify the value of u so that the result would not be reliable. Another approach can be developed by substituting Eq. (7) into Eq. (6) leading to cos u ¼ ½ðgND =gw Þ1=2 2 1:

ð8Þ

The measurement of gND leads to the computation of cos u as well as gs. Carrillo et al. (1999) experimentally verified the validity of these relationships. Therefore cos u can be determined for soils with u greater than 908 without extended exposure of the soil to water.

7. Water entry pressure head ð6Þ

Selecting gl so that cos u ¼ 0 ðu ¼ 908Þ; which is the ninety degree surface tension, leads to

gs ¼ gND =4:

6. Contact angle (u greater than 908)

ð7Þ

Therefore the rather simple procedure of measuring gND leads to a measurement of gs. This factor is a justification for reporting results in terms of gND rather than the MED index or volumetric ethanol percentage. The numerical value of gs is useful for predicting the behavior between the solid and various liquids. For example, the liquid – solid contact angle is zero when gs equals gl (Eq. (6)). If gs is greater than gl; u is less than zero, which is geometrically impossible. Under this condition, spreading of the liquid over the surface occurs. Miyamoto and Letey (1971) graphically depicted the relationships between gs, u, gl and the spreading coefficient.

When u is greater than 908, pressure must be applied to force the water into the soil. The pressure head required to initiate water entry into the soil is a function of both u and the soil pore radius (Eq. (1)). This water entry pressure head (sometimes referred to as breakthrough pressure) has relevance to field cases because it affects infiltration. Carrillo et al. (1999) described a laboratory apparatus for measuring the water entry pressure. Basically the apparatus consists of a tube into which the soil material can be packed. The soil is retained at the bottom of the column by a screen or other materials that allows air to escape from the soil. Two electrodes are placed to protrude into the soil just below the soil surface. One electrode is connected to a 1.5 V battery and the other is connected to a data logger input. A pressure transducer is placed above the soil surface and its output monitored with a data logger. Water is applied at a constant flow rate to the column. The depth of the water, as measured by the pressure transducer, is recorded as a function of time. The electrodes are

Characterizing the degree of repellency

shorted out when the water penetrates the soil and the signal is transferred to the data logger input. The data logger simultaneously collects the voltage and pressure as a function of time. The water entry pressure head is the pressure when the voltage signal appears on the data logger input. To avoid preferential infiltration along the container walls, the walls should be coated with a Teflon based dry film lubricant or other material to make the walls very water repellent before adding the soil.

8. Summary The degree of soil water repellency can easily be measured in the field or laboratory by measuring the ninety degree surface tension, gND. Both the solid – air

55

surface tension, gS, and the water – soil contact angle, u, can be computed from gND by using Eqs. (7) and (8). These data provide information on the initial water repellency of the soil. The degree of water repellency changes with time after contact with water. Measurement of the water drop penetration time, WDPT, provides information on the stability of the repellency, measurement of both gND and WDPT provides information on the degree and stability of water repellency. The water entry pressure head, which is a function of both water –soil contact angle and pore size, is an important parameter for interpreting infiltration and water flow stability in the field. This measurement can be made in the laboratory by an apparatus such as described above. Development of a convenient reliable method of making this measurement in the field is important.


Chapter 6

Sessile drop contact angle method J. Bachmanna,*, A. Elliesb and K.H. Hartgea a

Institute of Soil Science, University of Hannover, Herrenhaüser Str.2, D-30419 Hannover, Germany b Instituto de Ingeneria, Agraria Universidad Austral de Chile, Casilla 567, Valdivia, Chile

Abstract Existing methods for determining the contact angle as a measure of the water repellency of soils are either indirect, or cumbersome and time-consuming. Our objective was to develop a new method that is simpler than the existing procedures like the capillary rise method and that still yields accurate and reproducible results. To this end, we applied the so-called sessile drop method. To obtain the needed plane sample surface and to reduce geometrical effects, a single layer of air-dry soil particles is sprinkled on double-sided adhesive tape. Eight droplets of deionized water are placed carefully on this horizontal particle layer. Immediately after sample preparation, the contact angle at the three-phase boundary was measured with a goniometer-fitted microscope. In order to test the new method, contact angle measurements were carried out with wettable soil material that was hydrophobized, and with soils showing a wide range of natural water repellency. Both wettable silt and sand were made hydrophobic with Dimethyldichlorosilane and diluted with untreated soil to obtain a defined variation in the solid surface tension. Contact angle measurements showed a nonlinear decrease with increasing amounts of wettable particles in the 40– 908 contact angle domain. Generally, the contact angle decrease was predictable and could be described with an empirical model. For volcanic ash soils showing a wide range of repellency, the contact angles of different, narrowly sieved soils fractions (,20 mm, 20– 38 mm, 38– 63 mm, and 63– 100 mm) were within the range of 25– 1108. The impact of the different sieved soil fractions on the contact angles was found to be small. Measurements performed with independent replicates showed good reproducibility across the whole range of contact angles.

1. Introduction Wettability of soils is a property that has been investigated frequently because of its influence on water movement, particularly on infiltration. The wettability of solids cannot usually be determined directly. Therefore, a number of indirect methods have been developed. Wettability is often obtained from indirect measurements which represent the affinity of the porous media to adsorb water. The * Corresponding author. Fax: þ 49-511-762-5559. E-mail address: [email protected] (J. Bachmann). q 2003 Elsevier Science B.V. All rights reserved.

most frequent methods are the water drop penetration time test, the measurement of infiltration rate using ring infiltrometers, and the molarity of a water – ethanol – liquid required for a droplet to infiltrate into the soil within 10 s (Wallis and Horne, 1992). These methods are easily applied, but they are indirect and lead to relative numbers. The distinction between repellent and nonrepellent soils is somewhat arbitrary. A further disadvantage of these methods is that they do not consider hysteresis effects. Other methods like the determination of the contact angle from water retention curves or capillary rise experiments are time consuming and cannot be used for extremely repellent soils. For these soils, a

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J. Bachmann et al.

positive hydrostatic pressure is necessary to initiate water flow into the pore system. This is timeconsuming and difficult to arrange (Fink and Myers, 1969). A means of more directly assessing the wetting properties of a solid surface would be the measurement of the contact angle (Adamson, 1990). For measuring precise contact angles a homogeneous and absolutely plain surface is needed. To obtain a sufficiently large plain surface, Hartge et al. (1986) and Ellies and Hartge (1994) suggested the measurement of sessile drop contact angles on plane glass slides buried in the soil. Wetting angles against water were measured on the thin coatings that were developed on the wettable glass surface with a special microscope. Values were considerably smaller, however, than those measured for dry soil. Since it is impossible to have a sufficiently large flat area for a direct angle measurement on single soil particles, it was attempted to keep the unevenness of a plain, which is given by the form of the soil grains, homogeneous (Bachmann, 1988). Therefore, the measurements were performed on thin layers of narrowly sieved fractions which were fixed on an adhesive tape. This approach is comparable with the technique of Valat et al. (1991) who used compressed pellets. In this paper, the development of the sample preparation technique for this sessile drop method is discussed. To measure the impact of chemically heterogeneous rough surfaces on the contact angle, mixtures of silt and sand fractions were prepared which were made hydrophobic to varying degrees by the use of Dimethyldichlorosilane. The results obtained across this large range of particle sizes (20 – 200 mm) were analysed with empirical models based on the Cassie equation which was developed for random small scale heterogeneity (Drehlich, 1997). Further, attempts were made to analyse the range of measurable contact angles in the low angle domain. The variations in the liquid surface tension allowed a comparison to be made between the observed linear relationship between cos u and sl on ideal smooth surfaces with nonideal rough and heterogeneous surfaces. Finally, the sensitivity and reproducibility of the measurements and the influence of the size of the sieved soil fractions were tested by measurements

on soils showing a large variation in natural water repellency.

2. Theory If a drop of water is placed on hydrophobic or partly wettable surfaces it does not spread completely. It assumes a shape that depends on the relation between the free energies of the three involved surfaces. The angle that it develops at the threephase line is called the wetting or contact angle u (Fig. 1). It depends on the relation between the three interfacial energies (s ); liquid – vapour (slv), solid – vapour (ssv), and solid –liquid (ssl). This relation is known as Young’s equation and is formulated in the following equation

slv cos ui ¼ ssv 2 ssl

ð1Þ

Since Young’s equation is strictly applicable only to completely uniform and plain surfaces, two modifications have been applied to consider more realistically the properties of solid surfaces. On rough surfaces, the observed contact angle differs from the angle for an ideal smooth solid surface (Adamson, 1990; Hazlett, 1992). The observed contact angle (uobs) is smaller than the ideal (intrinsic) angle (u ) as long as this is below 908 and larger if the intrinsic angle (u ) is above 908. Therefore, a correction factor (r ) was introduced (Eq. (2)). It is defined as the ratio between the actual and the apparent or projected area. This leads to the so-called Wenzel equation: cosðuobs Þ ¼ r cosðuÞ

ð2Þ

which implies further, that the precision of the contact angle measurement depends on the magnitude of the contact angle. In order to measure small contact

Fig. 1. Shape of a small droplet on an ideal uniform surface with a medium water repellent surface.

59

Sessile drop contact angle method

angles accurately, the surface used must be much smoother than when large contact angles are measured (Zisman, 1964). The extent of roughness has the smallest impact in the important transient region between medium and strong repellency around 908. Chemical heterogeneity of the solid may further alter the observed contact angle according to Eq. (3), where f1 is the fraction of the surface having intrinsic contact angle u1 and f2 is the fraction of the surface having intrinsic contact angle u2 cosðuobs Þ ¼ f1 cosðu1 Þ þ f2 cosðu2 Þ

Table 1 Texture of natural repellent volcanic ash soils (topsoil) Sampling site

3. Materials and methods Two groups of soil materials were used in this investigation: (1) Volcanic ash soils with different ages and showing a large range of natural water repellency. This material originates from two sites near Valdivia (southern Chile, annual mean temperature 128C, mean precipitation 2500 mm). The Andosols are younger, less wetting and contain less clay than the Humults. Details are given in Table 1. From these samples specific fractions were obtained by sieving: , 20, 20 –38, 38 – 63 and 63 – 100 mm. (2) Laboratory quartz sand and silt subsoil (Chorizon) from a Weichselian loess under agricultural use (Table 2). Because of their narrow particle size distribution these materials were not sieved. Part of both was made hydrophobic by coating the grains with Dimethydichlorosilane (Shaw, 1975). 50 ml per kg was applied on the sand and 90 ml per kg on the silt. Untreated sand and silt, respectively, were added to the treated material to give samples with 100, 95, 85, 80, 70, 60, 50, 35, 20 and 10% of treated solid surface.

Silt (%) (2–63 mm)

Sand (%) (63–2000 mm)

Ultisols

Profile 1 Profile 2 Profile 3 Profile 4

63 66 62 61

24 22 26 27

13 13 13 13

Andosol

Profile 1 Profile 2 Profile 3 Profile 4 Profile 5 Profile 6

23 23 23 20 21 18

66 67 66 70 70 72

12 11 11 11 10 10

ð3Þ

The so-called Cassie equation (Eq. (3)) is considered as an empirical approach which describes the apparent contact angle on a chemically heterogeneous surface (Yekta-Fard and Ponter, 1992). The uobs is the angle that can be observed with a microscope and in the following simply denoted as u.

Clay (%) (,2 mm)

Samples were prepared for measurement by sprinkling air-dry sieved material on an adhesive tape of about 6 cm2 area which was fixed on a smooth microscope glass slide. Grains were loaded with 100 g weight for about 10 s. The slides were then knocked carefully to remove surplus grains. This procedure was repeated twice before starting measurement of the contact angle. Immediately after sample preparation the contact angles were measured. Eight 2 ml water drops were placed on the sample surface using a microsyringe. Thus the minimum drop diameter was at least 20 times larger than the average grain diameter. For the sand-samples 10 ml drops were produced. Deionized water was used and mixtures of ethanol – water with a surface tension (slv) between 25 and 76 mN/m. To assess the effect of evaporation, time dependent contact angle measurements were made on the sand and the silt samples (100 and 50% hydrophobic grains), and on three volcanic ash soil samples. Within 40 min, eight contact angle measurements Table 2 Texture of the two soil materials used for hydrophobization Soil

Particle size fraction (kg £ kg21) £ 100 Particle size class (mm) ,2 2–6 6– 20 20–63 63 –200 200–630 630–2000

Sand 0.0 Silt 2.6

0.1 1.4

0.2 15.1

0.2 70.0

8.5 6.0

90.5 4.8

0.4 0.1

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J. Bachmann et al.

(10 replicates at each time step) were taken. For these soils, the drop sizes were also varied in order to assess the range of stable, drop size independent contact angles. Maximum and minimum drop sizes varied by a factor of four; the reference size was 10 ml for the sand, and 2 ml for all other samples. Contact angle measurements were performed with a microscope which was fitted with a goniometer scale (Burghardt, 1985). Readings were taken at the threephase contact line. Accuracy of the measurements was approximately ^ 1.58 within the range of 10– 1708. On the samples from volcanic soils, 16 readings of angles were taken from the eight drops within 90 s. Readings of the angles on the sand and silt (five drops, 10 readings per sample) were performed twice. First, 10 readings within one minute after placing the drops and second 10 measurements per sample after the fifth minute. All contact angle measurements were performed at a relative humidity of 50% in the laboratory. Preliminary results had shown that the selected drop sizes were within a stable range of contact angles compared with larger or smaller drops. For these drop sizes it was assumed that gravity effects were not significant (Carroll, 1992). Additionally, the sand and silt samples with 100% hydrophobic grains were used for the standard water drop time penetration time test (WDPT). Soil storage and drying and humidity during the measurement was the same as for the soils used for the contact angle measurement. Three drops (drop volume 50 ml) were placed on the soil surface and the time for the second drop was reported.

4. Results and discussion Measuring the contact angle of sessile drops on rough, heterogeneous solid surfaces involves some difficulties. Air entrapment at the liquid –solid interface can cause drastic heterogeneity of the surface. For this reason the droplets were observed with a microscope and air inclusion was observed and noted. Fig. 2 shows the top view of the three phase line of a droplet (drop volume 20 times larger than used for standard contact angle measurements) on the coarse sand and on the silt surface. With the exception of

some very small air inclusions at the solid – liquid interface of the sand no considerable air bubbles were observed. The contact angle for 100% hydrophobic grains is about 958 for the sand, and 78 for the silt; the contact angle for the untreated grains (sand and silt) was assumed to be 08. For water the intrinsic contact angle measured on hydrophobized smooth glass surfaces is about 92– 938. The uppermost photograph on Fig. 2 shows the sand with contact angle 95% (100% hydrophobic sand grains, Fig. 3). The contact angle for the silt is 708, measured on 90% hydrophobic silt grains (Fig. 4), and the third photograph shows complete wetting which corresponds to a nonmeasurable contact angle (10% hydrophobic grains, Fig. 4). This figure shows also the homogeneity of the coating of soil particles and a few small local deformations of the three phase contact line for the silt with contact angle 708. The observation indicates that the surface is wetted nearly completely, even for contact angles greater than 908. The penetration of a liquid of a droplet into the porous surface is different from that of liquid connected with an infinite reservoir. It was found that the critical contact angle value, which distinguishes between penetration and nonpenetration, depends on the drop size, and can be much higher than 908 due to the positive pressure within the drop compared to the atmospheric pressure. Practically complete penetration into a capillary can be achieved for contact angles up to about 1148 for small drops (Marmur, 1988). Fig. 3a shows contact angle of the sand plotted as a function of the ratio of hydrophobic grains to untreated wettable grains. The contact angles for mixtures between 0 and 100% hydrophobic particles are close to the graph predicted with Eq. (3). This agrees with the results of other authors who have worked with chemically heterogeneous surfaces. A comprehensive review paper on contact angle dependence of chemical heterogeneous surfaces is reported by Drehlich (1997). The lower plot (Fig. 3b) shows the calculated ratio of the area covered by the grains related to the area of the slide. For the considerably finer particle fraction 20 –63 mm (Fig. 4), the situation is similar for the mid and high contact angle domain. On more hydrophilic mixtures, which corresponds to contact angles smaller than 308, the liquid spreads probably due to the increased horizontal capillarity


61

Fig. 2. Top view of the three-phase line of a droplet on nonwetting coarse sand (above), on nonwetting silt (centre), and the horizontally wetting front on wettable silt (below).

forces of the surface (Marmur, 1992). It is likely that for this particle fraction the contact angle measurements are restricted to values larger than 408 degrees. To spread the low angle domain, Chassin et al. (1986) pointed out that for wettable soils the sessile drop method can probably extended to lower angles when the vapour phase is replaced by a second liquid phase. From Figs. 3 and 4 it can be seen that values tend to decrease as time elapses between placing the drop and

reading of the contact angle. The WDPT confirms generally the measured contact angles. The WDPT for the sand with 100% hydrophobic grains (CA 958) was . 3600 s, for the silt with 100% hydrophobic grains (CA 788) a WDPT of 28 s was measured. According to Dekker (1998) the sand can be rated as extremely water repellent and the silt as slightly repellent. Another option to achieve a defined variation of the surface tensions is to decrease the liquid surface

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J. Bachmann et al.

Fig. 3. Contact angle of sand as a function of the proportion of hydrophobized grains (a) and area covered by particle coating as related to the area of prepared sample on the tape (b). Means and standard deviation from five drops and 10 readings for two times of readings after placing the drop.

tension slv and to keep the solid surface tension ssv constant. Fig. 5 shows the cosine of the contact angle u as a function of the surface tension of the liquid. The critical surface tension where the liquid wets the solid completely can be found at the intersection of the regression line and the zero contact angle line ðcos u ¼ 1Þ and is denoted by sc. In general, a rectilinear relation was found according to Eq. (1) between the cosine of the contact angle and the surface tension slv of organic liquids or mixtures of liquids (Zisman, 1964). Zisman stated a value of the critical surface tension of 24 mN/m for CH3-groups, which is close to the experimental value found on the hydrophobic smooth glass surface (Fig. 5). For the rough silt and sand surfaces (100% hydrophobic grains) the value of the critical surface tension was 5 mN/m greater which could probably be the effect of roughness or the effect of an incomplete silane

coating. Generally, the data measured on nonhomogeneous rough surfaces show for liquid surface tensions smaller than 40 mN/m a linear relationship comparable to measurements on the smooth glass surface. This result confirms further the linear relationship between cos u and slv found earlier by Watson and Letey (1970) for porous media. The contact angles measured on the naturally water repellent soils cover a wide range of 25– 1108 (Figs. 6 and 7). On rough surfaces consisting of material for which, if smooth, the equilibrium contact angle would be different from 908, the slopes of the asperities will be a very important factor in determining the effective equilibrium contact angle (Eick et al., 1975). The data measured on the soil fractions with different particle diameter indicate, however, that the means of the angles deviate within a range smaller than 208. The combined impact of grain size, grain shape, microroughness, and particle size on the apparent contact


63

Fig. 4. Contact angle of silt as a function of the proportion of hydrophobized grains (a) and area covered by particle coating as related to the area of prepared sample on the tape (b). Means and standard deviation from five drops and 10 readings for two times of readings after placing the drop.

angle of soils was, therefore, small for most cases. This result was found to be valid for all soils of different texture and all degrees of repellency (Fig. 6). Since standard deviation as shown in Figs. 3– 5 are large, reproducibility was tested by comparing means from groups of eight drops on two separate preparations (Fig. 7, A and B). Almost all the data points were close to the 1:1 line, indicating that data from different samples of the same material was reproducible. Despite the reproducibility of this method, it is evident that the analytical character of contact angle measurements on ideal homogeneous and smooth surfaces cannot be transposed to our samples due to the effect of roughness and chemical heterogeneity of the solid surface. The results, however, illustrate that reproducible apparent contact angles can be obtained with the same small amount of time for a wide range of repellency and surfaces. Several investigators have shown that the advancing contact angle of microscopic drops is comparable to the initial contact angle

Fig. 5. Contact angles of rough grainy (sand: below, silt: centre) and smooth material (glass microscope slide: above) with aqueous ethanol solutions of varying surface tension. All materials were hydrophobized with Dimethydichlorosilane. Arrows mark the critical surface tension of close packed methyl groups.

measured on macroscopic drops that were placed on the surface (Jung, 1992; Liukkonen, 1997). Good (1993) reported that the contact angle of sessile drops are nearer to the advancing angle, which is measured if the drop volume is increased, compared with the receding angle observed when the drop volume is decreased. The difference of angles measured within 1 min and after 5 min as shown in Figs. 3 and 4 might be considered to represent the transition towards the receding contact angle due to evaporation. We found that during evaporation the contact angles decreased

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Fig. 6. Contact angles of volcanic ash soils in relation to grain size of selected fractions. For profile numbers see Table 1.

Fig. 7. Reproducibility of measurements shown by the mean values of angles measured at eight drops on one slide (A) plotted against the mean of a new preparation (B).


linearly with time until the drop evaporates completely. Thus constant contact angles were not observed during the evaporation process, and the solid – liquid interface area remained constant. Therefore, it was concluded that the receding angles were small or not observable (Herzberg et al., 1970). This result confirms as well, that the solid surface should be wetted without considerable air entrapment. This conclusion was drawn because the formation of a small contact angle hysteresis, e.g. observable receding angles, indicates the formation of composite interfaces due to air entrapment (Johnson and Dettre, 1964). Good and Koo (1979) found that the contact angle formed by a drop of liquid varies with drop size below a critical drop diameter. Our investigations show that the drop angle varies approximately ^ 38 when the drop size was increased or reduced by a factor of four. This result indicates a stable configuration between the scale of the heterogeneities and the size of the drop.

5. Conclusions The importance of surface roughness and chemical heterogeneity on wettability is obvious. Quantification of both effects is difficult and for heterogeneous soil particles practically impossible. A way out of this dilemma is to prepare quasi-smooth surfaces by limiting the heterogeneity of the grains. This was obtained by isolating narrowly sieved fractions and fixing a monolayer of these grains on the plane surface provided by an adhesive tape. The size of the selected fraction did not greatly influence the results (Fig. 6). Compared with earlier investigations of smooth glass slides which had been buried in soil profiles (Hartge and Ellies, 1995), the contact angles measured here were generally higher (Ellies and Hartge, 1994). Our results with different grain fractions and with liquids of different surface tensions show, that contact angles can be measured reproducibly even in the range

65

considerably smaller than 908. As our results indicate, the contact angle variation due to a reduced surface tension of the solid surface on rough surfaces followed an equation developed for microscopic heterogeneity on a much smaller scale compared with our system. The observed contact angles obtained from measurements on the soils with natural water repellency were within the wide range from 25 to 1108. The results imply that the sessile drop method yields a wider range of values compared with the water drop penetration time test which has a sensitivity range in the narrow interval around 908 (King, 1981). Because of the wide contact angle range and the same time demand for the whole range the goniometric measurement is convenient to apply. An advantage of the new method is that no chemicals are used and effects like the replacement of molecules from the surface structure or adsorption of vapour or liquid are avoided. On the other hand, the mechanical impact due to sample preparation is small. According to King (1981), soils can be carefully sieved without greatly altering the repellency. Comparable mechanical strain as imposed by the pellet preparation described by Chassin et al. (1986) is avoided. Another advantage of the technique presented here is that only a few grams of soil are needed to prepare a sample. Therefore, investigations on the variation of water repellency can be performed even on a very small scale. These advantages, however, are counterbalanced by the fact that this method is not suited to the measurement of actual water repellency at different moisture states as they appear in situ. However, this measurement can be performed at relative humidities between 0 and 100% and can therefore be considered as a method for indicating the potential repellency. Further work should include investigations on the effect of temporary drying after sampling, of storing between collection and measurement and of the influence of microaggregation in the selected grain size fraction, and the measurement of the receding contact angle.


Chapter 7

Water-entry value as an alternative indicator Z. Wang*, L. Wu and Q.J. Wu1 Department of Environmental Sciences, University of California, Riverside, CA 92521, USA

Abstract Soil water-repellency is an increasingly important consideration in hydrology. In this paper, we relate the degree of soil water-repellency and wettability to the critical water-entry value of a soil. A water-ponding method was used for simple measurement of water-entry value in repellent soils. A tension – pressure infiltrometer method was demonstrated for measuring water-entry value in both repellent and wettable soils. The measurement techniques were used to detect a sudden breakdown of repellency under a sufficiently high water pressure. Experimental results have proven that the water-entry value, in terms of soil water potential, is positive in repellent soils, and negative in wettable soils or soil conditions. The water-entry value is shown to be an easily measured indicator of repellency or wettability that provide an assessment of hydraulic effects of soil physical, chemical and biological properties.

1. Introduction Soils containing large amount of hydrophobic materials (plant litter and residues, organic fertilizers and pesticides, etc.) may become water repellent or less wettable. The degree of repellency and wettability is traditionally judged using the water – solid contact angle (g ). A soil is classified as being water repellent if g . 908 and water wettable if g , 908: However, due to gradual breakdown of soil waterrepellency and the granular soil surface condition, direct measurement of the contact angle has not been possible. Presently, many indirect methods are used to measure soil water-repellency. The simplest and most common and practical method used to measure water-repellency is the water drop penetration time (WDPT) test (Van’t Woudt, * Corresponding author. Tel.: þ 1-909-787-6422, fax: þ 1-909787-3993. E-mail address: [email protected] (Z. Wang). 1 Present address: Department of Biological System Engineering, Washington State University, Pullman, WA 99164, USA.

1959, 1969; Letey, 1969). Three drops of distilled water from a standard medicine dropper are placed on the smoothed surface of a soil sample, and the time that elapses before the drops are adsorbed is determined. A soil is considered to be water repellent if the WDPT exceeds 5 s (Bond and Harris, 1964; DeBano, 1969d), which reflects the gradual breakdown of the soil water-repellency. Based on this method, Dekker and Ritsema (1994b) classified the Holland soils into five repellency classes: (1) wettable soil for WDPT , 5 s; (2) slightly water repellent soil for WDPT ¼ 5 – 60 s; (3) strongly water repellent soil for WDPT ¼ 60 – 600 s; (4) severely water repellent soil for WDPT ¼ 600 2 3600 s; and (5) extremely water repellent soil for WDPT . 3600 s: Another common method for characterizing soil water-repellency is the alcohol percentage (AP) test (Letey, 1969; Watson and Letey, 1970). Water containing increasing concentrations of ethanol is applied in drop form to the surface of soil samples until a concentration is reached where immediate infiltration occurs. A high concentration indicates

q 2003 Elsevier Science B.V. All rights reserved.

HYDROL 7—28/2/2003—15:05—BABU—140 – MODEL 3 – br,ed

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severe water repellency. While the water drop penetration time was designed to measure the gradual breakdown of repellency, the alcohol percentage test was used to induce and measure the instantaneous breakdown of repellency. In this paper, we use water-entry value as an alternative indicator of soil water-repellency. By first imposing a low hydraulic pressure at the soil surface to prevent initial wetting of the soil and then increasing the pressure, a critical pressure (or water-entry value) was observed at the instantaneous breakdown of repellency or the start of infiltration. A water-ponding (WP) method was used for measuring the water-entry values of repellent soils, and a tension-pressure infiltrometer (TPI) method for measuring waterentry values in both wettable and repellent soils. Sample experimental results are presented to show the effects of soil initial moisture and organic matter contents on the change of water-entry value.

2. Hydraulics of repellency and wettability 2.1. Water-entry and air-entry values The concepts of entry or bubbling pressures are used in fluid mechanics to characterize the start of fluid – fluid displacement in a porous medium (Bear, 1972). When a wetting fluid starts to displace a nonwetting fluid initially saturating the porous medium, the displacement capillary pressure (or suction) is called the wetting-fluid bubbling or entry value. Inversely, when a nonwetting fluid ðg . 908Þ starts to displace a wetting fluid, the threshold capillary pressure is called the nonwetting-fluid bubbling or entry value. The capillary pressure (hc) is defined as hc ¼ hnw 2 hw

ð1Þ

where hw is the wetting fluid pressure at the fluid – fluid interface, and hnw is the nonwetting fluid pressure at the interface. In the unsaturated zone, water is the wetting fluid and air is the nonwetting fluid. In a hydrophobic porous medium, however, water becomes a nonwetting fluid and air is the wetting fluid. If the soil air pressure hnw below the wetting front is zero (i.e. at the atmospheric pressure), the capillary pressure is given by hc ¼ 2hw

ð2Þ

Thus, the water-entry value (hwe) and air-entry value (hae), in terms of capillary pressure heads, are equivalent to the negative of soil water potential. In this paper, we choose to express the entry values in soil water potentials for conveniences of hydraulic considerations. The water-entry value (hwe) is hitherto referred to as the critical soil water potential (hw) at which water starts to displace air in the porous medium. The air-entry value (hae) is then the critical soil water potential (hw) at which air starts to displace water in a porous medium. Therefore, the infiltration process involves water-entry at the wetting front and the drainage process involves air-entry at the soil surface. Detailed analyses of the entry values were also described by other authors (Bear, 1972; Hillel, 1980; Hillel and Baker, 1988; Jury et al., 1991; Kutilek and Nielsen, 1994). Since water-entry value is the threshold for infiltration, and air-entry value for drainage, entry values can be estimated from the soil water retention curves (SWRCs). Although many authors have proposed various methods for estimating the values of hwe and hae based on SWRCs (Bouwer, 1964, 1966; Brooks and Corey, 1966; Mein and Larson, 1973; Morel-Seytoux and Khanji, 1974; Neuman, 1976; Brakensiek, 1977; Morel-Seytoux et al., 1996; Wang et al., 1997), these methods were found to result in very similar average values of hwe and hae (Mein and Larson, 1973; Wang et al., 1997). According to the most recent study of Wang et al. (1997), the values of hwe and hwe corresponding to the inflectional capillary pressure, hpc ; on the wetting retention curve and the drainage retention curve, respectively, can be determined using 1=n 1 n21 p hc ¼ a nðm þ 1Þ 2 n þ 1 ( 1=n m =a for m ¼ 1 2 1=n ð3Þ ¼ 1=a for m ¼ 1 2 2=n The corresponding water saturation, Spe ; on the retention curves can be calculated using n21 m Spe ¼ 1 2 nðm þ 1Þ ( ð1 þ mÞ2m for m ¼ 1 2 1=n ð4Þ ¼ 0:5m for m ¼ 1 2 2=n where, a, m and n are parameters of Van Genuchten (1980) model for SWRC.


Water-entry value as an alternative indicator

The above SWRC method gives indirect estimations of static entry values for wettable soils. The entry values for a wettable soil can also be directly measured using a pressure infiltrometer method (Fallow and Elrick, 1996). In their method, the soil was initially saturated under positive pressures, then was gradually desaturated until air entry into the infiltrometer. This was followed by water entry into the soil. However, the water-entry value of the soil at the initial water content could not be detected in the process. In a recent experiment, Wang et al. (1998b) found that the water-entry values for repellent soils were positive ðhwe . 0Þ: An initially repellent soil was not wetted until a critical depth of ponding was reached. This critical ponding depth was considered the waterentry value of the repellent soil, as shown on a positive wetting retention curve in Fig. 1. Since soil water-repellency disappears when the soil is initially saturated or at a high initial water content (Dekker and Ritsema, 1994b), the drainage retention curve of a repellent soil should be qualitatively the same as that for a wettable soil. The water-entry value (hwe) is clearly affected by soil texture, structure, initial water content and the contact angle (g ). In wettable soils ðg , 908Þ; hwe was considered being equal to the

69

capillary rise (Kutilek and Nielsen, 1994). In repellent soils, hwe should then be equivalent to the capillary drop. 2.2. Potential and actual repellency and wettability A soil is less wettable when it is dry and contains organic matters. The contact angle increases with the increase of organic matter content. However, the repellent soil becomes wettable when soil water content is above a critical value. The potential water repellency was defined and measured on dried samples, whereas the actual water repellency was measured on samples with an initial water content (Dekker and Ritsema, 1994b). Dekker and Ritsema (1996b) also showed that the potential repellency increased with the drying temperature in the oven. The soil became more repellent under a drying temperature of 658C than under 258C. Both the soil water-repellency and the critical water content were dependent on the hydrophobic organic matter content in the soil. For convenience of integrated analyses along with other soil properties (e.g. dry bulk density), we recommend here to measure the potential soil water wettability and potential soil water-repellency on samples dried under 1058C for at least 24 h. The dried samples need to be cooled down to room temperature before measurement for potential water-entry values. The actual soil water-repellency and wettability are measured under other conditions with initial soil water contents.

3. Methods for measuring water-entry value 3.1. The water-ponding method for measurement of soil water-repellency

Fig. 1. Soil water retention curves for a wettable and a repellent soil. The hwe and hae denote water-entry and air-entry values of the porous soils, respectively.

In this method, the repellent soil is packed into a transparent tube (2 – 5 cm i.d.), as shown in Fig. 2a. The inside wall of the test tube should be treated with repellent materials (e.g. Teflon dry film lubricant) before packing to prevent preferential edge flow down the tube. The soil sample should be placed on a porous plate or cheesecloth to prevent air entrapment during the test. The soil surface must be leveled and covered with a filter paper or cheesecloth under the porous plate to prevent soil surface disruption by water flow.


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Fig. 2. WP method for measurement of water-entry value (hwe) of a repellent soil (contact angle g . 908Þ : (a) water ponding on soil surface; (b) equivalent capillary depression method; and (c) Ring insertion in the field.

By increasing the ponding depth of water on the soil surface (Fig. 2a), one will notice a critical depth at which water suddenly starts to infiltrate into the soil. This critical depth is the water-entry value, hwe, of the repellent soil. The magnitude of hwe indicates the degree of water repellency. An equivalent method is to lower the transparent tube into the water, as shown in Fig. 2b, the water-entry value equals to the critical capillary depression at which water starts to infiltrate into the soil. The in situ water-entry value of a repellent field soil can be measured by using an insertion ring (5 – 10 cm i.d.), as shown in Fig. 2c. Water is added to soil surface inside the ring until the water-entry value is observed. 3.2. Tension-pressure infiltrometer method The water-entry value can also be used to evaluate wettability of a soil. In this case, the soil –water interface is initially provided with a sufficiently high suction to prevent the soil from being wetted. The suction is gradually reduced until water enters the soil. For the purpose of measuring potential and actual wettability ðg , 908Þ in terms of water-entry value at any initial water content, a TPI was designed based on designs of tension infiltrometer (Perroux and White, 1988) and Guelph pressure infiltrometer (Fallow and Elrick, 1996).

As shown in Fig. 3, the TPI is composed of five parts: a transparent Mariotte reservoir R1, a transparent tension tower R2, a transparent Vinyl extension tube, a cylinder-shaped infiltration head, and a porous disk glued to the bottom of the infiltration head. This design of TPI is slightly different from the design of a tension infiltrometer (Perroux and White, 1988) where the infiltration disk was a perforated plastic plate wrapped with a layer of Nylon membrane. Since the Nylon cloth was easily broken or clogged at the soil surface, we used a porous ceramic disk in the places of the plastic plate and the Nylon cloth. The capillary airentry value of the ceramic disks (5 cm in diameter) was in the range of 30– 70 cm of water height. The TPI can be hung on a tripod with height adjustment mechanisms to maintain a minimum and constant pressure at infiltration surface (disk –soil interface). The flexible extension tube was used to adjust the infiltration head to achieve good contact between the soil and the infiltration disk. The soil water pressure at the infiltration surface is regulated in the tension tower by adjusting the height e1 of the bubbling tube relative to e, the vertical distance between the air tube and the disk surface. When the bubbling tube is set at the “base line” defined by e1 ¼ e; the pressure at the infiltration surface is zero. When e1 . e; a negative pressure head (suction) is imposed, and when e1 , e; a positive pressure head (ponding) will result. The water



71

of water reservoir R1 using a syringe attached to valve V2, until water level, z2, in the bubbling tube decreased close to (but not exceed) z2 ¼ z1 2 e 2 c; where c is the air-entry (bubbling) value of the ceramic disk; and (4) turn off valve V2. Once the infiltrometer head is placed at the soil surface, water will not infiltrate due to the condition h0 , hwe . By gently moving the bubbling tube upward or releasing air through a small needle, soil water potential at the infiltration surface increases. One will notice a critical value of zp2 and zp1 (Fig. 3) at which water starts infiltrating with a noticeable flow rate. The critical value hp0 ¼ e 2 ðzp2 2 zp1 Þ is the water-entry value (hwe) of the soil (Fig. 3). The value of hwe for hydrophobic (or water repellent) soil, although could be easily measured using the WP method (Wang et al., 1998b), can also be measured using TPI following the above procedure. An insertion ring (Fig. 2c) should be attached to the disk head and inserted into the soil. The critical value of hp0 ¼ e 2 ðzp2 2 zp1 Þ for a repellent soil is positive.

3.3. Example measurement results

Fig. 3. Tension–Pressure Infiltrometer method for measurement of water-entry value (hwe) to quantify the soil water wettability or repellency. V1 is a two-way valve and V2 is a three-way valve.

pressure head (h0) at the infiltration surface is calculated as h0 ¼ e 2 e1 ¼ e 2 ðz1 2 z2 Þ

ð5Þ

where z1 is the water level in the tension tower and z2 the water level in the bubbling tube (Fig. 3). The procedure to measure the wettability (waterentry value) of a soil using a TPI starts from a high negative initial pressure at the soil surface. The specific procedures include: (1) push down the bubbling tube to the bottom of the tension tower; (2) hold the infiltrometer head in the air (i.e. not in contact with or directly above the soil surface), turn off valve V2, and turn on valve V1 in sequence (Fig. 3). Water will flow out from the infiltration disk with a decreasing rate; (3) extract air out of the head space

Both repellency and wettability vary with soil type and with state variables such as initial water, clay and organic matter contents, soil pH values and temperature or fire experiences, the presence of fungal mycelium, and others (Letey et al., 1975). Example measurement results using WP and TPI methods are presented here to demonstrate the effects of initial soil water, clay and organic matter contents on repellency and wettability of selected soils. Three types of oven-dried soils, a water-repellent coarse sand, a water-wettable fine sand, and a waterwettable loamy sand (Table 1) were used for determining their potential repellency or wettability. Properties of the three-layer Ouddorp repellent sands were described in detail by Ritsema et al. (1993). The silicon fine sand was purchased from a local store. The loamy sand was taken from an agricultural field in Bakersfield, California. The bulk samples of Bakersfield soil were taken from two neighboring plots: one treated with 50 tons per hectare of dairy manure, the other was not treated with any organic amendment. The water-entry values (hwe) of the repellent sands were measured using the water


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Table 1 Properties of the packed porous media used for measurement of water-entry value Medium type

Water repellent sandsa 1st layer (humose topsoil) 2nd layer (transitional) 3rd horizon (bottom) Water wettable sand Initially dry Initially wet Wettable loamy sand Manure treated (50 ton/ha) Without manure a

Dry bulk density gd (g/cm3)

Initial water content u0 (cm3/cm3)

Organic matter content OMC (% wt.)

Clay content CC (%)

1.41 1.54 1.59

0 0 0

20 4 ,5

,3 ,3 ,3

1.52 1.52

0 0.31

0.15 0.15

1.61 1.69

0 0

1.12 0.89

1.6 1.6 8 10

Water-entry potential hwe (cm H2O)

Water drop penetration time (min)

12 7 2

.60 10 –60 1–10

211 223

0 0

213 225

0 0

Sands of Ouddorp, The Netherlands.

ponding method (Fig. 2a). The entry values of the wettable soils were measured using the TPI method (Fig. 3). As shown in Table 1, the repellent sands exhibited positive water-entry values (hwe), and the wettable soils had negative values. Fig. 4 shows the effects of initial water content on the magnitude of water-entry value. The initially dry silicon sand exhibited a water-entry value hwe ¼ 211 cm of water (Fig. 4a), whereas the initially wet sand had hwe ¼ 223 cm of water (Fig. 4b). These two values show the strong dependence of wettability on the initial water content. This also indicates that the unsaturated flow will be more likely to enter the initially wet sand rather than the initially dry sand. As shown in Fig. 5, soil wettability is also severely affected by the organic matter and clay content. The manure treated loamy sand was less wettable (hwe ¼ 213 cm; Fig. 5a) than the same soil without manure treatment (hwe ¼ 225 cm; Fig. 5a). This also reveals that flow in soils are affected by chemical and biological properties. Such compound effects are reflected in the water-entry value.

4. Discussion and conclusions We presented a hydraulic method to measure repellency and wettability of a soil or any other porous medium. An initially low water pressure is imposed at the soil surface. By gradully increasing the source

pressure head, a critical pressure, the water-entry value, is realized at which water starts to infiltrate, overcoming the repellent forces in the soil. The waterentry value is a hydraulic representation of the water drop penetration time. Two simple methods, the WP and TPI method, were developed for quick measurement of soil water-repellency and wettability. Experiments were also conducted to demonstrate the effects of initial soil water, clay and organic matter contents on repellency and wettability. The magnitude of water-entry value reflects the combined effects of various soil properties and state variables on water mobility in the soil. It is a hydraulic indicator of soil water-repellency or wettability. The water-entry value has been used in many simulation models for predicting infiltration, identifying the onset of wetting front instability, and calculating the size and speed of fingered preferential flow (Morel-Seytoux and Khanji, 1974; Neuman, 1976; Wang et al., 1997, 1998a). The concept can also be directly used to reduce runoff and increase infiltration in repellent soils by applying a greater depth of water than the water-entry value in the field. We tend to believe that the potential soil waterrepellency and wettability (water-entry value of dry soil) is mainly affected by soil organic matter and clay contents as well as the dry bulk density. Thus, the water-entry value may be empirically predicted utilizing the soil survey data. The actual soil water wettability (water-entry value of wet soil) is



73

Fig. 5. Water infiltration into: (a) a manure treated loamy sand; and (b) the loamy sand without manure treatment, showing the potential effects of organic mater and clay contents on the magnitude of water-entry value. Fig. 4. Water infiltration into: (a) an initially dry sand; and (b) an initially wet sand, showing the effects of initial water content on water-entry value.

altered from the potential water-entry value due to different initial water saturation. This can also be predicted using a functional relation based on

theoretical and experimental analyses. The concept of water-entry value can be directly used in hydrologic models. It might also be used in design and practice of surface irrigation where high ponding of water is necessary to enhance infiltration in repellent soils.



OCCURRENCE AND HYDROLOGICAL IMPLICATIONS


Chapter 8 Soil wettability in forested catchments in South Africa D.F. Scott* Department of Earth and Environmental Sciences, Okanagan University College, 3333 College Way, Kelowna, BC, Canada V1V 1V7

Abstract Earlier studies in South Africa had shown that water repellency in the soils of timber plantations was associated with a greater risk of overland flow and soil erosion on mountain slopes. This paper reports on a follow-up study to determine how prevalent water repellent soils are in the forestry areas of South Africa, and to what extent this phenomenon is associated with specific vegetation types. Soils from a representative series of forestry sites around South Africa were sampled from beneath each genus or plantation type and the range of local vegetation types. These soils were dried at low oven temperatures and then subjected to a series of tests of soil wettability, namely, water drop penetration time, infiltration rate, critical surface tension and apparent advancing contact angle as determined by the equilibrium capillary rise test. Water repellency is common in dried soils from timber plantations. The dominant variation in repellency is explained by the different vegetation types: soils beneath eucalypts are most repellent, followed by those beneath wattle (Acacia species), indigenous forest and pine. Soils beneath grassland and fynbos scrub were least likely to show repellency, perhaps because regular fires remove plant litter and thus the potential for hydrophobic substances to develop. Soil characteristics explained very little of the variation in repellency. Organic carbon content was weakly correlated with higher repellency, but organic carbon content and soil texture added little explanation to models that first accounted for variation in vegetation type and point of origin. These results are broadly the same regardless of which method of measuring repellency was used. However, the critical surface tension test was far superior to the others in terms of information gained, speed, efficiency and statistical utility of the resultant scores.

1. Introduction Earlier studies in South Africa had shown that severe wildfire in some pine-afforested catchments induced water repellency in the soils, and that the degree of repellency was positively related to fuel loads during the fire (Scott and VanWyk, 1990). This repellency was associated with a greater risk of overland flow and increased soil erosion. It was also apparent that severe water repellency in soils was not confined to burned sites, and that the degree of repellency was linked to vegetation type (Scott, 1991; * Tel.: þ1-250-762-5445; fax: þ 1-250-470-6005. E-mail address: [email protected] (D.F. Scott). q 2003 Elsevier Science B.V. All rights reserved.

Scott and Schulze, 1992b; Scott, 1994). Therefore the objective of this study, undertaken in 1989, was to determine how prevalent water repellent soils were in the forestry areas of South Africa, and to what extent this phenomenon was associated with specific vegetation types and soil characteristics. 1.1. Background to the study In most soil physical studies soils are assumed to be completely wettable because of the strong attraction between the soil particles and water molecules (DeBano, 1981). It is not uncommon though for some soils to show resistance to wetting, i.e. that they are water repellent, hydrophobic or hard-to-wet. This

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D.F. Scott

condition may be noticeable in the dry state only with certain soils or at all stages of wetting (DeBano, 1981). In most cases water repellency in soils can be attributed to coatings on the soil particles of hydrophobic substances of organic origin. One of the more common sources of these organic skins are fungal mycelia (Bond and Harris, 1964; Jex et al., 1985) which are particularly noticeable in lawns and pastures as the so-called “fairy-ring” phenomenon, causing dry circles of grass where the soils are nonwettable (DeBano, 1981; Dekker and Ritsema, 1996b). Water repellency is also commonly associated with certain vegetation types or plant species, such as citrus orchards (Jamison, 1946; Bishay and Bakhati, 1976) and some Californian desert plants, where repellency is confined within the drip zone of the plants (Adams et al., 1970), to chaparral vegetation (DeBano et al., 1967; Holzhey, 1969a) and some eucalypts (Bond, 1964; McGhie and Posner, 1980; Burch et al., 1989; Doerr et al., 1996). If water repellency is caused by a coating on soil particles it follows that soils with a low specific surface area (surface area per unit of mass), i.e. coarse textured soils, should develop the phenomenon more readily. Thus sand is often particularly repellent, as found in Florida (Jamison, 1946), Australia (Roberts and Carbon, 1972), Egypt (Bishay and Bakhati, 1976) and the Netherlands (Dekker, 1988), and as observed in the coastal sands of Zululand and the Cape Flats in South Africa. Other factors positively related to the degree of water repellency are the amount of organic matter in the soil (Van’t Woudt, 1959; Scholl, 1971), the age of the vegetation (period since last fire) through controlling the build-up of plant litter on the soil (Teramura, 1980), and the dryness of the soil (Gilmour, 1968; Singer and Ugolini, 1976; Grelewicz and Plichta, 1983) which may result in the seasonal appearance of water repellency. Finally, as mentioned earlier, soil heating, such as may occur during fire, has been found to intensify water repellency in the soil (DeBano and Krammes, 1966; Dyrness, 1976; John, 1978; Shakesby et al., 1993). In this study, representative soils from the main timber-growing areas in South Africa were sampled from beneath plantations of different timber genera and the adjacent natural vegetation, and subjected to a range of standard wettability tests. Results from these

tests are presented, some comment is made on the various tests used and the implications of the results are discussed.

2. Methods

2.1. Soil wettability and its measurement Wetting is a complex phenomenon, the theory of which is dealt with in the chemistry and physics of surfaces. Water repellency or soil wettability is not an absolute state; factors such as surface chemistry, surface roughness and porosity may all influence perceived repellency, which also varies with soil wetness and temperature, and possibly also atmospheric humidity (Hammond and Yuan, 1969; King, 1981). Hence the wettability of a soil is not static. Consequently, there is no single complete method for measuring repellency. There is no universally accepted or absolute measure of repellency, and results obtained from different studies are therefore not necessarily directly comparable. Relative scales, such as the repellency index proposed by Watson and Letey (1970) and used in this study, may be set so as to be meaningful for a particular sample of soils and thus may differ between different studies. For these reasons, and in the light of earlier experience in measuring water repellency, this study employed a battery of tests of soil wettability, rather than a single test, as different tests highlight different aspects of the repellency and a fuller picture of soil wettability is presented by multiple measures. The intention in using a range of tests of wettability was not to repeat previous comparisons of techniques, but simply to improve the representativeness of the conclusions drawn. To remove any variation caused by differences in initial wetness, all soils were air dried or dried at low oven temperatures (70 –1008C). The phenomenon being measured, therefore, is what Dekker and Ritsema (1994b) termed “potential repellency”. Under the typically sharp seasonal drought conditions of South Africa, though, it is likely that surface soils do reach this air-dry condition in most years.

Soil wettability in forested catchments in South Africa

2.2. Sampling of soils Soil samples were taken from 10 forestry sites around South Africa that broadly represented most of the major timber-producing soils. At each point, a typically thin cover (, 50 mm) of plant litter and duff were removed and a disturbed sample of roughly one kilogram was taken from the top 50 mm of mineral soil. Soil samples were taken from beneath each of the local plantation tree genera as well as beneath grassland or other indigenous vegetation at that location. In the case of plantations, the chosen sites had supported that tree type for at least twenty years. In South Africa the predominant timber plantation trees are eucalypts (mostly Eucalyptus grandis ), pines (Pinus patula, P. elliottii, P. taeda and P. radiata ) and black wattle (Acacia mearnsii ), and all have been established into short vegetation of grassland or scrub (fynbos). The actual timber species grown depends on the geographical location; all cover types are not represented at each location. The native vegetation at each location varies. In the summer rainfall region, i.e. the eastern seaboard and escarpment areas (locations 1 – 8 in Table 1), the native vegetation is sub-tropical, fire sub-climax grassland, which is seasonal except on the coast. The grasslands are maintained by annual or biennial burning. In the Mediterranean type climatic zone of the Western Cape Province (locations 9 and 10 in Table 1) the native vegetation is fynbos, a fire-maintained sclerophyllous scrub unique to the southern tip of Africa. The fire cycle in fynbos is around 10 years, though highly variable. Throughout the sampling area, native evergreen forest occurs in isolated fire refuge sites. Where forest occurred in close proximity to the sampling points, soils from beneath this cover type were also sampled. At each of the 10 locations, under as many vegetation types as were represented (Table 1), soil samples were collected as near to each other as possible (usually within 50 m), so as to reduce the variability in soil properties which was not due to the vegetation supported by that soil. Uniformity of soils from one location was determined by observation in the field; at four locations additional samples were taken to account for inclusion of local variations in conditions. In total 38 samples representing different combinations of location, vegetation type and soil

79

were included in the analysis. The origin (location), parent material, soil types, organic carbon content and texture class of the sampled soils are summarised in Table 1. The forestry areas of South Africa are the high rainfall areas; the soils are typically highly leached, freely draining and dominated by kaolinitic and sequioxidic mineralogy. While clay mineralogy was not measured specifically, it is not expected to vary substantially between sites. The general geographic locations of the sample sites are shown in Fig. 1. 2.3. Soil analyses and tests In the laboratory, the soil samples were passed gently through a 2 mm sieve and dried. Sub-samples were submitted for standard laboratory analyses. Organic carbon was determined by the Walkley – Black method. Particle size distribution was determined by sieving of the sand fraction and the pipette method used to separate coarse (20 – 50 mm) and fine (2–20 mm) silt fractions from clay (, 2 mm). Sieve openings of 0.5, 0.25, 0.106 and 0.053 mm were used to separate coarse, medium, fine and very fine sand fractions. The soils were then subjected to four soil wettability tests. The range of tests used was expected to provide complementary information on soil wettability. 2.3.1. Water drop penetration time Wettability is most easily conceived of as the speed with which a water drop penetrates a soil, or is absorbed. Anything less than immediate absorption indicates less than perfect wettability. The water drop penetration time test in reality often measures persistence or stability of repellency, as drops which initially are not absorbed may enter the soil after some period of time (Watson and Letey, 1970; DeBano, 1981). In the first test, water drop penetration time (WDPT) was the time, measured in seconds to a maximum of 300, for a water drop to be absorbed into a smoothed surface of the soil. For each measurement, the mean of six drops across the surface of the prepared soil was recorded. A second and analogous test used a miniature ring infiltrometer as proposed by King (1981). Here, the time for a known depth of water to infiltrate was

80 Table 1 The origin and some properties of a range of soils from forestry regions in South Africa. The mean of organic carbon content and derived specific surface area are shown with their standard errors in brackets. The location codes cross-refer to Fig. 1 and the text (S.F. ¼ State Forest) Location

Geology

Soil form and approximate {FAO equivalent soil group}a

1. Biesievlei

Bergvliet S.F., Mpumalanga Cathedral Peak, KwaZulu–Natal Drakensberg Ceylon S.F., Mpumalanga Jonkershoek, S.W. Cape

Nelspruit Granites

Magwa {Humic Ferralsol} Inanda {Humic Ferralsol} Inanda {Humic Ferralsol} Sweetwater {Cambic Umbrisol}

MacMac S.F., Mpumalanga Ntabamhlope, KwaZulu–Natal Drakensberg Richmond, KwaZulu–Natal Midlands Saasveld, Southern Cape Windy Hill, KwaZulu–Natal Midlands KwaMbonambi, Zululand Coast

Lyttleton Dolomite

2. CP

3. Cey 4. Jk

5. MM 6. Nt

7. Rd

8. Sd 9. WH

10. ZC a b

Beaufort Shale

Timeball Hill Shale Cape Granite

Texture class

Specific surface areab (m2 g21)

Vegetation cover types sampled

7.5 (2.6)

Loam

4.57 (0.86)

7.8 (1.1)

Silty clay loam

6.16 (0.46)

Eucalyptus grandis, Pinus elliottii, grassland E. grandis, Pinus patula, (2) grassland (2)

4.0 (0.7)

Silty clay loam

7.08 (0.14)

6.3 (1.4)

Silty loam

4.55 (0.65)

Silty clay loam

8.40 (0.52)

Organic carbon (%)

Eucalyptus saligna, Pinus taeda, grassland Pinus radiata, Eucalyptus cladocalyx fynbos (scrub) E. saligna, P. elliottii (2), grassland (2) Eucalyptus fastigata, Acacia mearnsii, grassland

Inanda {Humic Ferralsol} Magwa {Humic Ferralsol}

11.2 (2.1) 6.2 (2.2)

Silty clay

8.39 (0.68)

Ecca Shale

Inanda {Humic Ferralsol}

10.0 (1.2)

Silty clay

9.08 (0.39)

E. grandis, A. mearnsii, grassland

TMS and Saasveld schist

Nomanci {Cambic Umbrisol} Nomanci {Ferralic Cambisol}

6.8 (2.0)

Loam

2.47 (0.27)

6.2 (1.8)

Loam

3.95 (1.43)

P. radiata (2) native forest (2) E. grandis, P. patula, A. mearnsii, grassland

Fernwood {Albic Arenosol}

1.2 (0.03)

Loamy sand

0.76 (0.13)

Beaufort Shale

Table Mountain Sandstone (TMS) Quaternary coastal sands

From the South African (SCWG, 1991) and FAO classifications (FAO-UNESCO, 1974). Estimated from the particle size distribution by a formula given by Hillel (1980).

E. grandis (2) P. elliottii, grassland, native forest (2)

D.F. Scott

Location code


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defined as not instantaneous, but a penetration time of between 1 and 2 s. The test was repeated up to six times across the prepared surface and the CST that was representative of the soil recorded.

Fig. 1. The locations of the sites in South Africa from which soils were sampled for repellency testing. The location codes are used in Table 1 and the text.

measured, and expressed as an infiltration rate (IR). The ring was made from perspex tube with an inside diameter of 25 mm, with the lower end of the ring sharpened so as to displace the soil outwards. Before use the rings were dipped in a solution of paraffin wax in xylene and allowed to dry, thus rendering them water repellent to avoid an edge effect. The ring was pushed 10 mm into the soil surface and a measured depth of water (10 mm) was added. The depth of water absorbed (to a minimum of 5 mm) was divided by the time taken for its absorption (to a maximum of 15 min). 2.3.2. Critical surface tension Resistance to wetting can be overcome by reducing the surface tension of the fluid. The wettability of a soil thus can be characterised by its so-called critical surface tension (CST) which is the highest surface tension to readily wet the soil (Watson and Letey, 1970). This same method has been termed the molarity of ethanol droplet (MED) test when the molarity of the aqueous ethanol drop rather than its surface tension is reported (King, 1981). The test measured the CST, being the highest surface tension (N m21) which readily wets the soil. This was measured using a range of aqueous ethanol solutions of varying molarity and hence surface tensions (completely wettable soil will be readily wet at zero molarity of ethanol). Ready wetting was

2.3.3. Liquid– solid contact angle The wetting angle between a liquid and a solid, formally termed the liquid –solid contact angle, is also used to describe the wettability of the solid’s surface. In porous media this contact angle is determined from a capillary tube model and refers to the apparent angle the water meniscus makes with the pore wall (DeBano et al., 1967). In soil – water systems the contact angle is usually assumed to be zero, though laboratory determinations of the apparent advancing contact angle seldom show this for even readily wettable soils. The fourth test was measurement of the apparent advancing contact angle by the equilibrium capillary rise method of Letey et al. (1962a). Prior to being packed with soil, glass columns, 500 mm long with an inside diameter of 25 mm, were coated with a thin film of paraffin wax and then air-dried to make the glass water repellent. Detailed but unreplicated measurements of the rate of capillary rise of water were taken (up to the assumed equilibrium height reached at 24 h), as these data give a good indication of the effects of water repellency on the hydraulic properties of a soil. The first three tests were repeated three times on sub-samples of each soil, but the capillary rise tests were performed only once for each soil of which a large enough sample was available. For the first three tests, soil samples were placed in small bowls and the surface smoothed by gentle patting with the bottom of a glass beaker to reduce the gross roughness of the surface. Another measure of repellency, the repellency index (RI) was derived by dividing WDPT by the CST (Letey et al., 1975). This index has meaningless units, but combines the results of the two tests to provide an extended range of repellency ratings along a single axis. 2.4. Data analysis The repellency scores for the various soils were analysed by the general linear model (GLM) procedure of the SAS statistical package (SAS Institute, 1985). The GLM procedure is appropriate for

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D.F. Scott

unbalanced designs and allows the use of continuous and class variables as predictor or explanatory terms. The unbalanced design resulted from the unequal numbers of and different types of vegetation at each sampling location. Harper and Gilkes (1994) showed that the combination of numerous soils characteristics better explains the variation of repellency than single variables, and that multi-variable regression could be used successfully to predict the risk of repellency at the scale of a regional soil survey. Therefore, as a retrospective exercise, multiple regression analysis was also performed on the repellency scores using organic carbon and soil texture variables as predictor terms. After both types of analyses the model residuals were checked for random distributions. Analysis of variance and regression analysis were performed on the dependent variables CST and ACA only. The WDPT scores provide little discrimination amongst all non-wettable soils: the distribution of this variable tends to be strongly bi-modal with large numbers of either low or very high scores for wettable

or non-wettable soils with few scores in the middle of the scale. Similarly, the derived RI has a strong bimodal distribution. The infiltration rate (IR) scores were all concentrated at the low end of the scale and provided little discrimination between the various samples (Figs. 2 –4). These variables, WDPT, RI and IR, have distributions that are far from normal, that could not be normalised by transformations and were, therefore, not used in the analyses of variance and regression modelling.

3. Results 3.1. Simple correlations between variables The correlations between the different measures of repellency in soils (the dependent variables) and between these measures of repellency and some possible predictor variables are of interest (Table 2) as surveys of this scope are seldom undertaken.

Fig. 2. The water repellency of soils in the forestry regions of South Africa, as measured by four different tests and summarised by vegetation types. (a) Lower values of water drop penetration time, (b) infiltration rate, (c) critical surface tension and (d) higher values of apparent contact angle indicate stronger repellency. Boxes define the 25th and 75th percentiles positions, the bar inside the box shows the median score and the whiskers the 10th and 90th percentiles.


83

Fig. 3. The water repellency of soils in the forestry regions of South Africa, as measured by four different tests and summarised by the origin (location) of samples. Boxes define the 25th and 75th percentiles positions, the bar inside the box shows the median score and the whiskers the 10th and 90th percentiles.

Good, though not perfect, correlations were obtained between the different measures of repellency, indicating that while all methods may show a soil to be repellent different methods often highlight either different aspects of or provide more information on the soil’s response to wetting. Based on the hypothesis that repellency is the result of a coating on the soil particles of a hydrophobic organic substance, soil with a low specific surface area (coarse texture) is expected to have a greater likelihood of being repellent. In this study the relationship between texture and repellency is very weak; in the few cases where the correlation is significant the correlation coefficient is nonetheless close to zero (Table 2). Similarly, the proportion of sand in a soil was not correlated with the repellency rating. In short, it was found that soils of any texture could be water repellent. The level of organic carbon was positively related to the repellency rating, displaying significant correlations with all measures of repellency (Table 2).

3.2. Repellency scores The results of the four tests of soil wettability are summarised by vegetation type in Fig. 2, by sampling locations in Fig. 3 and by soil surface area class (texture) in Fig. 4. In each figure the same plotting symbol is used; the bottom and top of the boxes define the 25th and 75th percentile positions, the bar inside the box shows the median score, while the ends of the whiskers show the 10th and 90th percentile positions. In all the plots it is apparent that, generally, there are a wide range of scores within most of the plotting categories. Though there are minor differences between the results of the different tests, the same general pattern of water repellency emerges. In Fig. 2a, the WDPT scores range across the whole scale for each vegetation type except fynbos (which had few samples). Nonetheless, it is fairly clear that eucalypt soils are generally hard to wet (median WDPT of 300 s) while soils beneath pines, fynbos and grass are seldom repellent (median

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D.F. Scott

Fig. 4. The water repellency of soils in the forestry regions of South Africa, as measured by four different tests and summarised by classes of specific surface area. Class 1 are the coarsest soils (loamy sands) and Class 6 the heaviest (silty clays)—see text for details.

Table 2 Simple correlations between five measures of water repellency and some potential predictor variables for a sample of South African forestry soils. Repeat and auto-correlated comparisons have been omitted

WDPT CST (critical surface tension) ACA (apparent contact angle) Height of capillary rise of water IR (infiltration rate) Specific surface area of soil Sand fraction Clay fraction Organic carbon content

20.88** 122 0.78** 41 20.76** 41 20.67** 123 20.14 NS 123 20.06 NS 123 20.14 NS 123 0.49** 123

CST

ACA

IR

RI

1 20.76** 41 0.74** 41 0.78** 122 0.26** 122 20.02 NS 122 0.26** 122 20.52** 122

1

20.67** 41 20.07 NS 41 20.10 NS 41 20.01 NS 41 0.44** 41

0.62** 41 1 0.14 NS 123 20.14 NS 123 0.14 NS 123 20.41** 123

0.79** 41 20.77** 41 20.66** 122 20.14 NS 122 20.08 NS 122 20.14 NS 122 0.52** 122

RI, repellency index ¼ WDPT/CST; NS, not significant, i.e. a $ 0:1; * * a , 0:01; Key: Pearson’s r/probability . lrl under Ho: r ¼ 0 / (number of observations)

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WDPTs of 10, 2 and 1 s, respectively). A similar result can be interpreted from the infiltration rate scores (Fig. 2b), though here the wettability of soils from beneath pine and fynbos is lower than that of soils from beneath grass. The CST scores provide the clearest picture of differences between soils of different vegetation types (Fig. 2c); there is a steady increase in wettability from the eucalypts to grass. The same pattern is repeated with the ACA test (Fig. 2d) where the median apparent contact angle is 878 for eucalypt soils and falls steadily through the various vegetation types to a median of 748 for grassland soils. Where the repellency scores are summarised by location (Fig. 3) it is immediately obvious that values have broad ranges at most single locations. The reason for this is that in most cases the different vegetation types at each location induce repellency at various levels. The exceptions are the Cey and Sd locations where the soils are generally wettable or repellent, respectively, as illustrated by the WDPT, IR and CST plots (3a –c). The results are also summarised by soil “specific surface area” classes. The class boundaries were selected to allow similar numbers of observations in each group: Class 1 , 0:95 m2 g21 ; Class 2 ¼ 0:95 – 3 m2 g21 ; Class 3 ¼ 3 – 4:2 m2 g21 ; Class 4 ¼ 4:2 – 6:4 m2 g21 ; Class 5 ¼ 6:4 – 8 m2 g21 and Class 6 . 8 m2 g21 : When the repellency results are plotted against these classes (Fig. 4), ranging from loamy

sands (Class 1) to silty clays (Class 6), it is again clear that scores vary over a wide range within each texture class. There is no trend across the texture range, and it appears that texture of the soil alone offers little explanation of a soil’s likelihood to develop repellency. By the ACA test (Fig. 4d) the Class 2 and 6 soils (loams and silty clays, respectively) are most repellent (median values of 86.5 and 83.4, respectively). By the CST test the Class 2 and 6 soils are again most repellent (median values of 38.8 and 40.8 N m21), while the other four texture classes have median scores clustered between 49.1 and 54.5 N m21. 3.3. Analysis of variance The results of the analyses are shown in Table 3. Sources of variation, other than the main effects of vegetation and location, are only included in the table of results where they were statistically significant. There was no significant interaction between location (or lithology) and vegetation, so main effects were tested with the residual of the full model. Different results were obtained with the different measures of repellency (Table 3). In models for both dependent variables (CST and ACA) there was a portion of unexplained variance, though the CST scores were much more successfully modelled, with 91% of the variation explained, as opposed to 65% of the variation in the ACA scores

Table 3 The results of the analysis of variance of two measures of water repellency for a large sample of South African forestry soils Dependent variable

Model R 2

Source of variation

Critical surface tension (109 obs., 5 missing values)

0.91

Locations Vegetation (treatments) Replication within experimental units Sand content Residual error

Apparent contact angle (36 obs., 2 missing values)

0.65

Locations Vegetation (treatments) Residual error

a

NS, not significant; *ða . FÞ , 0:05; **ða . FÞ , 0:01. Test not valid without replication.

df

Mean square

a.F

F

9

754

7.1

**

5 76

1281 38

12.1 0.4

** NS

1 17

462 106

4.4

9

70

5 19

113 37

*

a

3.05

*

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D.F. Scott

(Table 3). For both dependent variables, vegetation type explained most variance and this was a significant effect in both models (Table 3). Without replication of the ACA measures the test of location effects was not valid. However, it is apparent from the distributions of the ACA variable in Fig. 4d that location is not a source of great variation in the results. The only significant differences between vegetation types in terms of ACA scores, by Duncan’s multiple range test, is between the means for the two short vegetation types, grass and fynbos, as opposed to and the other vegetation types, all tree types. For the dependent variable CST, location (which generally relates to the geological type) was a secondary but significant effect. Sand content (or specific surface area or clay content individually) provides some additional explanation but is barely statistically significant (Table 3). Organic carbon content, as an additional term, was not a statistically significant predictor. Duncan’s multiple range test shows that the mean CST scores for grass and fynbos (63 and 61 N m21, respectively) are significantly higher (i.e. less repellent) than those of the other vegetation types, though the difference between fynbos and pine soils, with a mean CST of 51 N m21, is not significant. 3.4. Multiple regression models Various possible regression models using the soil variables of organic carbon content (OC), specific surface area (SfA), sand and clay content as possible predictors of ACA and CST were tested. Only OC content was a significant ða , 0:05Þ predictor of ACA, though the model ACAð8Þ ¼ 74:3ð^2:2Þ þ 0:94ð^0:29ÞOCð%Þ only explains 24% of the variance in the ACA values. CST scores are more successfully predicted by the same group of soils characteristics. The organic carbon and specific surface area were both significant predictor terms in the model CSTð1023 N m21 Þ ¼58:7ð^2:2Þ 2 2:4ð^0:27ÞOCð%Þ þ 1:8ð^0:3ÞSfAðm2 g21 Þ which explained 46% of the variance in the CST scores. A similar degree of explanation could be

obtained from models where clay or sand content was substituted for surface area, but in all cases it is the OC content that explains most of the variation. 3.5. The effect of vegetation type In virtually all areas, as determined by four different measures (Fig. 5), soils under eucalypts were most repellent, with a median WDPT of 300 s, median CST of 0.038 N m21 and median ACA of 878. Critical surface tension values (Fig. 5a) above 0.065 N m21 would indicate a wettable soil, while values less than 0.045 N m21 indicate a repellent soil. By this classification grassland soils at three locations, MM, WH and ZC, two of which had sandy soils, were somewhat repellent. By contrast, soil from beneath eucalypts at only one location, Cey, was wettable, and at two others, Rd and ZC, the soils were somewhat repellent. A similar pattern emerges from the results of the other tests. At only one location, Cey, did soils from beneath eucalypts have a reasonable infiltration rate, a mean of 1 mm s21 (Fig. 5b), and even this was much lower than in the same soil beneath grassland. Except for one location, WH, soils from beneath eucalypts had the highest apparent contact angles, which relates to the very low or negative capillary rise in the eucalypt soils (Fig. 5c and d). A generalised ranking of the repellency developed under different vegetation types (Fig. 5) would be that grassland and fynbos soils are wettable, pine soils are somewhat repellent, soils beneath native forest and wattle vary from somewhat repellent to repellent, and eucalypt soils are repellent. The soils from fynbos sites are closest to the grassland soils in being least repellent, and they also share the characteristic of being more regularly burned. This regular removal of plant litter may in part explain the weak development of hydrophobic substances. The one grassland soil which had moderate to strong repellency was from the Windy Hill (WH) site. This particular soil was coarsetextured and from a fire-refuge site (subject to little burning), allowing a build-up of plant litter over many years. Typical of the effect of water repellent soil on water movement is the resistance to capillary rise. Amongst other things, capillary rise is dependent on the contact angle between the soil and water. Where

Soil wettability in forested catchments in South Africa Fig. 5. The effect of vegetation type on four measures of soil wettability, within each of the eight native grassland locations where timber types have been planted. (Note that only a subset of the vegetation types grow at each location.)

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D.F. Scott

Fig. 6. Time plots of the height of rise of water up columns of soils from two locations in KwaZulu– Natal province, South Africa: (a) Cathedral Peak and (b) Windy Hill near Wartburg. “Water Head” indicates the depth to which the columns were immersed in water.


this angle is very high, capillary rise will be restricted. The capillary rise curves for the same soils under different vegetation types (Fig. 6a and b) illustrate this effect of contact angle. Actual capillary rise occurs only once water has risen above the level of the water in which the columns are standing. At both sites, Cathedral Peak (a) and Windy Hill (b), therefore, no real capillary rise took place in the eucalypt soils. Despite the positive head, penetration of water into the soil columns did not reach the level of the head over the two days of measurement. This resistance to the entry of water into the soil columns, despite a positive head, was typical of the eucalypt soils. The capillary rise in pine soil samples was also depressed below that of grassland, but to a minor degree. 3.6. The effect of soil texture It is apparent from the correlations in Table 2, and the results of the analysis of variance (Table 3) and regression that the risk of water repellency is not determined by soil texture, though it may be a minor contributory factor. Some fine-textured soils, such as the silty clay loam from MM and the silty clay at Rd (Table 1), were highly repellent (Fig. 5). The silty clay loam derived from shales at Cey though showed no tendency to repellency (Fig. 5). There was a tendency though for sandy soils beneath grass to show greater repellency than heavier soils beneath grass, e.g. WH and ZC versus Nt and Rd.

4. Discussion 4.1. On the different measures of repellency Water drop penetration time is a useful screening test in that it is a quick and easy test for the presence or absence of repellency. But in soils where repellency is well developed WDPT is unable to provide any distinction between different repellent soils. The related infiltration rate test was similarly unhelpful on the large proportion of repellent soils where there was essentially no measurable infiltration over the duration of the test. This test had the additional disadvantage of requiring a larger soil sample and much more time and effort than the WDPT test. The infiltration rate test might have been more useful for a

89

range of less water repellent soils. The contact angle measurement by means of equilibrium capillary rise was a very demanding test in terms of time, space, equipment and size of soil sample. Because in running the test a head of 150 mm had been imposed on the soil columns, the method provided a very graphic illustration of the effect of repellency on the hydraulic behaviour of the soil, and the height of rise of water was very indicative of repellency in soils of similar texture. However, the derived apparent contact angles have a fairly narrow range (67 –948) and did not provide great discrimination between soils. The critical surface tension test had none of the disadvantages of the above tests: it is quick, easy and cheap to run, provided a good range of normally distributed values and a high level of discrimination between soils of different wettability. Because the test is quick and simple it is also easy to replicate and to do repeated measurements. The RI that combines WDPT and CST did not add any value to the CST scores alone because of the strongly bi-modal distribution of the WDPT scores. The different methods used here are reasonably correlated and, by and large, each would have given the same general result in this study. It therefore would make sense, generally, to use just the easiest and most informative CST test. 4.2. Effects of vegetation The results show that vegetation was the primary determinant of water repellency in a range of different soils. The reason for the differences between the vegetation types was not explored specifically. But two factors are suggested as possible explanations: firstly, the genus-related chemistry of the plant litter itself and, secondly, the fire-free interval during which litter accumulates. The eucalypts are known for the high levels of oils in their leaves and the soil surface below eucalypts typically has a low cover of herbaceous plants. These factors seem to relate to the litter of eucalypts producing organic leachates that inhibit plant growth beneath their canopies, perhaps by means of inducing repellency in the soils. The leachates from the litter of wattle trees (Acacia mearnsii ) has a less obvious source of hydrophobic substances than the eucalypts, though the ground beneath wattle plantations is, similarly, well known to be fairly bare.

90

D.F. Scott

In the case of the native vegetation covers, the period of litter build-up between fires is thought to be a factor. In the indigenous, evergreen forests there is the longest fire-free period during which litter accumulates, while both fynbos and grassland are fire-maintained vegetation types and fires occur at regular intervals. During these fires litter is consumed and this is thought to reduce the potential for hydrophobic substances to develop in the decomposing plant litter. Of these two factors, the chemistry of plant litter appears overall to be more important than the role of a fire-free interval. 4.3. Influence of soil characteristics The available soil characteristics, organic matter content and texture, were unable to explain the bulk of the variation in repellency (46% of CST and 27% of ACA scores). This was not altogether in contrast to the findings of Harper and Gilkes (1994) where a model incorporating organic carbon and clay content explained just 47% of the variation in WDPT scores; addition of reactive iron as a predictor variable in their model provided a total of 63% explanation of the variation. Repellency has frequently been associated with coarse textured soils (Jamison, 1946; Roberts and Carbon, 1972; Bishay and Bakhati, 1976; Dekker, 1988). In this study, though, soil texture did not play a big role in determining the risk of repellency. The reasons for this are not clear, though the influence of vegetation was perhaps just much stronger and overshadowed any texture effects. Alternatively, the typically well-developed micro-aggregation in heavier soils might have allowed them to present a lower active surface when wetting from the dry state. Although the role of texture was hardly significant overall, there was evidence that texture does play a role. The sample of indigenous forest soils were generally highly repellent, but were also all coarse, and the only grassland soils to have well developed repellency were those that had a coarse texture. 4.4. Implications of the results The results indicate that water repellent soils are a common feature of South African forestry soils, at least when in the dry state. Because repellency is more

pronounced when a soil is dry, its presence may not always be noticeable in field conditions. Also, it is unlikely that water repellent soils will occur in a continuous layer in the soil. Observations showed that repellency is usually poorly developed or absent at certain spots in a given locality, where infiltration and percolation can occur at high rates. Such points may be alongside rocks, disturbed soil, old root channels or other macropores. Consequently, subsoils may appear to be normally wetted after a rainstorm, while overlying soils are unexpectedly dry. Channelling of water to preferred pathways as a result of water repellent soils may not lead to surface wash or erosion while there is a reasonable ground cover of plant litter or where the slope is gentle. Ground cover provides added opportunities for rain water to be detained and retained at the point where it falls, reduces the velocity of any surface flow that may develop, and can trap soil which is eroded by rainfall and overland flow. The most obvious effect of water repellent soils is that they impede infiltration and percolation in the soil, which may result in the generation of overland flow and the restriction of percolation to preferred pathways in the soil profile (Burch et al., 1989; Van Dam et al., 1990; Ritsema and Dekker, 1996b). Especially when repellency is highly developed, as observed in soils under eucalypts, water may be channelled to preferred paths for rapid and deep percolation. At depths below those normally classed as the agricultural soil, large and deep-rooting trees can exploit the water which is not available to shallowly rooting plants. Allison and Hughes (1983) found, with the aid of tracers (stable isotopes of oxygen and hydrogen), that rain water on eucalypt savanna in semi-arid South Australia percolated to depths of at least 12 m below the surface, whilst rain on adjacent agricultural lands planted to cereals, with a much lower water use, had not penetrated more than 2.5 m in the same time (17 years). These authors suggest that eucalypts channelled water into root channels that acted as macropores for water transport to the water table. A similar situation has been observed in an experimental eucalypt planting near Greytown in the KwaZulu –Natal midlands, South Africa. A neutron moisture instrument was used to follow the wetting fronts below ponded water into dry soil. In addition to


the slow and gradual wetting from the surface that was expected, there was a simultaneous and rapid increase in wetness at the bottom of the profile (Boden, 1992). In this case the author suggests that large cracks in the soil profile, caused by the desiccating effect of the eucalypt plantation, provided the channels for the rapid transport of water. Revegetation of sites previously supporting eucalypts may be difficult because of the persistence of water repellency in the soil. Delayed revegetation would leave the site exposed to erosion for longer. There are indications that this has been the case on certain sites cleared of eucalypt vegetation on Table Mountain in the Western Cape Province of South Africa (personal observation).

5. Conclusions The water drop penetration time and infiltration rate tests did not prove to be very useful, particularly in that they could not distinguish between degrees of stronger repellency, and both produced strongly nonnormal distributions. Determining apparent contact angle by equilibrium capillary rise provides an integrated illustration of the effects of repellency on the hydraulic properties of the soil, but the range of ACA scores was narrow, being limited to between 67 and 948 for readily wettable to severely repellent soils. This ACA test is also extremely demanding in terms of time, facilities, effort and size of soil sample. The critical surface tension test was the most useful and efficient of those tried in this experiment. It is quick,

91

easy and cheap to run, provides a good range of normally distributed values and a high level of discrimination between soils of different wettability. Water repellency is a common feature of the soils of timber plantations in South Africa. Plantations of eucalypts (Eucalyptus spp.) and wattle (Acacia mearnsii ) and indigenous evergreen forest in general, relative to other vegetation types in the forestry regions of South Africa, induce a high level of water repellency in the soil beneath them. Soils of all texture classes are vulnerable to the development of water repellency. This is true for the range of soils sampled, and appears to occur in all the major timber production areas in South Africa. Strong repellency develops without the heating of soils during fires. Thus these sites have a higher risk of overland flow and soil erosion when the sites are cleared of ground cover, such as after a fire. Soils beneath pine plantations do not have very high levels of repellency, but may have more chance to develop extreme repellency following wildfire than the other plantation types, the soils of which already show high levels of water repellency in the dry state. Because water repellency is more pronounced when a soil is dry, its presence may not always be noticeable in the field. Also, surface storage of rain water in the plant litter on the forest floor may disguise the fact that infiltration and percolation are impeded. Consequently, water repellent soil may not be a problem until canopy and ground cover are removed during clear-felling, or as a result of a fire. Once surface storage capacity is removed and the soil is exposed to drying, the site is at risk of overland flow occurring during rainstorms, leading to soil erosion and reduced soil water replenishment.


Chapter 9

Soil water repellency in arid and humid climates D.F. Jaramilloa, L.W. Dekkerb, C.J. Ritsemab and J.M.H. Hendrickxc,* a

Profesor Titular, Universidad Nacional de Colombia, Medellıń, Colombia b Alterra, Green World Research, Wageningen, The Netherlands c New Mexico Tech, 801 Leroy Place, Socorro, NM 87801, USA

Abstract Soil water repellency generally tends to increase during dry weather while it decreases or completely vanishes after heavy precipitation or during extended periods with high soil water contents. These observations lead to the hypothesis that soil water repellency is common in dry climates and rare in humid climates. The study objective is to test this hypothesis by examining the occurrence of soil water repellency in an arid and humid climate. The main conclusion of this study is that the effect of climate on soil water repellency is very limited. Field observations in the arid Middle Rio Grande Basin in New Mexico (USA) and the humid Piedras Blancas Watershed in Colombia show that the main impact of climate seems to be in which manner it affects the production of organic matter. An extremely dry climate will result in low organic matter production rates and, therefore, less potential for the development of soil water repellency. On the other hand, a very humid climate is favorable for organic matter production and, therefore, for the development of water repellency.

1. Introduction The worldwide occurrence of soil water repellency has been well established by different authors. At present, areas where water repellent soils are known to occur include Australia (Roberts and Carbon, 1971, 1972), Canada (Dormaar and Lutwick, 1975), Colombia (Jaramillo and Herroń, 1991), Egypt (Bishay and Bakhati, 1976), India (Das and Das, 1972), Italy (Giovannini and Lucchesi, 1984), Japan (Nakaya, 1982), Mali (Rietveld, 1978), New Zealand (Wallis and Horne, 1992), Poland (Grelewicz and * Corresponding author. Tel.: þ1-505-835-5892; fax: þ 1-505835-6436. E-mail addresses: [email protected] (J.M.H. Hendrickx), [email protected] (D.F. Jaramillo), l.w.dekker@ alterra.wag-ur.nl (L.W. Dekker), [email protected] (C.J. Ritsema). q 2003 Elsevier Science B.V. All rights reserved.

Plichta, 1985b), Portugal (Doerr et al., 1996), South Africa (Scott and Van Wijk, 1990; 1992), Spain (Imeson et al., 1992), The Netherlands (Dekker and Jungerius, 1990; Hendrickx et al., 1993), and the USA (DeBano, 1981). Although the cited studies cover many different climatic conditions, there are relatively few studies available discussing soil water repellency under conditions of extremely dry or humid climates. Yet, this is of interest since soil water repellency generally tends to increase during dry weather while it decreases or completely vanishes after heavy precipitation or during extended periods with high soil water contents (e.g. Ritsema and Dekker, 1994b; Ritsema et al., 1997a). The best method to prevent water repellency on golf greens is to irrigate frequently and to maintain high soil moisture levels (Cisar et al., 2000), while reduced irrigation practices have resulted in

94

D.F. Jaramillo et al.

an increase in water repellency (Snyder et al., 1984; Wallis et al., 1989b). These observations lead to the hypothesis that soil water repellency is common in dry climates and rare in humid climates. The study objective is to test this hypothesis by examining the occurrence of soil water repellency in an arid and humid climate.

2. Materials and methods Soil water repellency is determined in this study with the empirical Water Drop Penetration Time (WDPT) test described by several investigators (e.g. Krammes and DeBano, 1965; Letey et al., 1975; King, 1981; Dekker and Jungerius, 1990). Three drops of distilled water from a standard medicine dropper are placed on the smoothed surface of a soil sample, and the time that elapses before the drops are absorbed is determined. Using the WDPT test on dried samples in the laboratory gives the persistence of the potential water repellency while its use on field-moist samples yields the actual water repellency (Dekker and Ritsema, 1994b). Different classification systems for water repellency are used (King, 1981; Dekker, 1998). A soil is considered wettable if the penetration time is less than 5 – 10 s; slightly water repellent if the penetration time is less than 60 – 90 s; and strongly water repellent at longer penetration times. This study has been conducted at two sites: one in the arid climate of the Middle Rio Grande Basin around Socorro (New Mexico, USA), one in the humid Piedras Blancas Watershed near Medellıń (Colombia).

the Rio Grande (elevation around 1400 m above sea level). In July 1994 soil samples have been collected in the mountains under pine trees; in the desert at locations covered with grasses, Creosote bush, Junipers, and Mesquite; in the riparian areas at locations with grasses, Cottonwood trees, and Salt cedars. At each site a 1 m long transect was sampled using 100 cm3 soil cores with a diameter of 5 cm (Fig. 1). Twenty samples were taken at depths 0 –5, 7– 12, and 15 –20 cm. The samples have been dried in the laboratory for several days at 408C. In the field the actual water repellency was determined using the Water Drop Penetration Test; after drying the same test was used for determination of the potential water repellency. In August 1998, actual water repellency has been checked in the field near riparian grasses, Cottonwood trees, Salt cedars, desert grasses, Junipers, Creosote bush, and Mesquite.

2.1. Arid Middle Rio Grande Basin Mean annual precipitation in the Middle Rio Grande Basin is 200 mm which is about one order of magnitude less than the mean annual potential evapotranspiration of 2100 mm. As a result of this large precipitation deficit the soils generally are dry with the exception of short periods (days to weeks) during the monsoon season in July and August and during winter in December and January. Soil water repellency has been studied around Socorro in mountainous terrain (elevation 2000 m above sea level), the desert, and riparian areas along

Fig. 1. Sampling a transect of the soil in the arid middle Rio Grande Basin with 100 cm3 steel cylinders.

95

Soil water repellency in arid and humid climates

2.2. Humid Piedras Blancas Watershed In Colombia, water repellency has been studied in the Piedras Blancas Watershed which is located 17 km East of Medellıń and covers an area of 2911 ha. The climate is very humid with mean annual precipitation of 2011 mm and mean annual potential evapotranspiration of 1104 mm resulting in a precipitation surplus of about 900 mm per year. The rainfall distribution is bimodal with two rainy seasons: one from March to May, one from September to November. It rains on 182 days. The mean average temperature is 148C while the mean monthly relative humidity varies from 74 to 98%. The monthly weather data in Table 1 show that only the month January has a distinct precipitation deficit while March to November is characterized by a precipitation surplus. The watershed is located at 2340 – 2680 m above sea level and consists of saprolites overlain by volcanic ash soils. These have been classified as Eutric and typic Fulvudand and Alic and Typic Hapludand. As a result of the wet weather the soils are generally moist to wet. Numerous field observations revealed that water repellency occurs with higher frequency and with greater intensity in plantations of Pinus species and Cupressus species and is less common under native vegetation. For this study we have selected 10 – 34 years old stands of Pinus patula which together cover an area of 115 ha. A total of 78 soil samples were taken from randomly selected locations which results in about 1 sample for each 2 ha. The samples were taken from the topsoil layer which is in contact with the litter layer. Before gathering the sample, the actual water repellency was determined using the WDPT test for a maximum time of 240 s (Fig. 2).

The samples were air dried in the laboratory, sieved through a 2 mm screen, and homogenized in Petri dishes. Five drops of distilled water were placed on each sample. Water drop penetration time was observed for a maximum of 10,800 s.

3. Results and discussion 3.1. Arid Middle Rio Grande Basin The results of the WDPT tests for the Middle Rio Grande Basin are presented in Table 2. Water repellency occurs frequently in the desert under Juniper and Mesquite brushes but has not been observed under Creosote brushes or in desert grasses. In riparian areas, water repellency is observed under Salt Cedar brushes and Cottonwood trees as well as in riparian grasses. In the mountain areas, water repellency is very common under Pine trees. The data confirm the occurrence of water repellency in the arid climate of the Middle Rio Grande Basin, but they also demonstrate the rare occurrence of severe water repellency (WDPT 600– 3600 s) and the absence of extreme water repellency (WPDT . 3600 s). Another characteristic feature is the shallow depth of the water repellency. Under Junipers water repellency penetrated to a depth of 7– 13 cm while the maximum depths under other vegetation types did not exceed 5 cm. The data in Table 2 have been corroborated over the years through many field observations by co-author Hendrickx and his students. Water repellency in the desert is confined to the soil area underneath Juniper and—to lesser extent— Mesquite. During precipitation, the water repellent toplayer causes the water to infiltrate through a few

Table 1 Mean monthly air temperature, precipitation, potential evapotranspiration, and relative humidity in the Piedras Blancas Watershed during the years 1983– 1991

Temperature (8C) Precipitation (mm) Potential ET (mm) Relative humidity (%)

Jan

Feb

Mar

Apr

May

June

July

Aug

Sep

Oct

Nov

Dec

14.6 51 84 84

14.8 83 84 83

15.0 131 96 83

15.3 209 91 86

15.4 243 97 83

15.3 143 96 82

15.3 130 105 79

15.4 168 107 79

15.1 222 96 81

14.4 317 86 85

15.6 227 81 86

14.7 87 81 84

96


Fig. 2. Actually wettable (N), and actually water repellent (R) soil areas in the surface layer of the humid Piedras Blancas Watershed. Table 2 Actual and potential soil water repellency measured in July 1994 and August 1998 in the arid middle Rio Grande Basin Vegetation

Date

Soil

Depth (cm)

n

Actual and potential soil water repellency (percentage of samples) ,5 s

Desert Grass

July 1994

Sand

Juniper

July 1994

Sand

Creosote Mesquite Riparian Grass

Aug 1998 Aug 1998 Aug 1998 July 1994

Sand Sand Sand Loam

Salt Cedar

Aug 1998 July 1994

Loam Loam

Aug 1998

Loam Sand Algae Loam

Cottonwood

Pine

July 1994

Aug 1998

Sand

0–5 7–13 15–20 0–2 2–5 7–13 15–20 0–2 0–2 0–2 0–2 2–5 7–13 15–20 0–2 0–2 2–5 7–13 15–20 0–2 0–2 0–2 0–2 2–5 7–13 15–20 0–5

20 20 20 20 20 20 20 10 10 10 20 20 20 20 10 20 20 20 20 10 10 5 20 20 20 20 10

5–60 s

60–600 s

600–3600 s

Act.

Pot.

Act.

Pot.

Act.

Pot.

Act.

100 100 100

100 100 100

– – – 85 60 5

– – –

60 100

– – – 15 40 35

– – –

50 100

– – – 100 100 50 20

100 30 80 100 100 100 100 25 90 100 100 100 10

20 100 100

85 100 100 100 15 85 100 100

65 100 100

– – –

80

70 20

15

75 10

35 15

40 100 100 80

Pot.

– –

50 – – –

50 30 30

45 5

100

25

Soil water repellency in arid and humid climates

wet patches and as a consequence to reach a relatively great depth. Therefore, the overall effect of water repellency in the desert seems to be enhancement of water conservation which is beneficial for the desert shrubs. 3.2. Humid Piedras Blancas Watershed The first observations of water repellency in the Piedras Blancas Watershed were made in 1986 during a survey by the senior author to evaluate the effect of Pinus patula on physical and chemical soil properties. Soil of the water repellent layer could not be wetted for texture determination by “feel”. Subsequent field visits revealed that water repellency occurs with higher frequency and with greater intensity in plantations of Pinus sp. and Cupressus sp. and is less common under native vegetation (Fig. 2). The WDPT results in the 10 –34 years old stands of Pinus patula demonstrate the common occurrence of soil patches with an extreme potential and (at least) a strong actual water repellency (Table 3). A typical water repellent soil profile consists of a top layer of fresh needles (^ 6 cm), a layer of partially decomposed litter (^ 10 cm), the water repellent layer (^ 6 cm), and wettable subsoil. After removal of the needles and litter one can clearly distinguish the dry spots with water repellent soil and the wet spots where water infiltrates into the subsoil (Fig. 2). Due to the excessive precipitation surplus the wettable subsoil is almost all the time found to be moist. The water repellent layer remains rather thin between 4 and 13 cm. This observation differs from Table 3 Actual and potential soil water repellency measured under stands of Pinus patula in the humid Piedras Blancas Watershed near Medellıń (Colombia) Potential water repellency (s)

.10800 7200– 10800 3600– 7200 1800– 3600 900–1800 23 2

Samples n

(%)

64 3 4 2 3 1 1

82 4 5 3 4 1 1

Actual water repellency (s)

.240 90–240 30–90 ,10

Samples n

(%)

71 2 1 4

91 3 1 5

97

observations by Dekker and Ritsema (1994b) who found water repellent layers to persist to depths of 45 cm. Several explanations may explain this difference in depth of water repellent layer. One is the time it takes for a water repellent layer to develop. However, since no correlation was found between the age of the Pine stands and the thickness of the water repellent layer this explanation appears without merit (Jaramillo, 1992). Another explanation may be that the large amount of precipitation reduces the thickness of the water repellent layer. A third factor may be the steep slopes in the watershed that induce lateral interflows in the wettable layer which will even out soil water content differences. The relatively thin thickness of about 6 cm of the water repellent layer combined with the high precipitation rates and lateral movement of water on the steep slopes completely eliminates the negative effects of water repellency reported by other investigators (e.g. Dekker, 1998). The Pine stands are healthy while spontaneous regeneration of Pine trees as well as of native vegetation in the stands seem not to be hindered in any way by the water repellent topsoil. 3.3. Comparison of arid and humid conditions The data in Tables 2 and 3 clearly demonstrate the occurrence of water repellency under extremely dry and wet conditions. However, the observations in the arid Middle Rio Grande Basin indicate that water repellency is not as common as might have been expected on the basis of the dry conditions. On the other hand, the observations in the humid Piedras Blancas Watershed indicate that even under very wet conditions water repellency does occur. Therefore, these observations are another confirmation that the principal cause of water repellency is organic matter (e.g. Dekker, 1998). The patchy occurrence of soil water repellency and the relatively shallow water repellent layers found in the arid Middle Rio Grande Basin appear caused by the absence of organic matter due to the arid climate. As a matter of fact in the Middle Rio Grande Basin the occurrence of water repellency seems to increase where the conditions for primary production become more favorable. In the riparian areas the vegetation has more water available than in the desert as a result

98


of occasional flooding or capillary rise from shallow water tables. In the mountains, slightly more precipitation and a lower potential evapotranspiration also result in a higher water availability than in the desert. The higher primary production in the riparian areas and in the mountains may explain the more common occurrence of repellency in these areas as compared to the desert. Even in the desert, water repellency is correlated with the litter layers found under Junipers and Mesquite brushes. The favorable growth conditions in the humid Piedras Blancas Watershed result in an abundance of organic matter. Therefore, the conditions are beneficial for water repellency to occur as has been confirmed by the observed high actual and potential water repellencies (Table 3). However, the wet conditions seem to limit the development of deep water repellent layers that have been observed in more moderate climates.

4. Conclusions The main conclusion of this study is that the effect of climate on soil water repellency is very limited. The main impact of climate seems to be in which manner it affects the production of organic matter. An extremely dry climate will result in low organic matter production rates and, therefore, less potential for the development of soil water repellency. The shallow depths of water repellent soil layers and the patchy occurrence of water repellency in the arid Middle Rio Grande Basin seem to confirm this. On the other hand, a very humid climate is very favorable for organic matter production and, therefore, the development of water repellency. This seems confirmed by the extreme water repellency found in the Pinus patula stands of the humid Piedras Blancas Watershed.

Chapter 10

Water repellency in dunes along the Dutch coast L.W. Dekker*, C.J. Ritsema and K. Oostindie Alterra, Green World Research, P.O. Box 47, 6700 AA Wageningen, Netherlands

Abstract Depth, degree and spatial variability of water repellency were examined in the surface layers of dune sands along the coast of the Netherlands. Soil samples were collected at six depths of up to 50 cm at 865 dune sand sites in nature reserves. The potential water repellency was measured on dried samples using the water drop penetration time (WDPT) test. The vegetation at the sites consisted of marram grass, buckthorn, grey hair grass, pine, oak, other grasses and heather. The 5190 samples were dried at the laboratory, after which the potential water repellency was measured using the WDPT test. About 60– 70% of the samples taken at several depths in the young dunes with a sparse vegetation of marram grass were wettable, whereas the other samples were slightly to strongly water repellent. The samples taken at a depth of 0– 5 cm in the surface layer at the sites with different vegetations were all strongly to extremely water repellent. At all of these sites, the severity of water repellency decreased with depth. The decrease was most evident at the grey hair grass sites. No significant differences in severity of water repellency were found between the samples taken under a cover of buckthorn, pine and oak, any of the grasses and heather. The large variability over short distances in the water repellency and water content of the soil in the dune sands is shown by the intensive sampling of soil blocks at the Ouddorp, Westduinen, Schoorl and Zwanenwater sites. Drier as well as wetter soil areas were visualized in contour plots of the soil water content distributions in transects from the blocks. Large differences in wetting capacity between samples taken at several depths at the Ouddorp site were assessed by measurements of the wetting rate. In all cases, wetter samples wetted faster than their drier counterparts.

1. Introduction Water repellency of surface horizons is often found in soils that frequently dry out and are not cultivated. When dry, they resist or retard water infiltration into the soil matrix. The simplest way to recognize water repellency is by applying a drop of water on the surface of a fairly dry soil. If upon initial contact with the soil, the water “beads up” into a spherical shape * Corresponding author. Tel.: þ31-317-474267; fax: þ 31-317419000. E-mail address: [email protected] (L.W. Dekker). q 2003 Elsevier Science B.V. All rights reserved.

instead of quickly being absorbed into the soil, the soil is water repellent. Repellent soils can be found in many parts of the world, under a variety of climatic conditions (DeBano, 1981; Wallis and Horne, 1992; Doerr et al., 1996), and may occupy large areas, such as the sandy soils of South Australia, Western Australia, and Victoria (Roberts and Carbon, 1972; Blackwell, 1993). Water repellency of soils in the USA has been described by numerous researchers (e.g. Jamison, 1946; DeBano, 1981; McNabb et al., 1989). In the Netherlands, slightly to extremely water repellent topsoils occupy large areas of sand, loam, clay, and peat soils (Dekker and Ritsema, 1994b,

100

L.W. Dekker et al.

1995, 1996c,d, 1997). The surface layers in sands of nature reserves, including the coastal dunes, often exhibit strong to extreme water repellency (Dekker and Jungerius, 1990). Many workers have recognized the importance of various plant species in contributing towards water repellency in soils. Jamison (1946) associated water repellency with citrus trees, Bond (1968) with perennial pastures, Van’t Woudt (1959) with heath vegetation and coniferous trees, DeBano (1969d) with the various chaparral brush species, and Crockford et al. (1991) with dry sclerophyll eucalypt forests. Water repellency can also be found in grasslands, agricultural lands, sports turfs and on golf greens (DeBano, 1981; Danneberger and White, 1988; York and Baldwin, 1992b). The cause of water repellency in sandy soils was found by Bond and Harris (1964) to be an organic coating on the sand grains produced by the growth of fungi. DeBano (1969d, 1981) identified a direct contribution of partially decomposed plant parts to the development of water repellency in soils. Infiltration rates into dry water repellent soils can be considerably lower than those into dry wettable soils, which can lead to runoff and erosion on hill and steepland soils (Bridge and Ross, 1983; Jungerius and Dekker, 1990; Witter et al., 1991; Wallis and Horne, 1992). After prolonged rainfall, however, the soil will start to wet, generally resulting in a very wet surface layer on top of a still dry subsoil. Water may flow laterally through this surface layer, the so called “distribution layer”, to provide water to places where vertical flow paths are formed, the so called “fingers” or “tongues” (Ritsema and Dekker, 1994b; Ritsema et al., 1993, 1997b, 1998b). These vertically directed preferential flow paths facilitate the rapid movement of water and solutes to the groundwater (Ritsema and Dekker, 1995). The purpose of the present study is: (i) to investigate the occurrence, distribution, and depth of water repellent layers in the Dutch coastal dune sands; (ii) to measure the severity of water repellent layers in relation to the current vegetation; and (iii) to draw attention to the hydrologic consequences of water repellency. Most importantly, it is hoped that the publication of the results will stimulate debate and research in an insufficiently recognized field.

2. Climate In the Netherlands, the average annual precipitation is 765 mm, which is evenly distributed over the year. Potential evaporation averages 690 mm/year. Mean monthly temperatures vary between 1.78C in January and 17.08C in July. During the growing season there is a small precipitation deficit; in autumn and in winter, a precipitation surplus.

3. Materials and methods

3.1. Sampling coastal dune sand area Soil samples were collected at 865 sites, distributed throughout the dune sand areas along the west coast of the Netherlands, including the North Sea Islands (Fig. 1). The samples were taken with an auger at depths of 0– 5, 5– 10, 10– 20, 20– 30, 30 – 40 and 40– 50 cm. Sparse marram grass (Fig. 2) grew at 148 sampling sites, whereas grey hair grass and mosses occurred at 104 sites (Fig. 3). Here, 93 sites had a vegetation consisting of buckthorn, 188 sites had pine and oak, and 66 sites had heather, whereas at 266 sampling sites the dune sand was covered by different grass types. The 5190 samples collected were dried at the laboratory at an oven temperature of 658C, after which the persistence of potential water repellency was measured using the WDPT test. After drying, the samples were kept at a constant temperature of 208C and a relative air humidity of 50%, for at least two days, to allow the samples to equilibrate with the ambient air humidity, before the WDPT test was performed. The test involved three drops of distilled water being placed on the surface of a sample, after which the time required for infiltration was recorded. Five classes of water repellency were distinguished, based on the time needed for the water drops to penetrate into the soil: wettable, non-water repellent (infiltration within 5 s), slightly water repellent (5 – 60 s), strongly water repellent (60 – 600 s), severely water repellent (600 s to 1 h), and extremely water repellent (more than 1 h).

Water repellency in dunes along the Dutch coast

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Fig. 1. Schematic map of the Netherlands showing the distribution of coastal dune sands and the four sites studied.

3.2. Soil block sampling at four dune sand sites To study the variability of soil water content over short distances, and to obtain two- and threedimensional water content patterns, samples were taken from four soil blocks in dune sands at Ouddorp, Westduinen, Schoorl and Zwanenwater (Fig. 1). The dune sands of these sites are classified as mesic Typic Psammaquent (De Bakker, 1979). The Ouddorp site is grass-covered, is in use as pasture, and has not been tilled for at least several decades. The other three dune sands are situated in nature reserves. The Zwanenwater site is covered with a grassy vegetation, the Westduinen site with grey hair grass and mosses, and the Schoorl site by pine. Organic matter contents of 8.9 –40.4 w% were found in the thatch layer of the Ouddorp and

Zwanenwater sites, and in the uppermost layer, including some litter, of the Schoorl site. Grey sand at depths of 9 –24 cm at the Ouddorp, and at depths of 7 – 26 cm at the Zwanenwater site, contained 0.7 – 1.3 w% organic matter, whereas the yellow sand, deeper in both profiles, and from 7 cm downwards at the Westduinen and Schoorl sites, contained only 0.2 –0.6 w% organic matter. The soils were sampled at different depths, using steel cylinders (100 cm3) with a diameter and height of 5 cm. The cylinders were pressed vertically into the soil, emptied into plastic bags and used again. The plastic bags were tightly sealed to minimize evaporation from the soil. The field-moist soil in the plastic bags was weighed, the actual water repellency was measured, and water content and dry bulk density of the samples were calculated after drying at 658C.

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Fig. 2. Sparse marram grass vegetation in a foredune along the North Sea coast.

Sampling dates and depths of the layers sampled are indicated in Table 1. At the Ouddorp site a large soil block was sampled and a total of 1680 samples were collected; 240 samples were taken at seven depths, in a regular grid of 40 £ 6 samples. At the Westduinen, Schoorl, and Zwanenwater sites, smaller soil blocks were sampled and in these cases 75 samples were taken at each depth, in a grid of 15 £ 5 samples.

Measurements of the actual water repellency were performed immediately after assessment of the wet weights. The samples were divided into two groups: wettable (, 5 s) and water repellent (. 5 s). The actual water repellency of the samples from the Ouddorp site was measured to be up to more than 6 h. The samples from depths of 0 – 5 cm at the Ouddorp and Westduinen sites were split into two 2.5 cm parts.

Fig. 3. Dune sand occupied by grey hair grass and mosses.

Water repellency in dunes along the Dutch coast Table 1 Mean dry bulk densities and soil water contents at several depths at the four dune sand sites Depth (cm) Bulk density (g/cm3) Soil water content (vol%) Minimum Mean Maximum November 1995 ðn ¼ 240Þ 0.90 18.6 1.42 1.9 1.42 1.5 1.49 0.7 1.49 1.0 1.48 1.0 1.47 3.9

30.5 6.2 4.6 4.4 3.4 3.8 7.4

38.3 14.3 11.4 10.0 7.9 9.0 9.6

Westduinen, 16 August 1996 ðn ¼ 75Þ 0–5 0.88 4.9 7–12 1.49 1.1 14–19 1.54 0.9 21–26 1.53 1.0 28–33 1.53 2.3 35–40 1.54 3.9

11.1 3.1 2.8 4.0 5.4 6.0

23.7 6.1 10.6 7.2 7.4 8.3

Schoorl, 4 November 1996 ðn ¼ 75Þ 0–2.5 0.61 17.3 2.5–5 1.38 7.3 7–12 1.51 4.4 14–19 1.49 1.4 21–26 1.53 1.0 28–33 1.53 0.7 35–40 1.58 0.6

37.3 14.7 7.0 4.8 2.7 1.9 1.3

71.7 41.5 10.7 10.9 6.8 6.6 4.6

Zwanenwater, 30 October 1996 ðn ¼ 75Þ 0–5 0.28 22.8 7–12 1.21 2.8 14–19 1.46 1.2 21–26 1.48 1.0 28–33 1.48 2.2 35–40 1.48 4.7

34.1 10.8 4.1 4.3 6.3 7.9

43.7 22.6 9.0 9.1 10.6 12.6

Ouddorp, 28 0–5 9–14 19–24 30–35 42–47 55–60 69–74

After drying at 658C, the potential water repellency of the samples was measured at the conditioned laboratory up to more than 6 h. 3.3. Two- and three-dimensional visualization of soil water content Contour plots of the soil water content distributions were obtained using the Genstat 5 statistical package, release 2 (Lane et al., 1987). For the soil block from the Westduinen site, the water content values were ordered according to a predefined matrix

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system. This data set was used as a basis for visualizing the water content patterns. Visualization was done using the IRIS Explorer modular visualization software environment, on a SGI Indigo workstation (Ritsema and Dekker, 1996b, 1998; Ritsema et al., 1997a, 1998a). In addition to the visualization of a three-dimensional isosurface, intersecting horizontal and vertical planes were also depicted in the graph.

3.4. Wetting rate measurements on samples from the Ouddorp site Resistance to wetting was determined by measuring the wetting of field-moist samples collected at depths of 0 –5, 7.5 –12.5, 29 –34, 42.5 –47.5, and 50– 55 cm at the Ouddorp site. The sand samples were taken with the help of steel cylinders (100 cm3) with a height and diameter of 5 cm. These samples, within their steel cylinders, were subjected to a constant pressure head of 2 2.5 cm water applied to the bottom of the sample (Dekker et al., 1998). The experimental set-up was designed in such a way that water content increments of 0.2 vol% were recorded automatically over a period of one week.

4. Results 4.1. Influence vegetation type on water repellency in coastal dune sands Fig. 4 shows the severity of potential water repellency of the 5190 samples taken to a depth of 50 cm, distributed over 865 sites throughout the coastal dune sands. A remarkable feature is the slight to strong water repellency of at least 26% of the samples taken from the surface down to a depth of 50 cm in the yellow sands of the foredunes, which are sparsely vegetated with marram grass and have a low organic matter content. On the other hand, potentially wettable sand at depths of 0– 5 cm was only found at the sites with marram grass, whereas at sites with different vegetation types the surface layer was always strongly to extremely water repellent. At the sites with different vegetation types the severity of water repellency decreased with depth, which is

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Fig. 4. Relative frequency of the degree of potential water repellency of soil samples taken at six depths on numerous locations with selected vegetation types.

associated with the reduced amounts of organic matter, the source of the hydrophobic materials. Although even at depths of 30– 40 cm most sites exhibited water repellency, with the exception of the sites with a vegetation of grey hair grass and moss, where 70% of the samples at that depth were wettable. Thus, the water repellent layer under a gray hair grass cover was generally found to be thinner than that under buckthorn, pine, heather, and various grass vegetations. The more rapid decrease of water repellency with depth under the grey hair grass cover is also due to lower organic matter contents at shallow depths. 4.2. Spatial variability of potential water repellency The severity of potential water repellency differed from place to place under similar circumstances as regards the vegetation type. For example, potential water repellency varied from wettable to strongly water repellent in the topsoil and subsoil of the dune sands with a sparse marram grass vegetation (Fig. 4). Wettable to extremely water repellent sand was established at depths of 5– 30 cm at the locations

with grey hair grass. The locations with buckthorn, pine, heather, and grass vegetations also showed major variations in the severity of potential water repellency, especially in the layers at depths of 5 – 50 cm. The spatial variability of potential water repellency was also high for samples taken within short distances of each other in the soil blocks of the Schoorl (pine), Westduinen (grey hair grass), Zwanenwater (grass) and Ouddorp (pasture) sites. For example, WDPTs ranged from 60 s to more than 6 h over a horizontal area of only 0.19 m2 at depths of 7– 12 cm at the Westduinen site (Fig. 5). The Ouddorp soil block, with horizontal planes of 0.6 m2, also showed a wide range of WDPT values (Fig. 6, right-hand panel). As shown in this diagram, the most severe potential water repellency was found in the 9 to 35 cm zone. Deeper in the profile, potential water repellency decreased, although it could still be detected in the 69– 74 cm layer. It seems likely that the most extreme water repellency found in the 9– 14 cm soil layer obstructs infiltrating wetting fronts most effectively, causing instability at this depth.


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Fig. 5. Relative frequency of the degree of potential water repellency of samples ðn ¼ 75Þ taken at several depths at the Schoorl, Westduinen, and Zwanenwater sites.

4.3. Actual water repellency and critical soil water content Water movement can be severely restricted by the dry water repellent dune sand topsoils. Rain that falls on the surface of water repellent sand does not penetrate evenly but moves downwards through narrow channels, leaving the intermediate soil quite dry and causing considerable variation in the moisture content and actual water repellency of the sand. Fig. 6 (left-hand panel) shows the frequency of various classes of actual water repellency for all field-moist samples from Ouddorp (1920 in all), indicating that soil water repellency was restricted to the 2.5 –60 cm zone at the moment of sampling. The highest degree of actual water repellency was found in the 9 – 14 and

19– 24 cm depth soil layers. Even at depths of 55 – 60 cm, actual water repellency was distinct in approximately 60 of the 240 samples. Water contents of the layers sampled at the Ouddorp, Westduinen, Schoorl and Zwanenwater sites ranged from 0.6 to 71.7 vol% (Table 1). Some of the samples, with relatively low water contents, taken at depths of 2.5 –60 cm at Ouddorp, at depths of 21 – 40 cm at Schoorl, at depths of 0 – 26 cm at Westduinen, and at depths of 14 – 26 cm at the Zwanenwater site, were determined to be actually water repellent, whereas all or nearly all samples from the other depths were actually wettable. Fig. 7 shows the relative frequencies of actually water repellent soil samples from the four dune sand sites at the moment of sampling.

Fig. 6. Relative frequency of the degree of actual and potential water repellency of samples ðn ¼ 240Þ; at several depths in the dune sand at Ouddorp, sampled on 28 November 1995.

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Fig. 7. Relative frequency of the actually water repellent soil volume at several depths at the four dune sand sites.

A “critical soil water content” exists above which a water repellent soil layer becomes wettable (Dekker and Ritsema, 1994b). These critical soil water contents were determined for each layer on the basis of the complete set of individual WDPT measurements

carried out on field-moist samples from the Ouddorp site during several studies (Dekker and Ritsema, 1994b; Ritsema and Dekker, 1996b). Soil samples in each layer were divided into actually wettable and actually water repellent, and critical soil water contents

Fig. 8. Critical soil water contents versus depth for the Ouddorp site, indicating actually wettable soil to the right, and actually water repellent soil to the left of the line.


were established, making use of the measurements of the present and previous studies (Fig. 8). At water contents to the left of the line, the soil is water repellent; to the right it is wettable. Water repellency depends strongly on water content, which varies with depth. Therefore, water repellency will affect water flow mainly in initially dry soil, and its influence will be less significant under wetter soil conditions. 4.4. Actual versus potential water repellency Spatial variations in the degree of potential water repellency might be caused by a heterogeneous distribution of water repellent humic substances

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within the soil. In contrast with the actual water repellency, which may change rapidly in time because of changing soil water contents, the potential water repellency is a more or less time-independent soil property, as much time is needed to change the quantity and/or quality of the water repellent humic substances within a volume of soil. The left-hand diagram of Fig. 9 shows that all actually wettable field-moist samples from depths of 2.5– 47 cm at the Ouddorp site became slightly to extremely water repellent after drying. The right-hand diagram at the top of Fig. 9 shows, however, that the potential water repellency of the actually water repellent samples was significantly greater.

Fig. 9. Relative frequency of the degree of potential water repellency of (A) initially wet (actually wettable) and (B) initially dry (actually water repellent) samples from several depths at the four sites.

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The actually water repellent dune sand samples from the Westduinen, Schoorl, and Zwanenwater sites also exhibited significantly greater water repellency than the actually wettable ones after drying, as can be seen by comparing the right-hand with the left-hand diagrams in Fig. 9. 4.5. Moisture patterns and spatial variability in soil water content Visual observation of wetting patterns in trenches dug in the water repellent dune sands revealed that water moved downwards through narrow channels, leaving the adjacent soil volumes dry and causing considerable variation in soil water content (Fig.10). The channels, tongues or preferential flow paths offer less resistance to wetting due to a lower degree of water repellency or to ponding on the surface in shallow surface depressions where the hydrostatic pressure aids water entry. The variation in water content and the occurrence of irregular wetting patterns can be easily established by intensive soil sampling, as is shown for all four sites (Table 1). The mean water contents of layers sampled in the four soil blocks varied between 1.3 and 37.3 vol%. The highest water contents were found in the surface layers, which have higher organic matter contents and lower dry bulk densities.

The difference between minimum and maximum water content was often high for all soil layers sampled (Table 1). The variation in water content within short distances is shown by means of contour plots of horizontal and vertical planes for the Schoorl, Zwanenwater and Westduinen sites (Fig. 11). Wet spots and dry spots can be distinguished in the contours of the top views, whereas wet preferential flow paths and adjacent dry soil bodies are evident in the side views. The water content distribution within the soil block sampled at Westduinen, covered by grey hair grass and mosses, was visualized three-dimensionally (Fig. 12). In addition to a water content isosurface, a horizontal cutting plane at a depth of 20 cm and a vertical cutting plane were also visualized. The color key indicates water contents ranging from 0.6 to . 8 vol%. Distinct fingerlike patterns were detected in this soil block, sampled on 16 August 1996. Visualizing different water content isosurfaces allowed us to derive an optimum water content at which the finger flow patterns showed up clearly. Fig. 12 shows the 5 vol% moisture isosurface. Higher moisture contents were observed above and within the “fingers”, while drier soil, as low as 0.7 vol%, was found in between the fingers. The fingers formed at a depth of around 5 cm, which corresponds roughly

Fig. 10. Characteristic moisture patterns in a horizontal plane at a depth of 5 cm in a dune sand covered by grey hair grass and mosses.


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Fig. 11. Contours of the volumetric soil water contents in horizontal and vertical planes at the Schoorl site on 4 November 1996, at the Zwanenwater site on 30 October 1996, and at the Westduinen site on 16 August 1996. The top views of the sites are situated at depths of 2.5–5, 7–12, and 14–19 cm, respectively.

with the boundary between the humose topsoil and the underlying dune sand, which contains less organic mattter. 4.6. Resistance to wetting of field-moist samples The wetting rate of field-moist samples was measured for samples taken at depths of 0– 5, 7.5 – 12.5, 29 –34, 42.5 – 47.5 and 50 – 55 cm at the Ouddorp site. At each depth, samples were taken in duplicate, one in a wet finger and the other in between the fingers in the drier sand. The initial water contents of these samples are indicated in the diagram of Fig. 13. It is evident

from the curves in the diagram that wetting is faster and results in higher water contents, when the initial water content of sand is greater. The dry samples from depths of 7.5– 12.5, 29 –34 and 50 – 55 cm, with initial soil water contents ranging from 1.0 to 2.2 vol%, did not wet at all during the 7 day experiment.

5. Conclusions The surface layer of nearly the whole coastal dune sand area in the Netherlands is water repellent during

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Fig. 12. Three-dimensional soil water content distribution with intersecting horizontal and vertical planes in the soil block excavated at the Westduinen site on 16 August 1996. Values in the legend indicate volumetric water contents in vol%. The red color indicates dry soil with a water content of less than 1 vol%, while the purple color refers to a water content of around 7 vol%.

Fig. 13. Volumetric water content versus time of field-moist dune sand samples taken at several depths at the Ouddorp site, and placed at a constant pressure head of 22.5 cm water applied to the bottom of the samples.


dry spells. Wettable sand was only found in the yellow sands of the foredunes, which are sparsely vegetated with marram grass and have a low organic matter content. The samples taken at a depth of 0 – 5 cm in the surface layer at the sites with different vegetations were all strongly to severely water repellent. At all these sites, the severity of water repellency decreased with depth. The decrease was most evident at the sites covered by a vegetation of grey hair grass and mosses. No significant differences in water repellency were found between the other sites, which were covered by buckthorn, pine, oak, other grasses and heather. The severity of potential water repellency differed from place to place under similar circumstances as regards to vegetation type. For example, the locations

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with buckthorn, pine, heather, and grass vegetations showed major variations in the severity of potential water repellency, especially at depths of 5 – 50 cm. The spatial variability of potential water repellency was also high for samples taken within short distances of each other, as is shown with the soil blocks of the Schoorl (pine), Westduinen (grey hair grass), Zwanenwater (grass) and Ouddorp (pasture) sites. The actually water repellent dune sand samples from these sites exhibited significantly greater potential water repellency than the actually wettable ones. The variation in soil water content and the occurrence of irregular wetting patterns can be easily established by intensive soil sampling.


Chapter 11

Water repellent soils on UK golf greens C.A. York* and P.M. Canaway STRI, Bingley, West Yorkshire, BD16 1AU UK

Abstract Water repellent soils have been identified as a major problem in the management of golf greens in the UK for over 60 years. The cause of this problem has provoked much speculation, but prior to this work, no research into the possible cause of water repellent soils in the UK had been completed. One of the commonly believed links with water repellent soils on UK golf greens was the activity of basidiomycete fungi. This was proposed as a possible causal factor because the symptoms expressed on the turf above affected soils, were similar in many instances to those symptoms expressed by the activity of superficial fairy rings. Since it was impractical to study superficial fairy rings, it was decided to observe other basidiomycete fairy rings (Type 1 fairy rings) to see if any water-repellence could be identified as being associated with them. Three of these rings, caused by the fungus Marasmius oreades (Bolt ex. Fr) Fr., were studied on each of the two different sites. Soil samples were removed at intervals from the centre of the rings, across the obvious symptoms of the rings (i.e. the zone of dead grass bordered on both sides by a zone of stimulated grass growth) and beyond, into the uncolonised soil. These samples were taken to the laboratory, allowed to air dry and were then tested to determine relative levels of water repellence. It was found that on the ‘outside’ of the fairy rings where the fungus had not yet colonised, the soils were less water repellent than they were in the other zones of the rings (i.e. the dead zone and the inner zone). In the region of the dead zone of these fairy rings, the soil was very water repellent. This may have been expected because the fungus was present in this area in large quantities and the fungus itself repels water. However, of particular interest, were the results from the inner part of the ring where the fungus had been present in the past, but where it no longer colonised the soil. In these soil samples, the rootzone soil was still very water repellent. It was concluded from the study that it was possible for basidiomycete-type fungi to effect water repellence on soils through which they have passed. Thus, this may be at least a contributory factor to the development of severely water repellent soils on UK golf greens.

1. Introduction Fairy rings are not an uncommon problem on areas of amenity turf but they cause few management problems unless they occur on the highly maintained areas such as golf greens (Fig. 1). A wide range of soil fungi are capable of producing fairy ring symptoms but generally, they can be classified as basidiomycetes. The symptoms expressed by the ring forming fungi * Corresponding author. Tel.: þ44-1274-565131; fax: þ 44-1274561891. E-mail address: [email protected] (C.A. York). q 2003 Elsevier Science B.V. All rights reserved.

depend on the specific fungus present and the rootzone through which the fungus is growing. Irrespective of the fungus causing the ring, the method of its development is thought to be common. It is considered that all rings develop from a single fungal spore (Kozelnicky, 1974) or mycelial fragment which produces radial mycelial growth (Kozelnicky, 1974; Smith et al., 1989). The fungus grows in a centrifugal manner with the actively growing tips always at the outer edge of the colony. The fungus decomposes the soil’s organic matter, releasing nutrients required for its metabolism and making nutrients available to the plants. This ultimately

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Fig. 1. Type 1 fairy ring on the edge of a golf green.

results in the observed stimulation of grass growth. The mycelium of Marasmius oreades (Bolt ex. Fr) Fr. also produces cyanogenic compounds that damage plant roots and facilitate further colonisation of the stressed plant (Filer, 1965; Kozelnicky, 1974). The older mycelium towards the centre of the ring, eventually dies, caused in part by the depletion of available nutrients and the build-up of toxic metabolites produced during its period of growth (Smith et al., 1989). The areas of the soil which support high concentrations of the fungal mycelium, i.e. the

identifiable dead zone of the rings being studied, tend to show lower levels of available moisture when compared with the adjacent non-infected zones. When densely packed, the fungal mycelium forms a barrier which is impervious to water penetration. This reduces the water which is available for uptake by the plant (York, 1995). On many golf greens with dry patch, the affected areas appear as a ‘mosaic’ of ribbons/arcs which closely resemble the symptoms of superficial fairy rings (Fig. 2). These superficial fairy rings are usually

Fig. 2. The ‘mosaic’ effect of dry patch on a golf green.

Water repellent soils on UK golf greens

associated with non-fruiting or shyly fruiting basidiomycetes (Smith et al., 1989). However, due to the inability to isolate fungi from areas with dry patch (Wilkinson and Miller, 1978), the formal association between the two conditions has remained speculative. Since the fungi that cause superficial fairy rings are known to be basidiomycetes and the same class of fungi result in the fairy rings described above, it was decided to use identifiable ring forming fungi to prove whether or not basidiomycetes had the potential to produce the symptoms of water repellence identified in areas of dry patch. The type of ring selected for this study, therefore, were those formed by M. oreades. This fungus produces fairy rings with characteristic features; a ring (zone) of dead grass that is bordered on both its inner and outer edge by a ring (zone) of stimulated grass growth.


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The centre of each of three fairy rings selected at each site, was determined by measuring the radius through 3608. A marker pin was placed at the centre of the ring and a tape measure taken from the centre across the radius of the ring (Fig. 3). Sampling locations were then identified across the radius by placement of marker pins through each of the inner, dead and outer zones. At Ganton Golf Club, the three fairy rings selected for this study had radii of 2.40, 1.50 and 2.20 m and at Shipley Golf Club, the radii of the three rings were 1.10, 1.50 and 1.20 m. In all the cases, the rings were sampled at regular (0.15 m) intervals such that soil was removed from the inner, the dead and the outer zones of the fairy rings. The only exception to this sampling interval was that for the three rings studied at Shipley Golf Club, the sampling interval was reduced to 0.05 m across the dead zone of the rings. This was done because the dead zones of these rings was narrow and a sampling interval of 0.15 m would have given only one sample location within the dead zones.

2.1. Sampling sites and intervals 2.2. Sampling technique Three entire M. oreades fairy rings were selected for study at each of two sites. Site one (the practice area at Ganton Golf Club, SE 982 778) had a sandy loam rootzone and site two (the practice area at Shipley Golf Club SE 109 390) had a loam rootzone.

Intact soil cores of 0.03 m diameter and 0.12 m depth (Ganton Golf Club) or 0.08 m depth (Shipley Golf Club) were taken from the fairy rings at regular intervals (detailed in Section 2.1) using zero

Fig. 3. Type 1 fairy ring showing the line for the sampling locations.

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contamination tubes. The JMC zero contamination tubes (Clements Associates Inc., Iowa, USA) were used with PTEG copolyester liners, 0.023 m in diameter and 0.15 m in length, to remove intact cores from the soil profile without disturbance and minimising loss of moisture from the soil by sealing the tubes with plastic end covers (York and Lepp, 1994). 2.3. Presence of active fungal mycelium A plate sampler was used to remove soil ‘sections’ of 0.08 m width £ 0.15 m depth £ 0.01 m thickness from each of the sample locations (detailed in Section 2.1) across the three rings. These soil sections were placed inside plastic bags and stacked horizontally to facilitate transport of intact samples from the field site to the laboratory. These soil samples were then incubated in chambers kept at high humidity to determine the presence of active fungal mycelium. The location of the active mycelium across the radius of the rings is shown in bold in the results tables. 2.4. Moisture content Soil cores taken from each of the sample locations (detailed in Section 2.1) were sectioned at certain depth intervals (0.02 or 0.01 m intervals, for Ganton and Shipley golf course, respectively) and their mass recorded. The samples were then air-dried at 208C for 1 week before being re-weighed and the percentage loss of moisture determined (York and Lepp, 1994). 2.5. Organic matter content The air-dried soil samples used to determine the percentage moisture content, were oven dried before being bulked according to their sampling location (i.e. inner, dead or outer zone of the rings) and placed in a muffle furnace at 4008C for 6 h to burn off the organic matter. The grass was cut from the surface of the cores prior to ignition so that only the soil organic matter was quantified. The mass of the samples was recorded prior to and after burning and the percentage change in mass determined (York and Lepp, 1994).

2.6. Severity of water repellence To quantify the severity of the soil’s waterrepellence within each zone and with depth, soil cores were sectioned at intervals (0.02 or 0.01 m intervals for Ganton and Shipley golf course, respectively), air-dried, milled and sieved to # 1.3 mm particle size to remove stones and the root/thatch material from the samples. The sieved soil was levelled in Petri dishes (0.085 m diameter). A range of aqueous ethanol concentrations (between 0.0 and 7.0 M) were prepared from 99.7– 100% ethanol. Drops of each ethanol solution (36 ml) were in turn placed on the surface of the levelled soil using a Pasteur pipette and their penetration time recorded. Ethanol drops which required over one minute to pass into the soil layer were recorded as . 60 s. As a means of standardising the method, a time period of 10 s was allowed for the complete penetration of each ethanol solution. Ten replicate drops of each ethanol concentration were placed on the soil surface and the penetration times recorded. The minimum ethanol drop concentration (MED) which penetrated the soil in less than 10 s (mean of 10 drops), was the value of the MED ascribed to that sample. This method is based on that of King (1981). 2.7. Data analysis All data were analysed by analysis of variance at a significance level of P # 0:05: Percentage data were checked for normality prior to analysis and transformed using the arcsin transformation to conform to the requirements of the analysis. Below the results tables, the least significant difference (LSD) values are given for the parameters analysed.

3. Results 3.1. Presence of active fungal mycelium The recorded growth of the fungi in each sample location of the rings, is shown in the following tables. The bold numbers in Tables 1 – 6 inclusive represent active fungal growth.

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Table 1 Mean values of moisture content (%) at distances from the inner edge of the dead zone and with depth through the soil profile, in rings sampled at Ganton golf course Depth (m)

0.00– 0.02 0.02– 0.04 0.04– 0.06 0.06– 0.08 0.08– 0.10 0.10– 0.12

Mean moisture content (%) at given distances (m) from the inner edge of the dead zone 20.45

20.30

20.15

0.00

0.15

0.30

0.45

22.25 9.85 7.34 6.05 6.48 7.77

25.94 9.58 6.95 5.91 6.12 7.51

21.58 6.68 4.49 4.36 4.82 7.82

10.27 5.69 5.57 6.23 6.50 6.96

14.21 7.36 8.02 6.42 6.90 7.68

22.36 11.25 9.99 9.30 6.87 7.54

27.29 17.55 15.47 12.60 9.33 11.02

LSD distance from the inner edge ¼ 3:570; LSD depth ¼ 3:305; negative values of distance indicate the inner zone; positive values of distance indicate the outer zone; bold numbers indicate the presence of active mycelium.

3.2. Moisture content The mean values of moisture content (%) recorded across the rings at Ganton and Shipley golf courses are given in Tables 1 and 2, respectively. At both golf clubs, the analysis shows that there is a significant difference in moisture content of the soils both with distance from the inner edge of the dead zone and with depth through the soil profile. At Ganton Golf Club, the results show that the moisture content at 0 m (the inner edge of the dead zone) is significantly lower than that recorded for soils at ^ 0.30 and ^ 0.45 m either side of that location. The moisture content at 0.45 m from the dead zone is significantly higher than that of soil at all other locations across the rings. With regard to depth, the moisture content at 0– 0.02 m was shown to be significantly higher than at all other

depths. At Shipley Golf Club, the soil moisture content was shown to be significantly highest in the outer zone of the ring (distances of 0.25 and 0.40 m from the inner edge of the dead zone) and significantly lowest in the region 0.00 –0.05 m (the dead zone itself). With depth, analysis shows that the soil moisture content progressively decreases. Values of moisture content were not significantly different either between depths of 0.02 and 0.04 m or between depths of 0.04 and 0.06 m. 3.3. Organic matter content The mean values of organic matter content (%) recorded within the inner, dead and outer zones of the rings at Ganton and Shipley golf courses, are given in Tables 3 and 4, respectively. There was no

Table 2 Mean values of moisture content (%) at distances from the inner edge of the dead zone and with depth through the soil profile, in rings sampled at Shipley golf course Depth (m)

0.00– 0.01 0.01– 0.02 0.02– 0.03 0.03– 0.04 0.04– 0.05 0.05– 0.06


20.30

20.15

0.00

0.05

0.10

0.25

0.40

31.02 24.71 22.39 20.08 18.79 17.82

32.89 23.38 20.17 19.58 17.89 16.52

29.04 21.14 17.67 17.89 15.34 15.81

28.88 19.06 14.69 18.28 12.99 13.13

21.53 15.35 12.38 15.05 11.51 14.06

27.03 17.91 19.47 18.62 17.37 18.03

38.64 29.68 27.04 24.58 24.91 20.43

35.36 33.33 28.46 25.21 23.80 23.76


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Table 3 Mean values of soil organic matter (%) within the inner, dead and outer zones and with depth through the soil profile, in rings sampled at Ganton golf course

Table 5 Mean values of MED at distances from the inner edge of the dead zone and with depth through the soil profile, in rings sampled at Ganton golf course

Depth (m)

Depth (m)

0.00– 0.02 0.02– 0.04 0.04– 0.06 0.06– 0.08 0.08– 0.10 0.10– 0.12

Mean soil organic matter content (%) within each zone sampled Inner

Dead

Outer

27.14 6.88 5.19 4.85 4.10 4.27

19.18 7.58 5.99 5.53 5.16 5.06

18.45 10.00 7.41 6.51 5.35 4.73

LSD zone ¼ NSD, LSD depth ¼ 2:246; bold numbers indicate the presence of active mycelium.

significant difference in the level of organic matter recorded in the sampled zones at Ganton golf course. Significant differences were, however, identified with depth. Analysis shows that the 0.00– 0.02 m interval contained significantly higher levels of organic matter compared with all other depths. From 0.04 to 0.12 m, there were no significant differences between the recorded levels of organic matter. At Shipley golf course, the results show that there was a significantly higher level of organic matter in the outer zone compared with the other two sampled zones. As at Ganton Golf Club, significant differences were identified with depth. There were significant differ-

Table 4 Mean values of soil organic matter (%) within the inner, dead and outer zones and with depth through the soil profile, in rings sampled at Shipley golf course Depth (m)

0.00– 0.01 0.01– 0.02 0.02– 0.03 0.03– 0.04 0.04– 0.05 0.05– 0.06

Mean soil organic matter content (%) within each zone sampled Inner

Dead

Outer

24.44 16.59 13.14 11.84 11.17 13.54

22.67 17.33 15.00 13.39 12.08 11.66

25.21 21.27 18.08 16.34 14.97 14.04

LSD zone ¼ 0:983; LSD depth ¼ 1:391; bold numbers indicate the presence of active mycelium.

0.00–0.02 0.02–0.04 0.04–0.06 0.06–0.08 0.08–0.10 0.10–0.12


20.30

20.15

0.00

0.15

0.30

0.45

4.50 4.17 4.08 3.37 2.73 1.75

4.50 4.08 3.65 3.42 2.28 1.38

4.75 5.00 4.42 4.58 3.67 2.58

4.50 5.08 5.50 5.17 4.38 3.62

5.17 5.58 5.50 5.25 4.28 3.50

5.17 3.67 3.82 3.50 2.53 1.83

4.75 1.08 1.00 0.42 0.08 0.00


ences among the top three depth intervals (0.00 –0.01, 0.01– 0.02, 0.02– 0.03 m) and all were significantly different from the depths below 0.03 m.

3.4. Severity of water-repellence The mean values of MED recorded across the rings at Ganton and Shipley golf courses are given in Tables 5 and 6, respectively. The results show that at both golf courses, there are significant differences in MED values ascribed to the soil samples both with distance across the rings and depth through the soil profile. At Ganton Golf Club, the highest MED values were recorded at distances between 2 0.15 and 0.15 m from the edge of the dead zone. The lowest MED values recorded were for the samples taken from the outer zone at a distance of 0.45 m from the edge of the dead zone. With increasing depth through the profile, the mean MED values are seen to decrease. At Shipley Golf Club, significantly higher MED values were recorded at distances between 2 0.15 and 0.10 m from the edge of the dead zone, compared with all other sample locations. The lowest MED values were identified at the edge of the outer zone, i.e. 0.40 m from the edge of the dead zone. The depth intervals which showed the highest levels of water repellence were between 0.03 and 0.05 m.

119


Table 6 Mean values of MED at distances from the inner edge of the dead zone and with depth through the soil profile, in rings sampled at Shipley golf course Depth (m)

0.00– 0.01 0.01– 0.02 0.02– 0.03 0.03– 0.04 0.04– 0.05 0.05– 0.06

Mean MED values at given distances (m) from the inner edge of the dead zone 20.45

20.30

20.15

0.00

0.05

0.10

0.25

0.40

0.58 0.50 0.73 1.08 1.42 1.05

0.88 1.20 1.40 2.37 2.18 2.06

1.97 2.10 3.32 4.08 5.00 5.25

4.92 5.00 5.50 5.25 5.25 5.25

5.17 5.08 5.50 5.00 4.67 3.88

5.17 4.58 4.58 4.50 4.50 4.50

1.83 1.75 1.75 1.50 1.50 0.00

0.70 0.48 0.17 0.00 0.00 0.00


4. Discussion The results show that the dead zone of the rings at Ganton and Shipley golf courses were of roughly comparable width. However, the depth of the rings varied due to the difference between the rootzones. The rings at Ganton golf course grew to at least 0.12 m depth whereas the rings sampled at Shipley golf course grew to no more than 0.06 m (York, 1995). Although there were some differences with regard to the level of organic matter present in the different zones of the rings at each course, these differences were slight and shown to be significant in Table 4 caused by the inclusion of the large amount of thatch in the upper part of the profile. The moisture content of the soil, below the thatch layer, showed similar trends on both courses sampled. Soil from the outer zone contained significantly higher moisture levels compared with the other two zones. Soil from the dead zone contained moisture levels, which were either not significantly different from, or significantly lower than, those recorded for the inner zone. The MED values recorded at Ganton golf course show that the soils of the outer zone (excluding the 0.00 –0.02 m depth which contained mainly thatch material) are not water repellent, but that soils of the dead zone were

strongly water repellent (with similar exclusion). This was likely to be caused by the presence of active fungal mycelium within this zone of the ring. The mycelium itself is highly water repellent. In the inner zone, however, the soil was water repellent, even though no active fungal mycelium was present at the time of sampling. The results from Shipley golf course follow a similar trend but the degree of waterrepellence is not as evident as in the soils of the other site. 5. Conclusions The results of this work indicate that, in soils where M. oreades rings have previously been active, something remains in the soils which renders them water repellent. The observed water repellence cannot be linked to the level of organic matter present in each of the fairy ring zones since, below the thatch layer, the levels of organic matter within each zone are not significantly different. This work implies that fungi do have an active role to play in the development of water repellence on UK golf courses but further work will need to be completed if we are to know exactly what causes this phenomenon to occur.


Chapter 12

Soil water repellency in the Natural Park of Donana, southern Spain F.J. Moral Garciaa, L.W. Dekkerb,*, K. Oostindieb and C.J. Ritsemab a

Departamento de Expresioń Gra´fica, Universidad de Extremadura, Centra de Elvas s/n 06071 Badajoz, Spain b Alterra, Green World Research, PO Box 47, 6700 AA Wageningen, The Netherlands

Abstract The occurrence and consequences of fire-induced water repellency have been studied in several regions of Spain during the last 13 years. The occurrence of water repellency formed under natural conditions, however, has only been described for a few areas in Spain since 1998. The purpose of the present study was to investigate the severity of naturally occurring water repellency in the sandy soils of the Natural Park of Donana in southern Spain. The persistence and degree of soil water repellency were measured on field-moist and dried sandy soil samples taken beneath pine trees. Around 50% of the field-moist soil samples taken at depths between 0 and 10 cm exhibited actual water repellency. Potential water repellency, measured after drying the samples at 60 8C, showed for 68% of the samples slight to extreme water repellency. The organic matter content was found to be positively related with persistence and with degree of potential water repellency. Evidence was found for low infiltration rates and irregular wetting of the soil as a consequence of water repellency.

1. Introduction The problem of soil water repellency has been recognized in various parts of the world (DeBano, 2000b; Jaramillo et al., 2000), including Spain (Cerda`, 1993, 1998; Moral Garcia, 1999). The effects of fireinduced water repellency on infiltration, runoff and erosion of soils have been studied in several regions of Spain by Sevink et al. (1989), Imeson et al. (1992), Calvo and Cerda` (1994), Diaz-Fierros et al. (1994), Soto et al. (1994), Cerda` et al. (1995, 1998), and Soto and Dıáz-Fierros (1998). Also the formation of fireinduced water repellent soil, its chemical nature, and its effect on some soil physical and chemical properties have captured the attention of numerous scientists * Corresponding author. Tel: þ31-317-474267; fax: þ 31-317419000. E-mail address: [email protected] (L.W. Dekker). q 2003 Elsevier Science B.V. All rights reserved.

in Spain (Almendros et al. 1988, 1990, Soto et al., 1991; Soto and Dıáz-Fierros, 1993; March et al., 1994; Martinez-Fernandez and Diaz-Pereira, 1994; Molina et al., 1994). As far as we know, Cerda` et al. (1998) and Moral Garcia (1999) were among the first who described the occurrence and consequences of water repellency formed under natural conditions in soils of Spain. Naturally occurring water repellency is generally attributed to hydrophobic organic matter coating soil particles or accumulating in the soil environment (Roberts and Carbon, 1972; Franco et al., 1994; Piccolo and Mbagwu, 1999). Sources of these hydrophobic materials may include: accumulated plant-derived organic matter (decomposing roots and plant tissues and root exudates), plant-derived waxes and organic acids, fungal hyphae, and microbial organic acids and polysaccharides (Holloway, 1994; Neinhuis and Barthlott, 1997; Hallet and Young,

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F.J. Moral Garcia et al.

1999; Passialis and Voulgaridis, 1999; Czarnes et al., 2000; Hallett et al., 2001a). Passialis and Voulgaridis (1999) showed, for instance, that pine needles and bark contain strongly hydrophobic substances, and according to Rumpel et al. (1998) the organic matter from the roots of pine and several other trees may contribute to the hydrophobicity by releasing aliphatic compounds. The current theory proposes that naturally occurring, plant-derived compounds with amphiphilic properties (fatty acids, fulvic acids, humic acids) undergo conformational changes whereby the hydrophilic regions of the molecule orient themselves towards the surface of the soil particle and the hydrophobic regions orient away from the soil surface into the adjacent pore space. In a wettable soil, the hydrophilic regions orient themselves towards the soil solution and the hydrophobic moeities on the molecules interact with each other and with other organic matter. Infiltration rates into dry water repellent soils can be considerably lower (Fig. 1) than those into dry wettable soils, which can lead to runoff and erosion on hill and steepland soils (Moral Garcia, 1999). After prolonged rainfall, however, the soil will start to wet, generally resulting in an irregularly wet surface layer on top of a dry subsoil (Fig. 2). The purpose of the present study is: (i) to investigate the severity of naturally occurring water repellency in the sandy soils of the Natural Park of

Fig. 2. Irregular wetting pattern in the dry sandy soil under the pine trees after a rain event.

Donana in southern Spain, and (ii) to examine the relationships between the organic matter content and the degree and persistence of soil water repellency.

2. Materials and methods 2.1. Study area The study area is located in the Natural park of Donana, which is part of the National park of Donana, situated in southern Spain, south of Sevilla. The actual and potential water repellency were measured on samples taken in a sandy soil at two sites. One site was covered by pine trees and the other site was a bare sandy soil. The climate is Mediterranean, with hot, dry summers and moderately cold, moist winters. Mean annual precipitation is 511 mm. The annual rainfall distribution shows a dry season lasting from June to September and a wet season from October to April. Interannual variability is high with minima and maxima of 277 and 1007 mm, respectively. Droughts with a duration of more than 2 years are a common feature, with an occurrence of once in the 7 – 8 years (Cerda` et al., 1998). 2.2. Organic matter measurement

Fig. 1. Ponding of water in the Natural Park of Donana, due to the low infiltration rate of the water repellent surface layer.

The organic matter content was determined for 46 samples from the pine site. The organic matter content was measured by drying the samples for 1 day at

Soil water repellency in the Natural Park of Donana, southern Spain

105 8C and igniting the dried samples for 4 h at 650 8C. The weight difference between the dried and ignited samples was taken as the organic matter content. 2.3. Water drop penetration time (WDPT) test The persistence or stability of water repellency of the soil samples was examined using the WDPT test (e.g. Dekker and Ritsema, 1994b). Using a standard medicine dropper, three drops of distilled water are placed on the smoothed surface of a soil sample, and the time that elapses before the drops are absorbed is determined. We measured the soil water repellency of all samples under controlled conditions at a constant temperature of 20 8C and a relative air humidity of 50%. In general, a soil is considered to be water repellent if the WDPT exceeds 5 s (Dekker, 1998). Using the WDPT test on dried samples gives the persistence of the ‘potential’ water repellency. In the field, some of these samples were dry, but others were moist. Therefore, we also checked the persistence of the ‘actual’ water repellency of field-moist samples. These measurements were divided into two classes: wettable or non-water repellent (, 5 s) and water repellent (. 5 s). The persistence of the potential water repellency was measured after removing the samples from the oven, after drying at 60 8C during 1 week. The WDPT tests were deferred for at least 2 days to allow the samples to equilibrate with the ambient air humidity and temperature of the laboratory. In the present study, an index was applied allowing a quantitative classification of the persistence of potential soil water repellency as described by Dekker and Jungerius (1990). Thus five classes of repellency were distinguished based on the time needed for the water drops to penetrate into the soil: Class 0, wettable, non-water repellent (infiltration within 5 s); Class 1, slightly water repellent (5– 60 s); Class 2, strongly water repellent (60 –600 s); Class 3, severely water repellent (600 – 3600 s) and Class 4, extremely water repellent (more than 3600 s). The severity of the potential water repellency measured on dried soil samples, is considered to be the most appropriate parameter for comparing soils with respect to their sensivity to water repellency (Dekker and Ritsema, 1994b), because differences in water content are wiped out.

123

2.4. Alcohol percentage test The fundamental principles underlying the process of wetting show that a reduction in the surface tension of a solid substance to be wetted reduces its wettability. Conversely, a reduction in the surface tension of the applied liquid increases wettability. The liquid –solid contact angle is dependent on the surface tension of the liquid. In general, when the surface tension of the liquid decreases, the liquid – solid contact angle will also decrease. The liquid surface tension which wets a soil material with a 908 contact angle was proposed as an index of water repellency by Watson and Letey (1970). This 908 surface tension can easily and quickly be measured as follows. A series of aqueous ethanol solutions producing different surface tensions is prepared. A drop of each solution is applied to the soil surface, and the penetration time recorded. If the surface tension of the liquid applied to the soil is lower than the 908 surface tension, the drop will penetrate rapidly. If the surface tension is higher than the 908 surface tension, the liquid applied will be slightly retarded in penetration. Five seconds was arbitrarily chosen as reference time (Letey et al., 1975; Richardson, 1984). We measured the degree of water repellency of 46 samples (later used for the determination of the organic matter content), using the following alcohol percentage test. We used bottles with solutions containing 1, 2, 3, 4, 5, 6, 8, 10, 12.5, and 15% and with increments of 2.5 to 35% of ethanol on a volume basis. The degree of water repellency of a sample is the lowest alcohol percentage of the solution that penetrates the soil in 5 s or less. Alcohol percentage tests were conducted on the dried samples, thus measuring the degree of potential water repellency.

3. Results and discussion 3.1. Actual water repellency Soil samples were regularly collected between 20 March 1997 and 25 November 1998 beneath the pine trees at depths of 0 – 2.5, 2.5 –5, and 5 –10 cm. Thirty eight samples at depths of 0 –2.5 cm, 26 samples at depths of 2.5 –5 cm, and 29 samples at depths of 5– 10 cm of a total of 89, 46, and 65 field-moist

124

F.J. Moral Garcia et al.

Fig. 3. Relative frequency of wettable and water repellent fieldmoist samples, taken at depths of 0– 2.5 (n ¼ 89), 2.5–5 (n ¼ 46), and 5–10 cm (n ¼ 65) in the sandy soil beneath the pine trees.

samples, respectively, exhibited actual water repellency with WDPT values exceeding 5 s. Thus in total, respectively, 43, 57, and 47 % of the soil samples taken at those depths were water repellent in the field, as is illustrated in Fig. 3. 3.2. Persistence of potential water repellency A large number of soil samples from the pine site exhibited water repellency after drying. Thirty seven samples from depths of 0 –2.5 cm, four samples from depths of 2.5– 5 cm, and six samples from depths of 5 –10 cm, were actually wettable, but became water repellent by drying at 60 8C. The frequency distri-

bution of the persistence of potential water repellency for samples taken from three depths at the pine site is shown in Fig. 4. The variation in persistence of potential water repellency is high. At all depths, wettable, slightly, strongly, severely, and extremely water repellent soil samples have been measured. At the bare soil site, eight times six samples were collected at depths of 2.5 –5 cm, during the period between 11 October 1995 and 3 September 1996. Potential water repellency was measured after drying the samples at 60 8C. It is remarkable that also at this site without vegetation so many (25 of the 48) samples exhibited slight to extreme water repellency. The frequency distribution of the persistence of potential water repellency of the samples is given in Fig. 5. 3.3. Organic matter content and potential water repellency Accumulation of sufficient amounts of organic matter can induce water repellency in any soil, and water repellency increases in severity with increasing organic matter content (Harper et al., 2000). The positive relationship between organic matter content and persistence of potential water repellency of samples collected from the topsoil of the pine site is evident, as illustrated in the diagram of Fig. 6. Samples with less than 6% organic matter were slightly to severely water repellent, whereas all samples with higher organic matter contents exhibited severe to extreme water repellency. In contrast, a study of water repellency in transects in a dune sand of the Netherlands (Dekker and Ritsema, 1994b)

Fig. 4. Relative frequency of the persistence of potential water repellency of the samples of Fig. 3 after drying at 60 8C.

Soil water repellency in the Natural Park of Donana, southern Spain

125

Fig. 5. Relative frequency of the persistence of potential water repellency of samples (n ¼ 48) taken at depths of 2.5–5 cm in the bare sandy soil.

showed no relationship between the organic matter content and the persistence of potential water repellency. Also the degree of potential water repellency of samples from the pine site, measured by the alcohol percentage test, was positively related to the organic matter content (Fig. 7). For example, the alcohol

Fig. 7. Relationship between the organic mattter content and the alcohol percentage of samples (n ¼ 46) taken in the sandy soil beneath the pine trees.

percentage varied between 1 and 20 for all samples with an organic matter content of less than 6%, whereas the alcohol percentage varied between 20 and 35 for higher organic matter contents. This is in accordance with the study of water repellency in the dune sand of the Netherlands, which also showed a positive relationship between the organic matter content and the alcohol percentage (Dekker and Ritsema, 1994b).

4. Conclusions

Fig. 6. Relationship between the organic mattter content and the WDPT value of samples (n ¼ 44) taken in the sandy soil beneath the pine trees.

Naturally occurring water repellent soils were discovered in the Natural Park of Donana. Water repellency was not restricted to the topsoil beneath pine trees, but was also detected at places without vegetation. Severity of persistence (measured with the WDPT test) and of degree (measured with the alcohol percentage test) of the potential water repellency of soil samples taken beneath pine trees were positively related with the organic matter content of these samples. The infiltration rate of the topsoil appeared to be very low after dry periods, due to the occurrence of water repellency, and as a consequence rain events caused erosion and irregular wetting of the soil.


Chapter 13

Soil water repellency in northeastern Greece Apostolos K. Ziogasa, Louis W. Dekkerb,*, Klaas Oostindieb and Coen J. Ritsemab a

b

Democritus University of Thrace, Department of Civil Engineering, Vas Sofias1, Prokat, Xanthi 67100, Greece Alterra, Green World Research, Department of Land Use and Soil Processes, PO Box 47, 6700 AA Wageningen, The Netherlands

Abstract Many soils may be water repellent to some degree, challenging the common perception that soil water repellency is only an interesting aberration. When dry, water repellent soils resist or retard water infiltration into the soil matrix. Soil water repellency often leads to the development of unstable wetting and preferential flow paths. In the present study the persistence of water repellency was examined on samples from topsoils in Thrace, located in northeastern Greece, using the water drop penetration time (WDPT) test. The soil samples were collected from agricultural fields throughout the prefectures of Xanthi and Rodopi. Six sites were selected for intensive sampling of water repellency and soil moisture content in transects. Water repellency was measured on field-moist soil samples and after drying the samples at increasing temperatures, to study the influence of drying temperature on the severity of the persistence. Measurements of soil samples taken in agricultural fields under different crops e.g. winter wheat, tobacco, clover, olive groves, kiwi fruit, and vineyards, in the area of Thrace, revealed that 45% of the locations exhibited actual water repellency during dry periods. Drying of samples from the Sostis site resulted in wettable soil, whereas drying of samples from the Mitriko site increased repellency. Therefore, water repellency should preferably be measured on samples taken in the field under dry conditions in order to reveal and determine the highest degree of water repellency that might occur in the field.

1. Introduction Dry soils are normally easily wetted by rainfall and irrigation. If the attractive forces are neutralized or absent, e.g. because of the presence of a hydrophobic coating on sand grains or aggregates, soils are said to resist wetting and are considered to be water repellent and to exhibit hydrophobic properties. A water repellent soil will be defined as one which does not wet spontaneously when a drop of water is placed upon the surface. Water repellency has been observed in sand, loam, clay and peat soils all over the world (Wallis and Horne, 1992; Jaramillo et al., 2000; * Corresponding author. Tel.: þ31-317-474267; fax: þ 31-317419000. E-mail address: [email protected] (L.W. Dekker). q 2003 Elsevier Science B.V. All rights reserved.

Feng et al., 2002). Although an increasing number of researchers are aware of the occurrence and consequences of water repellency in a wide range of soils, it is still a neglected field in soil science for many soil scientists. On the contrary, DeBano (2000) stated that because of the many water repellent soils found during the last century, water repellency evolved from an isolated scientific curiosity to an established field of science. As far as we know, this one is amongst the first papers that describe the occurrence and consequences of water repellent soils in Greece. It has been recognized for many years that the water repellency of a soil is a function of the type of organic matter incorporated in it, and that certain organic matter induces water repellency in soils by several means. For example, organic substances

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A.K. Ziogas et al.

leached from plant litter can induce water repellency in sandy and other coarse-grained soils (DeBano, 1981), but also hydrophobic microbiological byproducts coating a mineral soil particle may induce wetting resistance (Bond and Harris, 1964; Chan, 1992). Mineral particles need not be individually coated with hydrophobic materials; intermixing of mineral soil particles with particulate organic matter, like remnants of roots, leaves and stems, may also induce severe water repellency (DeBano, 1969d; Bisdom et al., 1993). Generally, a soil might become water repellent after drying to some critical soil water content level. Below this level, the soil behaves as a water repellent soil with respect to water flow; however, if moist enough, the soil behaves again as a wettable soil. The change between these two states is generally thought to be caused by certain moleculair changes of all kinds of organic substances in soil (Bachmann et al., 2001b). Water repellency may dramatically affect water and solute movement at the field-scale, which has often been underestimated (Ritsema and Dekker, 1998; Bauters et al., 2000b). Water repellency and its spatial variability have been shown to cause nonuniform wetting and preferential flow, and as a consequence large variations in water content in many field soils (Dekker and Ritsema, 1994b; Ritsema and Dekker, 1994b; 1996b; Ritsema et al, 1998a). Up to now soils in Greece had not been surveyed for water repellency. Since other countries with similar climatic conditions, such as the southern parts of Spain and Portugal have extensive areas with water repellent soils, it is important to test Greek soils for water repellency. Therefore, the objectives of the present study were to investigate the occurrence of actual and potential water repellency in sandy and loamy topsoils of agricultural soils in Thrace, located in northeastern Greece.

2. Assessment of water repellency and critical soil water content Over the years many techniques have been developed to measure water repellency, during which time the understanding and definition of water repellency has evolved (Dekker and Ritsema, 2000).

One of the simplest and most common methods of classifying water repellency is the Water Drop Penetration Time (WDPT) test, already described by Van ‘t Woudt (1959). Three drops of distilled water are placed on the smoothed surface of a soil sample, using a standard medicine dropper and the time that elapses before the drops are absorbed is determined. Increasing the temperature of the water applied will reduce the surface tension, and in line with this, the time required for wetting will be reduced (King, 1981). Therefore, the temperature at the time of testing must be constant (Richardson, 1984). The relative humidity of the air in the laboratory also affects the penetration time of the water drops. Increasing the relative humidity increases the time that the drops remain on the surface (Doerr et al., 2002). We measured the soil water repellency of all our samples in the laboratory at a temperature of about 308C and a relative air humidity of around 45%. In general a soil is considered to be water repellent if the WDPT exceeds 5 s (Bond and Harris, 1964; DeBano, 1981; Dekker, 1998). We applied an index allowing a quantitative definition of the persistence of soil water repellency as described by Dekker and Jungerius (1990). Thus, seven classes of repellency were distinguished, based on the time needed for the water drops to penetrate into the soil: class 0, wettable, non-water repellent (infiltration within 5 s); class 1, slightly water repellent (5 –60 s); class 2, strongly water repellent (60 – 600 s); class 3, severely water repellent (600 – 3600 s); and extremely water repellent (more than 1 h), further subdivided into class 4, 1 –3 h; class 5, 3 –6 h; and class 6, . 6 h. We measured the water repellency of the soil samples after drying at 30, 65 and 1058C. The several temperatures were chosen to study the influence of drying temperature on the severity of the persistence of soil water repellency. Measurements on dried soil samples result in the “potential” water repellency (Dekker and Ritsema, 1994b). In the field, some of these samples were dry and water repellent, but others were moist or wet. Therefore, we have also checked the persistence of the “actual” water repellency of field-moist samples immediately after recording their wet weight. By measuring the water content of the samples, we could assess “critical soil water

Soil water repellency in northeastern Greece

contents” for the different depths of the intensively sampled transects. The soil is water repellent below, and wettable above these values.

3. Soil sampling and study sites Samples for testing water repellency were collected at depths of 0 –5 cm on 213 locations on sandy and loamy soils throughout the area of Thrace, mainly in the prefectures Rodopi and Xanthi, situated in northeastern Greece (Fig. 1) in the autumn and winter period between 31 October 1999 and 23 February, 2000 and during a dry period between 1 June and 1 September, 2000. The samples were taken under different crops, e.g. winter wheat, tobacco, clover, olive groves, kiwi fruit and vineyards. Water repellency was measured on the field-moist samples, and after drying the samples at a temperature of 658C. Six sites (Sostis, Mitriko, Abdera, Maggana (2 x), and Dialambi, were selected for sampling the soil water content and water repellency in detail (Fig. 1). The Sostis site is a vineyard and consists of a loamy soil. The Mitriko and Dialambi sites are pastures on sandy soils and the Abdera site is a coastal dune sand

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with grey hair grass. In Maggana there are two experimental sites on sandy soils, one with winter wheat and one with olive groves. All soils at these six sites were sampled at depths of 0– 5, 7 –12, 14 –19, 21– 26, and 28– 33 cm, using steel cylinders (100 cm3) with a height and diameter of about 5 cm. At each depth, 35 samples were taken at a close spacing over a distance of 1.8 m. The cylinders were pressed vertically into the soil, and layer after layer was sampled directly after removing the soil, to minimize evaporation of the soil sampled. The steel cylinders were emptied into plastic bags and used again, and the plastic bags were tightly closed. The wet soil in the plastic bags was weighed, dried for several days at 658C, and weighed again to determine the soil water content. Resistance to wetting was determined by measuring the wetting rate of field-moist samples. Samples were taken at depths of 0 –5 cm and 7– 12 cm at the Dialambi and Mitriko sites and remained in steel cylinders for this purpose. In the laboratory, the cylinders (100 cm3 content) were subjected to a constant pressure head of 2 2.5 cm water applied at the bottom of the sample (Dekker et al., 1998). The experimental set-up was designed in such a way that each increase in water content of 0.2 vol% was recorded automatically over a period of 1 hour.

Fig. 1. Area (grey) in Greece where soil samples were taken for testing the occurrence of water repellency. The locations of the study sites are indicated too.

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4. Results

4.2. The Sostis site

4.1. Investigated area

The actual water repellency measurements of fieldmoist samples taken in the loamy soil in the vineyard at the Sostis site on 20 October, 1999 are illustrated in Fig. 2. All samples in the topsoil at depths between 0 – 12 cm showed a slight to severe water repellency, whereas at 14 – 19 cm depth some, and at 21– 26 cm depth nearly all samples were wettable. All samples taken at depths of 28 –33 cm (data not shown) were wettable. The potential water repellency of the samples after drying at 308C showed an evident decrease in severity of water repellency, in particular at 0– 5 cm depth. A further decrease followed after drying the samples at higher temperatures. These results are entirely different from those found for soils in the Netherlands, where for several soils an increase in water repellency was found after drying the samples at higher temperatures (Dekker et al., 1998).

Field-moist soil samples from 42 of the 213 locations, taken during the autumn and winter period, exhibited slight to severe water repellency. It was remarkable that drying of the wettable samples did not result in potential water repellency, and that water repellency decreased after drying the actually water repellent samples. Numerous sandy, loam and clay loam soils in the area of Thrace in Greece appeared to be water repellent during dry spells. During the summer period of 2000 slight to extreme water repellency was determined at depths of 0 –5 cm on 95 of the 213 locations with crops like, winter wheat, clover, tobacco, and grass, as well as under kiwifruit, pine, olive, cherry trees and vineyards. After rain at several of these locations erosion and irregular wetting patterns in the soils had been observed.

Fig. 2. Relative frequency of the persistence of actual and potential water repellency measured on soil samples (n ¼ 35) taken at four depths in the vineyard of Sostis on 20 Oct. 1999.


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Fig. 3. Relative frequency of the persistence of actual and potential water repellency measured on soil samples (n ¼ 140) taken at two depths on 19 Jun. 1999, 21 Jan. 2000, 19 Sep. 2000, and 8 Dec. 2000 at the Mitriko site.

4.3. The Mitriko site

4.4. The Abdera site

Most samples taken at depths of 0– 5 cm in the pasture of the sandy soil Mitriko on four sampling dates exhibited slight to extreme actual water repellency (Fig. 3). The potential water repellency of the samples after drying at 308C was more severe in comparison with the actual water repellency. A further increase in severity followed after drying the samples at higher temperatures. On the other hand, for samples collected at depths of 7 –12 cm, only slight differences in water repellency were found between actual and potential water repellency after drying at different temperatures.

Nearly all samples taken at depths of 0 – 5 cm in the coastal dune sand, grown with mosses and grey hair grass, exhibited water repellency on the four sampling dates (Fig. 4). More than 50% of the samples taken on 19 April, 2000 were extremely water repellent. Large differences in severity of the actual water repellency occurred over the 1.8 m sampling distance on all sampling dates. The persistence of the actual water repellency at depths of 7– 12 cm was less severe when compared with the surface layer on the four dates.

Fig. 4. Relative frequency of the persistence of actual water repellency measured on field-moist samples (n ¼ 35) at two depths at the Abdera site on 12 Aug. 1999, 22 Sep. 1999, 7 Feb. 2000, and 19 Apr. 2000.

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Fig. 5. Bare spots in winter wheat grown on actually water repellent sand at the Maggana site.

4.5. The Maggana sites A water repellent sandy soil with an irregularly growing winter wheat crop was studied near Maggana (Fig. 5). The surface layer at bare spots was found to be severely water repellent, as illustrated in Fig. 5.

The actual water repellency measurements of fieldmoist samples taken in the topsoil at depths between 0–26 cm on 4 November, 1999 showed slight to extreme water repellency, whereas in the subsoil at 28–33 cm, 36% of the samples were slightly to strongly water repellent and the remaining part (64%) wettable (Fig. 6).

Fig. 6. Relative frequency of the persistence of actual water repellency measured on field-moist samples (n ¼ 35) at five depths in the sandy soil with winter wheat and with olive grove at the Maggana site on four sampling days.


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4.6. The Dialambi site

Fig. 7. Erosion of the water repellent sandy topsoil with grass cover at the Dialambi site.

The topsoil at depths of 0–26 cm and a part of the subsoil at depths of 28–33 cm were also slightly to severely water repellent on 24 November 2000 (Fig. 6). All field-moist samples taken in the sandy soil of the olive groves at depths of 0– 19 cm were slightly to extremely water repellent on 5 November, 1999 and on 20 November 2000, as is shown in the right hand diagrams of Fig. 6. A part of the samples at depths of 21 –26 cm exhibited slight to strong repellency on these days.

Infiltration rates into water repellent soils can be considerably lower than those into wettable soils. During rain events after prolonged dry periods, water repellency of the topsoil may cause surface runoff. An example of erosion of the topsoil due to water repellency was established in the sandy pasture of the Dialambi site (Fig. 7). The spatial variability in persistence of actual water repellency at depth of 0 –5 cm and 7– 12 cm at this site was high over short distances between 14 April, 1999 and 18 December, 2000 (Fig. 8). The temporal variability was high too, with the most extreme water repellency in both layers detected on 3 November, 2000, as shown in both diagrams. All samples from the surface (0 –5 cm) layer taken in the transects of the Dialambi site with soil water contents of . 15 vol.% were found to be wettable (Fig. 9). In contrast, all samples with a soil water content of , 9.3 vol.% were slightly to extremely water repellent with WDPT values between 5 and 60 s (class 1) and . 6 h (class 6). Between water contents of 9.3 and 15 vol.% wettable as well as water repellent conditions were encoutered. This soil water content zone was introduced by

Fig. 8. Relative frequency of the persistence of actual water repellency of samples (n ¼ 35) at depths of 0 –5 and 7–12 cm at the Dialambi site on seven sampling days.

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Fig. 9. Relationship between the soil water content and the persistence of the actual water repellency of samples taken at depths of 0–5 cm at the Dialambi site. The transition zone, with critical soil water contents is indicated with a grey tone.

Dekker et al. (2001d) as the “transition zone”. This means that the critical soil water content of the surface layer was variable and ranged between 9.3 and 15 vol.%, depending on wetting and drying history.

4.7. Resistance to wetting of field-moist samples The wetting rate of field-moist samples was measured for samples taken at depths of 0 –5 and 7– 12 cm at the Dialambi and Mitriko sites (Fig. 10). It is evident from the curves in the diagrams that wetting was slower and resulted in lower water contents at depths of 0 – 5 cm compared to 7– 12 cm after 60 minutes of wetting. The wetting rate of both samples from the Mitriko site was clearly higher in comparison with the Dialambi site.

5. Discussion and conclusions

Fig. 10. Increase in volumetric water content of field-moist samples taken at two depths at the Dialambi and Mitriko sites. The samples were placed at a constant pressure head of 22.5 cm water applied at the bottom of the samples during one hour.

Water repellency is an important, often neglected property of many soils, which has its greatest effect in relatively dry soils. It has serious consequences for the wetting of the soil. Infiltration rates into water repellent soils can be considerably lower than those into wettable soils. During rain events after prolonged dry periods, water repellency of the topsoil may cause surface runoff, especially in sloping areas. An example of erosion in a water repellent soil with grass cover in Greece is shown in Fig. 7. Thus, water repellency tends to increase runoff and erosion and decrease the volume of water absorbed by the soil. With increasing rainfall, water infiltration proceeds and finally starts to break through the water repellent layer by creating irregular wetting patterns


(e.g. Ritsema and Dekker, 1995). Water and solutes often flow in these soils through preferential flow paths, the so-called “tongues” or “fingers” (e.g. Ritsema and Dekker, 1996b). This phenomenon shortens solute travel time to the groundwater table, and therefore increases the risk of groundwater contamination (Ritsema and Dekker, 1998). The investigation in the area of Thrace revealed that 45% of the 213 locations showed actual water repellency during dry periods in the summer, and even about 20% of the locations exhibited slight to severe water repellency during the autumn and winter period. It is remarkable that in contrast with findings by Dekker et al. (1998) for samples in the Netherlands, drying of wettable samples at 658C did not induce water repellency, and drying of repellent samples even often decreased the severity of the persistence. Drying the samples at 308C resulted already in a significant decrease of the severity of water repellency for samples taken in the loamy Sostis site. Slight to strong actual water repellency was for example detected for the 35 field-moist samples from depths of 0– 5 cm, and all the samples but one were wettable after drying at 308C (Fig. 2). Samples from depths of 0– 5 cm at the Mitriko site behaved quite the opposite, with an increase in

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severity of the persistence after drying at higher temperatures (Fig. 3). On the other hand, only slight differences in water repellency were found between actual and potential repellency after drying at different temperatures for samples taken at depths of 7– 12 cm at this site. The temporal and spatial variability of the actual soil water repellency were found to be high at all depths sampled at the Abdera, Dialambi, and Maggana sites, as is illustrated with Fig. 4, 6, and 8. Assessments of water repellency of soils are commonly made on air-dried or oven-dried samples (Dekker et al., 2001d). The potential water repellency of samples dried at 30, 65, and 1058C in the present study, was often less severe and sometimes more severe than the actual repellency of field-moist samples. This suggests that the maximal persistence of potential water repellency measured on dried samples might underestimate or overestimate the level of repellency as it occurs in the field. The generally unpredictable response of the WDPT values to drying of wet samples does, however, suggest that samples should preferably be taken under dry conditions in order to reveal and determine the most realistic and highest degree of water repellency that might occur in the field.


Chapter 14

Soil moisture: a controlling factor in water repellency? S.H. Doerra,* and A.D. Thomasb a

Department of Geography, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK Telford Institute of Environmental Systems, Department of Geography, University of Salford, Peel Building, The Crescent, Manchester M5 W4T, UK

b

Abstract Water repellency (hydrophobicity) is known to be temporally variable. Most studies indicate that soils are most repellent when dry and least repellent or non-repellent (hydrophilic) when moist. In several studies, attempts have been made to establish a critical soil moisture threshold, demarcating water-repellent and non-repellent conditions. The reported thresholds vary widely and the exact relationship between hydrophobicity and soil moisture remains far from being understood. Using field and laboratory measurements, this study explores the effect of soil moisture on water repellency for Portuguese sandy loam and loamy sand forest soils. The results indicate that for these soils, repellency is absent when soil moisture exceeds 28%, but show that after wetting, repellency is not necessarily re-established when soils become dry again. It is thought that short-term and seasonal changes in soil water repellency are not simply a function of variations in soil moisture as indicated in the literature. It is suggested that, after wetting, re-establishment of repellency may also require a fresh input of water-repellent substances. The mechanisms of wetting and drying in water-repellent soils are discussed and associated hydrological implications are explored.

1. Introduction Hydrophobicity (water repellency) is not a static soil property but is known to follow short-term or seasonal variations. Hydrophobicity is generally found to be most extreme when soils are dry, declining and eventually disappearing as soils become wet (Bond and Harris, 1964; DeBano, 1971; Witter et al., 1991; Ritsema and Dekker, 1994b), although notable exceptions have been reported. Where hydrophobicity is associated with fungal or other micro-biological activity, hydrophobicity may increase initially with soil moisture due to the associated enhanced biological activity before disappearing as soils become wet * Corresponding author. Tel.: þ44-1792-295147; fax: þ 44-1792295955. E-mail address: [email protected] (S.H. Doerr). q 2003 Elsevier Science B.V. All rights reserved.

(Bond, 1960; Jex et al., 1985). Hydrophobicity is then believed to re-appear when soils become dry again (Roberts and Carbon, 1971; Tschapek, 1984; Wessel, 1988; Berglund and Persson, 1996). Thus a direct soil moisture/hydrophobicity relationship has been proposed and attempts have been made to establish ‘critical soil moisture thresholds’ demarcating hydrophilic and hydrophobic conditions (King, 1981; Dekker and Ritsema, 1994b; Soto et al., 1994). For example, Dekker and Ritsema (1996c) found that hydrophobicity generally ceases in Dutch clayey peat for soil moisture thresholds above 34 – 38%, whereas, for some Dutch dune sands, hydrophobicity was absent for soil moisture thresholds exceeding as little as 2% (Dekker and Ritsema, 1994b). This soil moisture-related aspect of hydrophobicity has important repercussions for land use planning

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and agriculture. For example, the detrimental effects of hydrophobicity such as increased runoff and soil erosion (Sevink et al., 1989; Imeson et al., 1992; Shakesby et al., 1993) or the formation of preferential flow (Ritsema et al., 1997a) which can lead to accelerated leaching of agrochemicals (Hendrickx et al., 1993) may not need to be considered during prolonged wet periods when soil moisture is above a critical threshold and thus soils remain hydrophilic. Also, where feasible, knowledge of the critical soil moisture threshold would allow more efficient irrigation regimes to be employed during periods of drought, which could keep soils sufficiently moist to avoid the onset of water-repellent conditions and their associated effects (Miller and Wilkinson, 1977). Generally, the overall hydrological consequences of hydrophobicity cannot be assessed fully without the knowledge of its temporal fluctuations. Critical soil moisture thresholds have usually been established by measuring hydrophobicity and soil moisture under different conditions in the field or on a range of field moist samples (Dekker and Ritsema, 1994b, 1996c; Soto et al., 1994), before and after airor oven-drying (Dekker and Ritsema, 1994b, 1996c) or by adding different amounts of water to dry soil (King, 1981; Berglund and Persson, 1996). Thus, the timing and the exact relationship of wetting and drying phases with hydrophobicity remain largely unexplored and to date, no detailed work has been conducted into the manner in which the change between repellent and non-repellent states occurs. This study explores the temporal aspects of hydrophobicity in relation to soil moisture on the same samples during a complete wetting and airdrying cycle under controlled laboratory conditions using sandy loam to loamy sand-textured Portuguese forest soils. Repeated hydrophobicity and soil moisture measurements taken after dry and wet periods on in situ soils in the field are also carried out to provide information on the applicability of the laboratory simulation to actual field conditions.


Pinus pinaster and Eucalyptus globulus. Soils are stony, shallow Umbric Leptosols (, 15 cm depth) overlying well-cleaved schists or Humic Cambisols overlying schist regolith (Perreira and FitzPatrick, 1995). They are of sandy loam to loamy sand texture containing , 6% clay (Table 1). The climate is wet mediterranean with a marked summer drought from June to September, and considerable annual rainfall (1300 – 1900 mm, increasing with altitude). An extensive field survey has shown that the soils are highly hydrophobic after prolonged dry summer conditions with very little spatial variation on both metre and millimetre scales (Doerr et al., 1998). Some of these soils have been subject to wildfires, which, however, have had no detectable effect on their hydrophobic properties. Further details of the soils and of the research area have been given in Doerr et al. (1996, 1998). Hydrophobicity assessments were carried out using the WDPT method (Letey, 1969). Initially airdried laboratory samples (# 2 mm fraction) and in situ surface soil samples were tested using the categories given in Table 2. To facilitate description, samples with a median WDPT of # 60 s are termed hydrophilic, and those . 60 s as hydrophobic1. In addition, the % ethanol method (Watson and Letey, 1970) was carried out using 3 s as a drop penetration-time threshold (Crockford et al., 1991) on air-dried samples prior to laboratory experiments to allow a wider comparison of the soils used here with levels encountered in other studies. The ethanol concentrations used were 0, 3, 5, 8.5, 13, 24 and 36 vol%. A comparison of both hydrophobicity assessment methods and further details of measurement and sample preparation techniques used are described in detail in Doerr (1998). 2.1. Laboratory experiments Thirty-five air-dry surface soil samples were selected encompassing soil from burnt eucalyptus (NE) and pine (NP; OP) land and from equivalent unburnt land (UE; XE and UP; XP) (Fig. 1) with 1

Field measurements and sampling were carried out in the foothills of the Caramulo Mountains, northcentral Portugal (Fig. 1) on land afforested with

WDPT thresholds distinguishing hydrophobic and hydrophilic conditions facilitate description, but are essentially arbitrary and vary between studies. The threshold used here has been the standard chosen in previous studies in the area.

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Soil moisture: a controlling factor in water repellency?

Fig. 1. Location of the study area and sites sampled.

Table 1 Typical particle size range of the fine earth fraction (#2 mm) of soils used in this study Size in mm % weight

, 0.002 5.6

0.002–0.063 45.2

0.063–0.125 13.1

0.125–0.25 7.2

0.25–0.5 4.4

0.5–1 7.9

1–2 16.6

Table 2 WDPT (s) intervals and descriptive labels used in this study Intervals

Hydrophilic

WDPT (lab.) WDPT (field)

,5 #60

10 #60

Hydrophobic 30 #60

60 #60

180 180

300 300

600 600

900 900

3600 .900

18 000 .900

.18 000 .900

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Table 3 Minimum and maximum soil temperatures (8C) (6 October 1993–25 August 1995) for bare and litter covered topsoil (0–2 cm) Soil temperature

Falgorosa burnt eucalyptus (bare)

Lourizela burnt pine (bare)

Falgorosa unburnt eucalyptus (covered)

Barrosa unburnt pine (covered)

Minimum Maximum Range

3.3 29.5 26.2

0.6 33.3 32.7

6.0 24.8 18.8

5.0 20.3 15.3

hydrophobicity levels (assessed in the laboratory on air-dry soil) ranging from WDPTs of , 5 s to . 5 h. Experiments were conducted at a laboratory temperature of 238C (^ 28C) which is similar to the highest summer temperatures likely to be experienced by litter-covered soils but below maximum temperatures of burnt soils in the area (Table 3).

2.1.1. The wetting phase To simulate a wetting phase, samples were exposed to a ponded layer of water for a maximum of 30 days during which their infiltration status was frequently checked. Clear plastic dishes (50 mm radius; 10 mm depth) were filled with 10 g of soil and 8 g of distilled water was carefully added to the smoothed surface in such a way that the soil surface was not completely covered with a water layer. The gap in the ponding layer was maintained in order to allow a pathway for air to escape during infiltration. A weight of 8 g of water was chosen as preliminary tests with hydrophilic samples from the study area had shown that this amount was sufficient to saturate a 10 g soil sample. The samples were then covered with lids to prevent evaporation. The clear dishes allowed visual determination of the progress in infiltration. A distinction was made between (a) dry samples where no wetting could be observed, (b) moist samples where some pore spaces were filled with water and (c) saturated samples where a continuous waterfront was visible at the bottom of the sample indicating complete infiltration. Although this qualitative approach was somewhat subjective, it was preferred to any alternative quantitative method, which would have resulted in physical disturbance of the sample. After completion of the wetting experiment, any excess ponding water,

together with any remaining surface water film, was removed using a hypodermic syringe and the samples were then WDPT tested. 2.1.2. The drying phase After the completion of the wetting experiment, sample dishes were left partly covered with a lid, allowing for slow air-drying of the soil samples. Within 2, 7, 21 and 42 days after the start of the drying phase, soil moisture was determined by weighing the sample dishes. The gravimetric2 soil moisture used here for the laboratory samples is expressed in % of the air-dried status prior to the experiment. The more commonly used method of oven-drying samples prior to analysis to arrive at near 0% absolute soil moisture was not used here in order to avoid (a) a heat-induced artificial enhancement of hydrophobicity and (b) an artificially low soil moisture content at the beginning of the experiment which is irrelevant to field conditions. The actual moisture content of air-dry samples under the laboratory conditions (compared to oven-dry at 1058C) is around 3% (average of 10 samples; range: 2.2 –5.2%). After each soil moisture measurement, the WDPT test was carried out at the sample surface in areas that had been covered by a water layer. These measurements could only be carried out on four occasions as each WDPT test involved the disturbance of part of the original sample surface. In cases where a hydrophilic crust had developed on a sample surface during drying, WDPT measurements were taken below this crust. The experimental design for the laboratory investigations is summarised in Fig. 2. 2

Dry bulk density of laboratory samples ranges considerably from 0.57 to 0.98 g/cm3. Thus the equivalent volumetric soil moisture content would range between ‘near half’ to ‘similar to’ the gravimetric content.


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Fig. 2. Experimental design of the wetting–drying cycle modelled in the laboratory.

2.2. Field investigations Field procedures included hydrophobicity assessments (WDPT test using the median of 20 droplets) during summer dry conditions or drying phases after rainstorms during spring and summer conditions. Measurements were conducted on each of 10 randomly located sampling sites for burnt eucalyptus (NE) and pine (NP) land and each of five sites for equivalent unburnt land (UE and UP). After in situ WDPT measurements, samples were taken to the laboratory for determination of the gravimetric soil moisture.

3. Results 3.1. Laboratory experiments 3.1.1. The wetting phase Of the 24 samples with initially very high levels of hydrophobicity (i.e. WDPT 5 h or . 5 h and 24 or 36% ethanol), 13 became saturated during the experiment; whereas, for the other 11 samples, infiltration was not completed within the 30 day measuring period, with 8 of them showing no water penetration at all (Fig. 3). The modal time category taken for infiltration for the 13 samples that became saturated was 5 – 20 h, although four samples became saturated in less than 1 h despite their high WDPT category. Thus, although about half of the highly hydrophobic samples resisted infiltration for more than 30 days, high initial WDPT and % ethanol values did not always indicate extreme resistance to infiltration. All 11 samples with an initial WDPT category of , 5 h became saturated during the experiment. Four

of the six samples with an initial WDPT category of 1 h became saturated within 1 h, whereas the remaining two samples were only saturated after 3 days. Data on soil hydrophobicity and gravimetric soil moisture threshold (as % of the air-dry weight) after saturation (or, in the case of the unsaturated samples, soil hydrophobicity after 30 days) are also presented in Fig. 3. All samples that became saturated during the experiment became hydrophilic with water drop penetration being effectively instant (WDPT , 5 s). For the moist, though unsaturated samples NE16 and NE24 (respective recorded soil moisture of 15 and 22%)3 no detectable reduction in hydrophobicity was found (WDPT . 5 h). Hydrophobicity of the samples with no visible infiltration after 30 days was also fully retained. Soil moisture for the saturated samples was in the range of 50 – 79% (average soil moisture 66%). As 8 g of water were added to 10 g of sample, which should have led to a soil moisture level of 80% after thorough wetting, the data indicate that evaporation losses during the wetting phase of the experiment averaged 1.4 g (14% soil moisture) with a maximum loss of 3 g (30%). Considerable differences are evident between pine and eucalyptus samples. Comparing samples with the same initial WDPT category of . 5 h reveals that the eucalyptus soils resist infiltration longer than pine soils. Only one of the 12 eucalyptus samples in this highest WDPT category became saturated during the experiment, whereas all six pine samples became saturated within 5 days. 3

Since these samples were not homogeneously wetted, the soil moisture level within the moist area at which the WDPT test has been conducted is likely to be somewhat higher than the 15 and 22% measured.

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Fig. 3. Laboratory wetting phase: WDPT and % ethanol levels before the experiment, visually observed infiltration status of 8 g water added to 10 g air-dry soil samples, and gravimetric soil moisture content and WDPT after saturation or, for unsaturated samples, after 30 days.

3.1.2. The drying phase Soil moisture status and hydrophobicity (WDPT) of the samples during the drying phase are given in Fig. 4. To allow a better distinction between the hydrophilic

(WDPT # 60 s) and the hydrophobic status (WDPT . 60 s) of the samples in this complex diagram, the data are shaded, with a darker shading indicating # 60 s and a lighter shading indicating . 60 s.


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Fig. 4. Laboratory drying phase: gravimetric soil moisture content and associated WDPTs. To facilitate the interpretation, hydrophilic and hydrophobic conditions correspond to dark and light shadings, respectively.

Gravimetric soil moisture was between 30 and 63% (average 49%4) after 2 days of drying with no 4 Soil moisture data are not complete for all samples throughout the measuring period. Average soil moisture is therefore based on the recorded data (see Fig. 4).

measurable increase in hydrophobicity. After 7 days, soil moisture ranged between , 1 and 12% with all but three samples being close to airdry (defined here as # 3% gravimetric soil moisture). At this stage a slight increase in WDPT from , 5 to 10 s was measured for five samples, but

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hydrophobicity was not restored in any of the samples. During the following weeks, no change in hydrophobicity was detected. Thus, hydrophobicity was not restored for any of the 24 samples that were saturated and were therefore hydrophilic at the beginning of the drying phase even after 7 weeks of drying. 3.2. Field hydrophobicity Hydrophobic conditions were measured for samples having up to # 28% gravimetric soil moisture, whereas hydrophobicity was absent above 28%. However, soil moisture thresholds and hydrophobicity levels do not follow a general pattern of increasing hydrophobicity with decreasing soil moisture for both pine and eucalyptus land (Fig. 5). During dry spells and following rainstorms, hydrophilic conditions were encountered for soil moisture thresholds ranging from 5 to 58%. This stands in contrast to the almost uniformly high levels of hydrophobicity measured in an extensive survey on the same land types in the study area during summer drought conditions (Doerr et al., 1998).

4. Discussion and implications 4.1. The wetting phase Both laboratory and field data confirm the existence of a critical soil moisture threshold during a wetting phase. For the laboratory simulation, soil moisture measurements could only be conducted directly after the wetting phase. However, the samples that did gain some moisture, but were not saturated after the wetting phase showed no decrease in hydrophobicity (NE16, 24 and 36), whereas a complete loss of hydrophobicity was observed in the samples that were close to saturation (soil moisture contents between 51 and 75%; Fig. 3). The field data indicate that for soil moisture contents of above approximately 28%, no surface soil hydrophobicity prevails (Fig. 5). Other studies have found soil moisture thresholds of 2 vol% for Dutch dune soils (Dekker and Ritsema, 1994b), 21% for medium textured soils in Spain (Soto et al., 1994), 38 vol% for Dutch clayey peats (Dekker and Ritsema, 1996c)

and up to 50 vol% for some organic soils in Sweden (Berglund and Persson, 1996). Furthermore, the laboratory experiment indicates that a considerable increase in soil moisture does not necessarily lead to a significant reduction in hydrophobicity as suggested in previous studies (Bond and Harris, 1964; Soto et al., 1994). The following principles might be considered in relation to the wetting of hydrophobic soils. The hydrophobic effect occurs when the surface tension of a surface is lower than that of water (72.75 dynes/cm at 208C) (Adam, 1963; Zisman, 1964). For this to apply, however, water has to be present in bulk (i.e. as a film or as droplets). Water vapour, in contrast, can move freely in a hydrophobic soil allowing soil water to be redistributed (Miyamoto et al., 1972). Miyamoto et al. (1972) also showed in various experiments that adsorption of water vapour is independent of the surface tension of the porous medium and it appeared that hydrophobicity only had an effect beyond the range of the maximum adsorption of individual water molecules. The adsorption capacity of a mineral surface for individual molecules, however, is small and further condensation of water onto surfaces can only take place in the form of droplets (Osmet, 1963) which would be constrained in hydrophobic soils. Provided not all particles in the soil are hydrophobic, other hydrophilic matter could absorb additional water distributed as vapour without affecting the actual hydrophobicity of the soil surface. This mechanism would, in relation to the wetting phase experiment and to findings in previous studies, explain why soils can gain a considerable amount of water while being hydrophobic and why, for the soils that gained moisture but remained hydrophobic throughout the experiment, it took some time (. 5 days) for a noticeable increase in soil moisture. Furthermore, the available surface area in finetextured soils exceeds by far that of coarse-textured ones and the above mechanism would explain the differences in the critical soil moisture threshold found in the different studies outlined in the previous paragraph as, at least in part, texture related. In addition, it might be that the organic matter content also affects the critical soil moisture threshold. The time elapsed until hydrophobicity broke down and complete infiltration occurred varied considerably in this experiment (, 1 h for some pine soils to no


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Fig. 5. Median WDPT classes of in situ surface soil and average gravimetric soil moisture. The median WDPT is derived from 20 drops on each of 10 (5) randomly located sampling sites for locations NE and NP (UP and UE). Measurements were made during drying phases after rainstorms or during summer dry conditions in April 1993, October 1993, April 1994 and August 1994. WDPT categories (s) are: 1, #60; (hydrophilic) and 2, 60 –180; 3, 180–300; 4, 300–600; 5, 600 –900; 6, .900 (hydrophobic).

breakdown (and infiltration) after 30 days for most eucalyptus soils), even amongst soils with apparently similarly high levels of hydrophobicity. A higher

resistance to the breakdown of hydrophobic conditions in eucalyptus soils could be one reason why overland flow responses during storm events

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following prolonged dry periods in the study area tend to be higher on eucalyptus than on pine land (Walsh et al., 1994). These differences might be caused by variations in the type of bonding and/or the density or type of organic molecules between pine and eucalyptus soils. This, however, must remain speculative until the physicochemical principles of hydrophobicity in soils are better understood. To date, even observations regarding the breakdown of hydrophobicity under natural conditions in any environment are scarce, though, for an Australian eucalyptus forest, it has been reported that several weeks of wet weather were necessary for hydrophobicity to disappear (Crockford et al., 1991). The differences in persistence amongst highly hydrophobic soils to complete wetting observed in the wetting experiment are not reflected by differences in WDPT or % ethanol values. They might have been reflected if the WDPT test could have been extended to several days or more. This, however, is impractical due to the evaporation of the droplets. Thus, the ability to discriminate different levels of resistance to infiltration by the commonly used WDPT or % ethanol tests on highly hydrophobicity soils is in question and should be addressed within future developments in the methods for hydrophobicity testing. 4.2. The drying phase Although very little is known to date on the reestablishment of hydrophobicity after soils have been thoroughly wet, previous studies indicate that hydrophobicity is readily re-established when the soil moisture falls below the critical soil moisture content upon drying (Tschapek, 1984; Soto et al., 1994; Dekker and Ritsema, 1996c). The following physical mechanism has been suggested for this behaviour. After the hydrophobic compounds have been detached from the mineral particles during the wetting of the soil (rendering the soil particles non-repellent) these compounds remain intact. When the soil moisture becomes sufficiently low during the drying process, their polar ends associate and interact through hydrogen bonds, forcing the molecules back into a position with the polar ends attached to the mineral surface and the non-polar ends orientated

outwards leading to the re-establishment of hydrophobicity (Tschapek, 1984; Ma’shum and Farmer, 1985; Valat et al., 1991). Contrasting this, the soils in the laboratory experiments remained hydrophilic (a) as soil moisture fell below the critical soil moisture content of # 28%, (b) when soils were close to air-dry (although a slight increase in WDPT was observed for some samples at this stage) and (c) several weeks after the soils had reached an air-dry status. Thus, it is clear that the reestablishment of hydrophobicity after wetting is not simply caused by soil moisture loss. In addition, the fact that hydrophobicity did not recover even after the samples had been dry for several weeks suggests that the re-establishment of hydrophobicity is also not due to some very slowly operating physical mechanism alone which restores hydrophobicity at low moisture contents. It could be argued that the laboratory experiment does not necessarily reflect field conditions as it represents essentially a flooding event where the water is not allowed to drain. The field data, however, also indicate that hydrophobicity does not necessarily re-appear during or just after soils dry out under field conditions. Hydrophilic soil conditions were found at all moisture levels measured during and after drying phases. Hydrophobicity does, however, occur after long dry spells and therefore some alternative mechanisms, which could lead to the reestablishment of hydrophobicity after drying but did not operate in the laboratory simulation, are considered here. It has been shown that oven-drying of thoroughly wetted samples can re-establish hydrophobicity to some extent, although not to its initial levels (Ma’shum and Farmer, 1985; Doerr, 1997). This suggests that at least some organic molecules are still capable of inducing hydrophobicity after its cessation during the thorough wetting of the soil. This partial reestablishment of hydrophobicity may indeed be associated with the re-arrangement of the organic molecules as suggested by workers such as Valat et al. (1991) or simply with the redistribution of waxes already present in the soil matrix as interstitial globules (Franco et al., 1995). It is argued here that such a re-arrangement or re-distribution is not only due to a decrease in soil moisture, but is likely to be associated with the energy input during heating. This phenomenon is known in the outdoor fabric industry.


It has been shown that the application of heat (e.g. tumble-drying or ironing) to fabrics initially treated with a water-repellent coating can restore their hydrophobic character to some extent (Gore and Associates, 1994). Furthermore, it has been established that oven-drying (at 65 or 1058C) of field-moist, but already hydrophobic samples can lead to an increase in WDPTs (Franco et al., 1995; Dekker et al., 1998). This may be an important contributing mechanism in areas where surface soil temperatures reach high levels. For example more than 508C have been reported for an Australian soil (Rose, 1968). The highest summer soil temperatures (averaged over the top 2 cm of soil) recorded in the study area were 33.38C for bare soil and 24.88C for covered soil (Table 3). For the very surface of bare soils in the study area, temperatures may well have reached close to those reported by Rose (1968) and could have potentially contributed to the re-establishment of soil hydrophobiciy. For the covered soils in this study, however, this mechanism is unlikely to operate. The fact that hydrophobicity re-appears at least some time after drying in the field and that hydrophobicity remained absent in the laboratory suggests that a process, not operative in the laboratory, causes hydrophobicity to become fully re-established. Heat might be an important factor, however, the observations on litter-covered soils in this study and the fact that heat failed to fully re-establish soil hydrophobicity in laboratory experiments (Ma’shum and Farmer, 1985; Doerr, 1997) suggest that heat has only a contributory role in re-establishing hydrophobicity. It might be that the organic molecules that cause hydrophobicity initially become fully re-attached to the soil particles through a mechanism triggered by micro-biological activity. Another conceivable mechanism is that hydrophobicity is restored by a new input of hydrophobic substances. Such an input could originate from: (1) mechanically eroded wax particles from plant leaves during rainfall (Neinhuis and Barthlott, 1997); (2) a release of substances from wax globules already present within the soil matrix (Franco et al., 1995), which might be triggered by heating and/or biological processes; or (3) a release of new substances from plants or micro-organisms. Field observations in the study area indicate that the establishment of hydrophobicity is, at least under eucalyptus, associated with activity within the root

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zone and does not depend on the presence of litter (Doerr et al., 1998). Considering this and the mechanisms outlined above, it is suggested here that the complete re-establishment of hydrophobicity after wetting is associated with a new input of hydrophobic substances mainly related to biological activity in or near the root zone during or after the soils dry out. Although the exact mechanism for the re-establishment of hydrophobicity in the study area is not known, it appears that the processes involved need ‘some time’ to fully restore hydrophobicity after drying. For the study area, it is not known how long this process takes. However, observations in cultivated sandy soils over a period of several years in SW Australia indicate that it can take 3– 4 weeks for hydrophobicity to be reestablished after the onset of a dry period (D. Carter, pers. comm.). The temporal aspects of hydrophobicity and associated factors thought to operate for the soils in the study area are summarised in a simple conceptual model in Fig. 6. 4.3. Implications for soil hydrology The findings of this study suggest that soils in the area may take up to 28% of gravimetric soil moisture while their surface hydrophobicity remains unaffected. This finding, together with the fact that most eucalyptus soils resisted infiltration for more than 30 days in the laboratory, suggests that hydrophobicity in the study area is not readily affected during the first rainfall event(s) following persistently dry weather, particularly in the case of eucalyptus stands. This is supported by other work in the study area, reporting that hydrophobicity was still present after 200 mm rainfall following 6 weeks of dry weather (Ferreira, 1996). Ferreira observed furthermore that hydrophobic conditions can prevail through a wet season in some areas. This has also been noted for soils under Mediterranean oak type forest in NW Spain (Imeson et al., 1992). On the other hand, after a prolonged wet period causing the cessation of hydrophobicity, the onset of hydrophobic conditions may be delayed after drying. Thus, the effects of hydrophobicity such as a reduced infiltration capacity may persist longer into wet periods, but may also not come into effect as readily after long wet periods as is

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Fig. 6. Schematic conceptual model of temporal variations in soil hydrophobicity associated with wetting and drying for the study region.

commonly considered, particularly if it takes as long as 3– 4 weeks for hydrophobicity to be restored as was observed in SW Australia. Although very little is known to date about the temporal variations of hydrophobicity in relation to moisture regime, other factors in addition to the particular critical soil moisture thresholds and the actual resistance to breakdown of hydrophobicity in soils are thought to affect the temporal variations in hydrophobicity: 1. Rainfall regime—hydrophobicity may only break down during a persistently wet period and also may only become fully re-established during prolonged dry conditions. 2. Biological productivity—provided that a new input of hydrophobic substances is required to reestablish repellency after wetting, its rate of reestablishment would depend on the biological productivity of the ecosystem. Thus its re-establishment, whether induced by fungi, roots or plant litter, may be most rapid in comparatively warm humid environments. 3. Spatial variations—spatial discontinuity in hydrophobicity may accelerate its breakdown as wetting is not restricted to contact with raindrops or

ponding surface water, but could, in addition, be achieved via adjacent and/or underlying wet hydrophilic soil zones. Considering the implications and factors outlined above, and the findings of previous studies in the area, there is little doubt that the spread of Eucalyptus globulus replacing Pinus pinaster stands and cultivated areas on private land throughout north-central Portugal has altered the hydrological properties of large areas of soil. Hydrophobicity is not only significantly more severe but also spatially less variable under eucalyptus compared to pine (Doerr et al., 1998), impeding its breakdown in wet periods. The findings of the present study suggest that even for similar levels of severity, hydrophobicity of eucalyptus soils may be intrinsically more resistant to breakdown. In addition, it is likely that hydrophobicity recovers more rapidly after breakdown under eucalyptus, associated with the higher biomass production and fresh organic inputs to the soil compared to pine. An understanding of these consequences is clearly important in order to optimise current land-management practices which affect, for example, flood prevention, groundwater recharge and soil erosion.


5. Conclusions This study challenges the established view of a simple two-way soil moisture/hydrophobicity relationship such that, after thorough wetting, hydrophobicity is readily re-established when soil moisture levels fall below a particular threshold. The operation of a ‘critical soil moisture threshold’ is not questioned for the soils investigated here. Such a threshold, however, can only be regarded as a limiting upper one and biological rather than soil moisture driven processes alone are thought to lead to the recovery of soil hydrophobicity after wetting in the study area. This implies, from a soil-management point of view, that soil hydrophobicity can be expected to remain absent as long as the soil moisture remains above a critical level. However, it cannot be assumed that it will be restored when soil moisture levels fall below that threshold. Thus, the temporal behaviour of soil hydrophobicity appears to be even more complex than currently believed, making the modelling of its effects

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extremely difficult particularly in climates with complex rainfall regimes (e.g. where the occurrence of dry periods is rather unpredictable). Hydrophobicity is usually regarded as a detrimental soil characteristic enhancing the risk of environmental degradation and reducing agricultural productivity. Thus, the fact that it may not at all times be present below a critical soil moisture threshold or even for dry soils can be considered as being advantageous as it indicates that the presence of hydrophobicity may be less frequent or of shorter duration than currently considered. This effect, however, may be outweighed by the high resistance of hydrophobicity to breakdown in some areas with highly hydrophobic soils. Before the temporal variations of hydrophobicity can be modelled and their effects addressed in more detail, however, more research into the principles underlying the temporal aspects of hydrophobicity, complemented by prolonged field monitoring is clearly needed.


Chapter 15

Wetting patterns in water repellent Dutch soils L.W. Dekker* and C.J. Ritsema Alterra, Green World Research, P.O. Box 47, 6700 AA Wageningen, The Netherlands

Abstract The present study suggests that many soils in the Netherlands may be water repellent to some degree, challenging the common perception that soil water repellency is only an interesting aberration. When dry, water repellent soils resist or retard water infiltration into the soil matrix. Soil water repellency can lead to the development of unstable wetting and preferential flow paths. The persistence of water repellency was examined in topsoils using the water drop penetration time (WDPT) test. The severity of water repellency measured on dried soil samples, the so-called “potential” water repellency, can be used as a parameter for comparing soils with respect to their sensitivity to water repellency. Measurement of the “actual” water repellency on field-moist samples determines the soil fraction excluded from direct solute and water flow. However, preferential flow is a dynamic process, which is why the ratio between water repellent and wettable soil is time dependent. The “critical soil water content”, below which the soil in the field is water repellent and above which the soil is wettable, was found to be a useful parameter in water repellency studies. Spatial and temporal variability in volumetric soil water content was studied in vertical transects by intensive sampling with 100 cm3 steel cylinders. Spatial variability in soil water content under grass cover was high, due to fingered flow. On arable land, vegetation and microtopography appeared to play a dominant role. This study provides examples of uneven moisture patterns in water repellent sand, loam, clay and peat soils with grass cover, and in cropped, water repellent sandy soils.

1. Introduction Normally, dry soils are easily wetted by rainfall and irrigation. The force of attraction between soil particles and water causes the water to lose its cohesiveness, i.e. the tendency to retain its droplet shape, allowing it to flow along the surfaces of the particles. The water thus disappears as a liquid drop, wetting the soil (Fig. 1, left-hand panel). If the attractive forces are neutralized or absent, e.g. because of the presence of a water repellent coating, the water remains as a droplet and the soil is said * Corresponding author. Tel.: þ31-317-474267; fax: þ 31-317419000. E-mail address: [email protected] (L. Dekker). q 2003 Elsevier Science B.V. All rights reserved.

to repel water (i.e. resist wetting). Such soils are considered to be water repellent and to exhibit hydrophobic properties, especially when they are dry. Before water will evenly infiltrate into or percolate through a soil, there must be a continuous film of water on the soil particles. Hence, the soil must first be wetted before water will flow. Any condition resisting the wetting of the soil particles will inhibit water infiltration (Fig. 1, right-hand panel). Although an increasing number of researchers are aware of the occurrence and consequences of water repellency in a wide range of soils, it is still a neglected field in soil science. In the Netherlands for example, the occurrence, distribution and hydrological problems of water repellent peat soils have been known for many years (Hooghoudt, 1950; Hooghoudt

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L.W. Dekker and C.J. Ritsema

Fig. 1. Water poured upon the surface of dry, wettable soil (left-hand side) infiltrates immediately, whereas it ponds on extremely water repellent soil (right-hand side).

et al., 1960), but the phenomenon of water repellency in sandy soils and heavy clay soils has only been recognized in the last decade. Although the occurrence of water repellent sandy soils in the Netherlands is widespread, it was scarcely mentioned in the literature before 1985 (Dekker, 1998). The objectives of research reported in the present study were to investigate the occurrence, thickness, and distribution of water repellent layers in sand, loam, clay, and peat soils in the Netherlands, and to determine their effect on wetting patterns, water flow, and spatial variability in water content in the top layers of these soils.

2. Origins of soil water repellency It has been recognized for many years that the water repellency of a soil is a function of the type of organic matter incorporated in it, and that the organic matter induces water repellency in soils by several means. Firstly, irreversible drying processes in organic matter can induce water repellency, mainly in the surface layers of peat soils, which are difficult to rewet after drying (e.g. Hooghoudt, 1950). Secondly, organic substances leached from plant litter can induce water repellency in sandy and other coarse-grained soils (DeBano, 1981). Thirdly, hydrophobic microbial

by-products coating a mineral soil particle may induce wetting resistance (Bond and Harris, 1964; Chan, 1992). Fourthly, mineral particles need not be individually coated with hydrophobic materials; intermixing of mineral soil particles with particulate organic matter, like remnants of roots, leaves and stems, may also induce severe water repellency (DeBano, 1969d; Bisdom et al., 1993). All primary parts of plants (except roots) are covered by a cuticle that constitutes the interface between plants and their environment. The cuticle is composed of soluble, hydrophobic lipids embedded in a polyester matrix (Holloway, 1994). The micro-relief of plant surfaces, mainly caused by epicuticular wax crystalloids, often results in effective water repellency (Barthlott and Neinhuis, 1997). Plants with water repellent leaves can be found in any habitat and with all life forms, with a clear dominance among herbs (Neinhuis and Barthlott, 1997).

3. Assessment of soil water repellency 3.1. Water drop penetration time test The fundamental principles underlying the process of wetting show that a reduction in the surface tension of a solid substance which is to be wetted reduces its

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Wetting patterns in water repellent Dutch soils

Fig. 2. Water repellency measurement (WDPT test) by placing three drops of water upon the surface of a soil sample and determining the time to complete absorption.

wettability. Conversely, a reduction in the surface tension of the applied liquid increases the wettability. The study of repellency development or its amelioration requires appropriate measurement techniques to be used. Ideally, the technique should be simple and inexpensive and should provide a rapid quantitative measure of practical significance. Over the years many techniques have been developed to measure water repellency, during which time the understanding and definition of water repellency has evolved (Krammes and DeBano, 1965; Watson and Letey, 1970; DeBano, 1981; Wallis and Horne, 1992). One of the simplest and most common methods of classifying water repellency is the empirical water drop penetration time (WDPT) test, already described by Van’t Woudt (1959). Three drops of distilled water from a standard medicine dropper are placed on the smoothed surface of a soil sample, and the time that elapses before the drops are absorbed is determined (Fig. 2). Increasing the temperature of the water applied will reduce the surface tension, and in line with this, the time required for wetting will be reduced, the actual reduction in time increasing with increased water temperature (King, 1981). Therefore, the temperature at the time of testing must be constant, for instance between 18 and 238C (Richardson, 1984). The relative humidity of the air in the laboratory also affects the penetration time of the water drops. Increasing the

relative humidity increases the time that the drops remain on the surface (Bisdom et al., 1993). We measured the soil water repellency of all our samples under controlled conditions at a constant temperature of 208C and a relative air humidity of 50%. In general, a soil is considered to be water repellent if the WDPT exceeds 5 s (e.g. Bond and Harris, 1964; DeBano, 1981). This allows soils to be qualitatively referred to as being either wettable or water repellent. Classification into these categories implies an “eitheror” situation, with a sharp demarcation line between the two properties. However, soil water repellency is a relative property, varying in intensity. We applied an index allowing a quantitative definition of the persistence of soil water repellency as described by Dekker and Jungerius (1990). They distinguished five water repellency classes, as shown in Table 1. Table 1 Classification of the persistence of soil water repellency Class

WDPT (s)

Nomenclature

0 1 2 3 4

,5 5–60 60–600 600–3600 .3600

Wettable; non-water repellent Slightly water repellent Strongly water repellent Severely water repellent Extremely water repellent

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3.2. Actual water repellency and critical soil water content Using the WDPT test on dried samples gives the persistence of the potential water repellency (Dekker and Ritsema, 1994b). In the field, some of these samples were dry, but others were moist or wet. Therefore, we also checked the persistence of the actual water repellency of field-moist samples from several layers of different soils. These measurements were done immediately after weighing their wet weight. The samples were divided into two classes: wettable or non-water repellent (, 5 s) and water repellent (. 5 s). Because we also measured the soil water content of these samples we could assess “critical soil water contents” for the distinct layers. The soil samples are water repellent below and wettable above these values.

4. Consequences of soil water repellency 4.1. Infiltration and erosion In general, water movement is initially severely limited in dry water repellent soils. Infiltration rates into water repellent soils can be considerably lower than those into wettable soils (DeBano, 1981). During rain events after prolonged dry periods, water repellency of the topsoil may cause surface runoff, especially in sloping areas (McGhie, 1980b). Water repellency is greater in dry than in wet soils, which may be the reason why runoff during the first rainstorm after a dry spell is larger than during later, comparable storms (Imeson et al., 1992). Jungerius and Dekker (1990) studied the influence of water repellency on erosion and sedimentation of sand for Dutch coastal dune sands. Dekker (1998) described the effects of a water repellent surface layer in sandy soils in the Veluwe area, in the eastern part of the country. Such layers were found to influence surface runoff towards fens, as well as erosion of sandy roads, and the sedimentation of sand and organic matter in lower places, including the fens (Fig. 3A and B). Thus, water repellency tends to increase runoff and erosion and decrease the volume of water absorbed by the soil. With increasing rainfall, water infiltration

proceeds and finally starts to break through the water repellent layer by creating irregular wetting patterns (Fig. 3C) and/or vertical flow paths into the subsoil. 4.2. Preferential flow and accelerated transport Knowledge of the movement of water and solutes through the unsaturated zone of field soils is essential for reliable predictions of pollution risks to groundwater and/or nutrient losses from agricultural soils. As intensive sampling is costly and time-consuming, characterization of the soil moisture state is often based on a limited number of samples or measurements, which may lead to an incomplete picture of the real flow mechanisms in field soils. So far, most models simulating water and solute transport through the unsaturated zone have assumed homogeneous infiltration and a subsequent downward movement of the wetting front parallel to the soil surface. This type of stable flow, however, is uncommon in field soils. Deviations are caused by a variety of mechanisms, which in general are related to specific soil properties or soil characteristics. Firstly, preferential flow (or bypass flow) of water and solutes may occur in wellstructured clay and/or peat soils owing to the presence of shrinkage cracks and/or channels left behind by decayed roots or soil fauna (biopores), providing pathways through which water and solutes migrate rapidly, essentially bypassing the buffering capacity of much of the unsaturated zone. Secondly, preferential flow may also occur in non-structured sandy soils, owing to the development of unstable wetting fronts (Raats, 1973). Perturbations in an initially flat wetting front may grow into “fingers” or “preferential flow paths”, instead of flattening out by lateral diffusion. This occurs if: (1) the hydraulic conductivity increases with depth, as is encountered in soils with a fine-textured layer covering a coarse-textured layer (Hill and Parlange, 1972); (2) air entrapment takes place during an infiltration event (Raats, 1973; Hillel, 1987); (3) the soil is water repellent (Raats, 1973; Dekker and Jungerius, 1990; Ritsema et al., 1998a). Wetting patterns in water repellent soils can be quite irregular and incomplete after rain (DeBano, 1969d; Ritsema et al., 1997a, 1998a,b; Ritsema and Dekker, 1998). Solute leaching related to preferential flow in water repellent soils has been studied by


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Fig. 3. Erosion of sandy roads (A), sedimentary sandy and humic materials in lower places (B), and irregular wetting (C) in the water repellent sands of the Veluwe area, the Netherlands.

applying dye solutions or other coloring agents to the soil surface (e.g. Van Ommen et al., 1988; Dekker and Jungerius, 1990; Hendrickx et al., 1993). Visual observations of wetting patterns in trenches dug in a

water repellent dune sand with grass cover by Dekker and Jungerius (1990) revealed that water moved downward through narrow channels, leaving the adjacent soil volumes dry and causing considerable

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Fig. 4. Preferential flow paths in a water repellent dune sand visualized by using a dyestuff staining.

variation in soil water content. They studied the penetration of rain into some dune sands by using a dyestuff staining technique. Trenches were dug after rains during autumn and winter and the pit faces were dusted with a dry mixture of 1% Rhodamine B in finely ground kaolinite until the faces were uniformly covered with a white powder. Within a few minutes, the wet areas developed an intense red color, while dry areas remained white. The patterns were photographed to provide a permanent record for comparison (Fig. 4). Once preferential flow paths have formed, the soil no longer impedes infiltration of water, so that additional precipitation tends to infiltrate through the existing preferential paths which have been wetted before. Thus, dry zones tend to persist due to their water repellent character and their low hydraulic conductivity. Field evidence of preferential flow of bromide through a water repellent dune sand soil, resulting in early arrival times and high bromide concentrations in the groundwater, were presented by Hendrickx et al. (1993) and Ritsema and Dekker (1998). From a management point of view, it is essential to know where and when preferential flow may be expected in field situations and to what extent it may accelerate water and solute transport, in order to develop consistent strategies to minimize environmental risk to groundwater and surface waters.

5. Moisture variability in water repellent soils Considerable soil water content variations and irregular wetting patterns in water repellent horizons have been reported by numerous authors (e.g. Krammes and DeBano, 1965; DeBano, 1981; Dekker and Jungerius, 1990; Hendrickx et al., 1993; Ritsema and Dekker, 1994b, 1995). In the next part of the paper we give specific examples of irregular wetting patterns and soil water content variations in water repellent sand, loam, clay and peat soils in the Netherlands. 5.1. Study sites and soil sampling The average annual precipitation in the Netherlands is 765 mm, and is evenly distributed during the year. Potential evaporation averages 690 mm/yr. During the growing season, there is a small precipitation deficit, in autumn and in winter a precipitation surplus. A water repellent dune sand soil (mesic Typic Psammaquent) with grass cover was studied near Ouddorp (Fig. 5) in the western part of the Netherlands (Dekker and Ritsema, 1994b). The variability of soil water content over short distances in a water repellent sandy black plaggen soil (sandy, silicious, mesic, Plaggept) with a maize crop was assessed near Zwolle (Fig. 5) in the eastern part of the country (Dekker and Ritsema, 1997). The occurrence of


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The wet soil in the plastic bags was weighed, dried for several days at 658C, and weighed again to determine soil water content. 5.2. Moisture variability in the dune sand

Fig. 5. Schematic map of the Netherlands showing the study sites.

fingerlike wetting patterns was studied in a water repellent silt loam soil (mesic Typic Fluvaquent) near Yerseke Moer (Fig. 5) in the southwestern part of the country (Dekker and Ritsema, 1995). Also grasscovered heavy basin clay soils in the Netherlands appeared to be water repellent (Dekker and Ritsema, 1996d). Moisture patterns and the variability in water content were assessed in a heavy basin clay soil (mesic, Typic Fluvaquent) with grass cover near Zaltbommel (Fig. 5). Examples will also be given of the variation in water content in water repellent topsoils of grass-covered peaty clay and clayey peat soils (mesic Typic Medihemist), assessed by Dekker and Ritsema (1996c). All the soils were sampled at several depths using steel cylinders (100 cm3) with a height and diameter of about 5 cm. At each depth, samples were taken in close order over a distance of 1.5 – 5.5 m. The cylinders were pressed vertically into the soil, and layer after layer was sampled directly after removing the soil, to minimize evaporation of the soil sampled. The steel cylinders were emptied into plastic bags and used again, and the plastic bags were tightly closed.

The potential water repellency measured on dried samples from five depths of the Ouddorp dune sand are summarized in Fig. 6. Nearly all the samples from 5 – 10 cm and 15 –20 cm, and 87% of the samples from 25– 30 cm depth, show extreme water repellency, with water drops remaining for more than 1 h on their surfaces. With increasing depth the dry soil becomes less water repellent and more variable. Even at the 45– 50 cm depth, 65% of the dried samples repel water. Water movement can be severely restricted by dry water repellent sandy topsoils. Rain that falls on the surface of water repellent sand does not penetrate evenly but moves downwards through narrow channels, leaving the intermediate soil quite dry and causing considerable variation in the moisture content of the sand. Fig. 7 shows the soil moisture contents of samples taken in the dune sand at four depths on 30 August 1988. Variability is extremely high, especially in the upper layers. The peaks represent the wet sand of preferential flow paths and the valleys the dry, principally, still water repellent sand. The horizontal lines in the five diagrams indicate the critical soil

Fig. 6. Relative frequency of the WDPT test on dried samples from five depths in the dune sand of the Ouddorp site. For each depth the persistence of potential water repellency was measured on 1000 samples.

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Fig. 7. Volumetric water contents of samples in 550 cm transections in the water repellent dune sand. The samples with contents above the horizontal lines (representing the critical soil water contents) are actually wettable, and those below the lines are actually water repellent.

water contents, which are different for each layer. Samples with water contents beneath these lines are actually water repellent, and samples with higher contents are actually wettable. The critical soil water contents for the 5– 10, 15 – 20, 25 –30, and 35 –40 cm layers are 4.75, 3.0, 2.5, and 2.0 vol%, respectively. In places where preferential flow paths have been formed, the soil becomes actually wettable, allowing the water to flow down through these paths. Between the preferential flow paths, dry, actually water repellent soil pockets or zones will persist. 5.3. Moisture variability in the black plaggen soil Man-made raised sandy soils in the Netherlands are classified as “brown” or “black” plaggen soils. When dry, the brown soils are wettable, but the black soils are water repellent (Dekker, 1998). On 26 September 1989, when maize crop had just been harvested, distinct moisture patterns were found in the sandy black plaggen soil at the Heino experimental

farm near Zwolle (Fig. 8). Due to interception and stemflow, water was funneled towards the roots, and thus concentrated in the maize rows. But distinctive wetting patterns were also formed between the maize rows, caused by rain water dripping to the ground from overhanging leaves. Microtopographical depressions further concentrated the water that dripped down. The difference in elevation between the top of the row and the bottom of the interrow was about 7 cm. Fig. 8 shows clearly wetter soil areas near the roots in the maize row and halfway between the maize rows. The transport of water took place mainly through these wetter portions. Side and downward movement in the wet soil portion was restricted because of the actual water repellency of the dry sand. As a consequence, the difference in soil water content was considerable. Fig. 9 shows the great variability of the soil water content for samples taken at depths of 10– 15 and 25– 30 cm on 5 October 1989, after the maize had been harvested. High soil water contents were found


Fig. 8. Soil moisture patterns (dark areas) in the water repellent sandy black plaggen soil near Zwolle, the Netherlands.

Fig. 9. Variation in soil water content at two depths over a distance of 18 m in the black plaggen soil near Zwolle.

159

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halfway between the maize rows (interrows) and under the maize rows. Low soil water contents were present in the dry, actually water repellent soil pockets or zones between the rows and interrows. 5.4. Moisture variability in the silt loam soil Fingerlike wetting patterns were visible in the silt loam topsoil of the Yerseke Moer site, where dry soil areas can always be found, even after large amounts of rain during the winter period (Fig. 10). The persistence or stability of potential water repellency, expressed by the WDPT, influences the wetting of the soil. Nearly all samples taken up to 15cm depth are strongly to extremely water repellent, and the persistence of water repellency decreases with increasing depth. However, even at the 30- to 35-cm depth, 30% of the samples show severe to extreme water repellency. In Fig. 11 locations of dry soil areas with low soil water content partly correspond with extremely water repellent soil parts and, on the other hand, wet fingerlike patterns are found in areas with low persistence. The variation in soil water content was found to be large in all layers of the trenches sampled (Dekker and Ritsema, 1995). Fig. 12 shows the volumetric soil water content of samples at three depths of the 15

January 1993 trench. The soil water content in these layers varied between 11.0 and 46.1 vol%. Because both actual water repellency and soil water contents were measured, critical soil water contents for different depths of the silt loam could be assessed. The critical soil water content at the 10- to 15-cm depth is about 25 vol% and at 20- to 25- and 30- to 35cm depths 20 vol%. These values are indicated in Fig. 12. All samples above the lines were actually wettable, and below, actually water repellent. 5.5. Moisture variability in the heavy clay soil Water repellency in the top layers of the heavy clay soil (more than 60% clay) occurred mainly owing to a coating on the aggregates (Dekker and Ritsema, 1996d). When the clay soil is dry, a major proportion of the water from precipitation or sprinkler irrigation may flow rapidly through shrinkage cracks to the subsoil, bypassing the matrix of the clay peds. However, preferential flow is not limited to macropore flow: irregular wetting patterns are also formed through the small pores of the matrix. Fig. 13 shows the spatial distribution of the volumetric soil water content in the 10 trenches sampled at the De Vlierd experimental farm near Zaltbommel. The soil profile on 31 August 1993, was relatively dry, and moisture patterns appeared to

Fig. 10. Dry soil areas still exist in the silt loam soil after a long, wet winter period. The depth of the excavation is 20 cm.


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Fig. 11. Contour plots showing the (A) potential water repellency and (B) volumetric soil water content in the silt loam soil.

be vertically oriented. After 43 mm of precipitation had fallen, the soil moisture content of the surface layer and of the layer between 25 and 35 cm of the 17 September 1993, trench increased from 25 –35 vol% to 35 –55 vol%. However, at depths between 10 and 25 cm a large part of the clay soil remained relatively dry, with soil water contents between 30 and 35 vol%

and occasionally less than 30 vol%. After an additional 52 mm of precipitation, vertically directed wetting patterns were formed, as shown in the diagram of the 11 October 1993, trench (Fig. 13). The diameter of these wetting patterns exceeded the width of individual cracks, indicating that the surrounding matrix was participating in the vertically directed

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Fig. 12. Volumetric water contents of samples in a 275-cm transection in the silt loam soil. The horizontal lines in the diagrams indicate the critical soil water content value, below which the soil is water repellent.

flow as well. From 11 October 1993, to the following sampling date, 1 February 1994, a total amount of 389 mm of precipitation was recorded. The clay soil clearly became wetter, with moisture contents of 50 vol% to more than 55 vol% in the surface layer, and moisture contents of 40 –55 vol% at depths between 10 and 35 cm. After 1 February 1994, the soil dried out to a large extent, as can be seen in the May 16 and August 16 trenches. With increasing rainfall, irregular wetting of the soil started again, often with the occurrence of similar wetting patterns as found in the 11 October 1993, trench.

5.6. Moisture variability in peat soils and resistance to wetting In the Netherlands many grass-covered clayey peat and peaty clay soils are susceptible to drought and difficult to wet after a prolonged dry period (Dekker and Ritsema, 1996c; Dekker, 1998). When dry, these soils are water repellent. Precipitation can flow rapidly through shrinkage cracks towards the subsoil, bypassing the matrix of the peat. However, trenches studied at several locations in the peat soils revealed that preferential flow was not limited to macropore flow:


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Fig. 13. Contour plots showing the spatial distribution of volumetric water content for all the trenches in the heavy clay soil near Zaltbommel.

irregular (Fig. 14) and fingerlike wetting patterns (Fig. 15) were also formed in the soil matrix. As a consequence of the irregular wetting patterns, variability in soil moisture content is high, as is illustrated for two layers at the Wilnis site in the diagrams of Fig. 16. We found large differences in soil moisture content in the peaty clay and clayey peat soils at all

sites and for all measurements (Dekker and Ritsema, 1996c). Water repellency is a time-dependent physical property of the soil, because the resistance to wetting of a water repellent soil will decrease over time. Topsoils of the six sites studied were actually water repellent during dry spells. For certain layers in

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Fig. 14. View of a vertical cross-section of the clayey peat topsoil at the Wilnis site after a rain event (dark areas are wet, light areas are dry).

Fig. 15. Fingerlike wetting patterns in the peaty clay soil at the Bodegraven site.

the topsoils of the Broek in Waterland and Joure sites, the critical soil water content, below which the soil is water repellent and above which it is wettable, was found to be around a water content of 34 –38 vol% (Dekker and Ritsema, 1996c).

Large differences in wettability exist between wettable and water repellent peat soils. Wetting is fast in wettable peat, whereas in actually water repellent peat wetting may be a very slow process. Resistance to wetting was determined by measuring

Fig. 16. Volumetric soil water content at depths of 3–8 cm and at depths of 10–15 cm in the clayey peat topsoil of the Wilnis site.


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Fig. 17. Increase in water content versus time of field-moist peat samples from the Joure and Broek in Waterland sites, placed at a constant pressure head of 22.5 cm water below the bottom of the sample.

the wetting rate of field-moist samples with varying water contents. For this purpose samples were taken at different depths at the Broek in Waterland and Joure sites in October 1995. These samples, in steel cylinders (100 cm3) with a height and diameter of 5 cm, were subjected to a constant pressure head of 2 2.5 cm water below the bottom of the sample. The experimental setup was designed in such a way that water content increments of 0.2 vol% were recorded automatically over a period of 1 week (Dekker and Ritsema, 1996c). Fig. 17 shows that the increase in water content versus time in field-moist samples originating from the two sites differed greatly. The water uptake during 1 week by the actually water repellent peat samples, with initial contents between 25.1 and 31.1 vol%, led to an increase in the soil water content of only 1.4– 3.6 vol%. By contrast, the increase in the water content of the wettable sample from Joure, which had an initial water content of 41.1 vol%, was 23.2 vol% over the same period of time. The wetting of the other

wettable samples was also very fast, but the water uptake decreased after 1 day, as the sample reached its equilibrium.

6. Discussion and conclusions Water repellency is an important, often neglected property of many soils, which has its greatest effect in relatively dry soils. It has serious consequences for the wetting of the soil. Infiltration rates into water repellent soils can be considerably lower than those into wettable soils. During rain events after prolonged dry periods, water repellency of the topsoil may cause surface runoff, especially in sloping areas. Thus, water repellency tends to increase runoff and erosion and decrease the volume of water absorbed by the soil. With increasing rainfall, water infiltration proceeds and finally starts to break through the water repellent layer by creating irregular wetting patterns. Water and

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solutes often flow in these soils through preferential flow paths, the so-called “tongues” or “fingers”. This phenomenon shortens solute travel time and increases the risk of groundwater contamination. The persistence or stability of water repellency can be measured using the water drop penetration time (WDPT) test. Measurements of water repellency on dried soil samples may give some information about the occurrence, depth, distribution, variability, degree and persistence of water repellency, allowing comparisons between soils. In some cases the actual water repellency was measured using the WDPT test on field-moist samples. The percentage of the actually water repellent samples indicates the soil fraction excluded from direct solute and water flow. Because we also measured the water content of the samples, we could assess critical soil water contents for the different depths of the intensively sampled trenches. The soil is wettable above, and water repellent below these values. Stemflow and microtopography were found to play an important role in the development of irregular wetting patterns in a maize-cropped, water repellent sandy black plaggen soil during rain events after a dry period. Wetter areas were established within the rows of the maize field, due to stemflow, as well as halfway between the rows, due to leaf drip and microtopographical depressions. The spatial variability of the water content of the soil was often found to be high.

Grass-covered heavy basin clay soils in the Netherlands are water repellent when they are dry. Water repellency in the top layers of these soils was mainly due to a coating on the aggregates. The variation in moisture content over short distances was found to be high during the period 31 August 1993 to 22 December 1994. When the clay soil is dry, a major proportion of the water from precipitation or sprinkler irrigation may flow rapidly through shrinkage cracks to the subsoil, bypassing the matrix of the clay peds. However, preferential flow is not limited to macropore flow: irregular wetting patterns are also formed through the small pores of the matrix. These preferred pathways are thought to form at places with cracks which receive relatively large amounts of water, due to rainwater moving over the surface and through the surface layer towards slightly lower places, the so-called “distribution flow”. Hence, the surrounding small pores in the matrix can be wetted as well, resulting in irregular wetting patterns. Preferential flow in peat soils may occur through cracks or biopores, bypassing the matrix of the peat, but, irregular, fingerlike preferential flow paths are also formed in the matrix. Owing to these typical wetting patterns, the water content of the soil varied over short distances at all sites and on all sampling dates.

Chapter 16

The impact of water-repellency on overland flow and runoff in Portugal A.J.D. Ferreiraa,*, C.O.A. Coelhoa, R.P.D. Walshb, R.A. Shakesbyb, A. Ceballosa and S.H. Doerrb a

Centro das Zonas Costeiras e do Mar, Departamento de Ambiente e Ordenamento, Universidade de Aveiro, 3810-193 Aveiro, Portugal b Department of Geography, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, Wales, UK

Abstract Soil water-repellency (hydrophobicity) is a widespread property of Eucalyptus globulus and Pinus pinaster forest soils in central and north littoral Portugal and is particularly severe during the summer dry conditions. This paper attempts to assess the impact of water repellency on overland flow and runoff generation at plot and catchment scales for two types of terrain with differing land management and degree of soil hydrophobicity: (i) highly hydrophobic land with regenerating eucalyptus forest following fire; and (ii) largely hydrophilic land on which deep-ploughing prior to planting eucalyptus seedlings had eliminated hydrophobicity. Overland flow responses were monitored over a 40-month period for two 8 m £ 2 m plots and streamflow was recorded continuously at gauging stations for two small catchments of predominantly regrowth eucalyptus and ploughed/planted eucalyptus, respectively. Soil hydrophobicity was assessed using the Water Drop Penetration Time (WDPT) test. Seasonal variations in the factors influencing plot overland flow response were assessed for each land management type using multivariate analysis. For the regrowth eucalyptus plot, overland flow generation was found to be negatively correlated with antecedent soil moisture in summer (suggesting that hydrophobicity-linked Hortonian overland flow is then dominant), but positively related to throughflow in winter (suggesting that saturation overland flow generation in a hydrophilic-phase soil was at that time the dominant mechanism). In the ploughed/planted areas, negative correlations with soil moisture were found neither in summer nor winter. Rainfall amount (and in winter also antecedent precipitation) were found to be the variables most strongly and positively related to overland flow volume. The plot results are compared with streamflow responses for the small catchments.

1. Introduction Consequences of water repellency (hydrophobicity) include reduced soil infiltration capacity and associated increases in overland flow and soil erosion (e.g. Burch et al., 1989b; Imeson et al., 1992; Scott * Corresponding author. Tel.: þ351-34-370200; fax: þ 351-34429290. E-mail address: [email protected] (A.J.D. Ferreira). q 2003 Elsevier Science B.V. All rights reserved.

and Schulze, 1992b; Scott, 1993; Shakesby et al., 1993), the magnitudes of which depend in part upon the severity and spatial variability of hydrophobicity (Jungerius and de Jong, 1989; Ritsema and Dekker, 1994b). Hydrophobicity can also strongly influence soil water movement within soils (e.g. Scott and Van Wyk, 1990; Lavabre et al., 1993; Ritsema et al., 1993). Sevink et al. (1989) noted that hydrophobicity can lead to a preferential distribution of soil moisture, with a high water content in the ectorganic layer,

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A.J.D. Ferreira et al.

beneath which is a dry water-repellent layer, which in turn is underlain by a moist, less hydrophobic layer. Hendrickx et al. (1993) reported unstable wetting fronts in water-repellent soils, contrasting with the homogeneous wetting front typical of hydrophilic soils. Water and solutes have been found to reach the water table more rapidly in water-repellent than hydrophilic soils, as a result of the enhancement of preferential flow pathways in the hydrophobic soil (Ritsema et al., 1993). Although much has been learned about how hydrophobicity affects soil moisture, its influence on the overland flow and streamflow generation has proven to be difficult to assess (both in terms of spatial and temporal significance) and distinguish (Shakesby et al., 2000). This paper attempts to explore the impact of hydrophobicity on runoff generation at the small plot and small catchment scales in forested terrain in northern Portugal that was burnt in 1986, but subsequently subjected to two contrasting land management practices: (1) regrowth of eucalyptus from burnt and sawn rootstock (characterised particularly in summer by severe soil hydrophobicity); and (2) terrain that has been deep-ploughed (thereby rendering the soil hydrophilic) and planted with eucalyptus seedlings. Overland flow data from small runoff/erosion plots and storm hydrograph data from two small catchments are used to explore differences in overland flow generation for winter and summer between these ‘hydrophobic’ and ‘hydrophilic’ terrain types.

2. Study area The study area is situated 10 – 15 km east of ´ gueda (408 350 N, 88 260 W) and forms a part of the A ´ gueda River catchment in the largely forested A foothills of the Caramulo Mountains in north-central Portugal (Fig. 1). The underlying geology is PreCambrian schist. Altitude ranges from less than 20 m ´ gueda and above mean sea level at the confluence of A Alfusqueiro rivers to more than 500 m on ridges and hill summits in the east. The area is deeply dissected and slope profiles are generally convexo-rectilinear with few or no basal concavities. Valley-side slope angles are typically around 208, but steeper in places. The climate may be described as Atlantic – Mediter-

ranean. Annual rainfall varies with altitude from 1300 to 1900 mm with a summer drought from July to September. On steeper slopes, soils are shallow (, 0.5 m), stony, sandy loam Umbric Leptosols (Pereira and FitzPatrick, 1995), comprising mainly sand (55 –80%) and silt (15 – 40%), and little clay (, 5%) (Doerr et al., 1996; Shakesby et al., 1996). These soils are weakly structured, usually with a dark brown surface horizon directly overlying the bedrock. The forests comprise plantations of Eucalyptus globulus Labill and Pinus pinaster Aiton. Forest management practices, stand age and character vary substantially over small areas, reflecting an intricate land ownership pattern. The failure to clear the highly flammable forest litter and undergrowth, an improved accessibility to the forests and a recent sharp decrease in annual rainfall in Portugal (Ferreira et al., 1998) have contributed to a marked increase in the forest fire frequency over the past two decades, which, together with the higher harvesting frequency of eucalyptus, has led to its increased dominance in recent years. Eucalyptus is generally planted following deepploughing (or ‘rip-ploughing’), which on steeper slopes is carried out in a downslope direction. Trees are harvested frequently (as little as every ten years) and can be allowed to regrow from the rootstock after felling or fire for 3 –4 cycles before rip-ploughing and replanting takes place. These two terrain types—‘rip-ploughed eucalyptus’ and ‘regrowth eucalyptus’— form the focus of this paper. They are characterised by important differences in soil properties. Long undisturbed eucalyptus forest soils, including those associated with regrowth eucalyptus, are generally highly hydrophobic throughout their depth (WDPT . 1 h) both on Eucalyptus globulus and Pinus pinaster land, although to a slightly lesser degree on the latter (Shakesby et al., 1993; Doerr et al., 1996, 1998). In contrast, deep rip-ploughing renders the soil almost uniformly hydrophilic until the eventual re-establishment of hydrophobicity around the root zone and beneath the litter layer of the planted eucalyptus trees (Doerr et al., 1998). Soil hydrophobicity in both pine and eucalyptus soils is subject to seasonal variation, being most severe in the summer drought periods and patchy or absent after prolonged wet conditions (Walsh et al., 1994; Doerr and Thomas, 2000).

The impact of water-repellency on overland flow and runoff in Portugal

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´ gueda Basin, Portugal. Fig. 1. Location of study sites and instrumentation in the A

3. Research methodology

3.2. The overland flow plots

3.1. Research design

The two small ð8 m long £ 2 m wideÞ overland flow plots were located in mid-slope positions 100 m apart on a southwest-facing 208 slope at 445– 450 m altitude in the Serra de Cima catchment. The rip-ploughed area around the first plot had a tree density of 1760 ha21, growing from seedlings at the beginning of the monitoring period to 2 m in height by 1992. Ground cover increased from near zero at the start to about 25% by 1992. For the regrowth eucalyptus stand in which the second overland flow plot was sited, multiple stumps were commonly regenerating from the same felled trunk. The density of trunks was 1664 ha21, that of stems 3824 ha21. These trees were more than 10 m high, and the canopy gave a 68.1% cover by the end of the measurement period (Ferreira, 1996). Overland flow at the two plots

To investigate the hydrological responses of the transiently hydrophobic regrowth eucalyptus and the hydrophilic rip-ploughed eucalyptus forested areas to rainstorms, runoff responses were monitored over a 40month period from October 1989 to January 1993 at two spatial scales by means of overland flow plots and small catchments located in both types of forest in an area burnt in 1986 (Fig. 1). Because of the known tendency for hydrophobicity to become pronounced after dry periods and disappear in wet weather, the approach ensured that the data record covered both winter wet and summer dry seasons. In addition, ancillary data on rainfall, throughflow, soil moisture and hydrophobicity were collected for use at the analytical stage.

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was recorded either weekly or following individual storm events throughout the monitoring period using tipping-bucket recorders, backed up by storage tanks. 3.3. Ancillary data collection Rainfall amounts and intensities were recorded with two siphoning autographic rainfall gauges installed at Falgueirinho and Castanheira in the study area (Fig. 1) and storage gauges close to the plots. Two throughflow pits were installed, one in each type of land cover, to give an indication of sub-surface lateral water fluxes. The pits were 30 cm wide, with 30 cm £ 20 cm metal blades inserted at the base of soil horizons A and C into their upslope faces. The throughflow intercepted by each blade was directed via short lengths of plastic guttering into individual storage containers, using a modified version of earlier designs for throughflow measurement (e.g. Knapp, 1973). Infiltration capacity was assessed at 10 – 14 sites in each land management type using a double-ring (10 and 20 cm in diameter), constant-head infiltrometer. The infiltration measurements were usually performed for 60 min but for several hours where infiltration capacity was very low. The final 5 min of the experiments, when the soil was assumed to be under saturated conditions, were used to derive the infiltration capacity (Ferreira, 1989). Soil moisture content was determined using two weighing lysimeters, consisting of a soil column placed within a PVC container 19.1 cm in diameter and 50 cm in length. Measurements were taken at each visit to the plot, normally every two days during dry periods, and two to three times a day during rainfall events. The soil was sampled where infiltration capacity measurements had been carried out (10 – 14 samples per plot). The dry bulk density and the organic matter content of the soil were determined. Ground cover, percent bare area, stone cover and vegetation cover were determined within each bounded plot systematically using a 1 m2 grid on eight occasions between 1989 and 1993. Sixty-four measurements were taken at each plot on each occasion. To measure water repellency, the Water Drop Penetration Time (WDPT) test (Letey, 1969) was used. Five sets of 20 drops were used for each plot, in

two contrasting conditions: (a) after 45 days without rain; and (b) following 200 mm of rain within a period of one week. Soils where the water drops penetrated in less than 1 min were classified as hydrophilic; soils where drop penetration occurred after 30 min were considered to be strongly hydrophobic. 3.4. Catchment flows Hydrological responses at the small catchment scale were assessed by recording and analysing streamflow at two adjacent catchments (Fig. 1) differing in terms of dominant land use. The Sernadinha catchment (0.33 km2; altitudinal range 295– 485 m) was predominantly regrowth eucalyptus (72.4% of the area), with the remainder comprising rip-ploughed eucalyptus (14.6%), regrowth pine (9.4%) and abandoned agricultural land (3.6%). In contrast, 46% of the Serra de Cima catchment (0.51 km2; altitudinal range 280 – 475 m) was ripploughed in 1987 – 1989 and planted with eucalyptus seedlings, with the remainder either regrowth pine (32.7%) or regrowth eucalyptus (21.3%). Both catchments were instrumented with continuous stage recorders since November 1988. Stage-discharge rating curves (Walsh et al., 1994), determined using volumetric and dilution gauge techniques were used to convert stage into discharge data. The autographic rainfall gauge at Falgueirinho is located at the headwater divide between the catchments. Finally, plot runoff responses were compared with catchment responses for two contrasting storm event periods: (a) after a prolonged dry spell, when any hydrophobicity would be best developed; and (b) following wet winter weather, where potentially hydrophobic soils can be expected to exhibit little or no hydrophobicity. 3.5. Analytical procedures To study the factors influencing overland flow generation at the plot scale for the two land uses during the summer and winter periods, a multivariate analysis was used. The models were built using a stepwise interactive procedure, in which the dependent variable was the overland flow amount (mm) since the previous measurement. The independent

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variables were selected at each step depending on their contribution to the explained variance and the corresponding significance level. The panel of independent variables comprised those related to rainfall amount and intensity and those related to antecedent soil conditions. The former comprised: (i) P, rainfall amount since previous measurement (mm); (ii) I, average intensity (mm/h); (iii) IMAX, maximum 5-min intensity (mm h21); and (iv) KE, total kinetic energy of rainfall (in J m22), which is a function of both amount and intensity. The antecedent soil condition variables were: (v) P48 and (vi) P168 rainfall, respectively, during the 48 and 168 h periods prior to the start of the measurement period (mm); (vii) SM, soil moisture content at the start of the period (% by volume); and (viii) TF, the throughflow flux recorded during the period (ml). It was reasoned that a strong throughflow response would indicate high antecedent soil moisture conditions and thus easier soil saturation and generation of saturation overland flow. The multivariate approach is clearly only exploratory, as the non-linear nature of runoff process responses conflicts with the linear assumptions of the model.

4. Results and analysis 4.1. Key soil properties: bulk density, organic matter, infiltration capacity and hydrophobicity The two land uses studied exhibit marked differences in soil properties relevant to hydrological processes (Table 1). Dry bulk density of the soil is significantly higher in the rip-ploughed soils than in the regrowth eucalyptus stands. This is partly due to

the higher root and other organic matter content near the soil surface in the regrowth eucalyptus. In the rip-ploughed areas, during ploughing the pre-existing litter layer was either mixed into the soil profile or swept downslope by the heavy machinery. Thus organic matter content of the regrowth eucalyptus stands is almost twice that of rip-ploughed stands. Ground cover in the regrowth eucalyptus was close to pre-fire levels both regarding litter cover and understorey vegetation. The mean infiltration capacity is significantly lower in the rip-ploughed soils (7.8 mm h21 ) than in the regrowth soils (22.3 mm h21). In both areas, capacities varied between the ridge and furrow but in contrasting ways. In the rip-ploughed stand, transport of fine sediment had occurred from the ridges to the furrows, causing lower infiltration capacities in the latter. In the regrowth eucalyptus, however, litter had accumulated preferentially in the furrows, where the highest infiltration capacities were recorded. The two soils differ in water repellency (Table 2). Owing to a lack of a litter layer, the total destruction of the previous soil structure and the incorporation of hydrophilic weathered bedrock material, the ripploughed soils were predominantly hydrophilic (WDPT less than 60 s), both under dry and wet conditions. For the regrowth eucalyptus stand, the uppermost mineral soil beneath the litter was highly hydrophobic in dry through to moderately wet (18.6% by volume) conditions, but Doerr and Thomas (2000) found similar soils to be hydrophilic or less hydrophobic after prolonged wet conditions. The mineral soil outcrops (a few sites on the plough ridges not covered by organic matter) are dominantly hydrophilic both under wet and dry conditions.

Table 1 Surface (topmost 10 cm) soil properties of the two land management types. Sample sizes were 10–14 on each land use-type in each case (SD ¼ standard deviation) Land-use type

Bulk density (g cm23) Mean

Regrowth eucalyptus Rip-ploughed eucalyptus Statistical significance of difference

0.83 1.10 , 0.05

SD 0.28 0.13 n/a

Ground cover (%)

Organic matter (%)

Mean

Mean

SD

Mean

13.2 7.1 , 0.05

9.6 0.9 n/a

22.3 7.8 , 0.05

79.8 7.7 , 0.01

SD 21.2 14.3 n/a

Infiltration capacity (mm h21) Range 5.0–63.6 1.7–24.5 n/a

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Table 2 Water drop penetration times of the mineral surface soil on regrowth eucalyptus and rip-ploughed eucalyptus land under wet and dry soil moisture conditions. Sample sizes were 100 applied water droplets in each land-use type on each occasion Land-use type

Antecedent conditions

Soil moisture (vol%)

Percentage of droplets in different water drop penetration time categories ,60 s

60–1800 s

.1800 s


Wet Wet

18.6 30.1

0 100 ,0.0001

10 0 ,0.005

90 0 ,0.0001


Dry Dry

7.9 12.1

0 70 ,0.0001

0 5 ,0.05

100 25 ,0.0001

4.2. Plot overland flow Overland flow during 1989 – 1993 averaged 7.8% of rainfall for the regrowth eucalyptus compared to 46.4% for the rip-ploughed plot (Table 3). Overland flow is not uniformly seasonally distributed. The rip-ploughed plot has a maximum (54.3%) during spring and a minimum (45.4%) during winter. At the regrowth eucalyptus plot, maxima were observed during winter (9.9%) and summer (7.5%), with lower values occurring during spring and autumn. 4.2.1. Rip-ploughed terrain In the case of the winter period, two variables provide the best explanation of overland flow variability in the rip-ploughed areas: the kinetic energy of rainfall and 48 h antecedent rainfall amount (Table 4). As both relationships are positive, these overland flow responses apparently increase with rainfall amount and intensity, and with wetter antecedent conditions. Table 3 Mean overland flow (as a percentage of rainfall) by season for the runoff plots for the period from October 1989 to January 1993 Land-use type

Overland flow (as % of rainfall) Summer

Regrowth eucalyptus Rip-ploughed eucalyptus

Autumn

Winter

Spring

Year

7.5

6.9

9.9

5.1

7.8

50.2

48.6

45.4

54.3

46.4

As the infiltration capacity in the furrows is very low, both rainfall amount and intensity (integrated in the kinetic energy variable) have an overwhelming weight in the model. Nevertheless, the antecedent soil moisture conditions, represented here by the amount of rainfall in the 48 h prior to the event, also have a positive correlation. High overland flow thus occurs during long and intense rainfall events following wet weather, when a greater percentage area of the soil becomes saturated. For the summer dataset, the model is somewhat different. Rainfall amount is the only variable with a sufficiently high correlation to warrant inclusion in the model, explaining 73.8% of overland flow variation (Table 4). As most summer rainfall occurs as intense thunderstorms, some variables related to rainfall intensity have limited ranges, thus perhaps explaining why they are not statistically significant in their influence on overland flow. 4.2.2. Regrowth eucalyptus The winter overland flow model resembles that in the rip-ploughed areas in that kinetic energy is the most important variable (Table 4). However, the contribution made by the throughflow variable is almost as great, with overland flow tending to be higher when throughflow is high and the soil is wet. In the summer, the model is very different: rainfall amount dominates the model, with a b weight of 0.957, but there is a significant negative correlation with the soil moisture (Table 4), implying that overland flow increases with antecedent soil dryness.

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Table 4 Best-fit stepwise linear regression equations for overland flow in winter and summer at the runoff plots under regrowth eucalyptus and rip-ploughed eucalyptus Land-use type and season

Best-fit equation

b weights

Rip-ploughed eucalyptus Winter Summer

OF ¼ 0:242 KE þ 0:035 P48 2 0:519 OF ¼ 0:572 P 2 0:432

0.854 (KE) 0.859 (P)

Regrowth eucalyptus Winter Summer

OF ¼ 0:004 KE þ 0:000387 TF 2 0:216 OF ¼ 0:083 P 2 0:05 SM þ 0:323

0.549 (KE) 0.957 (P)

r2

n (number of events)

0.153 (P48) –

0.789 0.738

91 47

0.400 (TF) 20.112 (SM)

0.751 0.914

56 36

OF ¼ Overland flow (mm), P ¼ Event rainfall (mm), KE ¼ Rainfall kinetic energy (J m22), P48 ¼ Rainfall in 48 h prior to event rainstorm (mm), TF ¼ Throughflow (ml), SM ¼ Soil moisture (% by volume).

4.3. Catchment flow A set of 29 runoff events was analysed, 15 of them occurring after dry spells, the others following very wet conditions. Generally, the partly rip-ploughed Serra de Cima catchment produced higher peaks and runoff amounts under wet conditions, whereas the regrowth eucalyptus catchment was characterised by higher runoff peaks and volumes following dry spells. The peak response time is significantly different ðP , 0:005Þ for the two catchments under both dry and wet conditions, though there are some events (both wet and dry) in which similar response times were recorded (Table 5). Runoff responses at both plot and small catchment scales were investigated in more detail for two short periods in April and December 1992 that followed dry and wet weather, respectively. Rainfall was just under 200 mm in a week in each case, but whereas the weather had been dry for 45 days prior to the April 1992 period, rainfall during the 45 days prior to the December 1992 period totalled 329.5 mm. Hourly rainfall and streamflow for the mainly regrowth eucalyptus Sernadinha catchment and the 46% ripploughed Serra de Cima catchment are shown for the two periods in Figs. 2 and 3. Overland flow at the two plots and throughflow responses recorded at intervals during each period are summarised in Table 6.

4.3.1. The April 1992 event (following dry weather) The first streamflow peak, which commenced at around 20 h following a peak rainfall intensity of

5 mm h21, was both earlier and larger at the regrowth eucalyptus Sernadinha catchment than at the rip-ploughed Serra de Cima catchment (Fig. 2). Measurements at 29 h showed that, although overland flow was significant (6.7%) at the regrowth eucalyptus plot, no throughflow had been generated (Table 6). This pattern of an earlier and larger streamflow peak for the regrowth eucalyptus than the rip-ploughed catchment was maintained for the other three storm events (Fig. 2), despite overland flow responses at the plot scale being much larger on rip-ploughed terrain (Table 6). Throughflow fluxes were larger in the three later storm events in the regrowth eucalyptus stand than in the first storm event and were high in the third and fourth peaks on rip-ploughed terrain but minimal in the first two events (Table 6). 4.3.2. The December 1992 event (following wet weather) The period opened with an intense (12 mm h21), but short-lived rainstorm around 9 h followed by a series of less intense periods of rain between 40 and 77 h. Streamflow responses to these rainfalls were modest, but whereas the lag times were similar for the initial event, the Serra de Cima catchment response was significantly quicker than that of the Sernadinha regrowth eucalyptus catchment for the later events (Fig. 3). Overland flow responses at the regrowth eucalyptus plot were very modest (3.2 and 0.5% recorded at 57 and 77 h) for these early events and throughflow fluxes were also minor (Table 6). Both throughflow and percentage overland flow

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Table 5 Storm runoff (streamflow), peak flow and response time to rainstorms with hourly intensities exceeding 3 mm h21 for the regrowth eucalyptus Sernadinha catchment and the rip-ploughed eucalyptus Serra de Cima catchment for events following dry and wet antecedent weather in 1991– 1992. Response time difference is the Sernadinha time-to-peak minus the Serra de Cima time-to-peak in hours Month of storm

Rainfall Amount (mm)

Intensity (mm h21)

Events following wet antecedent conditions January 1991 16.2 2.4 January 1991 91.2 nd February 1991 14.5 4.1 March 1991 55.3 5.2 November 1991 22.4 6.9 November 1991 24.9 9.8 November 1991 15.1 2.5 November 1991 20.7 3.2 April 1992 7.7 3.7 December 1992 10.6 3.2 December 1992 11.1 4.4 December 1992 8.0 3.1 December 1992 37.3 7.4 December 1992 24.1 11.0 Events following dry antecedent conditions February 1992 28.8 9.8 March 1992 29.7 5.4 March 1992 45.6 6.9 April 1992 39.6 11.0 April 1992 23.3 8.9 April 1992 10.1 3.7 April 1992 23.2 13.4 April 1992 24.0 17.4 May 1992 21.7 4.9 May 1992 14.6 5.9 May 1992 17.4 7.5 May 1992 18.5 18.5 June 1992 19.3 9.0 August 1992 8.7 6.7 December 1992 13.5 12.1

Sernadinha catchment

Serra de Cima catchment

Storm runoff (mm)

Peak flow (mm h21)

Storm runoff (mm)

Peak flow (mm h21)

7.5 45.0 3.5 3.7 2.7 2.0 1.3 4.1 1.0 0.3 0.4 0.8 0.9 11.3

0.63 3.50 0.20 0.52 0.26 0.15 0.07 0.36 0.12 0.03 0.15 0.13 0.09 2.82

7.8 57.5 8.7 3.8 6.0 3.2 2.2 5.5 1.5 0.4 0.6 0.9 4.0 16.9

0.63 3.00 0.42 0.55 0.47 0.20 0.12 0.38 0.16 0.04 0.10 0.14 1.23 2.40

þ2 þ3 0 0 þ1 þ3 þ1 þ3 0 24 23 27 21 25

0.13 0.06 0.29 0.58 0.40 0.18 0.96 4.99 0.04 0.04 0.03 0.02 0.03 0.01 0.13

0.3 0.3 1.4 2.0 1.0 1.1 1.6 10.6 0.1 0.1 0.1 0.03 0.2 0.01 0.3

0.03 0.03 0.14 0.43 0.36 0.16 0.79 2.73 0.01 0.01 0.01 0.01 0.01 0.00 0.08

þ1 þ3 0 þ1 þ1 þ3 þ1 0 þ3 þ2 þ2 0 þ4 0 0

0.7 0.5 1.6 2.3 1.0 1.0 2.1 9.6 0.3 0.3 0.3 0.04 0.3 0.03 0.6

responses were much higher in the main storm event (87 – 103 h) both at the regrowth eucalyptus and ripploughed plots, as the readings at 127 h demonstrate (Table 6). The two streamflow peaks of the main storm hydrograph lagged 8 and 6 h, respectively, after peak rain intensity at the Sernadinha regrowth eucalyptus catchment, compared with only 3 and 2 h, respectively, at the Serra de Cima rip-ploughed catchment. This was in contrast to the situation in April 1992.

Response Time Difference (h)

5. Discussion Mean overland flow at the regrowth eucalyptus (7.8% of rainfall) and particularly the rip-ploughed plot (46.4%) is high compared with in mature eucalyptus (1.7 – 2.2%) and pine stands (0.1%) in the study area (Ferreira, 1996). For the rip-ploughed plot, this can be ascribed to the changes in the soil structure and the disappearance of any vegetation or litter cover associated with deep ploughing and the subsequent


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Fig. 2. Hourly precipitation and streamflow hydrographs for the Sernadinha (5-year regrowth eucalyptus) and Serra de Cima (partly ripploughed eucalyptus) catchments for storm events during the April 1992 period following dry antecedent conditions. Arrows indicate the occasions when throughflow and plot overland flow were recorded (see Table 6).

compaction and erosion by rainfall and the development of a surface stone layer (Walsh et al., 1995). Overland flow is lower on the regrowth eucalyptus plot, because by 1989 – 1993 it had recovered from many of the impacts of the 1986 fire and had been undisturbed by ploughing. In the rip-ploughed eucalyptus plot, the multivariate analyses are consistent with the view that Hortonian overland flow was being generated in most events. Thus in winter the kinetic energy (KE) dominates the best-fit equation. The additional importance of the 48 h antecedent rainfall variable in winter suggests that antecedent moisture also seems to play a role on overland flow generation. This may mean that during

this period, in addition to the Hortonian overland flow in the impermeable furrows, saturation overland flow may be developing on patches of more permeable ground within the plot, thereby extending the spatial extent of overland flow. The fact that rainfall amount, rather than the kinetic energy and maximum rainfall intensity variables, is the dominant variable in summer may reflect the fact that most rainfall received in summer is intense and derived from thunderstorms, thereby not providing a sufficiently high range in intensity for intensity-related variables to be important in a multivariate model. In the regrowth eucalyptus plot, the contrast in multivariate model between winter and summer is

176


Fig. 3. Hourly precipitation and streamflow hydrographs for the Sernadinha (5-year regrowth eucalyptus) and Serra de Cima (partly ripploughed eucalyptus) catchments for storm events during the December 1992 period following wet antecedent conditions. Arrows indicate the occasions when throughflow and plot overland flow were recorded (see Table 6). Table 6 6 Overland flow and throughflow responses during the April 1992 and December 1992 rainfall periods Rainfall period and sub-period

Rainfall (mm)

Rip-ploughed eucalyptus

Regrowth eucalyptus

Overland flow (%)

Throughflow (ml)

Overland flow (%)

Throughflow (ml)

83.8 44.0 76.6 66.5

0.00 0.04 0.92 0.53

6.7 2.9 7.6 20.5

0.02 0.12 0.07 0.12

December 1992 period (after wet conditions) in hours 1–57 40.6 53.0 58–77 12.0 75.8 78–104 70.9 70.9 105–127 13.1 131.3

0.07 0.49 0.18 2.24

3.2 0.5 27.7 21.4

0.01 0.04 0.77 2.40

April 1992 period (after dry conditions) in hours 1–29 29.7 30–80 51.1 81–121 54.2 122–148 48.4


probably linked to hydrophobicity in summer and its absence in winter. In winter, the combination of positive relationships between overland flow amount and kinetic energy and throughflow variables suggests that: (a) saturation overland flow is the mechanism involved; and (b) the soil is predominantly hydrophilic during winter wet conditions, as reported in other studies (Walsh et al., 1994; Doerr and Thomas, 2000). In summer, however, although kinetic energy (reflecting rainfall amount and intensity) remains the dominant variable, the negative correlation of overland flow amount with soil moisture and the relatively high overland flow responses (averaging 7.5% of rainfall) are interpreted as indicating Hortonian overland flow resulting from very low infiltration capacity associated with the extreme hydrophobicity developed in the soil when dry. In the regrowth eucalyptus area, overland flow responses therefore increase as hydrophobicity intensifies with soil dryness. This is in contrast to dry, but hydrophilic soils, where infiltration capacities characteristically tend to be high when soils are dry and fall during rainstorms as soils become wetter (Jones, 1997). Some of these inferences are confirmed to some extent by the contrasts between slope and catchment runoff responses in the April and December 1992 periods (Figs. 2 and 3; Table 6). The throughflow and overland flow data for the two periods (Table 6) suggest that different mechanisms of overland flow production (Hortonian in April and saturation overland flow in December) resulting from surface soil hydrophobicity in April (following warm, dry weather) and its absence in December (following prolonged wet conditions) may be responsible. Thus in April the substantial overland flow (6.7 – 20.5% at 29, 80, 121 and 148 h) at the regrowth eucalyptus plot was accompanied by very little throughflow (0.017 – 0.124 l mm21 rain), whereas in December substantial overland flow production only occurred when throughflow had increased to 0.77 l mm21 in 104 h and 2.4 l mm21 in 127 h. This suggests that in April low infiltration capacities linked to intense hydrophobicity led to immediate Hortonian overland flow and short lag-times in peak streamflow response, whereas in December the hydrophilic nature of the soil meant that time was needed for localised soil saturation (and saturation overland flow) to occur and hence streamflow peaks were delayed.

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In the rip-ploughed areas, there was no such contrast in lag-times. This may be due to the soil being largely hydrophilic in both periods, such that the spatial mosaic of Hortonian and saturation overland flow (depending on local soil conditions) that is generated will depend on a similar combination of rainstorm intensity, amount and antecedent wetness factors in both periods. These findings have similarities to those reported by Burch et al. (1989), who investigated seasonal differences in hydrophobicity and runoff responses of a grassland catchment and a strongly hydrophobic forest catchment in Australia. In the light of the very high overland flow percentages (mean 46%, rising to over 70% in more intense storms or following very wet antecedent weather) recorded at the rip-ploughed plot, one would logically expect much higher peaks from the ripploughed catchment than were actually recorded. Much of the overland flow generated at the plot scale must therefore fail to reach stream channels. One reason for this could be the upslope position of most of the rip-ploughed stands in the Serra de Cima catchment and the presence (required by Portuguese law) of a buffer zone between rip-ploughed stands and stream courses. Overland flow produced at plots in this buffer land use was 3.2% of rainfall on average for the 1989 –1992 period (Ferreira, 1996). This buffer zone is generally covered by dense undergrowth and scrub, which impedes water movement and promotes infiltration. This mechanism may be less effective in wet winter conditions when the soil of the buffer zone may be saturated. This may explain why the streamflow responses of the Serra de Cima catchment were relatively higher compared with the Sernadinha catchment in the wet December period, whereas the reverse was true in the April 1992 period. It should be stressed that in both the regrowth eucalyptus and rip-ploughed areas, runoff process controls and patterns are also dynamically changing with increasing time since burning (in the regrowth eucalyptus areas) and time since ploughing (in the ripploughed areas). The impacts of hydrophobicity on overland flow in the regrowth eucalyptus area are likely to decline with time as the soil becomes more characteristic of mature forest and develops more routeways (provided by root and faunal passages) through the topsoil even when hydrophobic. In the rip-

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ploughed areas, the hydrophilic nature of the soil imparted by deep-ploughing is only temporary; the soil tends to become hydrophobic (when dry) as the planted eucalyptus trees grow and provide inputs of hydrophobic substances from litter and root development (Doerr et al., 1998).

6. Conclusions The data presented here suggest that in post-fire regrowth eucalyptus areas of northern Portugal runoff processes and their controls vary radically between hydrophobic-phase dry weather and hydrophilicphase winter wet conditions. After long dry periods, particularly during the summer, hydrophobicity plays an important role in influencing runoff responses in regrowth eucalyptus areas by enhancing the generation of Hortonian overland flow. In contrast, after prolonged wet weather, soils can become hydrophilic leading to throughflow and saturation overland flow, which takes more time to generate, being the main runoff-generating processes. The frequency of hydrophobic-phase responses is likely to be enhanced by the

increased frequency of low soil moisture (and hence high hydrophobicity) conditions that result from the high transpirational demand of eucalyptus vegetation (Scott and Lesch, 1997). There are no such contrasts in the rip-ploughed areas, where ploughing has rendered the soils hydrophilic throughout the year. In the rip-ploughed areas, overland flow is high throughout the year as a result of poor ground cover, a lack of organic matter, and soil compaction (because of raindrop impact) and stone cover development (through preferential erosion of finer material). Although much of the overland flow developed is Hortonian in character, the finding that overland flow increases also with antecedent wetness variables suggests that a truer picture is one of a localised spatial mosaic of both Hortonian overland flow and saturation overland flow. The relatively modest streamflow peaks generated in the catchment where much of the land had been subjected to rip-ploughing is ascribed to the infiltration of overland flow in the downslope buffer zones of scrub and undergrowth between the rip-ploughed eucalyptus stands and the stream channels.

Chapter 17

The erosional impact of soil water repellency: an evaluation R.A. Shakesby*, S.H. Doerr and R.P.D. Walsh Department of Geography, University of Wales Swansea, Singleton Park, Swansea SA2 8PP, UK

Abstract Soil hydrophobicity affects the susceptibility of soils to erosion in a variety of ways (e.g. increased aggregate stability, reduced infiltration capacity, enhanced overland flow), but there are problems concerning the overall assessment of its erosional impact. Three current problems are discussed: (1) poor isolation of hydrophobicity from other effects; and poor understanding of its overall impact; (2) areally, at slope and catchment scales; and (3) temporally, over periods of months or years rather than on a storm basis. These problems are highlighted with reference to the literature and to research in Portugal on highly hydrophobic forest soils. A conceptual model relating erosion risk to three key aspects of soil hydrophobicity (temporal contiguity, spatial contiguity and the thickness of any overlying hydrophilic soil) is presented in order to provide a framework for future research into hydrophobicity– soil erosion links.

1. Introduction The neglect of soil hydrophobicity in soil erosion research texts as a significant soil property in explaining soil loss (e.g. Kirkby and Morgan, 1980; Zachar, 1982; Pimentel, 1993; Morgan, 1995), even though it reportedly affects aggregate stability (Giovannini et al., 1983), reduces infiltration capacity (DeBano, 1971; Meeuwig, 1971a; Wallis et al., 1993), enhances overland flow (McGhie and Posner, 1980; Burch et al., 1989; Witter et al., 1991) and can increase both rainsplash erosion (Terry and Shakesby, 1993) and soil erosion (Osborn et al., 1964a; Megahan and Molitor, 1975), stems from a weak overall understanding of hydrophobicity – erosion links. Three major problems in our understanding of these links can be identified. First, where present, * Corresponding author. Tel.: þ44-1792-295236; fax: þ 44-1792295955. E-mail address: [email protected] (R.A. Shakesby). q 2003 Elsevier Science B.V. All rights reserved.

the influence of hydrophobicity on soil erosion may be implied but is rarely isolated successfully from other factors and quantified. Second, whether point measurements of hydrophobicity are important in explaining soil losses from areas ranging in size from small plots to entire catchments (which may contain hydrophilic areas, cracks or macropores intercepting overland flow) is often unclear. Third, hydrophobicity is usually transient, disappearing during prolonged wet periods: the extent to which overall overland flow generation and soil erosion are influenced by periods when the soil is hydrophobic remains largely unexplored. The aims of this paper are: (a) to explore critically the role that hydrophobicity plays in influencing overland flow and soil erosion with reference to both the literature and particularly to results obtained for highly hydrophobic Portuguese forest soils; (b) to present and discuss a tentative conceptual model of erosion risk in relation to the characteristics of

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hydrophobic terrain; and (c) to identify the research directions needed to improve the understanding of the links between soil hydrophobicity and erosion.

2. Research in north-central Portugal: background information Aspects of the impact of forest fires and management practices on soil degradation and soil erosion in Eucalyptus globulus and Pinus pinaster commercial ´ gueda Basin, north-central Portugal forests in the A (Fig. 1) have been studied by two of the authors with others (e.g. Shakesby et al., 1993, 1994, 1996; Walsh et al., 1994, 1995; Ferreira et al., 1997). That the highly hydrophobic nature of the soils must be important in explaining the patterns and processes of overland flow and soil loss was acknowledged early in this research programme, but later detailed analysis showed that its characteristics and those of the erosional responses were often not as had been anticipated. It was subsequent critical questioning of

´ gueda Basin, Portugal. Fig. 1. Location of the A

some of the assumed hydrophobicity –erosion links that led to the identification of the problems on which this paper focuses. Hence, throughout the paper, reference is made to this research as well as to the main body of the literature. ´ gueda Basin lies in a deeply dissected upland The A (20 –500 m) that rises from the coastal plain along the western coast of Portugal (Fig. 1). The area experiences a wet Mediterranean type climate with mean annual rainfall increasing from 1400 to 1900 mm with increasing altitude. Most rainfall results from winter depressions. The period from July to September is mostly dry and warm. The soils are shallow, stony Humic Cambisols and Umbric Leptosols (Pereira and FitzPatrick, 1995). They comprise mainly sand (ca. 55– 80%) and silt (ca. 15 –40%) and small amounts of clay (typically , 5%) overlying weathered schist bedrock or schist and quartzite regolith (Terry, 1992; Doerr et al., 1996). Organic matter contents of humic layers range from 12 to 29% on both pine and eucalyptus terrain (Clarke, 1996). The soils are highly hydrophobic in unploughed forest (both burnt and unburnt) under summer dry conditions, with water drop penetration times (WDPTs) often exceeding 5 h (Doerr et al., 1998). Following major fires in 1986, 1991 and 1992, hydrological response and soil loss were determined at various scales and field hydrophobicity was assessed at points and on micro-plots (Fig. 2). Rainfall simulation experiments were carried out in burnt pine forest two years after a 1991 fire using a rotating nozzle spray and a 1 m £ 1 m plot arranged in diamond fashion with opposite corners pointing upslope and downslope and connected to a datalogged tipping-bucket recorder (Walsh et al., 1998). Soil losses from burnt and unburnt pine and eucalyptus forests were monitored using small ð8 m £ 2 mÞ bounded plots in mid-slope positions on slopes of 14– 228. Over intervals ranging from a few days to months, soil removed from the plots was collected, air-dried and weighed. Overland flow was recorded using tipping-bucket flow recorders and collecting tanks (Shakesby et al., 1991; Walsh et al., 1995; Shakesby and Walsh, 1997). Monitoring of runoff and suspended sediment was carried out for small catchments of contrasting dominant land use and management (Fig. 2) (Walsh et al., 1992, 1995).

The erosional impact of soil water repellency: an evaluation

´ gueda Basin. Fig. 2. Types and scales of investigations into hydrophobicity and its erosional impact carried out in the A

181

182


3. The current problems 3.1. Problem 1: hydrophobicity has often been implicated as a major cause of enhanced overland flow and high erosion rates but its impact has rarely been isolated Few studies have quantified the impact of hydrophobicity on overland flow generation and soil erosion because of the difficulty of isolating its effect from those of other factors. Successful isolation has arguably only been achieved to date in comparatively simple soil – ecological systems. For example, in a series of studies conducted in dune sands in the Netherlands (e.g. Jungerius and van der Meulen, 1988; Jungerius and de Jong, 1989; Witter et al., 1991; Jungerius and ten Harkel, 1994), it was possible to compare overland flow and erosion on hydrophobic and hydrophilic terrain while other factors were controlled. The bare terrain of hydrophobic burnt soil offers in some circumstances a similarly comparatively simple system to that of dune sands. An example of successful isolation of the erosional impact of hydrophobicity is provided by the study of Osborn et al. (1964a) who set up six plots in newly burnt and hydrophobic chaparral soils on slopes of similar angle. Over a period of approximately four months during which 334 mm of rain fell, losses on the three untreated plots were almost fourteen times higher than on plots treated with a wetting agent. Fireinduced hydrophobicity has attracted considerable attention both in terms of its nature and its impact on soil characteristics and soil erosion (e.g. DeBano and Krammes, 1966; DeBano et al., 1970; Savage, 1974; DeBano, 1981, 1991; O’Loughlin et al., 1982; Pradas et al., 1994; Zierholz et al., 1995; Scott et al., 1998). Indeed, it has been the focus of probably the largest amount of research into hydrophobicity – erosion links. It is often implicated in explaining enhanced overland flow. For example, in a burnt pine forest in South Africa, an increase in the stormflow response to 7.5% compared with just 2.2% on comparable unburnt terrain was attributed by Scott and Van Wyk (1990) to saturation overland flow promoted by a hydrophobic layer at a depth of 10 mm. Hydrophobicity induced or enhanced by fire has also been implicated in explaining higher soil losses on burnt

land (e.g. Megahan and Molitor, 1975; Wells et al., 1979; Morris and Moses, 1987; Scott et al., 1998). DeBano et al. (1979), for example, reported 34 times as much soil removed on a 278 slope following a moderately intense prescribed burn in Californian chaparral than in an unburnt area. However, it has often proved difficult to isolate the effect of hydrophobicity from that of other fire-related changes recognised in burnt areas. These include the reduction in ground cover and increase in percent bare soil (Chandler et al., 1983; Chartres and Mu¨cher, 1989), the sealing of soil pores (Wells et al., 1979), the loss of water storage capacity in the litter and fermentation layers (Imeson et al., 1992), and the provision of more easily removed sediment (White and Wells, 1982; Giovannini and Lucchesi, 1983; Scott et al., 1998). ´ gueda Basin, factors both associated with In the A as well as unconnected to hydrophobicity have been found to influence overland flow and erosional patterns following forest fire (e.g. Shakesby et al., 1993, 1996; Walsh et al., 1994, 1995). For example, percentage overland flow on 19– 228 slopes over the period October 1989 – October 1990 measured for plots in unburnt eucalyptus and pine forest was found to be low (0.1 – 2.5%) compared with that for plots burnt in June 1986 (3.8 – 11.7%). Soil losses from plots were also found to be very low in the unburnt forests, being two orders of magnitude lower than rates for newly burnt terrain 2 – 3 years after fire. These differences were initially attributed in large part to fire-induced hydrophobicity, but subsequent work has demonstrated that hydrophobicity is in fact just as severe in unburnt as in burnt forest (Doerr et al., 1996, 1998). In addition to the storage capacity of the thick litter and vegetation covers in unburnt forest, a high density of bypass routes provided by, for example, plant and tree roots and soil faunal burrows is thought to allow much overland flow to infiltrate the hydrophobic soil (e.g. Walsh et al., 1995; Ferreira et al., 1997, 1998). Thus, although fire brings about changes that enhance overland flow and soil erosion ´ gueda Basin (e.g. removal on the burnt terrain in the A of the litter and vegetation covers, ready availability of loose sediment), the soils are already highly hydrophobic prior to the passage of wildfire which brings about no detectable change in this respect. The relationship between fire and hydrophobicity is,

183


therefore, different from that reported elsewhere (e.g. DeBano, 1981), in which fire induces hydrophobicity ´ gueda Basin, it on previously hydrophilic soil. In the A appears that hydrophobicity is essentially a ‘dormant’ factor in unburnt terrain and becomes ‘active’ only when fire causes other changes that enable it to have an important erosional impact. One way in which hydrophobicity seems to become an erosionally ‘active’ soil characteristic on ´ gueda Basin is by increasing the burnt terrain in the A the rate of supply of sediment available for transport by overland flow processes. Its effect on rainsplash detachment has been isolated in the laboratory. In rainfall simulation experiments, rainsplash detached soil particles from reconstituted hydrophobic (WDPT . 1 h) soil at rates, respectively, 85 and 59% higher from horizontal surfaces and from surfaces sloping at 158 than from comparable hydrophilic surfaces (Terry and Shakesby, 1993; Table 1). Observations showed that on the latter the soil rapidly became cohesive and developed a compact surface seal that limited the amounts of splashed sediment. In contrast, on hydrophobic soil, surface soil particles remained dry and easily detachable even beneath a water film. The drops, whether hitting the hydrophobic soil or water film, could continue to detach much larger amounts of

sediment than from hydrophilic soil. Although these processes have been observed on in situ Australian hydrophobic sandy soils (P. Blackwell, pers. comm.), the impact of hydrophobicity on rainsplash effectiveness has as yet not been satisfactorily isolated and quantified under field conditions. The uncertainty about the relative erosional influence of hydrophobicity compared with other soil characteristics is exemplified by results from plots ´ gueda Basin. Table 2 shows the greater in the A erosional effectiveness of overland flow from postburn eucalyptus and pine plots during a hydrophobicphase period (June 14 –July 4 1993; period 5), which was preceded by 16 days of warm dry conditions, than in the wetter, predominantly hydrophilic-phase winter/early spring wet seasons (periods 1– 3 and 7 – 8). For eucalyptus plots A and B, soil losses per unit overland flow were, respectively, 465.0 and 83.8 g mm21 in the hydrophobic period compared with 10.8 – 40.7 and 6.1 –44.9 g mm21 in the winter/ early spring periods. Similarly, marked differences for the pine plots occurred between relatively high values in hydrophobic-phase period 5 and low values in such winter/early spring periods for which overland flow and soil losses were recorded. For the eucalyptus plots, the remaining spring/early summer period (4)

Table 1 ´ gueda Basin on horizontal and sloping surfaces Comparison of rates of splash detachment for hydrophobic and hydrophilic soils from the A under simulated rainfall (modified after Terry and Shakesby, 1993) Rainfall (mm)

Soil surface horizontal

Duration (min)

Rates of splash detachment (g mm21)a Hydrophobic soil (A)

Hydrophilic soil (B)

3.5 3.9 4.1 10.0

10 10 10 20

0.333 0.436 0.769 0.362

0.169 0.254 0.434 0.189

1.97 1.72 1.77 1.92 1.85

4.4 4.4 7.7 7.7

15 15 20 20

0.418 0.316 0.385 0.433

0.253 0.230 0.280 0.223

1.65 1.37 1.38 1.94 1.59

Average Soil surface sloping at 158

Average a

Ratio of hydrophobic:hydrophilic splash detachment rates (A/B)

Rates of splash detachment are expressed as grams per millimetre of simulated rainfall (g mm21).

184


Table 2 Soil loss rates (measured in grams of soil removed per millimetre of overland flow) monitored for E. globulus plots burnt in August 1992 and P. pinaster plots burnt in July 1991 in inferred hydrophobic- and hydrophilic-phase periods (modified after Walsh et al., 1994) (Hi ¼ hydrophilic-phase; Ho ¼ hydrophobic-phase; slope angles, eucalyptus plots, 17–188; pine plots, 20–218; average soil depths, eucalyptus, 3 cm; pine, 24 cm) Period

1 (22 November–12 December 1992) 2 (12 December 92–5 March 1993) 3 (5 March–29 April 1993) 4 (29 April–14 June 1993) 5 (14 June–4 July 1993) 6 (4 July–21 October 1993) 7 (21 October 1993–28 January 1994) 8 (28 January–4 April 1994)

Inferred hydrophobicity status

Burnt eucalyptus Rainfall (mm)

Plot A (g mm21)

Plot B (g mm21)

Rainfall (mm)

Plot A (g mm21)

Plot B (g mm21)

Hi Hi Hi Hi Ho Hi Hi Hi

33.5 205.4 163.9 255.7 19.9 492.8 507.9 238.7

– 10.8 17.6 105.1 465.0 49.6 40.7 18.3

44.9 7.5 6.1 21.3 83.8 37.7 51.1 7.4

56.0 320.4 – – 30.8 – 672.0 319.8

26.7 8.3 – – 286.3 – 2.0 9.2

13.2 7.6 – – 238.1 – 2.1 5.6

and summer/early autumn period (6) have intermediate soil loss rates which might reflect short intervals of restored hydrophobicity during generally wettable soil conditions. The extent to which the higher soil loss rates per unit overland flow in hydrophobic-phase period 5 can be attributed to hydrophobicity alone, however, is unclear; all soils irrespective of degree of hydrophobicity tend to be more erodible when dry because of increased availability of loose particles, more effective aggregate breakdown processes and a decline in vegetation cover during prolonged dry periods (Morgan, 1995). Even where most or all soil losses on burnt land might be attributable to hydrophobicity, however, its erosional significance could be overemphasised in certain circumstances because its impact becomes a focus of interest at the expense perhaps of more ´ gueda Basin, soil losses important factors. In the A resulting from deep-ploughing (51 t ha21 year21 in the first year after disturbance), which in fact renders the soil hydrophilic (Shakesby et al., 1993), are up to about seventeen times higher than any of those recorded for hydrophobic burnt soil (3 –8 t ha21 year21 during the first, critical year following fire) (Shakesby and Walsh, 1997). Thus, given that ploughing is an established forest management practice, the overall erosional impact of even severe hydrophobicity may actually be limited to a comparatively subordinate role in this locality.

Burnt pine

3.2. Problem 2: scaling up from point measurements to slope and catchment scales Measurements of hydrophobicity, infiltration and overland flow are made at points or on small areas of slopes. Soil hydrophobicity is usually determined using the WDPT and molarity of an ethanol droplet (MED) tests (Watson and Letey, 1970) on soil samples in the laboratory and less typically on in situ soils in the field. Such tests, however, have tended to provide little insight into the spatial variability of hydrophobicity or its contiguity with depth unless labourintensive micro- and meso-scale variability assessments are incorporated (e.g. Dekker and Ritsema, 1994b). Rainfall simulation tests are undertaken on small plots (usually of one square metre or smaller), and even bounded plots from which overland flow amounts are monitored rarely cover more than 40 m2 of slope. Such plots have the disadvantage, therefore, that they do not necessarily provide a reliable indication of the probable behaviour of overland flow and transport of detached sediment on entire slopes and from catchments (Prosser and Williams, 1998). Infiltration on hydrophobic soils can be strongly dependent on the distribution of plants on a slope (Imeson et al., 1992; Cerda` et al., 1998). Depending on vegetation type, infiltration capacity can be either enhanced (if root channels provide pathways for water) or depressed (if the vegetation induces


hydrophobic areas around it). For example, DeBano (1981) argued that in burnt soils, where hydrophobicity is often confined to a well-defined, fire-induced subsurface layer, water moves laterally over the top of this layer until it encounters cracks or irregularities that allow infiltration through the hydrophobic layer into hydrophilic soil beneath. Animal burrows (Garkaklis et al., 1998) as well as root channels (Meeuwig, 1971a) have been implicated as pathways for infiltrating water through hydrophobic layers. In forest areas, large macropores have been viewed as important routes for infiltrating water (e.g. Sevink et al., 1989). It has been estimated, for example, that ‘hose type’ macropores remaining after roots have decayed can comprise up to 35% of the near-surface volume of a forest soil (Aubertin, 1971). Root holes were highlighted by Burch et al. (1989) to explain the lack of a measurable impact of highly hydrophobic soil conditions on runoff in a eucalyptus catchment in Australia. Less abrupt spatial differences in hydraulic conductivity resulting from soil hydrophobicity variations can cause restricted percolation via ‘fingered flow’ (Dekker and Ritsema, 1994b, 1995). An illustration of the patchiness of overland flow on hydrophobic soil is shown in Table 3 for burnt

Table 3 Comparison of overland flow responses in summer 1993 for 1 m2 rainfall simulation plots and 16 m2 erosion plots in P. pinaster ´ gueda Basin, Portugal, burnt in July 1991 (modified after forest, A Walsh et al., 1994, 1998) (slope angles, 18–218; average soil depth ¼ 24 cm) Rainfall

Overland flow

(mm)

(mm)

(%)a

Rainfall simulation plots (1 m2) 1 32.8 2 34.5 3 39.5

8.1 6.5 2.1

24.8 19.0 5.2

Erosion plots ð8 m £ 2 mÞ 1 30.8 2 30.8 3 30.8 4 30.8 5 30.8 6 30.8

1.0 2.0 3.2 1.9 1.6 1.2

3.2 6.3 10.4 6.0 5.2 4.0

a

Overland flow expressed as a percentage of simulated/natural rainfall.

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´ gueda Basin. Rainfall P. pinaster forest in the A simulation experiments of 32.5– 39.5 mm per hour were conducted on three 1 m2 plots following dry antecedent summer weather on highly hydrophobic burnt soil two years after fire. Overland flow percentages comprised two high values (19.0 and 24.8%) and one relatively low value (5.2%) (Walsh et al., 1998). On the adjacent slope, during the same summer dry period, overland flow percentages from six 8 m £ 2 m bounded plots following a similar sized natural rainfall event (30.8 mm) were all comparatively low (3.2 – 10.4%). Notwithstanding differences in simulated and natural rainfall characteristics, the results suggest that the size of the simulation plots was sufficiently small for them to be sited sometimes (two out of three cases) on uniformly hydrophobic patches of soil. The larger erosion plots, however, could be viewed as including hydrophilic patches, macropores or cracks allowing comparatively high infiltration. That macropores can provide preferential pathways for water in these highly hydrophobic pine forest soils is supported by Ferreira (1996), who recorded under dry conditions in burnt pine forest infiltration capacities as high as 250 mm h21 directly above the burnt-out root channels of dead tree stumps compared with values as low as 1.8 mm h21 on adjacent rootfree hydrophobic soil. He argued that such networks of burnt-out roots could provide a coherent drainage network through the highly hydrophobic soils. These observations, therefore, suggest that cracks, holes and/or hydrophilic patches could be critical in determining the erosion risk presented by hydrophobic terrain. Comparison can be made between a hypothetical catchment with a uniformly hydrophobic soil (Fig. 3a) and one where a similarly hydrophobic soil is interrupted by many hydrological ‘sinks’ in the form of cracks, macropores or hydrophilic patches (Fig. 3b). For the former, the Hortonian overland flow generated would be continuous and increase in depth downslope thus becoming potentially highly erosive. For the latter, on the other hand, erosion at the slope and catchment scales would be low, and possibly little different from that of a catchment with only hydrophilic soil, because overland flow generated on a hydrophobic patch would flow downslope only as far as the nearest sink and little would reach channels at the base of slopes. The possibility of ‘source’ and ‘sink’ areas

186


Fig. 3. The possible influence of variation in the spatial contiguity of hydrophobicity on Hortonian overland flow in a catchment with: (a) a uniformly hydrophobic soil; and (b) a hydrophobic soil interrupted by ‘sink’ areas in the form of hydrophilic patches, macropores or cracks. Overland flow (arrows) generated on hydrophobic areas is shown intercepted by sinks (shaded areas). Catchment-scale erosion risk would be much higher in (a) than in (b).

of overland flow in hydrophobic catchments has also been raised by other workers considering the erosional impact of hydrophobic soils at slope and catchment scales (e.g. Imeson et al., 1992; Cerda` et al., 1998; Prosser and Williams, 1998). Scaling up from point or micro-plot measurements of hydrophobicity to infer slope- or catchment-scale runoff patterns can be expected, therefore, to be prone to substantial error. 3.3. Problem 3: temporal variability in hydrophobicity-induced overland flow and soil erosion Laboratory analysis has shown that hydrophobicity disappears when soils exceed a particular moisture threshold (King, 1981; Soto et al., 1994; Dekker and Ritsema, 1996c). Under field conditions, hydrophobic soils typically alternate seasonally or over shorter intervals between repellent and wettable states in response to rainfall and temperature patterns (e.g. Roberts and Carbon, 1972; Crockford et al., 1991; Dekker and Ritsema, 1994b). Burcar et al. (1994), for example, observed that for two Sierra Nevada soil types near Lake Tahoe, USA, soil hydrophobicity was absent under moist spring conditions, reaching a maximum under very dry conditions. Similarly, the infiltration capacities of soils under eucalyptus forest in Australia were 0.75 –1.9 mm h21 when dry and hydrophobic, but 7.9 –14.0 mm h21 when wet and hydrophilic (Burch et al., 1989). These changes between repellent and wettable states clearly have

important implications for rainsplash detachment, overland flow generation and soil erosion, but there remain critical gaps in our understanding. ´ gueda Basin Field WDPT measurements in the A suggest not only that soil hydrophobicity is most severe in dry summer and least in wet winter conditions, but also that long antecedent dry conditions outside of summer can restore hydrophobicity (Walsh et al., 1994; Doerr and Thomas, 2000). This temporal variation in hydrophobicity has been implicated as a factor in explaining variations through time in percentage overland flow recorded for bounded plots (e.g. Shakesby et al., 1993; Walsh et al., 1994). Table 4 gives details of overland flow responses for seven monitored storm events between February 1993 and April 1994 on 17 –188 slopes in newly burnt (August 1992) eucalyptus forest. Overland flow responses in the comparatively modest storm events of 12 March and 27– 28 June 1993, which followed prolonged dry weather, were considerably higher (mean responses of 14.3 and 11.9%, respectively) than those recorded for the larger and more intense storms of 10– 11 February (mean response 9.8%) and 20 – 21 April 1993 (mean response 8.0%), which followed a shorter dry period and wet antecedent weather, respectively. It is inferred that drought-induced hydrophobic soil conditions prior to the March and June 1993 events resulted in reduced infiltration capacities and promoted greater Hortonian overland flow, whereas hydrophilic conditions associated with wetter soil prior to the February and April events led to high infiltration capacities, thus preventing the development of Hortonian overland flow and instead leading to smaller

187


Table 4 ´ gueda Basin during selected storms with hydrophobicOverland flow responses at six E. globulus bounded plots burnt in August 1992 in the A phase and hydrophilic-phase conditions (modified after Walsh et al., 1994) (HOF ¼ Hortonian overland flow; SOF ¼ saturation overland flow; slope angles ¼ 17– 188; average soil depth ¼ 3 cm) Storm date

10–11 February 1993 12 March 1993 20–21 April 1993 27–28 June 1993 15–16 October 1993 22 April 1994 23–24 April 1994

Rainfall

Antecedent weather

Amount (mm)

Max. intensity (mm h21)

42.0 18.9 25.4 19.9 42.4 20.7 37.2

9.2 7.5 9.9 8.6 11.6 9.0 9.0

10 Days since rain Dry (1 month) Wet Dry (16 days) Wet 12 days since rain Wet

percentages of saturation overland flow. This inference is to some extent backed up by the measurements of predominantly hydrophilic conditions and moderate soil moisture (13.6% (w/w)) immediately prior to the April 1993 event. Though patchy hydrophobicity (67% of sites recorded as hydrophobic, 33% as hydrophilic prior to the storm) may have contributed to the high overland flow response of the 22 April 1994 event, the main causes of the high responses of the 15 – 16 October 1993 and 23– 24 April 1994 events are more likely to be the wet antecedent soil moisture conditions and large storm magnitudes (37.2 – 42.4 mm). These would have led to widespread saturation of the shallow (average 3 cm deep) soil and generation of saturation overland flow, despite hydrophilic soil conditions. Temporal variations in hydrophobicity are therefore viewed in this case as only partly explaining temporal variations in overland flow response. After long dry periods, soils are highly hydrophobic and Hortonian overland flow is promoted. In wet periods, however, the soils lose their hydrophobicity. Depending on antecedent conditions and storm characteristics, saturation overland flow will be developed to varying degrees in the shallow soils. The wetter the antecedent conditions, the more likely and widespread saturation overland flow will be, but storms that follow only moderately wet antecedent weather (as in the February and April 1993 events)

Overland flow (%) Mean

Range

9.8 14.3 8.0 11.9 13.0 51.1 77.1

3.9–15.8 6.1–19.9 3.9–13.7 6.1–19.8 2.4–23.6 49.0–53.2 67.8–86.4

Actual (p) or inferred hydrophobicity status

Inferred overland flow response

Hydrophilic Hydrophobic Hydrophilic (p) Hydrophobic Hydrophilic Patchy (p) Hydrophilic

SOF HOF SOF HOF SOF HOF/SOF SOF

may consequently produce little overland flow. Much of this is inference, however. Little is known about the thresholds of soil moisture content and length and intensity of dry period involved in the development of hydrophobic conditions, the spatial dynamics of hydrophobicity, and the mechanisms and speed by which hydrophilic conditions develop in periods of wet weather. In addition to these problems with establishing links between hydrophobicity and enhanced overland flow, relatively little is known about the mechanisms and conditions controlling the ‘switch’ between hydrophobic and hydrophilic states. In any assessment of the erosion risk resulting from hydrophobicity, however, these should be important considerations. The poor knowledge base about these controls arises from the difficulty of simulating satisfactorily natural hydrophilic – hydrophobic change under controlled conditions, as exemplified by a simple laboratory ´ gueda experiment carried out on highly hydrophobic A Basin soils. E. globulus soil samples (WDPT . 5 h) were placed in perforated containers and immersed for several weeks to a depth of 30 cm, yet there was no sign of hydrophobicity loss or significant penetration of the water into the soil. The problem of breaking down soil hydrophobicity under controlled conditions was also encountered in the field. In simulated rainfall events on burnt and unburnt P. pinaster plots during dry summer conditions reported in Section 3.2, only patchy wetting of the uppermost few millimetres of

188


soil could be detected; the hydrophobic state of most of the soil, including much of the surface soil, remained intact (Walsh et al., 1998). This raises the question as to whether breakdown of hydrophobicity, particularly if severe, requires processes additional to downward infiltration. Wetting in severely hydrophobic soils may be a process at least partly dependent on capillary rise from saturated hydrophilic subsoil (Hendrickx et al., 1993). Alternatively, lateral wetting from macropores and cracks may be critical in helping to break down severe hydrophobicity. This problem of identifying the switching conditions and mechanisms is complicated further by the difficulty involved in assessing the actual resistance of the hydrophobicity of a bulk soil to breakdown. There is evidence that the commonly used methods for measuring the degree of

hydrophobicity may not necessarily be able to discriminate between even large differences in this resistance (see Doerr and Thomas, 2000). 4. A tentative conceptual model of soil erosion risk in relation to hydrophobicity There are, therefore, important gaps in our understanding of the impact of soil hydrophobicity on soil erosion risk. The foregoing review and the results of ´ gueda Basin soils indicate that other studies of A aspects of hydrophobicity than just its severity may need to be considered in combination rather than individually to derive an improved understanding of hydrophobicity –soil erosion links. To this end, a simple conceptual model is proposed (Fig. 4).

Fig. 4. A conceptual model of the effect of three soil hydrophobicity attributes (temporal variation, spatial contiguity and thickness of hydrophilic soil overlying hydrophobic soil of undefined depth) on erosion risk. Grey tones represent hydrophobic conditions and unshaded areas represent hydrophilic conditions. The degree of hydrophobicity is assumed to be unvarying. Other factors likely to affect erosion risk (e.g. vegetation cover) are not considered in the model.


In the model, factors other than hydrophobicity that are likely to influence soil erosion risk (e.g. vegetation cover, slope angle, rainfall characteristics) are assumed to be constant. For soils with a similar degree of hydrophobicity, erosion risk is depicted as varying according to three hydrophobicity-related variables: (1) its temporal regime (i.e. the percentage of time that hydrophobicity operates, which may, in part, be determined by the persistence of hydrophobicity); (2) the thickness of any hydrophilic layer overlying hydrophobic soil (which affects the water storage capacity of any wettable surface layer and therefore the time taken during a storm before overland flow occurs); and (3) its spatial contiguity (both in terms of the degree of ‘patchiness’ of the hydrophobic soil and the frequency of other ‘by-pass’ routeways provided by cracks and macropores). Thus the model proposes that soil erosion risk will tend to be at its greatest where a soil is most often in a hydrophobic state, where hydrophobicity is spatially uniform and not interrupted by hydrophilic patches, cracks and macropores and where the thickness of hydrophilic soil over the hydrophobic layer is minimal or absent. As is apparent from the preceding discussion, a database on hydrophobicity and erosion from a sufficiently varied range of environments required for quantification and substantiation of the model and its underlying tenets is not yet available. For example, it is implicit in the model that soil erosion increases with the size of hydrophobicity-induced overland flow responses and declines with an increase in the frequency of sinks; this may not always be the case. If overland flow is directed into a soil pipe network, soil erosion may nevertheless be large because the potential for tunnelling erosion may be high. The model is presented, therefore, in order to provide a conceptual framework of soil hydrophobicity – erosion links for future work to address in a more holistic manner than has hitherto tended to be the case.

5. Suggested directions of future research into the erosional impact of soil hydrophobicity The following lines of research are proposed as ways of advancing understanding of the erosional impact of hydrophobicity:

189

1. Further work on point measurements of hydrophobicity, even when spatial variation at the mesoand micro-plot scale (Fig. 2) is investigated, is unlikely to provide the most crucial evidence for improving understanding of the erosional influence of hydrophobicity at slope and catchment scales. Instead research needs to focus on the spatial contiguity of hydrophobicity in terms of the distribution of overland flow-generating areas (hydrophobic areas without cracks or macropores) and sinks (hydrophilic patches, cracks and macropores). Rainfall simulation experiments could play a useful role in this research particularly in tracing the preferential pathways of water into a hydrophobic soil (Booker et al., 1993), but it is important that the research design is constructed such that the mode of hydrophobicity breakdown (e.g. by ponding or capillary movement from adjacent or underlying wet soil) can be simulated and detected. A particularly useful task (though unfortunately also difficult) would be close monitoring of the three-dimensional response of a hydrophobic soil at the meso- (i.e. slope) scale to natural rainfall. In situ monitoring of soil moisture flux and repeat measurements of hydrophobicity through a range of wet and dry periods without disturbance of the soil (e.g. by using a time domain reflectometry (TDR) device (Ritsema et al., 1997b) and/or tracers (Hendrickx et al., 1988a; Van Ommen et al., 1988) could be expected to reveal both the temporal regime of hydrophobicity and its impact on soil water movement for a variety of situations. 2. A carefully organised research design involving nested scales ranging from micro- and meso-scales to the macro- or catchment scale would assist in predicting the spatial variation in the erosional impacts of hydrophobicity. 3. A better understanding of the environmental requirements (e.g. temperature, moisture, vegetation, litter, microbial activity) determining the cessation and re-establishment of hydrophobicity is needed to enable seasonal and longer-term predictions of soil hydrophobicity to be made. In particular, the environmental conditions that control when ‘switching’ occurs, the soil moisture content necessary, and also whether the phasechange operates similarly for wetting and drying phases are questions that need to be addressed.

190


Such focused research directions might help to determine the degree to which soil hydrophobicity merits recognition as an important factor in influencing soil erosion risk generally, rather than being regarded as an interesting but, for most situations,

irrelevant phenomenon. It might also help in the identification of the most cost-effective means of mitigating against the erosional effects of soil hydrophobicity where it presents an unacceptable hazard.

EFFECT OF FIRE ON WATER REPELLENCY


Chapter 18 The role of fire and soil heating on water repellency L.F. DeBano Watershed Management, School of Renewable Natural Resources, University of Arizona, Tucson, AZ 85721 USA

Abstract This chapter describes the heat transfer mechanisms operating as heat moves downward in the soil along steep temperature gradients during both wildfires and prescribed fires. The transfer of heat downward in the upper part of the soil is enhanced by the vaporization and movement of water and organic compounds. Available information on the changes in the chemistry of vaporized organic compounds is summarized and discussed. An operational theory describing the formation of a highly water repellent soil condition during fire is presented. The relationship between the formation of this fire-related watershed condition and subsequent surface runoff and erosion from wildland ecosystems is explored. Worldwide literature describing fire-induced water repellency is reviewed and summarized.

1. Introduction Fire-induced water repellency in soils has been a continuous concern of watershed managers since its identification in the early 1960s. The formation of water repellent soil, its chemical nature, and its effect on infiltration, runoff and erosion have all captured the attention of numerous scientists and managers worldwide. This chapter is intended to present only a short overview which describes: (1) the discovery of fire-induced water repellency; (2) the processes responsible for its formation; (3) the worldwide importance of this soil property; (4) the chemical nature of the substances producing it; and (5) the linkages between fire-induced water repellency and postfire hydrologic responses on wildlands.

2. Background The excessive soil erosion following wildfires in the mountainous environment of southern California, USA q 2003 Elsevier Science B.V. All rights reserved.

has captured the interest of both scientists and land managers for over a century (Sinclair and Hamilton, 1954). Further, the flooding and erosion problems have become increasingly acute over time as more and more people continue to occupy the floodplains immediately below the steep, unstable, and chaparral-clothed San Gabriel Mountains that surround Los Angeles and nearby cities. Fire is a frequent visitor in this area and wildfires have been estimated to denude these chaparral watersheds about every 25– 30 years (Biswell, 1974). Much of the mountainous terrain in southern California is administered by the USDA Forest Service. Part of the approach in managing these brush-clothed watersheds included obtaining a better understanding of how frequent wildfires affected the vegetation, soils, and hydrology of these areas. Although much was known about the vegetation (Horton, 1960) and hydrologic responses (Rowe et al., 1954) of these watersheds following fire, little was known about the specific effects fire had on soil properties other than that the loss of vegetation directly exposed the soil surface to raindrop impact.

194

L.F. DeBano

The reason for the decreased infiltration after fire was initially believed to result from the loss of protective plant cover during combustion and the plugging of soil pores by ashy residue remaining on the soil surface. The decrease in infiltration, however, was later found to be affected by a repellent layer formed during the fire. Various postfire treatments to revegetate and stabilize the soil were cooperatively evaluated by scientists assigned to the USDA Forest Service and the University of California during the late 1950s and early 1960s. One of the first studies tested the use of chemical treatments (soil stabilizers) to reduce postfire erosion (Krammes and Hellmers, 1963). During these soil investigations, it was concluded that soil wettability played an important role in postfire erosion (Osborn et al., 1964a) and that remedial chemical wetting treatment with wetting agents could potentially reduce postfire erosion (Osborn et al., 1964b). In addition to the studies on postfire remedial treatments, detailed research on the effect of fire on the soil resource was implemented. Through a series of both laboratory and field experiments, it was shown that water repellency on these erosive watersheds was created and intensified by the soil heating occurring during a fire (DeBano, 1966b; DeBano and Krammes, 1966). This soil condition dramatically reduced infiltration, created overland flow, and, as a result, accelerated erosion. This soil property had been overlooked in previous watershed investigations (Krammes and DeBano, 1965) because it was assumed that loss of cover and plugging of soil pores were the only processes responsible for postfire erosion.

3. Water repellency and fire After fire, water repellency is typically found as a discrete layer of variable thickness and spatial continuity found on the soil surface or a few centimeters below and parallel to the mineral soil surface. If found in mineral soil, water repellency is usually covered by a layer of severely burned soil or an ash layer. Creation of this water repellent layer was described as the “tin roof” effect by earlier watershed researchers.

A hypothesis describing the formation of a water repellent soil layer in soils was developed during the mid and late 1960s (DeBano, 1981). This hypothesis evolved as a product of numerous field observations, laboratory tests and field research studies. The results of preliminary field observations suggested that water repellency might well be an important factor responsible for the accelerated erosion experienced during the first few years following wildfires (Krammes and DeBano, 1965). An initial laboratory study showed that water repellency could be intensified by heating a soil – organic matter mixture in a muffle furnace at different temperatures for different lengths of time (DeBano and Krammes, 1966). It was hypothesized that a more efficient coating of mineral soil particles occurred at lower temperatures and for shorter periods of heating than in the case of longer periods of heating at higher temperatures that destroyed the organic substances responsible for the water repellency. Laboratory tests of changes in water repellency resulting from different times and temperatures of heating were combined with measured temperatures during prescribed fires and wildfires to develop the hypothesis describing how a water repellent layer is formed beneath the soil surface during a fire (DeBano, 1981; DeBano et al., 1998). According to this hypothesis, organic matter accumulates on the soil surface during intervals between fires (Fig. 1A). During these intervals, the upper soil horizons become water repellent due to the drying out of the mixture of partially decomposed organic matter and mineral soil. The addition of hydrophobic substances due to the leaching of decomposing plant parts on the soil surface may also contribute to the prefire water repellency. Fungal growth also is a dynamic source of hydrophobic substances, particularly in the organicrich upper soil horizons. The combination of combustion and heat transfer during wildfires produces steep temperature gradients in the surface layers of the mineral soil (Fig. 1B). During a fire, temperatures in the canopy of burning chaparral brush can reach over 11008C (Countryman, 1964). Temperatures can reach about 8508C at the soil – litter interface. But, temperatures at 5 cm in the mineral soil probably do not exceed 1508C because dry soil is a good insulator (DeBano et al., 1979). Heat produced by combustion of the litter layer on the soil surface vaporizes organic substances, which are then

The role of fire and soil heating on water repellency

195

Fig. 1. (A) Soil water repellency in unburned brush is found in the litter, duff, and mineral soil layers immediately beneath the shrub plants. (B) When fire burns, hydrophobic substances are vaporized, moving downward along temperature gradients. (C) After the fire has passed, a water repellent layer is present below and parallel to the soil surface on the burned area (adapted from DeBano, 1981).

moved downward in the soil along the steep temperature gradients until they reach the cooler underlying soil layers, where they condense. Incipient water repellency at different soil depths could also be intensified in place by heating, because organic particles are heated to the extent that they coat and are chemically bonded to mineral soil particles. Movement of hydrophobic substances downward in the soil occurs mainly during the fire. After fire has passed, the continued heat movement downward through the soil can re-volatilize some of the hydrophobic substances resulting in thickening the water repellent soil layer or fixing the hydrophobic substances in situ (Savage, 1974). The final result is a water repellent layer below and parallel to the soil surface on the burned area (Fig. 1C). The above described investigations also provided some general relationships between water repellency and soil temperature, which showed: (1) little change in water repellency occurs when soils are heated less than about 1758C (DeBano, 1981); (2) intense water repellency is formed when soils are heated between

175 and 2008C (DeBano, 1981; March et al., 1994); (3) destruction of water repellency occurs when soils are heated between 280 and 4008C (DeBano et al., 1976; Giovannini and Lucchesi, 1997; March et al., 1994; Savage, 1974). Further, the water repellent layer produced during fire can vary widely because of differences in fire and soil characteristics. Fire behavior, fire severity and temperature gradients developing in the soil during a fire, all affect the formation of a water repellent layer (DeBano et al., 1976; DeBano, 1981). Soil properties that affect water repellency include: amount and type of organic matter present (DeBano, 1981; Imeson et al., 1992; Doerr et al., 1998); soil texture (DeBano, 1981); soil water content (DeBano et al., 1976; Robichaud, 1996); and the general soil –plant environment.

4. Worldwide distribution The relationship between soil heating and water repellency was reported at the first international

196

L.F. DeBano

conference on water repellency (DeBano and Letey, 1969) in Riverside, California, and in earlier publications (DeBano, 1966b; DeBano and Krammes, 1966). As a result, the awareness of water repellency was heightened, and numerous reports in other wildland environments of the United States soon began appearing. During the two decades between 1960 and 1979, water repellency was reported in: ponderosa pine forests following fire in Arizona (Zwolinski, 1971; Campbell et al., 1977); mixed conifer forest in California (Agee, 1979); Arizona chaparral (Scholl, 1975); high elevation forests in the Cascades of Oregon (Dyrness, 1976); forest soils in upper Michigan (Reeder and Jurgensen, 1979); forested environments of the Sierra Nevada Range of Nevada and California (Hussain et al., 1969); the sagebrush type found in the Great Basin of USA (Salih et al., 1973); and several vegetation types throughout the western United States (DeBano, 1969a). Water repellent soils have also been reported in soils where large accumulations of fuels, such as logging residues, are burned (DeByle, 1973) and under camp fires (Fenn et al., 1976). Although most of the reports between 1960 and 1979 were from the United States, fire-induced water repellency was reported in New Zealand pumice soils (John, 1978), and in Japan (Nakaya et al., 1977a). The interest in the effect of fire-induced water repellency continued through the 1980s until the present time and in the USA it has been reported in: the Pacific Northwest (Boyer and Dell, 1980; McNabb et al., 1989), Idaho (Campbell and Morris, 1988), Nevada (Everett et al., 1995), and southern California and Arizona (Wells, 1982, 1987). Fire-induced water repellency was also continuing to capture the attention of scientists in other parts of the world, including: British Columbia (Henderson and Golding, 1983), southern Chile (Ellies, 1983), England (Mallik and Rahman, 1985), Italy (Giovannini and Lucchesi, 1983; Giovannini et al., 1983, 1987, 1988), South Africa (Scott, 1989), Turkey (Sengonul, 1984), Portugal (Walsh et al., 1994; Doerr et al., 1998), and Spain (Sevink et al., 1989; Almendros et al., 1990; March et al., 1994; Martinez-Fernandez and Diaz-Pereira, 1994; Molina et al., 1994). During the 1990s, detailed studies on fire-effects began addressing the effect of different fire intensities on soil heating (Valette et al., 1994) and water

repellency (Giovannini and Lucchesi, 1997). The effects of fire on overall soil quality (Giovannini et al., 1990; Giovannini, 1994) and aggregate stability (Molina et al., 1994) were also investigated. One study reported the relationship of soil hydrophobicity to depth and particle size in burned and unburned eucalyptus forests (Doerr et al., 1996). Interest was being focused on the spatial variability of water repellency (Doerr et al., 1998) and on the relationship between the spatial distribution of water repellency and the erosion potential produced during prescribed burning (Robichaud, 1996).

5. Chemistry of fire-induced water repellency in soils In the late 1960s and early 1970s, several studies were conducted in an attempt to identify the substances responsible for heat-induced water repellency (Savage et al., 1969b, 1972; Savage, 1974). The objective of this effort was to chemically characterize the hydrophobic substances causing water repellency so that chemical wetting agents could be specifically synthesized to more effectively counteract the extreme hydrophobic conditions produced during wildfires. Although non-ionic wetting agents were found to be effective treatments to reduce runoff and erosion in many cases, the rates and methods of application were being determined largely by trial and error when prescribing treatment for burned watersheds. Alterations of organic substances occur both during their volatilization and after they have condensed on mineral soil particles. The volatilized fractions released during the heating of organic matter from chaparral soils in California produced only a slight water repellency when added to non-repellent sand, but when this treated sand was heated to 3008C for 10 min it became highly water repellent (Savage et al., 1972). It was proposed that the substances moving from burning organic matter may have been produced by pyrolytic reaction rather than a simple volatilization of organic matter and that these substances were produced in the greatest quantities above 3508C (Savage et al., 1972). Although these pyrolytic substances themselves produced little water repellency, further fractionation produced


three fractions that were capable of causing water repellency in a wettable sand, particularly if they were heated for a few minutes at 200 –3008C. A detailed analysis of the three fractions producing water repellency showed that one was an aliphatic hydrocarbon that contained a large proportion of oxygen as carbonyl groups. From these experiments it was concluded that 50 – 95% of the substances moving from burning litter into sand were capable of causing water repellency (Savage, 1974). Humic and fulvic acids have been examined as possible sources of water repellency in both fire (Giovannini and Lucchesi, 1984; Almendros et al., 1988, 1990) and non-fire (Wallis and Horne, 1992) environments. Coordinated use of differential thermal analysis and infrared spectrophotometric techniques revealed that soil water repellency may be due to a fraction of the organic matter that had a low degree of humification and that was made up of a compound identified as an ester between phenolic acids and polysaccharide-like substances (Giovannini and Lucchesi, 1984). Another soil heating study showed that the oxygen-containing functional groups in organic matter were particularly sensitive to thermal treatment (Almendros et al., 1990). The overall changes detected in the humic acid fractions were used to develop a conceptual model, which showed that substantial amounts of humic acids were converted into alkali-insoluble substances that contributed to the soil humus fraction during natural fires. In summary, research on fire-induced water repellency has not revealed specific hydrophobic substances, nor have the precise changes occurring during heating been determined. This conclusion is not unexpected, however, because chemistry of the hydrophobic substances produced by heating of organic matter would be expected to be extremely complex due to the infinite number of organic compounds that can be acted upon by fire to produce organic substances responsible for fire-induced water repellency.

6. Effect on hydrologic processes and watershed responses The interest in infiltration, runoff, and erosion following wildfires developed simultaneously with

197

the effort directed toward understanding the mechanisms responsible for producing fire-induced water repellency. During the 1970s, consideration of postfire erosion resulting from fire-induced water repellency was restricted to a few erosional studies reported in the southwestern United States (Rice and Osborn, 1970; Cleveland, 1973; DeBano and Conrad, 1976). The interest in extending the principles of water repellency to erosion and hydrologic performance at a watershed level gained further worldwide attention during the 1980s and 1990s, with reports being published for: Australia (Topalidis 1984), Portugal (Shakesby et al., 1993; Walsh et al., 1994), Spain (Imeson et al., 1992; Diaz-Fierros et al., 1994), South Africa (Scott, 1989; 1993, 1997; Scott and Van Wyk, 1990; Scott and Schulze, 1992a), and the United States (Wells, 1981; 1987; Robichaud, 1996). During the course of the above studies, a general understanding of the effect of water repellency on individual hydrologic processes (e.g. infiltration, runoff and erosion) developed as a result of measurements that were taken on field study sites exposed to either natural or simulated rainfall. These studies were done on different sized areas that varied from small plots to large watersheds. 6.1. Hydrologic responses The hydrologic responses to water repellency most studied are: infiltration, runoff, rill formation, raindrop splash and streamflow parameters. The effect of a water repellent layer near, or at, the soil surface of burned watersheds has been fairly easy to model conceptually and test in the laboratory, but has proven extremely difficult to model physically under field environments because of the large temporal and spatial variability found under natural conditions on large-scale watersheds. The following discussion of individual hydrological processes first describes a conceptual framework; supplemental information obtained during laboratory and field experiments is then added to describe more closely the wildland environments. 6.1.1. Infiltration and runoff Some anomalies occur during infiltration into a water repellent soil during laboratory studies.

198

L.F. DeBano

One such anomaly is that the uptake of water during infiltration is slower at the beginning of infiltration and increases over time which is contrary to infiltration into a wettable soil where the converse is true (Letey et al., 1962b; DeBano, 1975). A second anomaly is that in water repellent soils, faster infiltration rates occur in moist soils compared to dry soils (Gilmour, 1968). This second anomaly arises because initial soil moisture content affects the initial severity of the water repellent condition and is related to the concept of “potential” and “actual” water repellency (Dekker and Ritsema, 1994b). The above relationship describes water flow when the soils are uniformly water repellent or wettable. However, when a soil profile contains a layer of water repellent soil beneath a thin wettable layer (as is often found on burned watersheds that contain a wettable ashy surface layer), the water repellent layer affects infiltration in much the same way as a coarse-textured layer would in a wettable soil profile. If the water repellent layer lies beneath a layer of wettable soil, the wetting front moves through the wettable layer rapidly until it reaches the water repellent layer, after which the infiltration rate drops to that of the water repellent soil. The infiltration rate remains depressed until the wetting front passes through the water repellent layer into the underlying wettable soil; then the rate begins to increase (DeBano, 1975). The depth to the water repellent layer also affects infiltration rates so that a layer near the surface is more effective in restricting infiltration than a deeper layer (Mansell, 1969). The idealized model of infiltration into a soil having a uniformly distributed water repellent layer, such as that described above, grossly oversimplifies field environments because of the large spatial and temporal heterogeneity of soil water repellency patterns and the soil surface microtopography. Studies on agricultural soils indicate that uneven microtopography of the soil surface and a heterogeneous spatial distribution of water repellency within the soil profile lead to a redistribution of surface water and concentrate water flow through the soil in discrete wettable soil fingers (Ritsema, 1998). The same differential flow undoubtedly occurs frequently in wildland soils because of the highly complex and variable spatial patterns found in natural environments.

6.1.2. Hillslope runoff and erosion Some debate still occurs about the importance of fire-induced hydrophobicity on runoff and erosion from small plots and watersheds. In a study on small hillside plots under eucalyptus forest in Australia, it was concluded that the fire-induced water repellency produced localized runoff and sediment movement only on hillslopes, but did not appreciably affect the performance of the entire watershed (Prosser and Williams, 1998). Another study of plots covered with 8-year-old scrub species in Spain showed that fire intensities affected erosion, and sediment delivery was 8 times greater on plots burned at high intensities than on unburned controls (Soto and Diaz-Fierros, 1998). Other plot studies suggested that the hydrologic responses to fire-induced water repellency depended upon soil dryness (Walsh et al., 1994). The increased runoff was attributed to an increase in the severity of water repellency at lower soil water contents during the dry season. 6.1.3. Watershed responses Predicting watershed responses by using information gained from conceptual models, laboratory studies, field observations and runoff and erosion data from small plots is extremely difficult because expanding these relationships to a watershed scale further increases the variability of these heterogeneous and highly complex natural systems. One useful technique for evaluating watershed responses to different treatments is to use paired watersheds with the control and treated watersheds having been calibrated against each for several years before and following a treatment (in this case, prescribed fire or wildfire). Reports of several studies done in South Africa illustrate how watershed level studies can be designed and the responses evaluated when studying watershed responses to fire-induced water repellency (Scott and Van Wyk, 1990; Scott and Schulze, 1992a; Scott, 1993, 1997). These studies involved coordinated measurements of streamflow response, sideslope erosion, and soil water repellency. The results of studies done on Pinus radiata watersheds that had been burned showed that during the first year following the fire, total streamflow, quick flow volumes, peak flow rates, and the watershed response ratio all increased as a result of the fire (Scott and Van Wyk, 1990). The second year responses were


somewhat less (Scott, 1997). Soil loss by overland flow from the plots during the first year following fire increased from 10 to 26 t/ha, and both suspended sediment and bedloads increased about four-fold following the fire. Wettability of the soils was decreased significantly and the most severe water repellency was found deeper in the soil. In a separate experiment in South Africa, the effect of prescribed fires and wildfires was studied on watersheds supporting indigenous native fynbos, P. radiata forest, and Eucalyptus fastigata forest (Scott, 1993). One of the two fynbos watersheds was prescribe-burned and the second burned by a wildfire during the wet season. Fynbos is a vegetation type found in South Africa that is dominated by sclerophyllous (evergreen and leathery-leafed) shrub species. Both of the forested watersheds were burned by an intense wildfire during the dry season. The forested watersheds experienced significant increases in storm-flows and soil loss. In contrast, the fynbos watersheds showed no change in storm-flow although annual flow increased 16% because of reductions in transpiration and interception. Water repellency measurements suggested that the storm-flow responses were partly generated by increased surface runoff into the stream channel that occurred as a result of reduced infiltration into water repellent soils on the hillslopes. A third study utilized a nested watershed design, supplemented with hillslope plots and water repellency measurements (Scott and Schulze, 1992a). This study was designed to evaluate the effects on stormflow and hillside erosion of a high-intensity wildfire that burned a eucalyptus forest. The fire markedly increased storm-flows and caused high soil losses from the hillslopes. The increased overland flow was linked to the widespread presence of water repellency. Measured soil losses of the hillslopes, however, were about five times that measured at the stream gaging stations because a healthy riparian area acted as an effective buffer, which trapped large amounts of eroded soil and ash. 6.2. Erosional processes Two important erosion processes occur following fire when water repellent soils are present—rill

199

formation and raindrop splash. Rill formation occurs when rainfall exceeds infiltration rates and surface runoff occurs. Soil material moved by rill erosion accumulates in channels at the base of steep slopes and remains there until increased stream-flow moves it downstream (Wells, 1987). The greatest movement of sediment occurs when rill formation is accompanied by sufficient sideslope runoff to move the debris stored in the channels. 6.2.1. Rill formation A striking feature on freshly burned watersheds during the first postfire rainstorms is an extensive rill network, which is related to water repellency (Wells, 1981, 1982, 1987). The sequence of rill formation follows several well-defined stages. First, the wettable soil surface layer is saturated during initial infiltration (Fig. 2A). The water infiltrates into the wettable surface until it encounters a water repellent layer (Wells, 1981). This process occurs uniformly over the landscape so that when the wetting front reaches the water repellent layer, it can neither drain downward nor laterally. As rainfall continues, water fills all available pore space until the wettable soil layer becomes saturated. Because pores cannot drain, pore pressures build up immediately above a water repellent layer. This increased pore pressure reduces intergranular stress among soil particles, and as a result, decreases shear strength in the soil mass and produces a failure zone at the boundary between the wettable and water repellent layers where pore pressures are greatest (Fig. 2B). Pore pressure continues to increase and shear strength decreases until it is exceeded by the shear stress of gravity acting on the soil mass. When this happens, a failure occurs and a portion of the wettable soil begins to slide downslope (Fig. 2C). If the soil is coarse textured, initial failure causes a re-orientation of the soil particles in the failure zone and causes them to momentarily lose contact with each other. The loss of intergranular contact further reduces shear strength and extends the failure zone downslope. When most of the soil grains lose contact, a condition develops in which the shearing soil is almost fluid. This fluid condition produces a miniature debris flow in the upper wettable soil layer, which propagates downslope to the bottom of the slope or until it empties into a channel.

200

L.F. DeBano

Fig. 2. Rill formation during rainstorms following fire involves: (A) saturation of the wettable soil surface; (B) a failure at the boundary between wettable and water repellent layers; (C) loss of the wettable surface layer, with the flow of water over the water repellent layer; (D) erosion of the water repellent layer; (E) erosion through the water repellent layer and infiltration into the underlying wettable soil; and (F) development of a well-defined rill (adapted from Wells, 1987).

Water in the wettable soil layer adjacent to the debris flow is no longer confined and can flow out into the rill formed by the debris flow and free-flowing water runs over and erodes into the water repellent layer (Fig. 2D). Flowing water confined to the rill still cannot infiltrate into the water repellent soil and, therefore, flows down the debris flow track as free water in an open channel (Wells, 1981). As the water flows down the track, turbulent flow develops, which erodes and entrains particles from the water repellent layer. The downward erosion of the water repellent rill occurs until eventually the flow cuts completely through the water repellent layer and begins infiltrating into the underlying wettable soil (Fig. 2E). Flow then diminishes, turbulence is reduced, and downcutting ceases. Finally the rill is stabilized immediately below the lower edge of the water repellent layer (Fig. 2F). The individual rills formed by the above process develop into a network that can extend the length of a small watershed. Observations of rills after

the first rainstorms on recently burned watersheds confirm that the downcutting of rills stops at the bottom of the water repellent layer (Wells 1987).

6.2.2. Raindrop splash Larger amounts of soil are moved by raindrop splash on hydrophobic soils compared to similar wettable sandy loam soils when they are exposed to different rainfall intensities, durations, and soil surface inclinations (Terry and Shakesby, 1993). Raindrop impact on hydrophobic soils produces fewer, slower-moving ejection droplets that carry more sediment to a shorter range than a wettable soil. Hydrologically, raindrop detachment is more effective on hydrophobic soils compared to wettable soils because soil surfaces having an affinity for water becomes sealed and compacted during a rainfall event which makes them increasingly resistant to splash


detachment. Conversely, hydrophobic soils remain dry, non-cohesive and easily displaced by splash when the raindrop breaks the surrounding water film.

7. Longevity of fire-induced water repellency The longevity of fire-induced water repellency depends on some of the same factors that affect its formation. Water repellency produced by low-tomoderate severity fires is usually of shorter duration than that produced by high severity fires. For example, water repellency produced by a low severity burn in late spring in the forests of southwestern Oregon began to allow infiltration at a nearly normal rate after the rains began to fall (McNabb et al., 1989). Dyrness (1976) found that wettability of soil on areas that burned at low severity recovered more rapidly than that of soils in severely burned areas. Wettability of soils on sites burned at either low or high severity approached that of an unburned soil by the sixth year after fire. Conversely, three years after passage of a fire on an experimental plot in Sardinia, subsurface layers (in which translocated hydrophobic matter had accumulated) showed these hydrophobic substances to be unaltered, but they were more strongly cemented because the translocated hydrophobic organic matter complexed with polyvalent ions (Giovannini et al., 1987).

8. Ameliorating fire-induced water repellency Tests on small plots that treated water repellency chemically with wetting agents appeared encouraging during the earlier studies on fire-induced water repellency that began in the early 1960s (Osborn et al., 1964a). Although the benefit – cost ratio of these early wetting agent treatments was favorable (Osborn et al., 1964b), the rapid increase in the prices of chemicals (particularly during the energy crisis in the 1970s) probably would have limited their use for wide-scale applications. Also, for unknown reasons, the treatment of entire watersheds on an operational scale was found to be unsuccessful (Rice and Osborn, 1970). Mechanical techniques used to break up the water repellent layer, such as discing or using a

201

“sheepsfoot” roller, are generally impractical when treating large steep landscapes that are burned during wildfires. The only practical solution to manage fire-induced water repellency on wildland areas appears to be the regular use of prescribed fire as part of a comprehensive fuels management program. To prevent hydrophobic conditions during prescribed burning, it is recommended that prescribed burning programs be implemented on a regular basis to minimize soil heating (Robichaud, 1996). Frequent burning would reduce the dead fuel loading on areas, allowing fire managers to conduct low severity prescribed burns that would produce less opportunity for creating heatinduced water repellency. The regular reduction of fuel loading would further reduce the risk of high severity wildfires occurring. Also, the prescribed burning could be scheduled when the moisture in the lower layers in the duff is high enough so that the fire does not consume the lower duff layers, which insulate the soil from surface heating.

9. Summary Much has been learned during the past three decades about a unique water repellent soil condition that is formed by wildfires. The severity of the water repellency depends on the combined interactions of soil properties and the soil heating regime developing during a fire. A hypothesis involving the volatilization and condensation of hydrophobic substances during soil heating has been developed by combining the results of laboratory experiments, field observations and controlled plot and watershed studies under field conditions. The precise chemical composition of the hydrophobic substances producing water repellency in soils has not been determined, perhaps due to the large number of organic compounds that can be altered by soil heating during a fire. The effect of the water repellent layer is manifested in several hydrologic processes, involving raindrop splash, rill formation and total watershed responses. The increased erosion by raindrop splash and rill formation has been well verified by controlled experiments. Watershed responses to water repellency are less clearly defined, but seem typically to include increases in quick flow and peak flow, larger watershed response ratios,

202

L.F. DeBano

and greater erosion and sedimentation rates. The longevity of the water repellent condition, if any, following cooler burning prescribed fires is less than a year, but water repellency can extend over several years if produced by a severe wildfire burning through large fuel accumulations during the dry season. Treatment of the postfire water repellency with wetting agents has not been successful on a watershed

scale, although small plot and laboratory studies have shown better infiltration and reduced runoff following treatment with non-ionic surfactants. The best overall management strategy to ameliorate this “hard-to-wet” soil condition seems to be an effective fuels management program which reduces fuel buildups and thereby minimizes the occurrence of severe wildfires, particularly during the dry seasons.

Chapter 19

Infiltration rates after prescribed fire in Northern Rocky Mountain forests P.R. Robichaud* USDA-Forest Service, Rocky Mountain Research Station, Forestry Science Laboratory, 1221 South Main Street, Moscow, ID 83843, USA

Abstract Infiltration rates in undisturbed forest environments are generally high. These high infiltration rates may be reduced when forest management activities such as timber harvesting and/or prescribed fires are used. Post-harvest residue burning is a common site preparation treatment used in the Northern Rocky Mountains, USA, to reduce forest fuels and to prepare sites for natural and artificial tree regeneration. Prescribed burn operations attempt to leave sites with the surface condition of a lowseverity burn. However, some of the areas often experience surface conditions associated with a high-severity burn which may result in hydrophobic or water repellent conditions. In this study, infiltration rates were measured after logging slash was broadcast burned from two prescribed burns. The two sites were in Northern Rocky coniferous forests of Douglas-fir/lodgepole pine and ponderosa pine/Douglas-fir. Simulated rainfall was applied to one-square meter plots in three, 30-min applications at 94 mm h21 within the three surface conditions found after the burn: unburned-undisturbed areas, low-severity burn areas and high-severity burn areas. Runoff hydrographs from the rainfall simulations were relatively constant from the plots that were in unburned-undisturbed areas and in areas subjected to a low-severity burn. These constant runoff rates indicate constant hydraulic conductivity values for these surface conditions even though there was variation between plots. Hydrographs from the rainfall simulation plots located within areas of high-severity burn indicate greater runoff rates than the plots in low-severity burn areas especially during the initial stages of the first rainfall event. These runoff rates decreased to a constant rate for the last 10 min of the event. These results indicate hydrophobic or water repellent soil conditions, which temporarily cause a 10– 40% reduction in hydraulic conductivity values when compared to a normal infiltrating soil condition. Since variability was high for these forest conditions, cumulative distribution algorithms of hydraulic conductivity provide a means to account for the inherent variability associated with these hillslopes and different surface conditions cause by fire.

1. Introduction Water infiltration is defined as the flow of water from the soil surface into the soil profile. The rate at which water is transmitted through soil is highly dependent upon the surface conditions. In forest

p

E-mail address: [email protected] (P.R. Robichaud)..

q 2003 Published by Elsevier Science B.V. All rights reserved.

environments, various surface conditions can exist and it is important to characterize these conditions and their effect on infiltration. Runoff from harvested and burned hillslopes varies from extensive to minor. The major determining factor is the amount of disturbance to the surface material which is usually organic debris (commonly referred to as duff or forest floor) that protects the underlying mineral soil. Disturbance may be from tree

204

P.R. Robichaud

harvesting operations, road building, or fire. All of these activities may impact the protective duff layer. Adverse effects on the duff layer by burning depend upon the severity of fire (Robichaud et al., 1993; Robichaud and Waldrop, 1994; Robichaud, 1996). Post-fire condition of the surface horizons are important because they determine the amount of mineral soil exposed to raindrop splash, overland flow and the development of water repellent soil conditions (DeBano, 1981). Observations from previous studies (Robichaud et al., 1993) suggest there are four different surface/hydrologic conditions to monitor which affect infiltration. These conditions are: (1) areas subjected to high-severity burns (possibly hydrophobic); (2) areas subjected to lowseverity burns; (3) areas with bare soil due to log dragging, log landings, skid trials, or roads; and (4) unburned-undisturbed areas. Numerous observations of water repellent soil conditions have been reported throughout the western USA and the world. Water repellency caused by wildfires has received the most attention in southern California chaparral (DeBano et al., 1967; DeBano and Rice, 1973), although it has been reported after forest wildfires (Megahan and Molitor, 1975; Dyrness, 1976; Campbell, 1977) and on rangelands (Richardson and Hole, 1978; Soto et al., 1994). In burned soils, severity of water repellency not only depends on soil texture, but is also related to fire intensity, antecedent soil-water content and fuel conditions (DeBano et al., 1976; Robichaud and Hungerford, 2000; Robichaud, 1996). Under field conditions, the water-repellent layer is usually not continuous, so irregular wetting patterns are common (Bond, 1964; Meeuwig, 1971a; DeBano, 1981; Dekker and Ritsema, 1995, 1996d). Water repellency induced by a low-to-moderate severity prescribed burn is usually of short duration. For example, in southwestern Oregon, soil wettability resulting from a late spring wildfire burn returned to near normal levels after the fall rains began (McNabb et al., 1989). After a late summer wildfire in the Oregon Cascade Mountains, Dyrness (1976) found that soil wettability in stands of lodgepole pine (Pinus contorta) experiencing burns of low-severity recovered more rapidly than soils experiencing burns of high-severity. By the

sixth year after the fire, wettability of the soils that experienced both low- and high-severity burns approached that of unburned soil. The most apparent hydrologic effect of hydrophobic soil conditions is the reduction of infiltration which can induce erosion by overland flow (DeBano et al., 1967). Infiltration curves reflect increasing wettability over time once the soil is placed in contact with water. Infiltration increases with time because the hydrophobic substances responsible for water repellency are slightly water soluble and slowly dissolve, thereby increasing wettability (DeBano, 1981). Researchers have documented persistence of hydrophobic conditions from weeks to years (DeBano et al., 1967; Holzhey, 1969a). In general, hydrophobicity is broken up, or is sufficiently washed away, within one to two years after a fire. The objective of this study was to determine infiltration characteristics of forest soils burned at different severities. These calculated hydraulic conductivity values provide important input parameters for use in current erosion prediction models that describe hydrologic responses for various surface conditions typically encountered in forest environments.

2. Methods 2.1. Field sites The first site, Slate Point (7 ha), was located on the West Fork Ranger District of the Bitterroot National Forest in western Montana, USA. This location has a Douglas-fir (Pseudotsuga menziesii)/lodgepole pine (Pinus contorta) forest. The habitat type is Douglasfir/twinflower (Linnaea borealis) (Pfister et al., 1977). Slopes within the study area range from 30 to 70% with a northern aspect. Elevation range from 1620 to 1780 m. The soils (83% sand, 12% silt, 5% clay with 33% gravel component) consist of a loamy skeletal mixed Typic Cryoboralf and a loamy skeletal mixed Dystric Cryochrept. Both were formed from weathered rhyolite. The second site, Hermada (9 ha), was located on the Idaho City Ranger District of the Boise National Forest in central Idaho, USA. This location has a ponderosa pine (Pinus ponderosa)/Douglas-fir forest.

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Infiltration rates after prescribed fire in Northern Rocky Mountain forests

The habitat type is Douglas-fir/ninebark (Physocarpus malvaceus) (Steele et al., 1981). Slopes within the study area range from 40 to 75% with northeasterly and southeasterly aspects. Elevations range from 1760 to 1880 m. The predominant soil (85% sand, 13%, 2% clay with 12% gravel component) is Typic Cryumbrept, loamy skeletal mixed derived from granitic parent material. 2.2. Field experiment Duff and fuel characteristics were measured with a geostatistical sampling scheme prior to each burn (Robichaud, 1996). The geostatistical sampling scheme used about three-quarters of the sampling points on a grid basis and the remaining sampling points were located close to the grid sampling points to obtain shorter distances between sampling points. To estimate duff thickness and duff reduction by the fire, eight steel pins (200 mm in length) were installed flush with the duff layer (forest floor surface) located in the corners and midpoints of an imaginary 1-m square centered at each sampling point. There were 20 sampling points at the Slate Point site and 30 at the Hermada site. The duff consumed during the fire was determined from the differences between the two surveys (pre- and post-burn measurement). After the spring burn at the Slate Point site, the area had a mosaic surface pattern indicating variable fire severity. Selected fire behavior parameters are provided in Table 1. This mosaic pattern gave a variety of surface conditions from white ashy (complete combustion) to blackened appearance with minimal destruction of the duff layer, indicating a moderate to light ground char fire as described by Ryan and Noste (1983), or a low- to high-severity burn as described by Phillips and Abercrombie (1987). The fall fire did not burn as expected at the Hermada site. The southern aspect was dryer than the northern aspect but fuel loadings and duff thickness were very variable spatially, thus making it more difficult to carry the fire. After burning the Hermada site, small areas appeared ashy white, whereas the majority of the burn area had a black appearance indicating light ground char (Ryan and Noste, 1983) or low-severity burn (Phillips and Abercrombie, 1987). Surface conditions after the burn were classified on type and severity of disturbance.

Table 1 Selected fire behavior parameters from the Slate Point and the Hermada prescribed fires Measurement Litter temp. (8C) Duff temp. (8C) Mineral soil surface temp. (8C) 3 mm below mineral soil interface (8C) 22 mm below mineral soil interface (8C) Lower duff moisture content (%) Upper duff moisture content (%) Fine fuel moisture content (%) Flame length (m) Fireline intensityd (kW m21) Ambient temperature (8C) Wind speed (km h21) Wind direction Relative humidity (%) a b c d

Slate Point

Hermada

633– 837 69– 612 n.a.a 38

429–915 187–217 119–187 n.a.

30

37–112

72b 42 9 2–6 1160–12,600 23 8–13 N 22

39c 71 18 1–3 260–2800 12 0–8 SE 36

n.a. indicates data not available. N ¼ 20 at the Slate Point site. N ¼ 30 at the Hermada site. Fireline intensity is calculated as: 258 £ flame length2.17.

The four surface conditions were unburned-undisturbed, burns of a low-severity (65 –100% ground cover remaining and a duff thickness between 5 and 20 mm), burns of high-severity (0– 65% ground cover remaining and a duff thickness less than 5 mm) and skid trails (high disturbance) areas (Robichaud et al., 1993; Robichaud, 1996) (Fig. 1). Skid trails were not used in this analysis. 2.3. Rainfall simulation Rainfall simulation plots were located randomly in each surface/hydrologic condition area several days after the burn. Fourteen rainfall simulation plots were located at Slate Point site and 11 at the Hermada site (Table 2). Adjustments to plot locations were made for access to water supply and electrical power. Because of fiscal and logistical constraints, rainfall simulation could not take place at each geostatistical sampling location. Since variability within each surface condition was high, as many repetitions as possible were completed as permitted by time and weather. At the Hermada site, most of the area

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P.R. Robichaud

Fig. 1. Ground cover amounts and duff thickness used to classify areas as low- and high-severity burns (Robichaud, 1996).

was subjected to a low-severity burn, efforts were made to locate a few plots in areas subjected to highseverity burn to be able to determine the effects of the different surface conditions. Simulated rainfall events were applied to 1 m2 plots with the USDA-Forest Service oscillating nozzle

rainfall simulator. These plots were bordered by 150 mm wide sheet metal inserted vertically 50 mm into the mineral soil. The simulator produced a mean rainfall intensity of 94 mm h21 (SD ¼ 5.5 mm h21). Each plot received three 30-min rainfall events. Event 1 (dry) was conducted with existing soil moisture

Table 2 Surface conditions, areas, total runoff and hydraulic conductivity values for the Slate Point and Hermada sites Surface condition

Slate Point Unburned-undisturbed Low severity burn High severity burn Non-hydrophobic Hydrophobic Hermada Unburned-undisturbed Low severity burn High severity burn Non-hydrophobic Hydrophobic a

Total area (%)

20 65 15

40 55 5

No. of plots

Total runoff (mm)

Hydraulic conductivity (mm h21)

Event 1

Event 2

Event 3

Kfora

2 8 4 2 4c

4 15 12

4 9 14

4 8 11

77–81 60–89

3 3 5 2 3

15 24 26

Khydrobib

30–84 23 –55 17 24 22

16 22 20

36–62 10–63 22–74 15 –40

Hydraulic conductivity values fitted from the rainfall simulation hydrographs during rain event 3. Hydraulic conductivity for hydrophobic soil conditions fitted from the rainfall simulation hydrographs during the first 10 min of Event 1. c Two of the four plots that were hydrophobic were located in areas subjected to the low-severity burns. All others were in areas subjected to high-severity burns. b


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condition. After Event 1, the plots were covered with plastic sheeting and Event 2 (wet) was conducted the following day. Event 3 (very wet) was conducted about 30 min after Event 2. This procedure provided three distinct antecedent moisture conditions. A covered trough at the lower end of each plot carried runoff (water and sediment) through an outlet tube for timed volume samples, collected manually in 500 ml bottles. These data were used to develop hydrographs, total runoff volumes and sediment yields (Robichaud, 1996).

categories using the Kolmogorov – Smimov onesample test for goodness-of-fit (McCuen and Snyder, 1986; StatSoft, 1995). This tests the null hypothesis that the cumulative distribution of a variable agrees with the cumulative distribution of some specified probability function at specified a-levels.

2.4. Analysis methods

Ignition techniques, fuel moisture and weather during the Slate Point burn produced an intense fire concentrated in the center of the unit, whereas the Hermada burn produced a low intensity fire (Table 1). Maximum temperatures within the duff were 69 – 6128C lasting 3 –8 min at the Slate Point site, whereas at the Hermada site maximum temperatures were only 119– 1878C in the duff. Spatially varied surface conditions occurred after both prescribed burns. Duff depths averaged 47 mm prior to the fire and 19 mm following the burn at the Slate Point site. Duff depths averaged 36 mm prior to the burn and 29 mm following the burn at the Hermada site. At the Hermada site, the harvest unit did not burn well due to high moisture conditions (71%) of the upper duff, high humidity (36%) and higher fine fuel moisture content (18%) (Table 1). The fires created mosaic patterns of duff consumption and some unburned areas. These spatial patterns are described in detail in Robichaud (1996). The burn sites were divided into three surface conditions: unburned, low severity and high severity surface conditions for rainfall simulation plot locations. The areas subjected to a low-severity burn retained 65– 100% of its original ground cover. The area subjected to a high-severity burn retained 0– 65% of its original ground cover (Fig. 1). At the Slate Pont site, approximately 65% of the area was subjected to a low-severity burn and 15% of the area was subjected to high-severity burn at the top of the slope, where the heat generated during the fire consumed most of the duff layer. Whereas at the Hermada site, approximately 55% of the area was subjected to a lowseverity burn and only 5% of the area was subjected to a high-severity burn which occurred on a southern aspect drainage depression (Table 2).

Hydrographs show the temporal variation in runoff rate (mm h21) collected at the outlet of the 1 m2 plot for three 30-min rainfall events. Runoff amounts can be calculated by the integration of the hydrograph. These hydrographs were used to calculate hydraulic conductivity values by the methods of Luce and Cundy (1994) which determine parameter values for kinematic wave-Philip’s infiltration overland flow equation from the runoff hydrographs. The best fit equation minimizes the error between the observed and synthetic hydrographs by an iterative process of adjusting the values for the sorptivity, conductivity and time to ponding under constant rainfall rate and duration, plot slope and size, and moisture contents. Inputs to saturated hydraulic conductivity (Kfor) values were fitted from the very wet events when the soil was saturated, since these estimates are more reliable than from the dry or wet events. However, hydraulic conductivity values were also estimated near the beginning of the first rainfall event for determining the hydrophobic hydraulic conductivity (Khydrobi) when the hydrograph had a peaked shape (Figs. 2c,d and 3). When this occurred, the synthetic hydrograph was fitted to the peaked portion of the runoff hydrograph to estimating Khydrobi. Mean hydraulic conductivity values between the surface conditions were compared by the least significant difference (LSD) at a ¼ 0:05 (StatSoft, 1995). Probability distribution functions were also used to define probabilities of occurrence for values of hydraulic conductivity for each site. Best-fit distribution algorithms were determined by testing various distribution functions (normal, gamma and exponential with an a ¼ 0:05) and various number of

3. Results and discussion 3.1. Fire descriptions

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P.R. Robichaud

Fig. 2. Hydrographs from the Slate Point site that were within: (a) areas that were unburned-undisturbed; (b) areas subjected to a low-severity burn; (c) areas subjected to a high-severity burn with a slight hydrophobic response; and (d) areas subjected to a high-severity burn with a hydrophobic response.

3.2. Slate Point On the unburned-undisturbed areas, runoff was minimal and constant (4 mm for each event) (Table 2 and Fig. 2a). This low runoff rate resulted because the protective layer of duff (100% ground cover) covering the mineral soil remained intact. The intact duff layer protects the mineral soil from both overland flow and raindrop impact, thereby preventing erosion and

increasing infiltration. The duff provided detention storage by allowing water to be released slowly into the underlying mineral soil resulting in high hydraulic conductivity values (77 – 81 mm h21). The duff material also acted as a lateral flow path for water moving downslope. An example of a hydrograph from a low-severity burn area indicates a relatively constant runoff rate for all three 30-min rain events (Fig. 2b). Hydraulic

Fig. 3. Hydrographs from the Hermada site that were within areas that were subjected a high-severity burn with a hydrophobic response.


conductivity was calculated as 72 mm h21 during Event 3. Total runoff collected, calculated from the area under the hydrograph, were 14, 12 and 12 mm for each successive rainfall event. In contrast, a hydrograph from areas subjected to a high-severity burn indicate high runoff rates during Event 1, decreasing to a constant rate for the last 10 min of each event (Fig. 2c). The shape of a second hydrograph indicates a hydrophobic soil condition was present because runoff decreases with time. Another hydrograph from the same site and surface condition indicates a similar hydrophobic response with a greater magnitude of runoff during the initial portion of the simulated rainfall event and the final runoff rate (Fig. 2d). 3.3. Hermada Portions of the Hermada site provided another example of a hydrophobic response to simulated rainfall with runoff decreasing with each successive rain event (Fig. 3). Runoff quickly reaches 67 mm h21 and then drops to 30 mm h21 at the end of Event 3. Hydraulic conductivity was estimated at 62 mm h21 at the end of Event 3. At the onset of rain, the hydrophobic hydraulic conductivity was estimated at 35 mm h21. Thus we can see how hydrophobic conditions vary as the soil profile becomes wetted and eventually responds as a normal infiltrating soil. Normal infiltration theory indicates that downward infiltration in an initially unsaturated soil generally occurs under the combined influence of suction and gravity gradients. As the water penetrates deeper and the wetted part of the profile lengthens, the average suction gradient decreases, since the overall difference in the pressure head divides itself along an ever-increasing distance. This trend continues until eventually the suction gradient in the upper part of the profile becomes negligible, leaving the constant gravitational gradient as the only force moving water downward. Since the gravitational head gradient has the value of unity (the gravitational head decreasing at the rate of 1 mm with each millimeter of vertical depth below the surface), it follows that the flux tends to approach the hydraulic conductivity as a limiting value (Hillel, 1982).

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3.4. Hydrophobic response When analyzing a hydrograph such as in Figs. 2c,d and 3, hydraulic conductivity was determined from Event 3, where runoff and infiltration are fairly constant. Data from the beginning of Event 1 represents the hydrophobic hydraulic conductivity. The difference between initial (hydrophobic) hydraulic conductivity and the final hydraulic conductivity when hydrophobic conditions were present. The hydrophobic hydraulic conductivity values were 10– 40% of normal saturated hydraulic conductivity. Only 4 out of 14 plots indicated hydrophobicity from Slate Point, and 3 out of 11 plots from Hermada thus indicating that hydrophobic conditions were not extensive especially since only 5% of the total area at the Hermada site was subjected to a high-severity burn. Since hydrophobic substances are water soluble, they can be broken down and destroyed with water, as evident by the declining hydrographs during the third rain event (Figs. 2c,d and 3). The timing or persistence of the hydrophobicity was not measured in this experiment, i.e. repeated rainfall simulation over weeks or months on the same plots was not performed. Researchers have documented persistence from weeks to years. In general, the hydrophobicity is broken up or is sufficiently washed away within one to two years after the fire. 3.5. Cumulative distribution of hydraulic conductivity Hydraulic conductivity varies within each surface condition (Table 2) and the means were not significantly different by LSD method at a ¼ 0:05: For example at the Slate Point site, there was little variation for the unburned-undisturbed hydraulic conductivity; a range of 60 – 89 mm h21 for the surface conditions with a low-severity burn; and a range of 30 –84 mm h21 for the surface conditions with a high-severity burn. Therefore, best-fit cumulative distribution algorithms were used to describe the range of hydraulic conductivity by using all measured hydraulic conductivity for a given field site excluding hydrophobic response conditions (Figs. 4 and 5). Cumulative distribution algorithms combined with spatial distribution (Robichaud and Monroe, 1997) provide methods for estimates of runoff and erosion from spatially-varied forest conditions.

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P.R. Robichaud

Fig. 4. Cumulative distribution function for hydraulic conductivity at the Slate Point site. Kolmogorov–Smimov d ¼ 0:27 at the a ¼ 0:05 level.

This agrees with the finding of Smith and Hebbert (1979), Moore and Clarke (1981) and Hawkins and Cundy (1987) that a single value for hydraulic conductivity for a site is not appropriate for forest conditions. Hydraulic conductivity values at the Hermada site had larger variations for all treatments and means were significantly smaller by the LSD method when compared with Slate Point (Table 2; Figs. 4 and 5). The differences were due to larger post-fire site variation since much of the site did not burn well as previously described. Overall

lower values are probably due to some surface crusting and sealing which have been reported for these soil types. This thin crust can be developed by the beating action of the raindrops, or as a result of the spontaneous slaking and breakdown of the soil aggregates during wetting (Hillel, 1982). This was common on south aspects which have thinner duff. Thus, a single cumulative distribution algorithm for each site should provide reasonable estimates of hydraulic conductivity. Water drop penetration times, WDPT, (DeBano, 1981) were measured during this study using

Fig. 5. Cumulative distribution function for hydraulic conductivity at the Hermada site. Kolmogorov–Smimov d ¼ 0:16 at the a ¼ 0:05 level.


the geostatistical design described in Robichaud (1996). These results showed greater repellency in areas subjected to high-severity burns (Robichaud, 1996). Since WDPT were not measured prior to each rainfall simulation, no relations can be made on the expected reduction in infiltration throughout the site based on WDPT. Thus, the reduction in infiltration described here needs additional field evaluation to determine in spatial distribution. 4. Conclusions Variable surface conditions are common in forest environments especially after prescribed fires. Smallscale rainfall simulation techniques provide a reliable method to determine hydraulic conductivity for these various surface conditions. Two prescribed burns were conducted and both produced variable infiltration rates related to burn severity. When hydrophobic conditions were present, marked changes in the runoff

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hydrographs over time allowed for the determination a hydrophobic hydraulic conductivity. When hydrophobic conditions occurred after a high-severity burn, the saturated hydraulic conductivity was reduced between 10 and 40% during the onset of simulated rainfall, thus Khydrobi ¼ 0:1 – 0:4Ksat : These hydrophobic hydraulic conductivity values recovered to near saturated hydraulic conductivity values by the third simulated rainfall event for all plots. In a forest environment, hydraulic conductivity varies by surface condition which is a function of the type and severity of disturbance. Within each surface condition there is also variability. Cumulative distribution algorithms provide a means to account for the inherent variability associated with these hillslopes and surface conditions. Cumulative distribution algorithms and spatial distributions of hydraulic conductivity should be used with erosion prediction models to predict surface runoff and erosion from forest environments.


PHYSICS AND MODELING ON WATER REPELLENT SOILS


Chapter 20 Physics of hydrophobic soils T.W.J. Bautersa, T.S. Steenhuisa,*, D.A. DiCarlob, J.L. Nieberc, L.W. Dekkerd, C.J. Ritsemad, J.-Y. Parlangea and R. Haverkampe a

Department of Agricultural and Biological Engineering, Cornell University, Ithaca, NY 14853, USA b Department of Petroleum Engineering, Stanford University, Stanford, CA 94035, USA c Department of Biosystems and Agricultural Engineering, University of Minnesota, St Paul, MN 55108, USA d Alterra, Green World Research, P.O. Box 47, 6700 AA Wageningen, The Netherlands e Laboratoire d’Etude des Transferts en Hydrologie et Environnement, BP 53-38041, Grenoble Cedex 9, France

Abstract Although it is generally well known that water repellent soils have distinct preferential flow patterns, the physics of this phenomenon is not well understood. In this chapter, we show that water repellency affects the soil water contact angle and this, in turn, has a distinct effect on the constitutive relationships during imbibing. Using these constitutive relationships, unstable flow theory developed for coarse grained soils can be used to predict the shape and water content distribution for water repellent soils. A practical result of this paper is that with a basic experimental setup, we can characterize the imbibing front behavior by measuring the water entry pressure and the imbibing soil characteristic curve from the same heat treated soil.

1. Introduction Soils which have hydrophobic properties (also called water repellent soils) can resist or retard surface water infiltration (Brandt, 1969a). Besides the retardation or resistance of surface water infiltration, water repellent soils have been associated with preferential flow (Jamison, 1945; Bond, 1964; Gilmour, 1968; Nissen et al., 1999). Preferential flow paths create spatial variability in soil moisture affecting plant growth (Dekker and Ritsema, 1994b). In addition, preferential flow allows much faster transport of water and solutes, therefore creating a greater risk of groundwater contamination. It is important to predict water distribution and flow processes in water repellent soils and to understand how porous media * Corresponding author. Tel.: þ1-607-255-2489; fax: þ 1-607255-4080. E-mail address: [email protected] (T.S. Steenhuis). q 2003 Elsevier Science B.V. All rights reserved.

theory developed for hydrophilic soils applies to hydrophobic soils. This paper focuses on the physical interpretation of the infiltration experiments in hydrophilic and hydrophobic sands with the same textural composition, and on the effect of hydrophobicity on the water – energy relationships and resulting wetting front pattern. The main difference between a hydrophilic and hydrophobic soil is the shape of the wetting front. Infiltrating water in a hydrophobic soil forms an unstable front with fingers. In hydrophilic soils, water can infiltrate as a flat horizontal stable Richards’ typewetting front. According to Milly (1988), the wetting front in a hydrophobic soil is unconditionally stable when Richards’ equation is used without hysteresis in the soil moisture characteristic curve (Nieber et al., 2000). This paper is divided into several parts. In the first part, two sets of infiltration experiments using similar sands with different hydrophobicities are described

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T.W.J. Bauters et al.

and the results presented. In the discussion, the effect of water repellency on the constitutive relationships is discussed using the results of the experimental sets. Then, the wetting front behavior as a function of water repellency is discussed and, finally, a method for measuring water repellency is presented.

2. Materials and methods 2.1. Laboratory experiments Two sets of infiltration experiments with water repellent sands were carried out in the Cornell High Energy Synchrotron Source (CHESS). The procedures and partial results of experimental set I were described earlier (Bauters et al., 1998) and are reanalyzed here. Experimental set II has recently been carried out and involves infiltration in chambers smaller than the diameter of the finger. In experimental set I, hydrophilic sand was made hydrophobic by adding no (or 0), 3.1, 5, 5.7, and 9% by weight of extremely water repellent octadecyltrichlorosilane (OTS) sand. OTS sand was prepared by mixing sand with an ethanol solution containing 48 g/l OTS. Water repellencies of the mixtures were obtained that ranged from wettable to extremely water repellent. Infiltrations were carried out and imbibing and drainage curves of the constitutive relationships were measured in a polycarbonate

chamber with interior dimensions of 45 cm wide, 57.5 cm tall, and 0.8 cm thick with 1 cm walls. The soil characteristic drainage curves were obtained by filling the chamber from the bottom with water. The water was turned off when the water level in the chamber reached the surface and the chamber was allowed to drain. After 24 h, the front panel of the chamber was removed and the sand at the right and left side of the chamber was segmented into 1 cm levels and moisture contents were determined by oven drying. The soil characteristic imbibition curves were also determined in the chamber; a constant head of 10 cm was connected to the bottom of the chamber and left connected for 24 h until equilibrium was reached. The chamber was again taken apart to section the sand and to determine the water saturation. In a separate experiment, the surface tension of the water was measured. More information is given in Bauters et al. (1998). In experimental set II, water was infiltrated into a 3.1 cm square, 85 cm long polycarbonate chamber. Two naturally water repellent sands were used: Ouddorp sand in which Dekker and Ritsema (1994b) and Ritsema et al. (1998a) noted fingered flow, and water repellent golf greens sand used as the surface layer in golf greens. Corresponding hydrophilic sands were made by “burning off” the organic matter by heating the soil to 6008C for 6 h. For all experiments, the sand was added to the chamber by pouring it continuously through a number of random-

Table 1 Infiltration experiment Type of sand

Treatment

Infiltration fluid (surface tension, dynes/cm)

Drainage water (surface tension, dynes/cm)

Wetting potential (cm)

Front moisture content (cm3/cm3)

Porosity

Golf greens

Heat treated Natural Natural (surfactant)

70.01 71.37 35.63

69.91 60.72 46.04

217.0 3.0 23.5

0.30 0.34 0.31

0.38 0.38 0.38

Ouddorp 4–8 cm depth

Heat treated Natural

65.10 65.10

62.41 51.28

224.0 NA

0.33 NA

0.44 NA



63.40 69.51

53.43 53.33

217.0 9.0

0.31 0.41

0.41 0.41



60.62 61.51

50.01 53.33

219.0 6.5

0.32 0.40

0.40 0.40

Physics of hydrophobic soils

Fig. 1. Experimental set-up, all measurements are in cm.

ized screens. Distilled water (and in one case a surfactant solution consisting of 1% surfactant solution containing Primer 604w made by Aquatrols) was added at a rate of 10 cm3 min21 through a hypodermic needle located near the sand surface. Table 1 gives the details of the set of infiltration experiments. Matric potentials were measured with four fast responding miniature tensiometers (Selker et al., 1992b) positioned flush with the wall, 10 cm apart and starting 22 cm from the top (Fig. 1). Moisture content and soil density were measured with high intensity X-rays tuned to a fundamental energy of 40.7 keV provided by the A-2 beam line at the CHESS. Most of the details of this setup are discussed in DiCarlo et al. (1997) and Bauters et al. (2000a). The only differences were that a Si(111) crystal was used instead of a Si(220) crystal for tuning the energy, and that the energies were recorded with xenon ion chambers which were more sensitive at high energies than the argon chambers employed by DiCarlo et al. (1997). The chamber was mounted on a movable x, y platform so that measurements could be taken at any position within the chamber. Both stationary and transect data were taken to document the moisture content. Stationary data were collected at the same height as the second tensiometer, 50 cm above the bottom of the chamber. The transect data Q

Fig. 2. Moisture content and matric potential at 15 cm depth for an infiltration experiment for a sand used in constructing golf course: (a) heat treated sand; (b) naturally water repellent sand; and (c) infiltration with Primer 604w (Aquatrols).

217

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were taken at a 1 cm interval from the bottom of the chamber till 65 cm height, positioned in the middle of the chamber. The drainage water was collected in 50 ml beakers and the surface tensions were measured with a Fisher Surface Tensiomat Model 21 where a platinum –iridium ring was suspended in the fluid to measure the apparent surface tension.

3. Results Infiltration in the partly hydrophobic sand in experimental set I resulted in unstable fingered flow,

while the hydrophilic sand (with the same textural composition) had the typical flat stable Richards’ type wetting front. For the water repellent soil, the finger tip was saturated with a matric potential equal to the water entry value. Both moisture content and matric potential decreased behind the tip. The wetting front in the hydrophilic soil was unsaturated and the pressure increased slightly behind the front (Bauters et al., 1998). Those patterns were used in the validation of the finite element simulation by Nieber et al. (2000). Because of the artificial nature of our “constructed” sands, we used naturally repellent sands in the next set of experiments. We also used chambers

Fig. 3. Moisture content and matric potential for the Ouddorp sand: (a) 20 –30 cm depth heat treated; (b) 20–30 cm depth naturally water repellent; (c) 60–70 cm depth heat treated; and (d) 60– 70 cm depth naturally water repellent.


that were smaller than the narrowest finger measured. Thus, for either a stable Richards’ imbibing front or an unstable imbibing front, a flat imbibing front was obtained and, thereby, the compounding influence of the imbibing front shape on the moisture content and matric potential was avoided. Experimental set II, with the 85 cm long and 3.1 cm square chamber, showed that the heat treated non water repellent golf greens sand had the typical stable Richards’ type imbibing front behavior (Fig. 2a). The moisture content at the imbibing front was 0.30 cm3/cm3, which is less than the saturated moisture content (0.38 cm3/cm3). The matric potential at the imbibing front was 2 17 cm. Both matric potential and moisture content increased slightly behind the imbibing front and is characteristic for stable Richards’ type imbibing fronts (Fig. 2a). The infiltration pattern for the “natural water repellent” golf greens sand was typical for unstable flow in water repellent sand despite that no finger formed and the water filled the whole chamber. Fig. 2b shows that the matric potentials were slightly positive at the imbibing front (3 cm) and then decreased behind the finger tip. The tip was saturated but the moisture content did not decrease behind the tip because, as is discussed later, the chamber was too short. When surfactants were added to the infiltrating water, the imbibing front had both hydrophobic and hydrophilic characteristics (Fig. 2c). The moisture content was 0.3 cm3/cm3 and the matric potential was 2 3.5 cm (Table 1). For the Ouddorp sand, the same striking differences were observed between the water repellent and hydrophilic sands as for the golf greens sand with the exception for the severely water repellent sand taken from the 4 –8 cm depth, where the water could not infiltrate even after 22 cm of water was ponded on top. This was surprising, as in the field the water infiltrated through this layer (Dekker, 1998). The water content and matric potential for the water repellent and heat treated soil for the 20 –30 and 60– 70 cm depths are shown in Fig. 3. The pattern is similar to the golf greens sand: In the partly hydrophobic soil, the moisture content was near saturation and the matric potential decreased behind the front (Fig. 3b and d); In the hydrophilic heat treated soil, the water infiltrated at moisture contents less than saturation and the matric potential slightly increased behind the imbibing front (Figs. 3a and c).

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4. Discussion The physical properties of water repellent soils (soil water characteristic curves and unsaturated conductivity) will be described first, followed by a discussion of how these properties relate to the wetting front patterns. The results from experimental sets I and II are used for illustrative purposes. 4.1. Soil water characteristic curves The matric potential, cm, in a pore with radius, r, is affected by the surface tension, s, the contact angle, a, and the specific weight of water, rw s ð1Þ cm ¼ 2 rw gR r ð2Þ R¼ cos a where R is the radius of the curvature of the water air meniscus, and g is the gravitational acceleration (Marshall et al., 1996). Surface tension is influenced by heat treatment and surfactant solution. Table 1 shows that in the golf greens sand the surface tension of the drainage water for the hydrophilic heat treated soil is higher than that for the partly hydrophobic “natural soil”. Interestingly, the surface tension of the drainage water for the natural soil, infiltrated with surfactant, increases. The surface tension of the drainage water for the Ouddorp sand is generally lower than the originally infiltrating water. Hydrophobicity also affects the matric potential through the contact angle (Eq. (2)). For contact angles less than 908, water infiltrates under a negative pressure. Small pores fill up first, followed by successively larger pores. For contact angles greater than 908, the matric potential of the infiltrating water in dry soils becomes positive. Now the big pores fill up first followed by the smaller pores. When the contact angle is 908, all the pores will fill up simultaneously. Fig. 4a shows a meniscus in a capillary tube with an uniform contact angle of 1358. In real soils, the surface is not uniformly covered with the hydrophobic material and there are individual small organic particles in the soil that contribute to the water repellency (Bisdom et al., 1993). To portray schematically the effect of pores with different

220


Fig. 4. Shape of meniscus between two plates: (a) uniform contact angle of 1358; and (b) one plate is hydrophobic ða ¼ 1808Þ and the other hydrophilic ða ¼ 08Þ:

contact angles, the meniscus between two plates, one hydrophilic ða ¼ 08Þ; and the other hydrophobic ða ¼ 1808Þ; is drawn in Fig. 4b. The water – air surfaces will be much more complex in soils and we will use an effective contact angle that has the “same surface energy as the real meniscus”. Obviously, the effective contact angle in Fig. 4b is 908, indicating that two glass plates can be replaced by a medium of which the average contact angle is 908 and the meniscus is without curvature. Using the principle of effective contact angle, the wetting soil water characteristic curves from experimental set I are scaled (Miller and Miller, 1956) (Fig. 5). One difficulty is that the contact angles in soils cannot be measured independently. Therefore, one point of each imbibing curve is used to calculate the apparent contact angle and we then investigate how the other points on the curve scale with that contact angle. The best identifiable point is the water entry value. This is the point where the largest pores fill up with water (Jury et al., 1991). For apparent contact angles smaller than 908 (i.e. where water

Fig. 5. (a) Predicted (lines) and observed (symbols) for the wetting curves of the same sands but with different degrees of water repellency (Bauters et al., 2000a); and (b) drying curves.

infiltrates at negative pressures), the water entry value is identified by “the knee” in the imbibing soil characteristic curve near saturation. For water repellent soils ða . 908Þ where the large pores fill up before the smaller pores, the corresponding water entry value is “the knee” near air dry. The apparent contact angles can be found as: cos ai ¼

s r c iw cos ar s i cwr

ð3Þ

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Physics of hydrophobic soils Table 2 The water drop penetration time (WDPT) for the different batchesa Water repellency (%)

Water entry (cm)

Apparent contact angle (8)

WDPT (s)

Description

0 3.1 5 5.7 9.0

27.5 23 0 1 4

0 67 90 98 122

,0.5 40 2400 .3600 .3600

Wettable (,5 sec) Slightly water repellent Severely water repellent Extremely water repellent Extremely water repellent

a The WDPT test consists of randomly applying a single water drop (0.05 ml) onto the sand surface and measuring the amount of time (in seconds) it takes to infiltrate the soil (Letey, 1969; King, 1981).

where the superscript r refers to the reference soil and the superscript i to the particular soil. The hydrophilic soil is used as the reference soil and assumes that the contact angle a ¼ 08: This assumption is not a requirement but makes the discussion easier. The “water entry values” and the calculated apparent contact angles with Eq. (3) for the sand with different degrees of water repellencies are given in Table 2. The wetting curve for the 5% OTS sand is intriguing as it straddles the zero pressure line and an average contact angle of 908 was taken. The imbibing curves for the partly hydrophobic soils can be derived from the hydrophilic soils by scaling with the contact angles from Table 2, as: For a , 908 :

ci ¼

s i cos ai r c ; s r cos ar

ui ¼ ur

ð4Þ

u i ¼ us 2 u r

ð5Þ

and for a . 908 :

ci ¼

s i cos ai r c ; s r cos ar

The calculated and observed imbibing curves are shown in Fig. 5a. The imbibing curves are generally well predicted except for the 9% OTS sand. A possible reason is that for the 9% OTS sand the capillary rise experiments might not be a good way to measure the imbibing loop of the soil characteristic curve, because of difficulty for water to enter the smallest pores. Once water repellent soils are fully wet, the hydrophobicity disappears and, thus, the drainage curves should be the same. Fig. 5b shows that for experimental set I this is, in general, the case except for the 0% OTS sand which has a greater air entry value than the water repellent soils. A possible reason

for the lower air entry value for the water repellent soils might be caused by the limited pressures during imbibing, which could result in a not fully imbibed medium, thus leaving the finer pores hydrophobic, which explains the lower air entry value. The artificially prepared sands of experimental set I had only a small portion of the water repellent grains. In many regards, they are similar to naturally occurring sands where there is only a relatively small amount of hydrophobic material (Bisdom et al., 1993). The golf greens sand of experimental set II was prepared from taking hydrophilic sand and mixing it with less than 0.5% sterilized organic matter. It is intriguing how such a small quantity of water repellent particles can affect the soil water behavior. To explain this, we note that in uniform sands each grain is surrounded by 10 – 12 grains (Hillel, 1980). Thus, for the 3.1% OTS sand, at most, 37% of the pore space between grains are affected by water repellent material. But, more significantly, more than half of the pore spaces are not affected by any water repellency and the water can move through these pore spaces easily, provided that they are connected. This was demonstrated by the water infiltration in the 3.1% OTS sand where the finger actually meanders, likely finding the least repellent soil. For the 5.7% OTS sand, up to 60% of the pore space is affected and for the 9% OTS sand, all the pore spaces would have at least one water repellent particle making the effective contact angle greater than 908. Table 2 shows that, indeed, the water entry value is þ 1 cm. The sand in experimental set I was not uniform. In Fig. 6, a hypothetical heterogenous sand was constructed. Approximately 3% of the grains (colored in solid black) are water repellent. It is obvious that

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Fig. 6. Porous media with 3% of the grains (in solid) which are water repellent.

many passages are affected and only 20 to 30% of the pore spaces in the top section are still available for water to flow unhampered. If the water repellent particles are doubled to 6% (the additional grains are hatched) in some horizontal cross sections all pore spaces have one or two grains that are water repellent. In order for the infiltration front to pass these layers, the pressure needs to be positive. Once a pore is filled it can conduct water easily. 4.2. Unsaturated conductivity It is generally assumed that the hydraulic conductivity is only a function of the moisture content

(Jury et al., 1991; Hillel, 1980). As long as the contact angle is less than 908, this is a reasonable assumption. However, when the contact angle becomes greater than 908, the large pores fill with water before the fine pores as for the more wettable soils. Thus, for soils with contact angles larger than 908, we expect to see a higher conductivity at low moisture contents than for the wettable soils. DiCarlo et al. (1999b) showed the conductivity increased nearly linearly with moisture when the contact angle was affected by oil in the medium. Thus, for water repellent soils, there is hysteresis in the soil conductivity curve because different pores are filled for the same moisture content during imbibing and drying.

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4.3. Stability of the imbibing front The stability of the wetting front for different repellencies can be understood heuristically as follows. For density-driven displacements, the front is unstable whenever the flux, q, is less than the soil’s unsaturated conductivity, K(u ) (Parlange and Hill, 1976): q , KðuÞ:

ð6Þ

Results of experimental sets I and II show that, for water repellent soils, directly behind the wetting front the saturation is very high. This leads to a high conductivity and the instability criterion is satisfied for infiltration rates less than the saturated conductivity. Note that in hydrophilic soils the moisture content adjusts itself so that q ¼ KðuÞ: 4.4. Moisture content and matric potential at the imbibing front What is not obvious when unstable fingering occurs, is why the tip of the finger in dry soil is always saturated. The difference in moisture content and pressure between the unstable and stable Richards’ imbibing fronts can only be explained when the pressure at the imbibing front varies slightly due to inhomogeneities or uneven distribution of water repellent particles. To illustrate this, let us assume that the moisture content at the imbibing front for the two different soils is the same as observed for the 0% OTS sand (i.e. 40% saturation). The matric potential for the 0% OTS sand is, then, 2 10.5 cm (D, Fig.7) and for the 3.1% OTS sand it is 2 4.5 cm (A, Fig. 7). When the matric potential hypothetically increases by 2 cm, the moisture content and matric potential relationship follows the main imbibing curve and the moisture content for the 0% OTS sand becomes 70% saturated (E, Fig. 7) and for the 3.1% OTS sand the moisture content becomes 100% saturated (B, Fig. 7). A smaller increase in matric potential will have the same effect but cannot be illustrated as well in Fig. 7. If the pressure is now decreased, a drying loop will be followed (Fig. 7). For the 3.1% sand, this means that the soil will remain saturated (C, Fig. 7) and for the 0% OTS sand, a secondary drying curve is followed and the moisture

Fig. 7. Relationship between matric potential and moisture content when the matric potential is first increased, and then decreased by 2 cm. This hypothetical perturbation is depicted for a hydrophilic (0% OTS soil) and hydrophobic soil (3.1% OTS soil).

content will become approximately 65% saturated (F, Fig. 7). Repeated changes in pressure at the wetting front will follow the same secondary drying loop (Liu et al., 1995). Thus, we have shown that a small change in pressure together with hysteresis in the constitutive relationships, is responsible for the saturated moisture content of an unstable imbibing front. 4.5. Moisture content behind the finger tip Selker et al. (1992c) noticed that the velocity, v, of the finger was constant. This was confirmed for water repellent soils by Bauters et al. (1998). Based on this constant velocity, v, the moisture content inside the finger can be expressed as u ðz 2 vtÞ: Since the matric potential and K(u ) are only dependent on u, we can define h ¼ z 2 vt; and show that (Selker et al., 1996; DiCarlo et al., 1999a):

dc nuw ¼ Kðuw Þ 1 þ dh

ð7Þ

Eq. (7) can be integrated to give the moisture content and matric potential behind the finger tip. Of interest is to find the length of the saturated tip, L. At a particular time, Eq. (7) can be rewritten for the saturated finger tip and, after rearranging, gives

224


the length, L, as: L¼

cw 2 ca nu 12 f Kf

ð8Þ

where the subscripts w and a refer to water and air, respectively. For water repellent soils, there is a large difference between the air entry value and the water entry value (Fig. 5a and b) compared to typical coarse grained soils, and we expect, therefore, long saturated tips as was observed for experimental sets I and II (Figs. 2 and 3). 4.6. Practical application We have shown that the water entry value can be used to scale the wetting branches for a water repellent soil from the same soil that is heat treated. This suggests that a method to characterize the wetting front behavior may consist of measuring the water entry pressure and the imbibing soil characteristic curve of the same heat treated soil in which all

the organic matter has been removed. These measurements are not difficult and require a segmented column, a Marriot bottle, a drying oven and a scale.

5. Conclusion Two sets of experiments were carried out with similar sands that had different degrees of water repellency. Flat Richards’ type of wetting fronts became unstable and formed fingers when the repellency increased. We found that soil physics theory developed for hydrophilic soils is valid for water repellent soils provided that the contact angle effect is included. Water repellency had a direct and predictable effect on the constitutive relationship during imbibing. This in turn affected the shape of the wetting front. A laboratory method is proposed for examining the instability of the front by measuring the air water entry value and the constitutive relationship of the particular soil after removing the organic matter.

Chapter 21

Solute transport through a hydrophobic soil B.E. Clothiera,*, I. Vogelera and G.N. Magesanb a

Environment and Risk Management Group, HortResearch, PB 11-030, Palmerston North, New Zealand b Manaaki Whenua—Landcare Research, PB 3127, Hamilton, New Zealand

Abstract Unsaturated infiltration into the Ramiha silt loam, an Andic Dystrochrept, follows the classic pattern. A rapid drop-off from a high flow rate, seemingly induced by capillary attraction, appears followed by an apparent steady-flow maintained by gravity at around 0.5 mm s21. Beyond 100 min, however, the infiltration rate climbs nearly linearly to exceed 4 mm s21 as the soil’s water repellency breaks down. This is only evident after a period that might exceed the observer’s attention span. The hydrophobicity in this case could be due to one, or a combination, of the many unusual characteristics of this soil—its low bulk density (0.8 Mg m23), its strongly aggregated nature, the presence of mycorrhizal fungi, its high organic matter content (16.5%), or the presence of allophanic clay (4%). Our measurements of infiltration into undisturbed cores of Ramiha silt loam were made with disc permeameters set at the unsaturated pressure head of h0 ¼ 240 mm: The permeameters contained a solution of electrolytic tracer (KBr) so that we could observe solute transport in this soil. Vertical three-wire rods for Time Domain Reflectometry (TDR) measurement were inserted directly through the base plate of the permeameter so that we could continuously monitor the soil’s changing water content and resident concentration of electrolyte. The TDR measurements revealed the transient behaviour of fingered preferential flow into this soil during the breakdown of hydrophobicity. At the conclusion of the experiment, the soil cores were sectioned to permit measurement of the profiles in the resident concentration of the invading chemical. Near the surface, at the conclusion of the experiment, the resident concentration of bromide was found to be exactly that of the invading solution. So, despite the initial water repellency of the soil, the infiltrating bromide solution was subsequently able to invade the entire pore space—once the hydrophobicity had dissipated. Classic theory would then seem capable of describing solute transport after the effects of water repellency had faded.

1. Introduction The Water Drop Penetration Test (WDPT) relies on the gradual breakdown of hydrophobicity to register a measure of the soil’s water repellency (Letey, 1969). However, little attention has been directed towards understanding the mechanisms and consequences that result from the loss of ephemeral hydrophobicity which can follow the wetting of the * Corresponding author. Tel.: þ64-6-356-8080; fax: þ 64-6-3546731. E-mail address: [email protected] (B.E. Clothier). q 2003 Elsevier Science B.V. All rights reserved.

soil surface (DeBano, 1969b; John, 1978). Thus there has probably been a diminution in the reliance of the WDPT as a criterion of repellency. Other more dynamic measures of hydrophobicity such as the Molarity of an Ethanol Droplet (MED) (King, 1981), or the Repellency Index (RI) that relies on measuring the intrinsic sorptivity (Tillman et al., 1989), would seem better able to describe the degree of hydrophobicity. Our interest here is to describe the transient behaviour of infiltration into an ephemerally hydrophobic soil, and to describe its consequences upon solute transport.

226

B.E. Clothier et al.

We did not set out initially to study water repellency. Rather we were interested, at first, in determining the relative roles of interpedal convection and intra-aggregate diffusion on solute transport and exchange in a strongly aggregated soil. Thus, an experimental requirement was to have sufficient waterborne tracer enter the soil so that we could better discriminate between the various transport and exchange process. So our measurements of infiltration were maintained until a cumulative infiltration of I < 80 mm had entered the soil. This often required observations up to 10 h, well exceeding that which might often be considered the attention span of an operator intent on measuring the soil’s hydraulic properties. It was through these extended observations that we came to record the breakdown of hydrophobicity. Our experimental technique relied on a disc permeameter, set at an unsaturated head and containing a cocktail of inert and reactive tracers, to record infiltration into the soil. A threewire TDR probe (Vogeler et al., 1996) inserted through the permeameter allowed us to observe water infiltration and electrolytic tracer penetration into the soil during the breakdown of hydrophobicity. Interpretations of certain of the results of these experiments in terms of determination of the exchange isotherm of reactive chemical have already been published (Clothier et al., 1996). Here we present new results relating to the temporal changes in the measured infiltration rate as the repellency breaks down, and we interpret the TDR traces in relation to the temporal pattern of water entry into, and inert solute transport through this ephemerally hydrophobic soil.

2. Materials and methods The soil on which we carried out our field tests, and with which we conducted laboratory experiments on undisturbed cores, is the Ramiha silt loam—an Andic Dystrochrept. The pedology of this soil has already been detailed by Parfitt et al. (1984). Clothier et al. (1996) described the set-up of the laboratory experiments we will use here, although they analysed them in terms of the transport and exchange of reactive 35S transport. Here we only

provide details that are salient to hydrophobicity and the transport of solute during the breakdown of repellency. The soil of loessial origin has a bulk density 0.82 Mg m23, and a strong nut structure in the A horizon. The smeary consistency suggests that allophane is present, and indicates that the soil is an admixture of loess and tephra. Allophane comprises 4% by weight of the soil in the top 700 mm. A 20,000 yr BP ash band occurs in the soil at a depth 1 m to record the rate of loessial accretion. The organic carbon content of the top soil is 16.5%. When a clod is broken, the surface soil breaks into 5 – 10 mm diameter spherical aggregates, many of which are found to be coated with the white hyphae of a mycorrhizal fungus. Infiltration experiments were carried out both in the field, and on undisturbed cores taken to the laboratory, using a disc permeameter set at the unsaturated head h0 of 2 40 mm. The permeameters used in the field were of disc diameter 200 mm, whereas those atop the undisturbed cores were 97.5 mm in diameter. In both cases, a 2 –10 mm depth of fine sand ensured good hydraulic contact between the permeameter and the surface of the soil that had been gently shaved flat at a depth of about 50 mm below the soil surface, after the pasture thatch and roots had been removed. The description of the procedure for the collection of the four undisturbed cores, and their set-up in the laboratory is described elsewhere (Clothier et al., 1996). The reservoir of the permeameters contained a solution of 0.1 M KBr to permit transient measurement of electrolyte invasion by TDR, and also so that a terminal sampling in the soil would reveal the final pattern of solute transport. The vertical arrangement of the three-wire TDR probe of length L ¼ 125 mm, directly through the base of the permeameter, was described by Vogeler et al. (1996). They also outlined the procedures we used here to interpret the TDR traces in terms of soil water content u, and the soil solution concentration of bromide ions. In their experiments with repacked Ramiha silt loam, they used this set-up to examine the exchange of invading Kþ ions with resident Caþ, however here we seek to observe the consequences of the breakdown of hydrophobicity in undisturbed cores on the transport through them of inert bromide tracer. At the end of the experiment all four columns

Solute transport through a hydrophobic soil

were sectioned, and five small cores of about 1 g were extracted in a row across the diameter of the column at depths of 5, 15, 25, 45 and 75 mm. These samples were individually analysed for the Br2 concentration using a bromide-specific electrode. The field soil, both at the time of sampling and column exhumation, had a gravimetrically measured volumetric water content un of around 0.5 m3 m23. From gravimetric sampling under all the permeameter experiments, at h0 ¼ 240 mm on the undisturbed cores, Clothier et al. (1996) found the soil to wet to u0 ¼ uðh0 Þ ¼ 0:62 ^ 0:04 m3 m23 : From 10 samples taken from the column immediately under the disc with the TDR rods through the permeameter (core 4), it was found that u0 ¼ 0:643 ^ 0:03 m3 m23 : The Repellency Index of the soil, i.e. the ratio of the water sorptivity Sw at h0 ¼ 240 mm to the ethanol sorptivity Se at the same h0, was also determined (Tillman et al., 1989; Wallis and Horne, 1992). The Sw was taken from the early-time infiltration of water into two of the undisturbed cores described above. Two smaller, undisturbed cores of radius 70 mm were used for Se. Glass permeameters of radius 68 mm were used to supply an 85% solution of ethanol to the cores so that the time-course of the cumulative infiltration I(t) could be monitored. Often the sorptivity is found by plotting the initial I(t 1/2) infiltration data, for at early times, the two-term Philip (1957b) infiltration equation I ¼ St1=2 þ Kt

ð1Þ

is dominated by the first term, and so the sorptivity S is found as the slope of the plot. Where soils are extremely repellent, since Sw ! 0; use of this equation becomes problematic. Nonetheless, if it were found that Sw < 0 then the Repellency Index would, through division by zero, indicate a very hydrophobic soil! So it is critical that good measures of the sorptivity be obtained. As Smiles and Knight (1976) showed, more robust estimates of the terms in the Philip two-term infiltration equation can be obtained when It 21/2 is plotted against t 1/2. The Philip (1957b) expression is therefore It21=2 ¼ S þ Kt1=2

ð2Þ

227

and a better estimate of S is found from the intercept of It 21/2(t 1/2). We used Eq. (2) to determine Sw and Se at h0 ¼ 240 mm:

3. Results and discussion Water repellency is known to determine the pattern of water entry into soil (Dekker, 1998), and thereby control the processes of flow and transport (Ritsema, 1998). Our interest here is to examine the breakdown of hydrophobicity in an ephemerally repellent soil, and to determine the consequences of this on flow and transport mechanisms. 3.1. Infiltration If the attention span of the operator of a disc permeameter were only 100 min or so, then they would observe a classic pattern of infiltration into the Ramiha silt loam (Fig. 1(a), inset). Both in the field, and with the two cores in the laboratory, at this unsaturated head of h0 of 2 40 mm, the infiltration rate i drops from what seems to be a capillarityinduced high of around 3– 5 mm s21, to an apparently steady-state flow of about 0.5 –0.7 mm s21. Care was taken to exclude transient effects due to the initial wetting of the thin layer of contact sand. So, after 2 h of work, such an operator would feel confident with the values of S and K that they might have derived from these I(t) observations. An operator with a keen eye might have been perplexed as to why such an open, nut-structured silt loam had apparently returned a hydraulic conductivity measurement in the range of just 1 –2 mm h21. Anyway, our primary interest was to decipher the relative transport roles of convection and diffusion in moving solute through this aggregated soil. Thus we required that more than just this I of 5 mm of solution enter the soil. So we left the permeameters on the soil for much longer. Soon after 100 min, a dramatic rise in the infiltration rate commenced. (Fig. 1(a)). After some 9 – 10 h, the infiltration rate is around 4– 5 mm s21, indicating that the apparent conductivity that might have been inferred, from the inset after 100 min, is an order of magnitude too low.

228


Fig. 1. (a) The time course of unsaturated infiltration rate i(t) from a disc permeameter ðho ¼ 240 mmÞ into two undisturbed cores of Ramiha silt loam, and a field site. The inset provides an expansion of the initial 100 min and reveals an apparent steady-state flow rate. (b) The infiltration data of Fig. 1(a) are here plotted to show the instantaneous rate of infiltration i against the cumulative infiltration I up to that time.

Appropriately, now the Ramiha silt loam can be found to have a conductivity at h0 ¼ 240 mm of around 20 – 30 mm h21. These infiltration rate data clearly record the breakdown of ephemeral hydrophobicity in this soil. Such data would seem rare, maybe because few operators had the incentive to extend their period of observation.

In Fig. 1(b), the i(t) of Fig. 1(a) are replotted as i(I) so that it can be seen that the breakdown of hydrophobicity occurred after just 5 mm of solution had infiltrated. For future reference, it is worth noting that the presence of TDR rods through the base of the permeameter on core 4 made no difference to the recorded pattern of infiltration. This is not surprising, for all the experiments were conducted at


229

Fig. 1 (continued )

the unsaturated head of 2 40 mm, such that preferential flow, or enhanced wetting created by the rods, would be unlikely. 3.2. Repellency index From the short-time data in the inset of Fig. 1(a), a plot of It 21/2(t 1/2) provides an Sw of 24 mm s21/2 (Fig. 2, Eq. (2)), a value that is very low for such a well-structured, medium-textured soil with u0 2 un < 0:15: The short-time, infiltration data obtained with the ethanol-filled, glass permeameters are also shown in Fig. 2. Use of Eq. (2) provides an Se of 0.48 mm s21/2. As Tillman et al. (1989) noted, for a wettable soil the ratio of Sw/Se should be 1.95, due primarily to greater surface tension of water. Should the measured ratio be lower than this, then there is dynamic evidence of some water repellency. Wallis and Horne (1992) thus defined the Repellency Index (RI) as being 1.95 divided by the measured Sw/Se, so that values of RI greater than unity indicate

hydrophobicity. Moderately repellent soils tend to have RIs between 20 and 40, and severely repellent soils have been found to have RIs of up to 80. Here our soil has an RI of 40, placing it at the bottom end of the class of severely repellent soils. However, here we have observed that after about 100 min, or some 5 mm of infiltration, this severe repellency breaks down, and the soil eventually becomes hydrophilic. Now we will explore the pattern of soil wetting during this breakdown, and its consequences on the solute transport of the bromide tracer. 3.3. Transient wetting With the arrangement of the TDR rods through the permeameter, we are able to measure continuously the average water content uðtÞ over the length of the rods ðL ¼ 125 mmÞ during infiltration I(t). Since at any time t we know how much water has entered the soil from the permeameter, viz. I, our contemporaneous

230


Fig. 2. The short-time, unsaturated infiltration data from the inset of Fig. 1(a) are here plotted as I=t1=2 against the square root of elapsed time t1=2 so that the intercept provides the sorptivity of water, Sw (Eq. (2)). Results from unsaturated infiltration experiments using ethanol in a glass sorptivity tube are used to provide the sorptivity of ethanol, Se.

TDR measure of u can be used to infer something about the shape of the profile-of-wetting u(x), with the depth being x. This permits answers to key questions. Is the shape of the profile fully rectangular, as might happen with a Green-Ampt soil that possesses a Dirac-d diffusivity function that is highly non-linear (Philip, 1969)? Or, is the profile of wetting much flatter, with there being just a gradual wetting behind the wet-front xf as might characterise a soil whose diffusivity is only weakly dependent upon u (Philip, 1969)? Here, for simplicity, we consider two synthetic profiles of wetting u(x) so that we can easily interpret our uðIÞ data to infer which pattern of wetting we must be observing with our combined permeameter-TDR infiltration device. We choose to do this simply by considering two analytical profiles that would delimit the bounds of likely behaviour for the wetting of soil during infiltration—a profile of rectangular wetting, and one where the profile is always a triangle (Fig. 3). Wetting of most soils would lie somewhere between these extremes (Clothier and White, 1982).

Fig. 3. Synthetic profiles of water content in soil, in which (a) the profile of wetting corresponds to a rectangle as would happen for a soil with a Dirac d-function diffusivity relationship, and (b) the wetting provides a triangular profile. The simple analytical expressions for these synthetic profiles provide a straightforward relationship between the cumulative infiltration I and the mean water content u measured by TDR over rod lengths L (Eqs. (6) and (10)).

231


3.4. Rectangular wetting

soil that wets according to Fig. 3(b)

For infiltration into the highly non-linear soil of Fig. 3(a), the profile of wetting can be written as u0 x # x f uðxÞ ¼ ð3Þ u x . x

IðtÞ un þ L uðtÞ ¼ 2 ð u u 2 0 2 un Þ L 0 4IðtÞ

n

f

so that the cumulative infiltration is given by ð xf ½uðxÞ 2 un dx ¼ xf ðu0 2 un Þ I¼

ð4Þ

0

In this case, TDR rods of length L will measure an average water content of u given by u0 xf þ un ðL 2 xf Þ xf # L L u¼ ð5Þ u0 xf . L

By elimination of xf from Eqs. (4) and (5), we obtain the expression for u½IðtÞ that would be observed for a soil that wets according to Fig. 3(a) u þ IðtÞ I # Lðu 2 u Þ 0 n n L uðtÞ ¼ ð6Þ u0 I . Lðu0 2 un Þ 3.5. Triangular wetting For infiltration into the weakly non-linear soil of Fig. 3(b), the profile of wetting can be written as un þ ð u0 2 un Þ 1 2 x x # xf xf u¼ ð7Þ u x . x n f

so that the cumulative infiltration is given by I¼

1 x ð u 2 un Þ 2 f 0

ð8Þ

In this triangular case, the TDR rods of length L will measure an average water content of u given by un þ xf ðu0 2 un Þ xf # L 2L u¼ ð9Þ 1 L x . L u u u u þ þ ð 2 Þ 1 2 f n 0 n 2 0 xf By elimination of xf from Eqs. (8) and (9), we obtain the expression for u½IðtÞ that would be observed for a

I # Lðu0 2 un Þ=2 ð10Þ I . Lðu0 2 un Þ=2

Eqs. (6) and (10) bracket the likely profiles of wetting that are possible. So via inverse interpretation of our measured uðIÞ; or uðtÞ; we can infer the profile in the changing pattern of wetting within the soil during the breakdown of hydrophobicity. The measured uðtÞ data for core 4 are presented in Fig. 4, along with the predictions of Eqs. (6) and (10). The measured rise in uðtÞ over the first 200 min is far greater than that predicted by either Eqs. (6) or (10)! It should not be physically possible, given our reliance on mass balance (Eq. (4)), to observe more water in the soil, than that which could have infiltrated. Instead, the TDR probes must be measuring a pocket of preferential wetting that is due to the local presence of a wetted finger around one, or more, of the three TDR rods. Given that infiltration is at an unsaturated head of 2 40 mm, it is unlikely that this finger had been created by disturbance of the soil. Rather it is likely that these TDR rods are recording the local passage of a wetted finger, a phenomenon found typical of infiltration into hydrophobic soils (Ritsema, 1998). There is initially a rapid rise in the TDR-measured water content as the rods ðL ¼ 125 mmÞ measure the passage of a ‘finger’. After about 100 min the measured rate of wetting slows. At around 165 min, water was noted to be freely dripping out of the base of the core at x ¼ 250 mm: The wetted finger must have reached the bottom of the core, and the pressure potential there eventually risen to zero. Now flow is being driven by a steady hydraulic-head gradient of 0.84 ( ¼ 210/250). At this time (Fig. 1(a)), the flow rate was just over 1 mm s21, whereas by the end of the experiment, the flow was around 4.5 mm s21. So over the period 165 –600 min, the breakdown of hydrophobicity has seen the hydraulic conductivity K0 (i.e. K½h0 ¼ 240 mmÞ rise nearly five-fold. The pattern of wetting within the soil during this period can be seen to possess a slope that is better predicted by Eq. (10), suggesting that the breakdown of hydrophobicity is achieved with a flat, triangular like profile

232


Fig. 4. The time course in the TDR measured water content in the soil (X) in relation to that predicted by the expression that assumes a rectangular profile (…, Eq. (6)), and by that which considers wetting to be in the form of a triangle (- - -, Eq. (10)).

of wetting. However, between 165 – 500 min, the increase in u is quite small (< 0.04), relative to massive increase in K0, so to account for this it would seem that there must also have been an acceleration of water flow inside the wetted fingers, as well. Maybe this is in response to a widening of the wetted fingers as the hydrophobicity breaks down (Ritsema et al., 1998a). In addition, this observation could be explained by the later arrival of slower-moving fingers. The rise in the soil’s volumetric water content during the breakdown of hydrophobicity appears gradual, and characterised by a triangular profile of wetting away from the proximal surface. However, during this period, the soil’s hydraulic conductivity changed dramatically. Now we examine the consequences of this hydraulic behaviour on the transport of the bromide tracer. 3.6. Solute transport Following the procedure developed by Vogeler et al. (1996) for the Ramiha silt loam, our contemporaneous TDR measurements of the bulk electrical conductivity and the soil’s volumetric water

content, can be used to infer the soil solution concentration of bromide averaged over the length of the rods. The results of this analysis are shown in Fig. 5 and there can be seen a nearly linear rise in the measured resident concentration of bromide throughout the experiment with core 4. If there were a piston-displacement invasion of solute tracer, then we would expect the bromide front xs to be at I/u0. Over the length of this experiment, I < 80 mm (Fig. 1(b)), and u0 ¼ 0:643; so we would expect for non-preferential flow that xs be 125 mm— i.e. the length of the TDR rods. Thus for such a uniform case, dispersion aside, we would expect the TDR-measured resident concentration, at the end of the experiment, to be the influent concentration of 0.1 M. However, as can be seen in Fig. 5, at the end of the experiment, we only record a resident concentration of 0.06 M, such that through highly dispersive processes of solute transport, we are ‘missing’ some 40% of the applied solute. Such data might, at first glance, suggest that the Ramiha is a mobile– immobile water soil (Clothier et al., 1998), with there being a small mobile domain, so that the linear rise in the resident concentration of bromide in Fig. 5 reflects interdomain diffusion.


233

Fig. 5. The time course in the TDR-inferred concentration of bromide in the soil solution.

The resident Br2 data measured at the end of the experiment (Clothier et al., 1998) are presented here in Fig. 6, for they reveal that at the proximal surface the resident concentration of bromide had finally risen to exactly the influent concentration. Therefore, at the end of the experiment, after hydrophobicity had abated, the invading solute was travelling through the entire pore space. At this stage, at this depth, there was no preferential flow. Clothier et al. (1998) considered that solute transport in this geometrically complicated and hydraulically complex soil, could be simply described using a fully mobile version of the convection – dispersion equation, as long as a very large dispersivity were used to smear the profile of solute invasion. A dispersivity l of the order of 20– 30 mm, would generate a flat profile shape such as that in Fig. 6, and would corroborate the linear rise in the temporal pattern of solute invasion Fig. 5. In Fig. 6, a piston invasion-front of bromide would be at 125 mm, however the dispersed nose we have observed in the solute concentration profile has lead to some 40% of the bromide preferentially moving beyond this xs. This supports our observation in Fig. 5. It is this

smearing Fig. 6, and the linear rise in the resident concentration Fig. 5 that a l < 20 mm would mimic. The dispersion process would describe the sum effect of many mechanisms; the nature of the aggregates, the breakdown of the hydrophobicity, the gradual wetting of the soil, and the acceleration of flow within the widening preferential fingers of wetting, and the slower penetration of some fraction of the fingers. Whereas the hydraulic functioning of this soil during the breakdown of ephemeral hydrophobicity appears to defy simple description in terms of classical soil physics (Fig. 1), solute transport would appear capable of prediction using the standard convection – dispersion equation with the wetted domain being fully mobile. However, an unusually large dispersivity is needed to account for the effect of soil structure and patterns of preferential flow (Magesan et al., 1995). 4. Conclusion Observation of unsaturated infiltration in the Ramiha silt loam beyond 100 min revealed a breakdown in hydrophobicity that saw the effective

234


Fig. 6. The measured depthwise profile in the resident concentration of bromide in the soil solution obtained by sampling at the end of the experiments. If the profile of bromide invasion were rectangular in shape, a mass-conservative calculation would place the tracer front at a depth of around z ¼ 125 mm (i.e. I/uo). Data from four cores have been combined here, and for each core a row of five samples were taken at each depth. Thus every data point comprises 20 samples, and the shaded bands provide the standard deviation.

hydraulic conductivity of the soil rise nearly fivefold. Possible reasons for this soil being ephemerally repellent are its strongly aggregated structure, the presence of mycorrhizal fungi, a high allophane content, or its elevated level of organic matter. The breakdown began some 100 min after the soil was first wetted, and was still not complete after 10 h. So whether or not this soil would exhibit repellency in the field would depend on the intensity and duration of rainstorms.

Despite being unable to provide a rational physical prediction of the process of ephemeral hydrophobicity, it was found possible to describe its impact on solute transport using the classical convective – dispersive approach. The entire wetted pore space could be considered mobile, although a large dispersivity was needed to predict the smeared profile of solute invasion caused by the multitude of preferential flow processes.

Chapter 22

Effects of water repellency on infiltration rate and flow instability Z. Wanga,*, Q.J. Wua,1, L. Wua, C.J. Ritsemab, L.W. Dekkerb and J. Feyenc a

Department of Environmental Sciences, University of California, Riverside, CA 92521, USA b Alterra, Green World Research, P.O. Box 47, 6700 AA Wageningen, The Netherlands c Institute for Land and Water Management, Katholieke Universiteit Leuven, Vital Decosterstraat 102, 3000 Leuven, Belgium

Abstract Laboratory infiltration experiments were carried out to quantify the effects of soil water-repellency on infiltration rate and the wetting front instability. A two-dimensional transparent chamber (41.5 cm wide, 50 cm high and 2.8 cm thick) was constructed for infiltration experiments using three water-repellent Ouddorp sands (The Netherlands) and a wettable silicon sand. The results showed that if the water-ponding depth (h0) at the soil surface was lower than the water-entry value (hwe) of repellent sands, infiltration would not start until the water drop penetration time (WDPT) is exceeded; and contrary to infiltration in wettable soils, the infiltration rate increased with time. However, infiltration could immediately start at any time when h0 . hwe : The wetting front was unconditionally unstable for h0 , hwe ; resulting in fingered flow. However, the flow was conditionally stable for h0 . hwe if the soil was not layered in a fine-over-coarse or wettable-over-repellent configuration, and if soil air was not compressed during infiltration. The occurrence of stable and unstable flow in repellent soils was consistent with the prediction based on a linear instability analysis. The findings can be used to improve irrigation efficiencies in water repellent soils, e.g. using high-ponding irrigation methods.

1. Introduction Many soils of the world are water repellent. They are difficult to manage and pose negative effects on agricultural productivity and environmental sustainability (Debano, 1969d; Letey, 1969; Bond, 1969a; van’t Woudt, 1969; Jamison, 1969; Holzhey, 1969a; Letey et al., 1975; Ritsema et al., 1993). The effects of water repellency on infiltration are not yet fully understood. Field observations have indicated that the rates of water infiltration into repellent soils are very irregular. The fingered by-passing flow is more * Corresponding author. Tel.: þ1-909-787-6422; fax: þ 1-909787-3993. E-mail address: [email protected] (Z. Wang). 1 Present address: Department of Biological Systems Engineering, Washington State University, Pullman, WA 99164, USA. q 2003 Elsevier Science B.V. All rights reserved.

likely to occur in repellent soils rather than in wettable soils (Raats, 1973; Philip, 1975b; Parlange and Hill, 1976; Glass et al., 1989b; Wang et al., 1998a,b). Fingered preferential flow causes uneven distribution of water in the crop root zone, and accelerates the contaminant transport to ground water. The purpose of this paper is to quantify the effects of soil water repellency on infiltration rate and flow instability. We apply the unstable flow theory to predict the onset of wetting front instability and the occurrence of fingering in the vadose zone of waterrepellent soils. Two-dimensional chamber experiments were carried out to study the dynamics of infiltration and fingering in three Ouddorp repellent sands of The Netherlands. The results are compared with infiltration and fingering in a wettable sand of the same texture.

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2. Theoretical analysis Little is known from previous publications about the law of infiltration into repellent soils. Experimental studies have indicated that infiltration into repellent soils is complicated by the occurrence of fingering in porous media. Many field and laboratory studies have shown that fingering occurs in both wettable and repellent soils, not only in structureless sandy soils but also in structured loam and clay soils, and under both rainfall and irrigation conditions (Parlange and Hill, 1976; Starr et al., 1978, 1986; Clothier et al., 1981; Diment and Watson, 1985; Jury et al., 1986; Hillel and Baker, 1988; Glass et al., 1988, 1989b; Butters et al., 1989; Kung, 1990a; Ghodrati and Jury, 1990; Baker and Hillel, 1990; Roth et al., 1991; Ghodrati and Jury, 1992; Selker et al., 1992a; Jury and Flu¨hler, 1992; Ritsema et al., 1993; Flury et al., 1994; Liu et al., 1994a; Nieber, 1996; Held and Illangasekare, 1995; Dekker and Ritsema, 1994b, 1995). Many investigators suspect that factors leading to fingering may include vegetation, microtopography, irrigation method, soil water-repellency, soil layering, and soil macropores. However, theoretical analyses suggest that fingering can be induced by the onset of flow instability at the wetting front between two qualitatively different fluids. Fingering may take place in two-fluid flow even if there is no porous structure, e.g. in cracks or the Hele – Shaw cells (Saffman and Taylor, 1958). It is then possible that many of the aforementioned factors can induce wetting front instability in different ways. The original linear instability analyses of Saffman and Taylor (1958) considering viscous and gravitational forces, and of Chuoke et al. (1959) and Wang et al. (1998a) later including capillary forces, resulted in theoretical criteria for the onset of instability at the wetting front. According to Wang et al. (1998a), the condition for the onset of instability at the interface of two immiscible fluids in a porous medium is Vþ

eðrw 2 rnw Þgk cos b elsp lka2 2 .0 ðmw 2 mnw Þ ðmw 2 mnw Þ

the nonwetting fluid, and e ¼ 21 for the reversed displacement); r is the density and m the viscosity of the fluids, g the acceleration due to gravity, k the effective permeability of the porous medium, b the angle between the gravitational direction and the direction of the flow, s p the effective macroscopic interfacial tension, and a the magnitude of a disturbance to the wetting front. During vertical infiltration ðb ¼ 0Þ of water into the vadose zone, the density and viscosity of air are negligible. Thus Eq. (1) can be reduced (Wang et al., 1998a) into lhwe l3 V lh l3 , , 1 2 we c c Ks

where Ks is the natural saturated conductivity, hwe the water-entry value of the porous medium, and c is a constant indicating the relative effects of the maximum wetting front perturbation and microscopic heterogeneity on flow instability. According to the experiments by Yao and Hendrickx (1996), c < 175000 if hwe is in cm of water height. Thus, it can be predicted that the downward infiltration wetting front is unstable in porous media with lhwe l , 42 cm; otherwise the wetting front is stable. Assuming a sharp wetting front for the initially stable infiltration flow, the infiltration rate V can be expressed as h þ hwe 2 h0 ð3Þ V ¼ Ks 1 2 af L where h0 is the water pressure head at the soil surface, haf the gauge air pressure head below the wetting front, hwe the water-entry pressure of the porous medium, and L the depth of the wetting front. In most fingering-prone sandy soils lhwe l , 10 cm; thus the capillary effect on wetting front instability is negligible. Substituting Eq. (3) into Eq. (2) while assuming lhwe l3 =c ¼ 0; one obtains two alternative criteria for predicting the onset of wetting front instability: V , Ks

ð1Þ

where V is the infiltration rate, the subscript w refers to the wetting fluid and nw the nonwetting fluid; e indicates the wettability of the driving fluid to the porous medium (e ¼ 1 for the wetting fluid displacing

ð2Þ

ð4Þ

and F ¼ h0 2 hwb 2 haf , 0

ð5Þ

Thus, any time when V , Ks ; or the net matrix potential difference (F) across the wetted layer is less than zero (i.e. opposing the downward flow of water),

237

Effects of water repellency on infiltration rate and flow instability

the wetting front is unstable resulting in fingering. Otherwise, the flow should be stable manifesting a uniform and sharp wetting front. Eq. (4) is the criterion of Parlange and Hill (1976), whereas Eq. (5) is identical to the criteria suggested by Raats (1973) and Philip (1975b). We refer to Eq. (4) as the velocity (V) criterion, and Eq. (5) as the pressure head (F) criterion in this paper. According to Eq. (5), wetting front instability can be induced by the individual or combined effects of three factors: (a) a decrease in surface pressure head h0, for instance during redistribution of water following infiltration ðh0 , 0Þ; (b) an increase in water-entry value hwe due to, for instances, the presence of a fine-over-coarse layering in the direction of flow, the occurrence of macropores ðhwe < 0Þ; and infiltration into water repellent soils ðhwe . 0Þ; and (c) an increase in soil air pressure below the wetting front. Diment and Watson (1985) confirmed fingering as caused by factor (a). Hill and Parlange (1972), Glass et al. (1991), Baker and Hillel (1990), and Selker et al. (1992a) focused on fingering in the fine-over-coarse layered soils. Fingering due to air entrapment was confirmed by White et al. (1976) with experiments in the Hele – Shaw cells and by Wang et al. (1998b,c) in a sandy soil. Numerous other experiments have indicated preferential flow due to soil macropores and a combination of the aforementioned factors. In the repellent soils, fingered flow was observed by Ritsema et al. (1993) and Hendrickx et al. (1993). According to Eq. (5), assuming haf ; 0; the unstable flow should occur when h0 , hwe : In other words, the flow is unstable when the surface pressure head is

lower than the water-entry value that is positive in repellent soils. Field soils are heterogeneous and layered. The topsoil is often macroporous or sometimes waterrepellent. The soil air can easily be entrapped during high-intensity rainfalls or ponded surface irrigation events. The soil surface is otherwise under non-ponding infiltration or drainage conditions resulting in negative water heads at the soil surface. All these natural conditions tend to induce unstable flow. Hence, fingering is more likely a common phenomenon rather than the exceptions in the field. 3. Experimental materials and methods The purposes of the experiments are to measure the rate of infiltration in repellent soils; to identify conditions for the occurrence of fingered preferential flow in repellent soil; to verify the accuracy of Eqs. (4) and (5) with respect to the observed unstable flow patterns; and to compare the results with infiltration rates and occurrence of fingering in a wettable soil. The effects of soil water repellency and natural air compression on infiltration rate and occurrence of fingering were also investigated. The Ouddorp water-repellent sands of The Netherlands (Ritsema et al. 1993) and a water-wettable silicon sand (Wang et al., 1998c) were used in this study. The sands were initially oven-dried under 1058C for 24 h and then placed in the open air for at least 2 days before use. Hydraulic parameters of the two sands are listed in Table 1. The saturated hydraulic conductivity was measured using

Table 1 Properties of the porous media used in this study Porous medium type

gd, Dry bulk density (g/cm3)

f, Total porosity for rs ¼ 2.65 (cm3/cm3)

Ks, Saturated water conductivity (mm/min)

hwe, Water-entry value (cm)

Water wettable sand Water repellent sandsa 1st Horizon (humose topsoil) 2nd Horizon (transition layer) 3rd Horizon (bottom layer)

1.52

0.43

15.4

29

1.41 1.54 1.59

0.47 0.42 0.40

a

Sands of Ouddorp, The Netherlands.

7.99 8.01 8.11

12 7 2

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Z. Wang et al.

the constant-head method. The water-entry value (hwe) of the wettable sand was measured using the tension-pressure infiltrometer (TPI) method (Wang et al., 2000a). The water-entry values of the repellent sands were measured using a water-ponding (WP) method (Wang et al., 1998c). A Plexiglas slab chamber was constructed for 2D visualization of the dynamic infiltration flow in the repacked soils. The inside dimension of the chamber was 2.8 cm thick, 41.5 cm wide and 60 cm deep. The soil air phase was treated in two ways. When the soil air is allowed free to drain from ahead of the wetting front through an air exit at the bottom, the system condition is refereed to as an “air-draining” condition. In contrast, when the air is allowed to escape only from the soil surface, the system is refered to as an “air-confined” condition. The soil air pressure head (haf) below the wetting front was measured using a water manometer. Control of soil surface water head (h0) and measurement of the infiltration rate (V) were achieved by the use of a tension-pressure infiltrometer (Perroux and White, 1988). Water was uniformly applied to the soil surface through an inverted T tube (Wang et al., 1998c). Six experiments for 12 different combinations of the two soils and two air-flow conditions were carried out (Table 2). The dry sand was packed into Table 2 Matrix of laboratory experiments Porous medium and air flow combination

A, Air-drained

B, Air-confined

1. Homogeneous sand 2. Homogeneous repellent sand (1st horizon) 3. Homogeneous repellent sand (2nd horizon) 4. Homogeneous repellent sand (3rd horizon) 5. Three-layer water repellent sands ðh0 , hwe Þ 6. Three-layer water repellent sands ðh0 . hwe Þ

I III

II IV

V

VI

VII

VIII

IX

X

XI

XII

the 2D chamber using a funnel-extension-randomizer assembly and a drop impact method (Glass et al., 1989b). When preparing the repellent soil with different horizons, care was taken to maintain a clear textural interface and good contact between the layers. The surface of the packed soil was carefully smoothed and levelled. After a complete packing and set-up installation, the infiltration was then initiated by simply turning on the TPI. The development of wetting fronts behind the transparent Plexiglas plate, the falling water level in the TPI, and the gauge air pressure change in the water manometer, were recorded using video and photo cameras.

4. Results and analyses The recorded changes with time t of the infiltration rate V, the water pressure head h0 at the soil surface, the air pressure haf ahead of the wetting front (in case haf . 0), and the water pressure difference F ¼ h0 2 hwe 2 haf ; are shown in Figs. 1 – 4. The advance of wetting fronts for the corresponding experiments are shown in Figs. 5– 8. 4.1. Infiltration into a homogeneous wettable sand Water infiltration into the homogeneous wettable sand, without air-entrapment, was stable as predicted since the rate of infiltration V was always higher than the saturated hydraulic conductivity Ks (Fig. 1a). The wetting front propagation was always stable manifesting a sharp wetting front (Fig. 5a). The stable flow condition was consistent with both the V and F criteria (V . Ks and F . 0). The air-confined infiltration in the wettable sand resulted in unstable flow as shown in Figs. 1b and 5b. The gauge air pressure haf abruptly rose at the start of infiltration. Then, air bubbles intermittently escaped from the soil surface, which led to fluctuations in haf and F as shown in Fig. 1b. When the instability criterion V , Ks was satisfied at about t ¼ 3 min and the criterion F , 0 satisfied at t ¼ 1 min; the wetting front became unstable at about t ¼ 2 min: The flow became fingered after t ¼ 5 min: Two fingers appeared in the limited chamber as shown in Fig. 5b.


239

Fig. 1. Variation of infiltration rate, V, surface water head h0, gage air pressure head, haf, and pressure head difference F ¼ h0 2 hwe 2 haf ; with respect to infiltration time t in a wettable silicon sand: (a) air-draining condition; and (b) air-confined condition. Ks indicates the soil’s saturated hydraulic conductivity.

4.2. Infiltration into homogeneous repellent sands Dynamics of water infiltration into separately packed three horizons of Ouddorp sands are shown in Fig. 2. In all the three repellent horizons, the infiltration rate V was initially zero despite the ponding depths (h0) at the soil surface. In the most repellent sand (top horizon), water started to infiltrate after a ponding time t exceeded 30 min (Fig. 2a) that is approximately the water drop penetration time of the repellent sands (Ritsema et al., 1993). Notice that the infiltration rate increased with time, which is contrary to the law of infiltration in the wettable sand (Fig. 1a). In the lesser repellent second and third horizons, the required water drop penetration time was about 5 and 2 min, respectively. The infiltration rates were very low. The instability criteria, V , Ks

and F , 0; were satisfied in the above three experiments, and the wetting fronts indeed became fingered. The fingered flow patterns in the second and third horizons are shown in Fig. 6a and b, respectively. The wetting front in the third horizon was initially stable corresponding to the satisfied stability criteria V . Ks and F . 0 as shown in Fig. 2c. Our experiments in the air-confined conditions indicated that soil air was not compressed due to very slow infiltration rate in the repellent sands. The flow patterns were almost the same as under the airdraining condition. The experimental results indicate that infiltration into the repellent sands was very slow and fingered in contrary to high-rate and stable infiltration in the wettable sand. When soil air was entrapped and compressed, flow in both the wettable and repellent

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Fig. 2. Infiltration into the Ouddorp water-repellent sands: (a) the most repellent first horizon; (b) less repellent second horizon; and (c) least repellent third horizon (symbols are as defined for Fig. 1).

sands behaved similarly with low infiltration rate and fingered flow patterns. 4.3. Infiltration into layered repellent sands under low-ponding heads The Ouddorp repellent sands were packed into the 2D chamber in a three-layer configuration. The most repellent sand (first horizon) was on the top 10 cm, the moderately repellent sand (second horizon) in the middle 20 cm, and the least repellent sand (third horizon) at the bottom 20 cm. For both the airdraining (experiment IX) and air-confined (exper-

iment X) conditions under low-ponding depths varying from 2 to 3 cm, the repellent soil was extremely difficult to wet up. Water did not start infiltrating until 40 min after ponding as shown in Fig. 3. A considerable amount of edge flow occurred along the side-walls of the chamber, causing the water table to rise from the bottom. A single finger appeared in the air-draining chamber (Fig. 7a) and three fingers in the air-confined chamber (Fig. 7b). Due to the extremely low infiltration rate, soil air was not compressed before the edge flow reached the bottom. During the fast upward wetting, soil air in both chambers was slightly compressed. Air bubbles broke


241

Fig. 3. Infiltration into layered Ouddorp water-repellent sands under low-ponding depths: (a) air-draining condition and (b) air-confined condition (symbols are as defined for Fig. 1).

through a very thin layer of the wetted top layer and escaped into the open air. 4.4. Infiltration into layered repellent sands under high-ponding heads For the above experiments the water-entry value (hwe ¼ 12 cm) of the top repellent sand was not exceeded, which resulted in a zero infiltration rate at the beginning of infiltration. In experiment XI and XII (Table 1), we applied a greater than 12 cm of ponding head at the soil surface to observe the effects of high ponding. The results of experiment XII are shown in Figs. 4 and 8. Infiltration started promptly at t ¼ 6 min when the ponding depth h0 exceeded 12 cm. The wetting front was stable for a short time between t ¼ 6 and 9 min (Fig. 8a) when h0 . hwe ; which is consistent with V . Ks and F . 0 for stable flow. As

soon as h0 was reduced below 12 cm at about t ¼ 10 min; the wetting front became unstable and fingered as shown in Fig. 8b. In this case, the soil air entrapment between the waters at surface and bottom of the soil could have been compressed, which could have accelerated the occurrence of fingering. The unstable flow again corresponded well to the instability criteria V , Ks after t ¼ 9 min and F , 0 after t ¼ 15 min:

5. Discussions and conclusion The initially dry repellent sands were difficult to wet. If the repellent sand was eventually wetted after a long time of wetting, the infiltration flow advanced through fingered paths, bypassing a large volume of soil in the top layer. In addition to this

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Z. Wang et al.

Fig. 4. Infiltration into layered Ouddorp water-repellent sands under high-ponding depths and the air-confined condition (symbols are as defined for Fig. 1).

study other rence 1993;

conducted in the laboratory, there are many studies that have also confirmed the occurof fingering in the field (e.g. Ritsema et al., Hendrickx et al., 1993; Dekker and Ritsema,

1994b, 1995; Ritsema and Dekker, 1994b, 1995). The size and speed of the fingered flow in repellent soils were successfully predicted by Wang et al. (1998a,b).

Fig. 5. Wetting front patterns during water infiltration into a wettable silicon sand: (a) stable flow with the air-draining condition (Fig. 1a, t ¼ 5 min) and (b) unstable fingered flow with the air-confined condition (Fig. 1b, t ¼ 19 min).


243

Fig. 6. Wetting front patterns during water infiltration into separate layers of Ouddorp water-repellent sands: (a) fingered flow in the second horizon repellent sand (Fig. 2b, t ¼ 50 min) and (b) initially stable flow and subsequently fingered flow in the third horizon sand (Fig. 2c, t ¼ 20 min).

Fig. 7. Wetting front patterns during water infiltration into a three-layer configuration of Ouddorp water-repellent sands under the low-ponding condition: (a) fingered flow with the air-draining condition (Fig. 3a, t ¼ 190 min) and (b) fingered flow with the air-confined condition (Fig. 3b, t ¼ 300 min).

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Z. Wang et al.

Fig. 8. Wetting front patterns during water infiltration into a three-layer configuration of Ouddorp water-repellent sands: (a) stable flow when the surface ponding depth h0 ¼ 14 cm; greater than hwe ¼ 12 cm; the water-entry value of the repellent sand (Fig. 4, t ¼ 7 min) and (b) fingered flow when h0 , hwe (Fig. 4, t ¼ 33 min). The soil surface was 10 cm below the inverted T tube.

The infiltration rates in repellent sands were very slow. Fingering occurred if the surface ponding head h0 was less than the soil water-entry value, hwe. However, the infiltration flow became stable if h0 . hwe : This principle was also found to be true for wettable sandy soils (Wang et al., 1998a). All of the observed unstable and stable flows were accurately predicted by the velocity criterion ðV , KS Þ (Parlange and Hill, 1976) and the pressure criterion F ¼ h0 2 hwe 2 haf , 0 (Raats, 1973; Philip, 1975b; Wang et al., 1998a). The findings here are significant for field water management in repellent soils. For example, the difficulties in wetting the repellent soils can be overcome by using the high-ponding surface irrigation methods (e.g. level-basin or deep furrow irrigation). The high-ponding methods will increase infiltration. However, since the surface water head

will become negative after the cessation of infiltration, the pressure head criterion of h0 , hwe can easily be satisfied, thus fingering will eventually occur during redistribution of infiltrated water. Summarizing existing reports on the occurrence of unstable preferential flow, the individual or combined effects of air entrapment, soil layering, soil macropores, surface desaturation, and soil water repellency, are responsible for fingering in the field. With the unavoidable effect of surface desaturation (redistribution following infiltration), field infiltration and drainage cycles will more likely result in fingering. Further research is urgently needed to incorporate the unstable flow theory into simulation models, since unstable flow is an important mechanism for preferential contamination of groundwater systems.

Chapter 23

Non-equilibrium model for gravity-driven fingering in water repellent soils: Formulation and 2D simulations J. Niebera,*, A. Sheshukova, A. Egorovb, R. Dautovb a

Biosystems and Agricultural Engineering Department and Army High Performance Computing Center, University of Minnesota, 1390 Eckles Avenue, Saint Paul, MN 55108, USA b Kazan State University, Universitetskaya 17, Kazan 420008, Russia

Abstract The instability of infiltrating flow is studied using the mass balance equation coupled with a first-order relaxation equation relating the rate of change of saturation to the difference between the dynamic water pressure and the saturation-dependent equilibrium water pressure. A numerical solution of the mass balance equation, based on a mass conservative scheme, is applied to the simulation of infiltrating flows in a vertical, two-dimensional plane region. Both water wettable and water repellent soils are considered in the analysis. The effect of water repellency is introduced by modification of the equilibrium saturation– pressure relationship, in which water repellency causes the relation to become flatter. Conditions of even slight water repellency are found to be sufficient to cause infiltrating flows to become unstable. A sensitivity analysis related to the width of the surface source shows that the number of fingers generated increases with increasing source width. The sensitivity analysis also indicates that the non-equilibrium model approach can provide a physically plausible reason for flows becoming stable when the surface flux becomes vanishingly small.

1. Introduction Preferential flow of water in soils impacts several soil attributes, including the capacity of soils to remove/treat harmful constituents, and the capacity of soils to provide water and nutrients to plants. Nieber (2000) describes the several recognized forms of preferential flow, among these being the process of gravity-driven unstable flow. The earliest reported observations of gravity-driven unstable flow were reported by Hill (1952) in a study of flow in chemical separation columns. At the present time there is still much interest in experimental and theoretical descriptions of gravity-unstable flow. * Corresponding author. E-mail address: [email protected] (J. Nieber). q 2003 Elsevier Science B.V. All rights reserved.

As described by Raats (1973), the process of gravity-driven unstable flow may occur when any of the following conditions are met: 1) the saturated hydraulic conductivity increases with depth; 2) the soil is water repellent; 3) air pressure builds up at the wetting front of infiltrating water. In addition to these conditions, recent analyses by Egorov et al. (2002) show that the conditions for instability of flow requires that non-equilibrium in the saturation – pressure relation exist. For all of these conditions, the wetting front slows and the front becomes unstable, manifested by the breaking of the front into discrete fingers. Several mathematical models have been developed to quantify the conditions for flow instability in soils (Raats, 1973; Philip, 1975b; Parlange and Hill, 1976, Diment et al., 1982; Glass et al., 1989b). Most of these

246

J. Nieber et al.

studies have provided a means to estimate finger width, but none of them dealt directly with the mechanism of finger formation. It is now recognized (Dautov et al., 2002) that there are two requirements for the successful modeling of finger formation. These requirements are that (i) the mathematical model must be capable of producing unstable perturbations and (ii) the growing perturbations have to be persistent. The numerical model presented by Nieber (1996, 2000) and Ritsema et al. (1998a) correctly incorporated the means for the persistence of fingers by using capillary hysteresis in the water retention function. That numerical model however was deficient in the method for the formation of flow instabilities, because it was based on the solution of the Richards equation, which is now understood to be unconditionally stable (Otto, 1997; Egorov et al., 2002). In the work by Egorov et al. (2002), the conditions for growth of infinitesimal perturbations imposed on a uniform wetting front are well quantified. They analyzed three possible mathematical models: the conventional Richards equation, a sharp front Richards equation (Selker et al., 1992c), and an extended Richards equation with a non-equilibrium saturation – pressure function (Hassanizadeh and Gray, 1993). They showed that the conventional Richards equation is unconditionally stable, while the sharp-front Richards equation is unconditionally unstable and yields an unstructured field of fingers. The conclusion was that neither of these models should be used to describe fingering. The nonequilibrium Richards equation model was found to be the only model capable of generating a structured field of fingers from a slightly perturbed uniform wetting flow. In a subsequent paper, Dautov et al. (2002) applied this non-equilibrium model to simulate single and multiple fingers in water wettable soils. Hysteresis in the saturation– pressure relation was incorporated in the model to provide persistence of the growing fingers. Raats (1973) listed soil water repellency as one of the possible causes of gravity-driven unstable flows in soils. This has been confirmed experimentally by numerous research articles including Jamison (1945), Hendrickx et al. (1993), Ritsema et al. (1993), Ritsema and Dekker (1994b), and Dekker and Ritsema (1996d). Modeling of unstable flow in water repellent soils has been reported by Van Dam

et al. (1990), de Rooij (1995), de Rooij and de Vries (1996), Ritsema et al. (1998a), Nguyen et al. (1999), and Nieber et al. (2000). In this manuscript we formulate a non-equilibrium model for flow in unsaturated porous media, and present a numerical solution scheme to solve the governing equations. This numerical solution is then applied to the simulation of two-dimensional flow in unsaturated soils. The conditions considered are for a well-graded sand that is either completely water wettable or slightly water repellent. It is shown that the condition of complete water wettability leads to a stable flow, while the slight water repellency leads to instability and persistent finger formation. The sand media studied by Bauters et al. (2000a) and Nieber et al. (2000) is used as a guide for quantifying the impact of water repellency on the saturation –pressure relationship of the porous medium.

2. Model formulation Unsaturated flow in porous media is conventionally modeled using the Richards equation when the air phase is assumed to be at ambient atmospheric pressure. This equation may be written in dimensionless form as

›s › 2 7 · KðsÞ7p þ KðsÞ ¼ 0 ›z ›t

ð1Þ

p ¼ PðsÞ

ð2Þ

where s is the effective saturation (0 # s # 1), p is the water pressure, K is the relative hydraulic conductivity, PðsÞ is the equilibrium pressure represented by the relationship between saturation and water pressure, and z is the vertical coordinate taken positive downward. The pressure and the spatial coordinates are normalized on air-entry pressure taken as 1/a, the relative hydraulic conductivity is normalized on its saturated value, Ks, and time is scaled on fð1 2 sr Þ=ðaKs Þ; where f is the porosity, a is the well-known van Genuchten (1980) parameter, and sr is the residual saturation. The system Eqs. (1) and (2) is completed by defining the hydraulic properties of the porous medium. In this paper we adopt the van Genuchten-Mualem model (van Genuchten (1980)), which may be expressed in

Non-equilibrium model for gravity-driven fingering in water repellent soils: Formulation and 2D simulations

247

the non-dimensional form as PðsÞ ¼ 2ðs21=m 2 1Þ1=n ; KðsÞ ¼ s1=2 ð1 2 ð1 2 s1=m Þm Þ2 ; m ¼ 1 2

1 n

ð3Þ

The recent results by Egorov et al. (2002) and Otto (1997) proved that any solution of the Richards equation is unconditionally stable. Therefore, the conventional model must be modified to be able to describe fingering phenomena in unsaturated porous media. Hassanizadeh and Gray (1993) suggested one possible extension of the Richards equation. This model takes into account dynamic memory effects by adding a mechanism of relaxation in the relation between water saturation and water pressure. The non-equilibrium mechanism of relaxation is manifest in the results of several experimental studies including Nielson et al. (1962), Topp et al. (1967), Smiles et al. (1971) and Wildenschild et al. (2001). The non-equilibrium model does not change Eq. (1), but replaces the relationship Eq. (2) with

t

›s ¼ p 2 PðsÞ ›t

ð4Þ

where t . 0 denotes the dimensionless relaxation coefficient function. The dimensional relaxation coefficient (m sec) is normalized on f=ða2 Ks Þ: Note that in general PðsÞ can be hysteretic and, therefore, two main hysteretic curves: p ¼ Pw ðsÞ (main wetting curve or MWC) and p ¼ Pd ðsÞ (main drainage curve or MDC), are incorporated in the model. These two curves divide the ðp; sÞ-plane into three domains: the main hysteretic loop H0 ; the domain Hw above the MWC, and the domain Hd below the MDC (Fig. 1). This hysteretic non-equilibrium model postulates (Beliaev and Hassanizadeh, 2001) that dynamic memory effects (relaxation) are significant only outside the main hysteretic loop, and it takes those effects into account using the same modification of the saturation – pressure relation as in Eq. (4). The relaxation equation is therefore given by

ti

›s ; ti s_ ¼ p 2 Pi ðsÞ; ›t

ðp; sÞ [ Hi ; i ¼ w; d

ð5Þ

As a result of this postulate it is assumed that inside the main hysteretic loop region H0 ; wetting/ drainage processes follow equilibrium scanning

Fig. 1. A typical closed-loop hysteresis diagram. The dashed lines represent the scanning drainage curves. The trajectory of the process follows points 1 (initial state), 2 (switching point), and 3 (final state).

curves. In this paper we use the hysteresis model of Mualem (1974) and restrict our attention only to the two-stage wetting-drainage process. Trajectories for the wetting stage locate within Hw ; while trajectories for the drainage stage will be limited to H0 : The nonequilibrium drainage domain Hd will never be visited for this two-stage process. As a result of the applied conditions scanning curves turn out to be the primary drainage scanning curves (Mualem, 1974), yielding the relations p ¼ Psc ðs; sp Þ

ð6Þ

or s ¼ Ssc ðp; sp Þ for ðp; sÞ [ H0

Ssc ðp; sp Þ ¼ Sw ðpÞ þ

sp 2 Sw ðpÞ ðS ðpÞ 2 Sw ðpÞÞ 1 2 Sw ðpÞ d

ð7Þ

where Ssc ; Sw ; and Sd are the inverse functions of Psc ; Pw ; and Pd respectively, and sp corresponds to the switching point (point 2 in Fig. 1). Eqs. (5) and (6) may be transformed to one equation by introducing the continuously differentiable

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functions p ¼ Pt ðsÞ and s ¼ St ðpÞ : ( Pw ðsðtÞÞ; s_ $ 0 ; Pt ðsÞ ¼ Psc ðsðtÞ; sp Þ; s_ , 0 ( Sw ðpðtÞÞ; p_ $ 0 St ð pÞ ¼ Ssc ðpðtÞ; pp Þ; p_ , 0 where sp ¼ max sðtÞ; and pp ¼ Pw ðsp Þ: With these definitions the saturation –pressure relations given by Eqs. (5) and (6) may be rewritten in terms of Pt as ( tw ; ðp; sÞ [ Hw ts_ ¼ p 2 Pt ðsÞ; t ¼ 0; ðp; sÞ [ H0 For this paper we have adopted the functional form for tw given by

tw ¼ t0 ðsÞðp0 2 pÞgþ ;

t0 ðsÞ ¼ t0s P 0w ðsÞ;

g . 0 ð8Þ

where ð·Þþ ¼ maxð·; 0Þ; p0 is the parameter referred to as the water entry pressure of the soil (Baker and Hillel, 1990; Selker et al., 1992c), and t0s is the relaxation constant. The function t0s P0w ðsÞ is unbounded at saturations approaching zero and unity. The selection of this form for the function is based on the homogenization analysis of Panfilov (1998) in his study of two-phase flow in double porosity media. The use of the term ðp0 2 pÞgþ imposes a limit on the growth of pressure during flow. Without this term the traveling wave solution presented by Egorov et al. (2002) yields non-physical behavior in the form of an unbounded pressure, as the initial saturation approaches zero. For more detailed information on the selection of Eq. (8) to represent the dimensionless relaxation time the reader is referred to Dautov et al. (2002). With the functional form for t0 ðsÞ given in Eq. (8) we finally have for the relaxation equation

tP_ t ¼ p 2 Pt ðsÞ

ð9Þ

where the relaxation coefficient is now defined as t ¼ t0s ðp0 2 pÞgþ ; and t0s ¼ 0 inside H0 : Note that in the actual numerical implementation of this approach, rather than setting t0s ¼ 0 inside H0 ; it is set to a small constant . 1023 : Using this small value serves to allow the description of processes

Fig. 2. Schematic of the computational domain with specified initial and boundary conditions.

inside and outside the main hysteretic loop by the same procedure, thus facilitating the use of the same algorithm in all computations. The system of Eqs. (1) and (9) is subject to the initial and boundary conditions that are shown in Fig. 2. They are t¼0:

s ¼ sin

ð10Þ

z ¼ 0; xl # x # xr : ð xb qz ¼ qin þ q0 ð1 þ h cosðvjÞÞdðj 2 xÞdx

ð11Þ

xa

z ¼ zd : x ¼ xl

qz ¼ qin ¼ Kðsin Þ and

x ¼ xr : q x ¼ 0

ð12Þ ð13Þ

where d is the Dirac function, xl ; xr ; and zd are the left, right and lower boundaries of the computational domain, while xa ; xb ; and L ¼ xb 2 xa are the boundaries and the length of the infiltration source. By Eq. (11) the infiltration flux q0 is perturbed with a sinusoidal function having amplitude h and frequency v. To assure that the process studied is a two-stage wetting-drainage process, we maintain the uniform gravity-driven background flux qin related to the initial saturation sin over both the lower boundary


and the portion of the upper boundary outside of the infiltration source. The implementation of the condition given by Eq. (12) is valid until the infiltrating front reaches the bottom boundary. 3. Discretization and numerical solution The system of Eqs. (1) and (9) with initial and boundary conditions (10) –(13) is solved numerically by applying a mass-conservative finite difference approximation and evaluating the temporal terms by a fully implicit first-order backward Euler scheme. Details of this numerical solution scheme are presented in Dautov et al. (2002). The resulting nonlinear algebraic equations may be written in matrix form as, s 2 s~ þ AðsÞH ¼ 0; Dt P 2 P~ ¼ p 2 P; tðp; sÞ Dt

p¼HþZ ð14Þ s ¼ St ðPÞ

where s, p, and H are the unknown vectors of nodal values of saturation, pressure, and hydraulic head fields, respectively, A is the symmetric 5-diagonal matrix with coefficients dependent on s, tðp; sÞ is a diagonal matrix, and tilde indicates values at the previous time-step. Note that the hydraulic head variable has been substituted into Eq. (1) to arrive at Eq. (14). The advantage of using the hydraulic head variable instead of the pressure head variable in the mass balance equation is that the convective term (third term in Eq. (1) resulting from gravity) causes some numerical dispersion. The use of the hydraulic head variable eliminates this source of numerical dispersion. The set of nonlinear algebraic equations given by Eq. (14) are solved by the following iteration procedure. Here k þ 1 indicates the current iteration level. (1) Let k ¼ 0; and set pk ¼ p~ ; sk ¼ s~, H k ¼ p~ 2 Z (2) Find Pkþ1 and skþ1 from the explicit relations

tðpk ; sk Þ

Pkþ1 2 P~ ¼ pk 2 Pkþ1 ; Dt

Aðskþ1 ÞH kþ1 þ Dkþ1 H kþ1 ¼ Dkþ1 H k 2

skþ1 2 s~ Dt

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ð15Þ

The diagonal matrix Dkþ1 is the approximation of the derivative of the vector-valued function ðs 2 s~ Þ=Dt with respect to p: ! S0t ðPkþ1 Þ d pDt þ tðp; skþ1 ÞP~ kþ1 ¼ D Dt dp tðp; skþ1 Þ þ Dt p¼pk

Upon solving for H, the pressure is calculated as pkþ1 ¼ H kþ1 þ Z: (4) Set k ¼ k þ 1; and go to step 2. Repeat the loop until convergence is reached. To obtain the solution for H kþ1 we invert the matrix of the system (15) using a preconditioned conjugate gradient method based on modified incomplete Cholesky factorization. We note that the resulting matrix Aðskþ1 Þ þ Dkþ1 of the system has good algebraic properties: it is symmetric, 5-diagonal, and positive definite. Within the iteration loop (1) –(4) the hysteretic state at a particular grid point is not changed, but instead iteration continues until absolute and relative errors in pressure and saturation between two consecutive iterations is less than a value 1. The change in hysteretic state is checked after convergence. To assess whether any saturation reversals have occurred at the end of each time step, we check for changes in the nodal saturations from the previous time step. If the sign of si;j 2 s~i;j at a node ði; jÞ has changed, and lsi;j 2 s~i;j l . 101; the hysteretic state for the node is changed. Our experience with this numerical solution has been very positive. We have found that the solutions are achieved very efficiently. For the two-dimensional problems solved here all of the solutions were performed on an 800 MHz Pentium III PC. Grid sizes were upwards of 20,301 nodes. All solutions were achieved within one-half hour run time. 4. Conditions leading to unstable flows

skþ1 ¼ St ðPkþ1 Þ

(3) Solve the linear system of algebraic equations for H

Based on the use of a traveling wave solution to Eqs. (1) and (9), Egorov et al. (2002) showed that unstable flow occurs when the relaxation parameter t is sufficiently large. Unstable flows are manifested

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by a non-monotonic distribution of saturation in the direction of the flow. Egorov et al. (2002) showed that it is possible for the relaxation parameter to be just large enough to produce a stable flow with a slightly non-monotonic saturation profile. However, for a value greater than a quantifiable critical value,tcr ; the non-monotonic flow will be unstable. A plot of the critical dimensionless relaxation time tcr and the degree of saturation at the trailing end of an advancing wetting front is shown in Fig. 3a. The plotted lines were derived for various values of van Genuchten parameters a and n. The corresponding retention curves derived from

Fig. 3. (a) Critical dimensionless relaxation time versus saturation at the trailing end of an advancing wetting front, and (b) Equilibrium saturation–pressure relationships for distinct n and a: 1 2 n ¼ 2; a ¼ 2; 2 2 n ¼ 3; a ¼ 5; 3 2 n ¼ 5; a ¼ 10; 4 2 n ¼ 10; a ¼ 20; and 5 2 n ¼ 20; a ¼ 40:

the van Genuchten equations are shown in Fig. 3b. Coarse textured soils, including sands and gravels, will tend to have values of n exceeding 3. Soils with narrow particle size distributions will also tend to have larger values of n. In contrast, fine textured soils and well-graded soils, will tend to have smaller values of n, in the range of 1.1– 2.5. The lines plotted in Fig. 3a show that for either low or high trailing saturations, the flow is likely to be stable because the critical value of t increases without bound as the trailing saturation approaches zero or unity. But in the range of trailing saturations between about 0.2 and 0.9, the flow is more likely to become unstable because the critical value of t is a minimum in that saturation range. The dependence of tcr on the value of the van Genuchten n is instructive with respect to the type of soil/porous medium that will promote unstable flows. The plots in Fig. 3a show that as the value of n increases, the function tcr ðsÞ decreases, leading to an increasing range of saturations over which instability will be possible. Referring to corresponding curves in Fig. 3b, we may conclude that coarser textured soils, and soils with narrow particle size distributions will tend to support unstable flows. To put the values on the plot in Fig. 3a into perspective, Hassanizadeh (1997) indicated that the dimensionless relaxation time likely ranges between 1 and 2000 for sandy soils. Comparing this range to the plots in Fig. 3a indicates that flows in sandy soils should tend to be unstable over a fairly large range of saturations. Typical values of dimensionless relaxation time are not available for fine texture soils, but current theory (Baliaev and Hassanizadeh, 2001) indicates that the values should be larger than those for sandy soils. Since fine textured soils will have smaller values of n, it is necessary to have a much larger value of relaxation time to produce instability. So while non-equilibria may be a feature common to all soils, it may not lead to instability. The above analysis will be applied in the following sections to explain why water repellency in a soil can cause flows in the soil to become unstable.


5. Simulation results The mathematical model and the numerical method derived in the preceding sections may be applied to study fingering in soils that exhibit significant non-equilibrium in the saturation– pressure relation. Fingering has been observed to occur in initially dry uniformly graded soils (Hill and Parlange, 1972; Glass et al., 1989b), while it does not tend to be manifested in well-graded wettable soils or initially moist soils (Liu et al. 1994a). Fingering has also been observed in well-graded water repellent soils (Ritsema and Dekker, 1994b; Bauters et al., 2000a). We hypothesize that the manifestation of fingering is related to the presence of at least the critical amount of non-equilibria in the saturation– pressure relation. At present there does not exist sufficient experimental measurements of dynamic saturation – pressure relations to determine whether this hypothesis is correct, so for now we will use a rational approach to quantify the degree of non-equilibrium of the saturation –pressure relation. Our rational analysis is based on the relaxation model given by Eq. (9), and we rely on the support of the experimental studies presented by Bauters et al. (2000a). The sand used in the experiments was fairly well-graded as described by Nieber et al. (2000). The saturation –pressure curves shown by Bauters et al. show that the degree of water repellency is strongly manifested in the wetting curves, while little effect is observed in the drainage curves. The wetting curve for the water wettable sand manifested characteristics of a fairly well-graded sand, but as the degree of water repellency increased, the wetting curves exhibited the flatter saturation– pressure curve characteristics of uniformly graded porous media. Also, as the degree of repellency increased, they found that the water entry pressure increased to zero and then became positive for the cases of highest water repellency. For this manuscript we will examine the effect of flatness of the saturation – pressure relationship on the stability of infiltrating flows. Based on experimental evidence of Bauters et al. (2000a), we conclude that as a porous medium becomes more water repellent, the corresponding saturation – pressure relationship becomes flatter. The flattening of the saturation– pressure relationship is quantified by an increase in the van Genuchten n parameter. According to

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the analysis presented in the previous section, the flatter the saturation– pressure curve, and thereby the larger the value of n, the more likely is the transition from a stable flow to an unstable flow. We are interested to determine the effect of saturation – pressure parameters on flow stability. This effect will be demonstrated by examining the flow from a point source. Through this example we will show that for sufficiently large dimensionless relaxation time coefficient, the flow will lead to a defined finger, while for smaller values the flow will be stable. After demonstrating this effect we will consider the generation and propagation of multiple fingers from a finite surface source, and we will show that the number of fingers generated depends on the width of the source. We note here that a sharp-front regime may occur for some range of the input parameters (i.e. very dry soils), and very fine grids are required to adequately simulate a steep change in saturation over the front. We designed the grids so that the sharp fronts would be represented by at least four nodes. With this in mind the input parameters used in the calculations were taken within a range that allowed us to deal with reasonable grids of less than 25,000 nodes. To specify the hysteretic non-equilibrium model, it is necessary to define the hydraulic properties of the medium such as KðsÞ; Pw ðsÞ and Pd ðsÞ; as well as tw as a function of parameters of the process. We used the van Genuchten –Mualem relationships given by Eq. (3), and specified the dimensionless inverse capillary length 1=ad ¼ 2=aw ; and pore-size distribution parameter nw ¼ nd : We will present results only for the case with g ¼ 1 and 1 ¼ 1027 since varying these parameters over a reasonable range while keeping the other parameters constant led to qualitatively similar behavior for all simulations. Parameters n and t0s are varied in the calculations to show the impact of the dimensionless relaxation time on stability. 5.1. Single finger from a narrow source The single finger is generated by infiltration over a small length L at the upper boundary with the flux q0 . 0: The applied flux at the soil surface shown in Fig. 2 is less than the saturated hydraulic conductivity so that q0 , 1: The parameters for the simulation were chosen to be, t0s ¼ 5; q0 ¼ 0:2; L ¼ 1 and sin ¼ 0:075:

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Fig. 5. Saturation isolines s ¼ 0.1 representing finger size for q0 ¼ 0:2; L ¼ 1; sin ¼ 0:075 and five distinct cases: 1 2 n ¼ 2; t0s ¼ 0; 2 2 n ¼ 2; t0s ¼ 5; 3 2 n ¼ 5; t0s ¼ 0; 4 2 n ¼ 5; t0s ¼ 5; and 5 2 n ¼ 10; t0s ¼ 5:

Fig. 4. Saturation (a) and pressure head (b) distribution for the case of a single finger: n ¼ 10; t0s ¼ 5; L ¼ 1; q0 ¼ 0:2; and sin ¼ 0:075:

The total flux is then Q ¼ q0 L ¼ 0:2: Since we expected only a single finger to form, the grid was refined horizontally in a vertical column in the vicinity beneath the location of the source. There the horizontal grid cell dimension was 2/3 of the dimension in the region outside the refined zone. The total grid was composed of 91 cells horizontally and 151 cells vertically. The saturation and pressure head distributions at t ¼ 106 are illustrated in Fig. 4. The saturation distribution is represented as a clearly defined finger. The width of this finger is determined by the flux Q. The morphology of the finger is defined by three distinct features. First, as illustrated in Fig. 4a the finger tip has a higher water saturation, stationary

finger core and a distribution layer (Ritsema and Dekker, 1994b). Second, once developed the distribution layer remains steady while the tip of the finger moves at a constant velocity without change in shape and the finger core length grows at a steady rate. Third, lateral spreading of the finger core is halted by the pressure inside the finger being smaller than the background pressure as shown in Fig. 4b. We emphasize that the main characteristics of the finger are uniquely designated by Q. For other simulation runs we performed we found that by imposing different values of Q the finger velocity, finger width, and saturation at both the tip of the finger and the finger core increase with Q. This scenario is fully consistent with the physical picture of the process presented by Liu et al. (1994a) and Nieber (1996). The distribution layer provides a steady water flux into the finger core. For the case of a narrow source shown in Fig. 4 the distribution layer is quite narrow. In Fig. 4a the distribution layer is identifiable by the higher saturation near the source location. The influence of water repellency of the soil on finger size is illustrated in Fig. 5 which shows the position of the s ¼ 0:1 isoline at t ¼ 106 for various


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Curves 2, 4 and 5 represent the solution where the non-equilibrium mechanism is included in conjunction with hysteresis in the saturation – pressure relation. For these cases the flow shows different degrees of non-monotonicity in the saturation profile. For case 2 the degree of non-monotonicity is only slight, and for this case the flow is stable. For cases 4 and 5 the degree of non-monotonicity is significant and the flow profile has all the characteristics of finger flow. It is interesting to note that while case 2 has the same value of t0s as cases 4 and 5, the small value of n in the first case causes the flow to be stable, while the larger value for the latter two cases leads to unstable flow. This comparison points to the importance of the shape of the saturation– pressure function on flow stability. A soil having a saturation– pressure function with relatively small n, even if the value of t0s is large, will not support unstable flows. In contrast, soils with large values of n will support unstable flows. The effect of water repellency on the formation of unstable flows can be directly assessed by comparison of case 2 and cases 4, 5. While for all three cases the value of t0s is the same, the value of n is very different among the three cases. In reference to the water repellent treatments described by Bauters et al. (2000a), we might look at case 2 as representing the water wettable sand, while cases 4 and 5 represent the same sand but with increasing levels of water repellency. Fig. 6. (a) Saturation and (b) pressure head along the central vertical line x ¼ 10 for the five cases presented in Fig. 5. The pressure head profiles are not shown for cases 1 and 2 because they both plot below the line of pressure head equal to 22.

values of the parameters n and t0s : Plots of the saturation profile along the central axis of the domain are shown in Fig. 6a, while pressure head profiles along the same central axis are shown in Fig. 6b. Curves 1 and 3 are for a solution with very small t0s and therefore represent the solution to the conventional Richards equation. For both of these cases the flow is diffuse even though increasing n reduces the lateral diffusion and makes the saturation profile steeper at the wetting front. However, there is no accumulation of water at the front and this coincides with the conventional theory described by the Richards equation.

5.2. Multiple fingers from a finite surface source In the previous section we demonstrated the apparent effect of water repellency on the saturation -pressure relationship and showed that this can lead to instability of gravity-driven flow. In the present section we will confine our analysis to the case of unstable flow only and focus on the effect of source width on finger number and finger velocity. For this analysis the following parameters are used: n ¼ 7; t0s ¼ 5: These are parameters that we know will produce wetting front instability. We will now show that by increasing the length L of the infiltration source an increasing number of fingers will form. The width and velocity of individual fingers will be shown to be affected by the length L. Two sets of conditions will be considered. In the first set we will keep the total applied flux Q constant

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while varying L. Thus as L increases, the surface flux Q intensity q0 ð¼ Þ will decrease. The value of Q was L set to 0.5 for this set. For the second set we will keep q0 constant as L is varied leading to increasing Q with increasing L. The value of q0 is set to 0.2 for this set. For all of these simulation runs the width of the region was 50 and covered by a uniform grid. For the first set the length of the flow region was 100, and covered by a grid of 101 by 201 cells. For the second set the length was 60, with a grid of 101 by 121. The initial condition for the flow domain is the same as that applied for the case of a single finger generated from a point source described in the last section. In this case however, we apply a highfrequency, low-amplitude sinusoidal perturbation ðh , 1022 Þ to the surface flux. This perturbed flux is described by Eq. (11). The small gravity-driven background flux is again applied to ensure that the flow is maintained as a two-stage process. The initial saturation sin for the first set was set to 0.075, while it was set to 0.1 for the second set of runs.

5.2.1. Constant total flux with varying source width The analysis of the case of a constant total flux with varying source width is considered first. The numerical solution results are displayed in Fig. 7 for four cases of source width. For the case of L ¼ 1; essentially the case of a point source at the surface, we get a single finger. The plot in Fig. 7a shows the single finger at a time of 210. For this value of L there is no obvious distribution layer because the finger has larger width than the width of the source at the surface. The finger has a high saturation at the tip of the finger, and the saturation decreases monotonically behind the finger tip. For the case of the source width with L ¼ 10; the initial source breaks into two fingers at a depth of z . 7 as shown in Fig. 7b. The flow into the single finger for L ¼ 1 is equal to Q ¼ 0:5; while the flow into each of the two fingers in Fig. 7b is approximately equal to Q ¼ 0:25 since the total flux at the surface is approximately split equally between the two fingers. The plot in Fig. 7b shows the position of the fingers at

Fig. 7. Finger formation as a function of the length, L, of the surface infiltration source for a constant total infiltration across the source surface. The constant infiltration rate Q ¼ q0 L ¼ 0:5 and n ¼ 7; t0s ¼ 5; sin ¼ 0:075:


a time of 288, about 30% longer time than for that shown in Fig. 7a. While the position is about the same as shown in Fig. 7a, the time is longer because the flow in the fingers in this second case is smaller. This result shows that the finger velocity is related to the flux in the finger. When the width of the surface source is set to L ¼ 20 the flow breaks into three fingers. The fingers on the left and the right side of the formation first formed when the distribution layer was at a depth of z . 12: With time the distribution layer moved downward between these two fingers and a third finger formed at a depth of z . 22: The flux in the two outside fingers is about equal, while the flux within the middle finger is somewhat higher. This difference in flux explains why the position of the middle finger is about the same with the other two fingers at t ¼ 375: This difference also shows why the core of the middle finger appears to be wider and to have a higher saturation than the two adjacent fingers. In the last case, the source width is L ¼ 30; and the simulation result is shown in Fig. 7d. Here the flow breaks into four fingers. These fingers first formed at different depths as the infiltrating front moved downward. For this case the tips of the fingers are at a lower saturation, and the saturation within the finger cores is significantly lower than the core saturation observed in the previous cases. The total applied flux of 0.5 is split among the four fingers, but not uniformly. The distribution layer focuses the applied flux to the different fingers in a manner that does not produce an equal partitioning of the flux to

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the fingers. The position of the fingers are shown at t ¼ 456: So for this case the fingers required more than twice the time to reach the depth arrived at in the first case where all of the flux was concentrated into one finger. 5.2.2. Constant flux intensity with varying source width For this set of cases the flux intensity at the soil surface is kept constant while the length of the surface source is varied. The total flux through the soil surface then increases linearly with the length of the source. The saturation distribution within the flow region for the various cases of surface source length is illustrated in Fig. 8. All plots are for the same time level equal to t ¼ 96: For the case with L ¼ 5ðQ ¼ 1:0Þ a distribution layer is present, but the flux through that distribution layer is concentrated entirely into the single finger shown. The higher flux in the finger, five times the flux in the single finger shown in Fig. 8a, leads to a much wider finger tip and finger core, and the saturation at the finger tip and within the finger core is also higher. The velocity of the finger propagation is higher as well due to the higher flux within the finger. The flow distributions resulting from the larger source widths of L ¼ 10ðQ ¼ 2:0Þ; L ¼ 15ðQ ¼ 3:0Þ; L ¼ 25ðQ ¼ 5:0Þ; and L ¼ 30ðQ ¼ 6:0Þ are illustrated in Fig. 8c through Fig. 8g. The simulation results show that as the source width increases, while keeping the surface flux intensity constant, the flow breaks into an increasing number of fingers. There is one finger

Fig. 8. Morphology of the finger flow affected by the length, L, of the infiltration source for the constant intensity q0 ¼ 0:2 and n ¼ 7; t0s ¼ 5; sin ¼ 0:1: All cases are for the same time t ¼ 96:

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for L ¼ 10; two for L ¼ 15; three for L ¼ 20; three for L ¼ 25; and four for L ¼ 30: The fluxes within the individual fingers for each of these cases is approximately Q ¼ 2, Q ¼ 1.5, Q ¼ 1.33, Q ¼ 1.67, and Q ¼ 1.5, respectively. The fingers for all of these cases have width, velocity, and saturation distribution characteristics very similar to those seen in Fig. 8b. The small difference in flux within the individual fingers makes them have slightly different width and velocity, but overall the characteristics are very similar. Another aspect of interest here is that the depth of the distribution layer as well as the penetration depth is similar for cases Fig. 8c through Fig. 8g. This contrasts with the set of runs shown in Fig. 7 where the distribution layer thickness increases and the finger penetration depth decreases as the surface flux intensity decreases.

6. Discussion We observed from the plots in Fig. 7 that the distribution layer increases in depth as the surface flux intensity, qo ; decreases. While we have not run extensive tests of the effect of the surface flux intensity, based on the results presented in Fig. 7 we hypothesize that as the surface flux intensity decreases toward zero, the depth of the distribution layer will increase without bound. This of course means that the flow will be stable within the scale of the soil profile of interest. We believe this hypothesis is supported by the experimental results presented by Yao and Hendrickx (1996). They presented experimental results wherein they tested the effect of surface flux intensity on flow instability. They applied surface fluxes in the range from relatively high values, qo . 0:01; to relatively low values qo . 0:0001: The relatively high values correspond to the values used in experiments by previous investigators (e.g. Glass et al., 1989b)). Yao and Hendrickx showed that for the relatively high fluxes, the flow within the sand columns was unstable, while for the relatively low values the flow was stable. We should note that the flow was apparently stable at the scale of the columns studied by Yao and Hendrickx. Had the columns been larger in diameter or length, perhaps the flow even at these low flux rates might have become unstable at a larger time or space scale.

The idea that flow will be stabilized when the surface flux is relatively small is supported by a rational analysis of the relaxation equation, Eq. (4). For relatively high fluxes, the ›s=›t term will be significant, and therefore the non-equilibrium will be important and may lead to unstable conditions. In contrast, as the flux decreases the ›s=›t term becomes vanishingly small, leading to the conclusion that p . P; a result that means the flow will be stable. For the numerical results presented in Fig. 7d, the applied flux is qo . 0:017: We made a preliminary test of the hypothesis of stabilization of flow at low fluxes by reducing the applied flux by a factor of 10 to qo . 0:0017: For this case the propagating moisture profile was slightly non-monotonic, but within the depth of the profile the flow remained stable. Relating back to the experiments of Yao and Hendrickx (1996), an intriguing idea that should be considered in future analysis is that even at very low fluxes, it may be possible that within very deep unsaturated zones there would be sufficient distance/ time and sufficient disturbances to the flow field for flows to eventually become unstable. The lower flux tested, qo . 0:0017; yielded a slightly non-monotonic profile. Perhaps if the simulation were carried further, the front might become unstable at a larger depth. To study this possibility it will be necessary to develop both analytical solutions as well as numerical simulations of large unsaturated zone systems. Many of the simulated fingers shown in the section 5 had tips with saturations less than 100%. In most experimental and conceptual descriptions of finger flow the porous media at the tips offingers are described as being fully saturated. The difference between these experimental and conceptual descriptions and the numerical results presented here may be due to any one or all of the following reasons. First, the parameters for the non-equilibrium model have not been fully determined from experimental data. Until those parameters are determined, it will not be possible to know whether the trends shown in our simulated results are relative or absolute. Second, the initial saturations for our numerical solutions were above residual. In most experimental observations of finger flow the initial saturation is set to the air-dry conditions. One expects to have higher saturations at the advancing front for the drier initial condition. Third, experiments where saturation at the tips


of fingers has been measured have been for relatively high surface fluxes. Perhaps experiments with lower surface fluxes would show lower saturations at the tips. The numerical simulations presented used a specific rational form of equation to represent the relaxation function. We included in that function the dependence of both saturation and pressure. To date only a very small amount of data have been presented in the literature upon which to base an analysis of the saturation and pressure dependence of the relaxation function. The data by Topp et al. (1967) and Wildenschild et al. (2001) were for drainage processes only. Smiles et al. (1971) sought to measure nonequilibrium conditions for wetting processes, in addition to making measurements for drainage processes. The results from all of these studies showed that non-equilibrium is significant for drainage processes, but Smiles et al. concluded that nonequilibrium is non-existent for the wetting processes they studied. This conclusion conflicts with the imbibition studies of Nielson et al. (1962). To further study the mathematical nature of unstable flow processes it is essential that much work be done to quantify the degree of nonequilibrium for both wetting and drainage processes, including primary wetting/drainage as well as for secondary wetting/drainage. The horizontal diffusivity experimental technique, like that described by Nielson et al. (1962), in combination with an inverse modeling approach may be useful to derive the relaxation function for wetting processes. To quantify the function for wetting and drying processes it may be best to use pressure cell methods. These are normally operated in a pressure control mode, whereby the pressure is set and the volume of outflow/inflow is measured. A better approach would be to control the rate of outflow/inflow, and measure the pressure. The approach described by Wildenschild et al. (2001) where a syringe pump is used to control the flow rate is recommended. With the ever improved accuracy and precision of sensors and instrumentation, in-situ methods in laboratory scale models and in the field may also provide the means to derive the non-equilibrium functions. Our experience with the numerical solution to Eqs. (1) and (9), as represented by Eq. (14), has shown that more work needs to be done to improve the ability

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to handle sharp fronts of saturation and pressure. Adaptive grid refinement methods will need to be adopted to make this improvement. Such improvements in solution efficiency will be necessary before extending the solution to three space dimensions.

7. Conclusions In this paper we have confirmed the conclusions of Egorov et al. (2002) and Dautov et al. (2002), that solutions of the Richards equation are stable, while the simulation of fingering can be achieved by incorporating both dynamic (relaxation) and static (hysteresis) memory effects into the solution of the equation for mass balance of flow. As shown by Dautov et al., the relaxation mechanism generates fingers while hysteresis leads to the persistence of the fingers. Other mechanisms for finger generation may exist, but these still need to be postulated and examined. Application of the numerical solution to examine the effect of water repellency on flow instability shows that the sensitivity of the saturation –pressure relationship to the degree of water repellency is sufficient to cause flows to be unstable. Water repellency affects the shape of the wetting curve of the saturation –pressure relationship. The change in shape is described in terms of a flattening of the relationship, and a concurrent increase in the water entry pressure. Even for slight water repellency a porous medium with well-graded particle size distribution can behave like a porous medium with more uniform grading. Detailed experiments are needed to further quantify the relaxation function adopted in this paper. That function has received significant attention from a theoretical standpoint, but much work remains to derive experimental data. Several approaches can be used to derive these experimental data, including sorptivity methods, volume controlled pressure cell methods, and in-situ laboratory/field methods. Additional work is needed to improve the numerical methods to solve the governing Eqs. (1) and (9), especially for the case of initially dry media where extremely sharp wetting fronts are involved. Adaptive grid refinement techniques are appropriate to address this issue.


Chapter 24

Modeling implications of preferential flow in water repellent sandy soils C.J. Ritsema* and L.W. Dekker Alterra, Green World Research, P.O. Box 47, 6700 AA Wageningen, Netherlands

Abstract Leaching risks of surface-applied agrichemicals in water repellent soils can only be quantified with an acceptable degree of accuracy if knowledge of the underlying processes, principles and an appropriate simulation model are available. The present study aimed to investigate water flow and solute transport processes in the unsaturated zone of a water repellent sandy soil, and to indicate consequences for modeling. Soil blocks with a length, width and depth of 1.2, 0.6, and 0.52 m, respectively, were sampled in the Ouddorp water repellent sandy soil to investigate three-dimensional soil water content distributions. Preferential flow patterns were clearly visible in soil blocks sampled after distinct rain events. Additional TDR measurements revealed that preferential pathways develop rapidly during severe rain storms, causing infiltrating water to be preferentially transported to the deeper subsoil. Further, preferred pathways recurred at the same sites during all rain events. Simulations with a twodimensional flow and transport model indicate that preferential flow paths will only form during infiltration into dry water repellent soils, i.e. in the range below the critical soil moisture content. Based upon the obtained results, indications are given on how to incorporate this preferential flow and transport process in current one-dimensional simulation models.

1. Introduction Water repellency in soils is currently receiving increasing attention from scientists and policy makers, due to the adverse and sometimes devastating effects of soil water repellency on environmental quality and agricultural crop production. Soil water repellency often leads to severe erosion and runoff, and rapid leaching of surface-applied agrichemicals. To begin with, the effects of soil water repellency on runoff generation were studied quite intensively. Osborn et al. (1964a) and Krammes and DeBano (1965) had already indicated that water repellent soil * Corresponding author. Tel.: 317-474266; fax: 317-419000. E-mail address: [email protected] (C.J. Ritsema). q 2003 Elsevier Science B.V. All rights reserved.

surfaces might seriously affect water infiltration and, as a result, might promote runoff in hilly regions. Since then, several studies around the world have dealt with this phenomenon (Krammes and Osborn, 1969; McGhie, 1980b; Burch et al., 1989; Witter et al., 1991; Crockford et al., 1991; Jungerius and ten Harkel, 1994). Secondly, it had already been shown by Jamison (1945, 1946) that large variations in water content exist in water repellent soils, indicating that infiltrating water followed specific pathways instead of moving in a planar wetting front. Van Dam et al. (1990) and Hendrickx et al. (1993) showed that water and solutes may travel rapidly through water repellent soils, bypassing large parts of the unsaturated zone. The unsaturated or vadose zone of the soil, particularly the biologically and chemically reactive topsoil,

260


acts as an important buffer for various chemical compounds. When water flow bypasses large parts of the unsaturated zone, the risk of pollution of any receiving water bodies, such as groundwater or surface water, might seriously increase. Theoretical considerations by Saffman and Taylor (1958), Hill and Parlange (1972), Raats (1973), Philip (1975a,b), Parlange and Hill (1976), Diment et al. (1982), Diment and Watson (1983), Hillel and Baker (1988) and Baker and Hillel (1990) already indicated possible conditions under which perturbations at an infiltrating wetting front might grow into ‘fingers’ or ‘preferential flow paths’ instead of flattening out by lateral diffusion. Based on theoretical considerations, Raats (1973) was the first to indicate that soil water repellency might lead to the formation of preferential flow paths. Field evidence of preferential flow of bromide through a water repellent sandy soil, resulting in early arrival times, and high bromide concentrations in the groundwater, was presented by Hendrickx et al. (1993). Attempts to model flow and transport in water repellent soils have been undertaken by Van Dam et al. (1990) and De Rooij (1995). In both cases, however, analytical approaches were used which neglected specific processes like hysteresis in the water retention function, which is of significance in water repellent media. More recently, Nieber (1996) and Ritsema et al. (1998a) presented a numerical two-dimensional approach for simulating formation and recurrence of preferential flow paths in water repellent media. Unraveling and understanding water flow and transport processes in water repellent field soils is essential if we are to arrive at a user-friendly, easy applicable simulation tool. Leaching risks of surfaceapplied agrichemicals in water repellent soils can only be quantified with an acceptable degree of accuracy if knowledge of the underlying physics and an appropriate simulation model are available. The aim of the present study is to investigate water flow and solute transport processes in the unsaturated zone of a water repellent sandy soil, with special attention to: † obtaining high-resolution soil water content distributions by sampling of soil blocks; † monitoring the formation of preferential flow paths during successive rain events;

† demonstrating the effect of antecedent soil water content on infiltration behavior; † indicating the implications for development of one-dimensional water flow models.

2. Materials and methods 2.1. Soil characteristics Field experiments were carried out on a grasscovered water repellent sandy soil, classified as a Mesic Typic Psammaquent (De Bakker, 1979), near Ouddorp in the southwestern part of the Netherlands. The soil consists of an approximately 10 cm thick humous surface layer, on top of non-calcareous fine dune sand to a depth of 75 cm, overlying calcareous fine sea sand. The organic matter content of the humous layer is approximately 20 wt%, while below a depth of 10 cm it is less than 0.5 wt%. The clay content of the soil profile is less than 3%. Potential water repellency in the upper 50 cm of the Ouddorp soil is extremely high, except in the shallow surface layer. Deeper in the profile water repellency is absent (Dekker and Ritsema, 1994b). The critical soil moisture contents below which a soil is water repellent and above which it is wettable, ranges between 25 and 2 vol% for the humous surface layer and the layer at 47 – 52 cm depth, respectively (Ritsema et al., 1997a). 2.2. Soil blocks In order to obtain three-dimensional soil water content patterns, two soil blocks, each with a length of 1.2 m, a width of 0.6 m and a depth of 0.52 m, were sampled at the Ouddorp experimental site on 20 November 1991 and 1 September 1992, respectively. Sampling took place close after a period of rainy weather. Each soil block was sampled using 5 cm wide and 5 cm high (100 cm3) steel cylinders. Sampling took place in a regular grid, with 20 by 10 samples per layer. A total of 7 layers were sampled per soil block, at depths of 0 –5, 7 – 12, 14.5 –19.5, 22– 27, 29.5 –34.5, 39– 44 and 47 –52 cm, yielding a total of 1400 samples per block. The sampling grid applied was designed on the basis of previous

Modeling implications of preferential flow in water repellent sandy soils

experiences within the same experimental field during attempts to optimize sampling strategies for detecting preferential flow paths (Ritsema and Dekker, 1996a). All samples collected were used for determining the volumetric soil water contents using the ovendrying method. Sample spatial coordinates and data on the related soil water contents were used for visualizing the three-dimensional water content patterns using the IRIS Explorer modular visualization software environment, on a SGI Indigo (R4000XZ) workstation (Heijs et al., 1996; Ritsema et al., 1997a). Techniques used included the visualization of three-dimensional iso-surfaces, combined with intersecting horizontal planes. Typical soil water content iso-surfaces were visualized for each soil block, in order to reveal fingered flow patterns most distinctly.

2.3. TDR measurements In order to measure volumetric water contents of the soil in time at different positions in the profile, an automated TDR measuring system was constructed (Ritsema et al., 1997b). The volumetric water content measurements were done automatically at 98 different positions using the commercially available TRASE model 6050X1 TDR device. The standard three-rod probes were installed horizontally in the wall of a pit, in 7 rows of 14 probes each, covering an area 2 m wide and 0.7 m deep. The probes were placed 15 cm apart in the horizontal direction (center-to-center distance) at depths of 4, 12, 20, 30, 40, 55 and 70 cm. The probes were connected to 7 multiplexer cards with 16 channels each. Every 3 h, the TDR device automatically started a measurement series, and the volumetric water content measured for each probe was stored with date and time information. One measurement cycle along the 98 probes could be done in around 20 min. Up to 20,000 separate measurements could be stored in this way, equivalent to a data set covering 25 days. The measurement data were retrieved from the TDR device once every three weeks using a laptop PC. In total measurements continued for around 8 months, in which almost 200,000 volumetric soil water content values were recorded, which were used to construct

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two-dimensional water content distributions for every 3 h time-step. In all, around 2000 graphs were made, of which a selection is presented in this study. 2.4. Numerical simulations Numerical simulations of Richards equation and the convection– dispersion equation were performed for both an initially dry and an initially field-moist soil at the Ouddorp site. The purpose was to demonstrate the effect of antecedent moisture content distribution on the pattern of water flow and solute transport in the soil profile. The water flow and solute transport were simulated using a two-dimensional finite element solution of Richards equation and the convection – dispersion equation, as described by Nieber (1996) and Ritsema et al. (1998a). The flow solution method uses quadrilateral elements with a specialized downstream weighting of the hydraulic conductivities between adjacent node points. Hysteresis in the water retention characteristic was incorporated in the numerical solution. The convection– dispersion process was solved using a particle tracking random walk method (PTRW) described by Abulaban et al. (1998). 50,000 particles were used to represent the solute mass.

3. Results 3.1. Soil blocks For each of the two soil blocks, the minimum, maximum and mean water content values per soil layer are listed in Table 1, together with the standard deviations and coefficients of variation. Both soil blocks contained a wet surface layer upon a relatively dry subsoil. Mean soil water content for the surface layer equaled 41.0 and 34.2% for the 20 November and 1 September soil block, respectively. Mean soil water content in the water repellent subsoil varied between 3.9 and 7.5%. Highest variations in soil water content were found in the zone between 14.5 and 19.5 cm depth for both soil blocks. Standard deviations ranged from 1.3 to 6.0%, and coefficients of variation from 12.3 to 87.2%.

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Table 1 Volumetric soil water content for each depth ðn ¼ 200Þ for both soil blocks Depth (cm)

Minimum (%)

Maximum (%)

Mean (%)

SD (%)

CV (%)

20 November 1991 0–5 19.5 7–12 1.6 14.5– 19.5 0.6 22–27 1.4 29.5– 34.5 1.1 39–44 1.2 47–52 1.2

52.4 20.0 17.4 16.4 11.0 13.7 13.9

41.0 6.7 5.0 5.2 4.0 3.9 5.6

6.0 4.8 4.2 3.8 2.7 2.7 2.9

14.6 71.6 84.0 73.1 67.5 69.2 51.8

1 September 1992 0–5 22.5 7–12 1.5 14.5– 19.5 1.0 22–27 1.2 29.5– 34.5 1.7 39–44 3.6 47–52 3.5

44.8 17.5 15.3 12.6 11.9 9.6 10.3

34.2 4.4 3.9 5.0 5.7 6.8 7.5

4.2 3.5 3.4 3.5 2.7 1.3 1.3

12.3 79.5 87.2 70.0 47.4 19.1 17.3

SD and CV denote standard deviation, and coefficient of variation, respectively.

Fig. 1 shows the three-dimensional soil water content distributions for each soil block. Each visualization is based upon 1400 soil water content measurements. Each image shows a specific soil water content iso-surface, depicting the preferential flow paths most clearly. Further, two horizontal planes have been visualized for each image, at depths of 17.0 and 49.5 cm below the soil surface. These depths coincide with the centers of the 14.5 – 19.5 and 47– 52 cm soil layers. From Fig. 1, it can be seen that preferential flow patterns were present in both soil blocks, and that the preferred pathways always started at a depth of around 10 cm. It has been shown earlier by Ritsema et al. (1997a) that as time progresses after a major rain event, preferential flow patterns gradually become irregular or even unrecognizable, due to processes like drainage, redistribution of the water, and evaporation. In the long term, soil water content differences between zones with and without preferred pathways might become minimal. However, even small differences in soil water content might influence water flow patterns during successive infiltration events significantly, as has been shown for instance by Ritsema et al. (1998a).

3.2. TDR measurements In order to illustrate the process of preferential flow path formation and recurrence, a selection was made of three pronounced rainy periods. Table 2 summarizes information about these rain events. The duration of the rainy periods was between 8 and 10 days, and the total rainfall ranged from 40 to 81 mm per event. Subtraction of the potential evapotranspiration from the rainfall quantities yielded a conservative estimate of the amount of infiltrated water during the selected periods. Estimated minimum infiltration varied between 36 and 79 mm (Table 2), with the most distinct event during the month of December. For each rainy period, the soil water content distributions measured within the TDR transect are shown just before, during (twice), and at the end (or after cessation) of the rainfall (Fig. 2). Volumetric soil water contents before the rain events (Fig. 2, left hand side) were generally between 0 and 10% for the water repellent subsoil, and up to 10 –25% for the humous layer, although there were some differences. No preferential flow patterns were present before the start of the rain events, but these emerged during all rainy periods. The preferential flow paths protruded through the water repellent layer and reached depths of around 60 – 70 cm. Further, the observed patterns indicate that preferential flow paths recur at the same locations during successive rain events. It has been shown by Ritsema et al. (1998a,b) that the water retention function of the Ouddorp sand is extremely hysteretic, characterized by a steep main wetting branch. This is typical of water repellent materials, and explains why vertical fingers can exist in dry soil (Nieber, 1996). The recurrence of fingers at the same locations can be attributed to the hysteretic water retention character of the Ouddorp sand. After the cessation of rainfall, the water content of the soil decreases throughout the profile, within as well as outside the fingers. After a certain period of time, water content variations in previous locations with and without fingers decrease, but differences do remain, due to their different wetting history. Even when the soil water content differences have become very small, water will still show a tendency to flow along the previous pathways, as the water content in these places is slightly higher than that in the immediate vicinity of the previous


263

Fig. 1. Spatial distribution of volumetric soil water content in two grass-covered sand blocks, each with a length of 1.2 m, a width of 0.6 m and a depth of 0.52 m. Three-dimensional iso-surfaces and two-dimensional horizontal cutting planes at depths of 17 and 49.5 cm have been visualized. Each image is based upon 1400 soil water content measurements. Soil blocks were sampled at Ouddorp, the Netherlands, on 20 November 1991 (A) and 1 September 1992 (B), respectively.

fingers. Even only slightly higher soil water content implies significantly larger hydraulic conductivities, as was illustrated by Ritsema and Dekker (1994b), promoting the flow of water through the wetter soil.

This process of preferential flow path recurrence in soils might continue for an unlimited period of time, except that human influence might change the pattern. The particular grass-covered experimental site used

264


Table 2 Rainfall duration and quantity, potential evaporation, and minimum net infiltration for three pronounced rainy periods during the period September 1994 until January 1995 at the Ouddorp experimental site Rainy period

Rainfall duration (days)

Rainfall amount (mm)

Potential evapotranspiration (mm)

Minimum infiltration (mm)

1 2 3

10 8 10

47 40 81

11 3 2

36 37 79

in the present study had not been tilled for several decades. If fields are used for the cultivation of crops, preferential flow paths might retain stable spatial positions for one growing season only. In the following growing season, tillage treatments and seed bed preparation might cause fingers to occur at other locations than the year before. Therefore, persistent spatial preferential flow patterns like those found in the present study might develop in, for instance, untilled agricultural fields, nature reserves and forests.

3.3. Effect of antecedent soil water content To evaluate the effect of initial soil water content upon the generation of infiltration patterns, a two-dimensional numerical simulation model (Nieber, 1996) was used. The soil (Mualem – Van Genuchten) parameters applied in the simulations are presented in Table 3. Note that the parameters are given for both the drainage and wetting branches of the three respective soil layers in order to account for hysteresis. Two scenarios were simulated, one with infiltration into initially dry and the other into initially field-moist Ouddorp soil. The flow domain was 50 cm wide and 50 cm deep, while the humous top layer, water repellent sandy layer and wettable subsoil thicknesses were arbitrarily set at 5, 15 and 30 cm, respectively. The interface between the humous top layer and the water repellent sand layer was assumed to be wavy. For both the initially field-moist and the initially dry cases, rainfall was set at a constant intensity of 1.0 mm d21. The initial conditions were similar for

Fig. 2. Spatial distributions of soil water content in the 2 m long and 0.7 m deep TDR trench before, during and after the major rainy periods selected.

265

Modeling implications of preferential flow in water repellent sandy soils Table 3 Soil parameters used in the model simulations Soil layer

us

ur

a drainage (m21)

n drainage

1. Humous

0.435

0.04

0.69

1.63

2.74

1.69

1.0

2. Water repellent Initially dry Initially moist

0.365 0.365

0.005 0.06

1.82 1.82

3.72 3.72

19.56a 2.64b

10.00a 3.72b

1.0 1.0

3. Wettable subsoil

0.41

0.02

2.93

3.02

11.65

1.92

1.0

a b

a wetting (m21)

n wetting

Ks (m d21)

Main wetting curves for the water repellent layer. Boundary wetting curves for the water repellent layer.

both simulations, except for the initial water content of the water repellent sand layer. The pressure head for the entire, initially dry water repellent soil was set at 2 200 cm and wetting occurred according to the main wetting curve (Table 3). For the initially fieldmoist soil, initial pressure in the water repellent layer was set at 2 100 cm, and wetting followed the boundary wetting curve (Table 3). Pressure heads for the humous top layer and the wettable subsoil were set at 2 500 and 2 100 cm, respectively, for both the initially dry and initially field-moist cases. To simulate solute transport, an instantaneous pulse of bromide was imposed at the top of the flow domain at the start of the rainfall. The solute was applied at a rate of 2.0 g m22. In the simulations, local dispersivities were set at 0.001 m for the longitudinal and 0.0002 m for the transverse dispersivity. The bottom boundary condition used in the simulations was free drainage, i.e. unit hydraulic head gradient. Figs. 3 –7 show numerical results for the evolution of an infiltrating wetting front and an applied bromide pulse in an initially dry (water repellent) and an initially field-moist (wettable) Ouddorp soil. Simulated water distributions in the soil profile for the initially dry Ouddorp soil are shown in Fig. 3 for different times. At the start of the simulation period, the domain above the wavy boundary (humous top layer) is wet enough to be wettable, while below this boundary, but above the interface between the two sand layers, the soil is water repellent. Following the start of the rainfall, the upper part of the profile wets until it becomes saturated. The water does not readily enter into the underlying water repellent soil, and will

not enter it until the pressure head reaches the water entry value of the water repellent soil. Once the water pressure head at the wavy boundary reaches the water entry value, the front progresses downward at the troughs of the boundary, forming preferred pathways. The preferential paths continue to progress downward until they reach the wettable bottom layer, at which point they rapidly drain into that layer. Large quantities of water are stored in the preferential flow paths and in the upper part of the top layer prior to the pathways reaching the bottom layer, so once they

Fig. 3. Numerical results showing the growth of fingered flow pathways during infiltration in an initially dry Ouddorp soil. Unit of time is day.

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Fig. 5. Computed bromide breakthrough curves at a depth of 50 cm (bottom flow domain) for the initially dry (unstable) and initially field-moist (stable) cases.

Fig. 4. Numerical results showing the heterogeneous bromide transport during infiltration in an initially dry Ouddorp soil. Unit of times is days.

reach that layer the movement is rapid. Steady-state flow is achieved within a few days after the preferential flow paths have reached the wettable layer. The distribution of solute through the initially dry Ouddorp soil is illustrated in Fig. 4, which shows that the solute transport is not gaussian, but instead shows a significant tailing of the solute distribution. An interesting feature of the distribution is the amount of solute that is trapped in the upper part of the domain in the vicinity of the place where the water enters the preferential flow paths. This solute is in a low velocity zone and moves out of that zone very slowly. This confirms field observations described earlier (Ritsema et al., 1993; Ritsema and Dekker, 1995, 1998), showing that significant portions of an applied bromide tracer remained trapped in the humous top layer for long periods. The breakthrough curve for the unstable case (initially dry soil) is displayed in Fig. 5 along with that for the stable case (initially field-moist soil). The breakthrough curve for the initially dry soil is sharp, and there is significant tailing due to the slow release of the trapped solute.

For the initially field-moist Ouddorp soil, computed water distributions in the 50 cm soil profile are shown for different times in Fig. 6. The progression of the wetting front is visible in these diagrams, but the maximum saturations are quite low because of the low rainfall intensity imposed. It can be observed that under these conditions, the wetting front progresses downward in a stable manner. The bromide transport is shown in Fig. 7. The distribution of bromide in the direction of flow indicates a more gaussian-like spreading, as would be expected under stable flow conditions. Apparently, the distribution of bromide during infiltration into the field-moist soil was affected to some extent by the presence of the wavy interface (Fig. 7). The breakthrough curve for the solute at the bottom of the flow domain is illustrated in Fig. 5. The shape of this breakthrough curve indicates that the dispersion process produces a gaussian distribution of the solute. Bromide arrival at the bottom of the flow domain is much slower than computed for the initially dry situation (Fig. 5). This means that when adsorbing compounds are surface-applied to an initially dry Ouddorp soil, these compounds will not only travel to the groundwater much faster than when they are applied to an initially field-moist soil, but the dose of these compounds entering the groundwater will be


267

Fig. 6. Numerical results showing the formation of a stable wetting front during infiltration in an initially field-moist Ouddorp soil. Unit of times is days.

Fig. 7. Numerical results showing the transport of bromide during infiltration in an initially field-moist Ouddorp soil. Unit of times is days.

268


much higher as well, due to the fact that significant parts of the topsoil (i.e. buffering capacity) will be bypassed by the fingered flow pathways.

4. Modeling implications Mathematical solutions to Richards equation are inherently stable in a physical sense (Milly, 1988), but when hysteresis in the water retention function is incorporated in two-dimensional numerical models, the mathematical solutions may yield unstable flow as a natural outcome (Nieber, 1996). This means that not incorporating hysteresis in a two-dimensional model for water repellent or coarse textured soils may result in seriously misleading outcomes, especially for solute transport prediction. Most simulation models are based on some form of Richards equation and fail to directly address the possible occurrence of unstable flow (Van Genuchten and Jury, 1987), especially the one-dimensional simulation models. However, most field studies, even in sandy soils (Steenhuis et al., 1996a; Ritsema, 1999), have shown that preferential flow is the rule rather than the exception, and may partly account for inaccuracies in the prediction of water and solute movement. Since water repellency can be plant-induced, and often occurs in field soils (Wallis and Horne, 1992), preferential flow may be more common than is presently thought. In our opinion, current models need to be adapted to account for the unstable flow phenomenon if they are to be employed to full benefit. Incorporating hysteresis in one-dimensional flow and transport simulation models might not automatically result in predictions comparable with the ones obtained with the two-dimensional numerical model presented here. To incorporate adequately the process of preferential flow in water repellent soils into for instance existing (two domain) one-dimensional flow and transport models, some basic assumptions can be made, and rules assigned. † Use can be made of the Richards equation, both for the preferential flow domain and the immobile surrounding matrix. † No daily or weekly inputs of rainfall amount can be used. Actual rainfall rates/intensities should form the basis for the calculations.

† With soil water contents above the critical level, soil water flow is horizontal uniform (stable flow). † In case soil water contents drop below the critical level in the potential water repellent zone of the soil profile, a zero flux condition should be imposed at the upper layer of this zone. This allows for the formation of a perched water table above the dry water repellent zone. † The saturated conditions above the water repellent zone might result in lateral flow (depending on layer anisotropy etc.) towards positions where preferential flow paths might form. † The diameters of the preferential flow paths might be calculated according to the relationship provided by Selker et al. (1991), showing a relationship between the texture of a soil and the flux towards the fingers divided by the saturated soil moisture content of a soil. The amount of fingers will depend on the actual intensity and duration of the rainfall. † With sufficiently high fluxes, soil water contents in the preferential flow paths will be near saturation. † Lateral diffusion from preferential flow paths towards surrounding dry soil will be minimal in extremely water repellent soils, and thus can be denied in first instance. † Preferential flow paths might disappear either as a consequence of prolonged drying of the soil profile (soil water contents within the fingers drop below the critical level), after extremely wet periods causing many fingers to be formed in the profile, or after a drastic rise of the groundwater table. † Additional model input will at least consist of values of critical soil water contents per layer, and anisotropy characteristics of the surface layer. Model output will consist of soil water content, pressure head, and solute concentration profiles for both the preferential flow domain and the surrounding immobile matrix. The ideas mentioned above will be used as a basis for adapting the well-known SWAP model (Van Dam et al., 1997) in order to develop an easily usable tool for predicting flow and transport processes in water repellent soils. It is foreseen that a first operational version of the improved SWAP model will become available at the beginning of the year 2000.


5. Summary Three-dimensional soil water content distributions in the Ouddorp water repellent soil clearly indicated the presence of wet preferential flow paths embedded in extremely dry soil. The preferential flow paths started just below the surface layer at a depth of approximately 10 cm, and protruded through the entire water repellent zone up to a depth of 60 –70 cm. The water content distributions obtained by using the automated TDR device showed that the formation of preferential flow paths depends on the wetting history of the soil and the rainfall characteristics. Preferred pathways developed rapidly during severe

269

rain storms, causing significant portions of the infiltrating water to flow preferentially to the deeper subsoil. The preferential flow paths recurred at the same sites during successive rain events, due to the extreme hysteresis in the soil water retention characteristics of the Ouddorp water repellent soil. The numerical results clearly indicated that perturbations at an infiltrating wetting front can either propagate or dissipate, depending on the antecedent soil water content of the soil. With initial soil water contents below the critical soil water content level, preferential flow paths do form during an infiltration event, while under initially wetter conditions stable flow can be expected to occur.


AMELIORATION TECHNIQUES AND FARMING STRATEGIES ON WATER REPELLENT SOILS


Chapter 25 Clay spreading on water repellent sands M.A. Cann* Primary Industries and Resources, Struan, P.O. Box 618 Naracoorte, SA 5271, Australia

Abstract Water repellence is a serious land degradation issue and a constraint to productivity on approximately two million hectares across South Australia. Sandy soils that have poor coverage of pasture and crops lend themselves to the risk of wind erosion and loss of yields. A technique that has once seen highly unproductive pasture country now producing crops is ‘clay spreading’. As with many discoveries, the ability of clay to counteract the effects of water repellent sand was unearthed by accident thirty years ago by Clem Obst, farmer at Mundulla, SE of South Australia. Today, approximately 37,000 ha of land in South Australia has been clay spread, of which 32,000 ha is in the SE of South Australia. The application of clay to water repellent sand increases the effectiveness of pre-emergent herbicides, amelioration of water repellency, improves the germination, establishment and yields of pasture plants and crops and nutrient and moisture retention in the topsoil. Water repellent sand pastures generally contain little, if any, legume content and produce as little as 400 kg/ha dry matter compared to clayed pastures (3250 kg/ha) which are sub-clover or lucerne based. Spreading clay has generally doubled cropping yields. The technique of clay spreading is continually being modified to enable landholders to attain maximum economic returns while ensuring the long-term sustainability of agriculture on water repellent sands.

1. Introduction Amelioration of water repellent soils by the addition of materials with high surface area has been demonstrated by several researchers (Roberts, 1966; King, 1974a; Bond, 1978; Ma’shum, 1988). Ma’shum et al. (1989) regarded clay dispersibility as a major factor influencing the efficiency of clay in reducing water repellence. Clay that disperses exposes a greater surface area (Ma’shum et al., 1989) available for wetting up. The mechanism by which clay interacts with the organic/sand surfaces has not been thoroughly investigated, however, Ward and Oades (1993) suggest that the clay spreads over

* Fax: þ61-8-8764-7477. E-mail address: [email protected] (M.A. Cann). q 2003 Elsevier Science B.V. All rights reserved.

the organic coated sand grains, masking the hydrophobic sand surface, and exposing a hydrophilic clay surface. Clay addition to water repellent sand has no effect on water repellence until it has been exposed to a wetting and drying cycle (Ward and Oades, 1993). The wetting phase allows the clay to disperse and during the drying phase the surface tension forces the sand and clay into intimate contact which allows for the resultant lowering of water repellence (Kemper et al., 1974; Ward, 1993). Roberts (1966) examined the effect of fine particle amendments and found that clay (dominantly kaolinite) significantly increased soil moisture content. The clay may also provide some needed nutrients (availability and retention) to the infertile sands, thus improving CEC, germination and pasture/crop production.

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M.A. Cann

Ward (1993) and Van Dam et al. (1990) report that clay applied to a water repellent sand will remain in the topsoil for an extended period of time and the even mixing of clay into the sand should act to reduce water repellence for several years (Ward, 1993). Anecdotal evidence from Obst (personal communication) suggests that a paddock clayed 30 years ago is as good today as when it was first clayed, if not better, due to improved management practises on a previously water repellent soil. Fig. 1 compares a typically poor quality pasture on water repellent sand (Fig. 1A) to a highly productive pasture after clay spreading (Fig. 1B). Clay rates being spread on farms across the SE vary from 100 – 250 t/ha clay on sandhills to 40 –100 t/ha on sand over clay flats. It is important that the clay be incorporated evenly into the top 10 – 15 cm of soil for greatest benefits. Trials have been instigated to investigate the long term effect of clay (rate and incorporation) on

alleviating water repellence. Farmer groups have trials that evaluate the changes in soil moisture, nutrition and production in an effort to improve long term sustainability of agriculture on water repellent sand.

2. Methods 2.1. Assessment of water repellence The Primary Industries and Resources, SA experimental site is located in the upper south east of South Australia. The soil description of the site is Basic, Arenic, Bleached-Orthic, Tenosol; medium, nongravelly, sandy, very deep (Isbell, 1996). The treatments within the experimental site were four rates of dispersible clay (0, 50, 100 and 150 t/ha), the clay was obtained from a property at Mundulla, in the SE; and two application management strategies (surface applied clay and incorporation of clay to a depth of 7.5 cm). The treatments were replicated three times. The site was established in June 1991 and the plots were sampled six times in the first year and periodically over the next seven years. Five samples per replicate were taken and pooled at every sampling, the average of the three replicates is recorded in this paper. The sampling involved taking core samples at depths of 0 –2.5, 2.5 –5.0, 5.0 –7.5 and 7.5 –10 cm. The soils were dried for 24 h at 458C and then lightly sieved , 1 mm. The degree of water repellence was assessed by the molarity of aqueous ethanol droplets (MED values) (King, 1981) which were absorbed by the soil in 10 s. Samples were tested at 208C, or the data adjusted to this temperature, using the relationship of King (1981). 2.2. Assessment of clay rate on dry matter production and soil moisture content

Fig. 1. Comparison of a typically poor quality pasture on water repellent sand (a) to a highly productive clover–serradella pasture after clay incorporation into water repellent sand (b).

The Bangham/Western Flat Agricultural bureau set up one trial in their district in the upper south east in 1997 to compare the effect of pasture growth on varying clay rates on water repellent sand. The soil description for this site is Basic, Arenic, Bleached-Orthic, Tenosol; medium, non-gravelly, sandy, very deep (Isbell, 1996). The treatments within

Clay spreading on water repellent sands

the experimental site were clay rates of 0, 40, 80, 160 t/ha of clay, where the clay was incorporated 10 cm into the sandy surface. Replication did not occur in this trial. The site was on an undulating sand rise and the plots ran perpendicular to the contours of the land. The site was sown with a mixture of clover and serradella. Three positions on each plot were used to compare the differences in dry matter and soil moisture content. The ‘crest’ position of the plot was on the crest of the sand rise, ‘midslope’ was midslope and ‘low’ was the lower slopes of the sand rise. Small cages were strategically placed at these positions and one square metre plots were cut for the assessment of dry matter for each treatment. Soil moisture content readings were taken from each treatment and position. Soil moisture content was measured using a CS615 Water Content Reflectometer. 2.3. Assessment of pH change with clay rate The Bangham/Western Flat Agricultural Bureau set up one trial in their district in the upper south east in 1995 to compare the change in cropping yields, water repellence and pH with varying rates of clay on water repellent sand. The soil descriptions for this site are Basic, Arenic, Orthic, Tenosol; medium, non-gravelly, sandy, very deep and Brown, Mottled-Subnatric, Eutrophic, Sodosol; medium, non-gravelly, sandy, deep (Isbell, 1996). The treatments within the experimental site were 0, 60, 100, 135, 180, 240 and 400 t/ha of clay. The clay was incorporated 10 cm and the plots were replicated twice. Soil analyses were carried out in Spring of each year. Ten samples per replicate were pooled, then subsampled, for analyses and the results were averaged for each treatment. Observation of change in soil pH is recorded in this paper.

3. Results 3.1. Effect of clay rate on water repellence Before clay application, MED values for the site averaged 3.42 for the top 7.5 cm and 2.90 from 7.5 –10 cm. The incorporation of clay significantly

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reduced the MED values within the first month of treatment (Figs. 2 –5), with 100 and 150 t/ha clay rates reducing water repellence levels to MED values of 1.2 within the top 7.5 cm of soil. The MED values have continued to remain at relatively low levels (King, 1981) since the site was established in 1991 with 150 t/ha of incorporated clay, whereas 50 and 100 t/ha have reduced water repellence, but not to the same degree. Water repellence within the 7.5 –10 cm soil layer has continually fallen with time. Water repellence within the top 2.5 cm with all treatments have increased slightly with time and may be attributed to improved organic matter levels within this soil layer. The nil clay plot indicates that water repellence in the top 7.5 cm of soil has increased slightly with time. In comparison, surface applied clay at 100 t/ha (Fig. 6) initially has had a significant effect on reducing MED values in the top 2.5 cm. However, with time water repellence has slowly increased. Although the trend of water repellence for soil at depths from 2.5 to 10 cm have decreased from MED 3.0 to 2.5, they rate as severe water repellency as described by King (1981). The corresponding incorporated clay amounts vary from MED 1.2 to MED 2.8 which rate moderate to severe on the water repellency scale (King, 1981). 3.2. Effect of clay rate on dry matter production The greatest effect of the clay on dry matter is shown in Fig. 7 where 160 t/ha clay has been applied. The dry matter content at the low position was 4300, 3000 kg/ha midslope and 3250 kg/ha on the crest of the rise. In direct comparison the nil clay effect on dry matter decreased from 1900 kg/ha at the low position to 430 kg/ha on the crest of the rise. Pasture composition is very important for the health of stock. It was observed that where nil clay was applied the site was predominantly silvergrass (low value for stock feed) at the low position and silvergrass and bare ground at the crest. In comparison, as the clay rate increased the percentage of clover and serradella persisting increased at the crest. The pasture was dominated with clover at the low and midslope positions of the sand rise with 160 t/ha clay.

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Fig. 2. Effect of no clay on water repellence over time.

Fig. 3. Effect of 50 t/ha of incorporated clay on water repellence over time.


Fig. 4. Effect of 100 t/ha incorporated clay on water repellence over time.

Fig. 5. Effect of 150 t/ha incorporated clay on water repellence over time.

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Fig. 6. Effect of 100 t/ha surface clay on water repellence over time.

Fig. 7. The effect of clay application rate on dry matter production at different positions on a sand rise.


3.3. Effect of clay on soil moisture Soil moisture content (% by volume) was measured in September 1997 to a depth of 10 cm (Fig. 8) and to a depth of 21 cm. The nil clay shows zero moisture at all positions on the sand rise as do the crest of the sand rise for 40 and 80 t/ha clay at the time of sampling. The 160 t/ha clayed strip has the highest readings of soil moisture on all positions of the sand rise. It is interesting to note that at the low position on the rise the moisture readings are lower than expected. However, this may be due to the higher production (Fig. 7) of pasture at this site. At a depth of 21 cm zero moisture was found at the crest with all of the treatments. The higher rates of clay exhibited moisture at the low and midslope positions (6% by volume). The nil clay treatment exhibited zero moisture at all positions on the sand rise. 3.4. Effect of clay on soil pH Soil pH increased by the addition of clay to the water repellent sandy topsoil. The sand pH (0– 10 cm) at the site averaged 6.3 (measured in water) before clay spreading. The pH of the clay used for

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spreading was 9.2. Fig. 9 shows that the ‘new topsoil’ pH has increased in the first year and the results vary over the following two years. The soil pH of the 400 t/ha of clay incorporation in the first year recorded higher than all the other clay application rates. It is expected that the soil pH will stabilise to 1/ 2 to 1 unit higher than the original sandy soil pH based on 200 t/ha of clay (anecdotal evidence from Obst, personal communication) over a period of a few years.

4. Discussion Although water repellent soils will almost always wet up at some stage of the season (Wetherby, 1984), the uneven infiltration of water leaves large areas of dry soil (Bond, 1964; Van Dam et al., 1990). The use of kaolinite clay on a very severe water repellent soil has been found to be the best clay in overcoming water repellency (Ward, 1993) due to the ability of the clay to remain ‘dispersed’ over the sand surface during the drying process. It is therefore quite fortunate that a large proportion of the water repellent soils in South Australia are underlain by kaolinite clay.

Fig. 8. The effect of clay application rate on soil moisture content (to a depth of 10 cm) at different positions on a sand rise.

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Fig. 9. Effect of clay application rate on soil pH over time.

In many areas of South Australia the addition of clay to water repellent sand has not only improved the wettability and soil moisture content of the soil but has also improved the topsoil pH from slightly acidic to neutral. The observed changes in water repellence from incorporating clay over the last 7 years are encouraging for the long term amelioration of this problem in South Australia. Claying water repellent sands has given farmers the opportunity to diversify from grazing to cropping and pasture rotations.

The positive effects that Obst (personal communication), and other farmers, reflect on over the past thirty years of claying water repellent sand can only reinforce the farming community’s need to apply this technology to the water repellent sand across the SE and South Australia. Many field days, be it machinery, farm walks or demonstration trials are used to promote clay spreading as a sustainable and economically viable method of long-term remediation of water repellence in sandy soils.

Chapter 26

Treating water repellent surface layer with surfactant L.W. Dekkera,*, K. Oostindiea, S.J. Kostkab and C.J. Ritsemaa a

b

Alterra, Green World Research, P.O. Box 47, 6700 AA Wageningen, The Netherlands Aquatrols Corporation of America, 5 North Olney Avenue, Cherry Hill, NJ 08003, USA

Abstract The objective of this study was to evaluate the effectiveness of the surfactant formulation Primerw604 for amelioration and management of soil water repellency in the surface layer (0– 5 cm) of a grass-covered dune sand. Primerw604 was applied three weekly between 22 April and 23 November 1999. During that period, soil samples were taken from the surface layer in the untreated and treated plot. Resistance to wetting was determined by measuring the wetting rate of field-moist samples. Measurements of water repellency revealed that applications of Primerw604 resulted in less persistent water repellency in the surface layer of the dune sand. At depths of 0 – 2.5 and 2.5– 5 cm the critical soil water content, below which the soil is actually water repellent in the field, was lowered significantly by the application of Primerw604, potentially due to coating of water repellent particle surfaces by the surfactant. This implicates that the surface layer in the Primer treated soil can dry to lower water contents than in the untreated soil before water repellency is induced. Primerw604 applications increased the wetting rate of field-moist samples from the thatch layer (0 –2.5 cm). This may result in a more effective wetting of the root zone during rain events and artificial supply of water (irrigation), and a reduction in runoff.

1. Introduction The problem of soil water repellency has been recognized in various parts of the world (Jaramillo et al., 2000) including the Netherlands (Dekker and Jungerius, 1990; Dekker and Ritsema, 2000) and has resulted for instance in serious land use problems in agriculture (Blackwell, 2000) and an ongoing management problem on sand-based turfgrass systems (Cisar et al., 2000). Water repellency may dramatically affect water and solute movement at the field-scale, and has often been underestimated (Bauters et al., 2000b). Water repellency and its spatial variability have been shown * Corresponding author. Tel.: þ31-317-474267; fax: þ 31-317419000. E-mail address: [email protected] (L.W. Dekker). q 2003 Elsevier Science B.V. All rights reserved.

to cause nonuniform wetting and preferential flow in many soils (Dekker and Ritsema, 1994b, 1996c, 2000; Ritsema and Dekker, 1996b; Ritsema et al., 1998b). Soil surfactants have been developed as a possible means for overcoming the problems caused by water repellent soils (Moore, 1981; Rieke, 1981; Dekker et al., 2000b; Kostka et al., 1997; Kostka, 2000). Wetting agents that have a strong affinity for the surface of hydrophobic soil particles and adsorb strongly at the soil surface will enhance infiltration rates at the soil surface interface. The objective of our study was to evaluate the effectiveness of the surfactant formulation (Primerw604) for amelioration and management of soil water repellency in a pasture on a native dune sand. The present paper describes the influence of the surfactant on reducing the severity

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of soil water repellency in the surface layer (0– 5 cm) and on increasing the wetting rate of the thatch layer (0– 2.5 cm).

2. Materials and methods 2.1. Field-soil and treatment The wetting agent was applied to a 25 m £ 5 m plot of a dune sand with grasscover near Ouddorp in the southwestern part of the Netherlands. An untreated adjacent plot was used for comparison. The soil consisted of fine sand with less than 3% clay to a depth of more than 3 m, and was classified as mesic Typic Psammaquent (Dekker, 1998). Organic matter contents of 18 and 10 w% were found at depths of 0 – 2.5 and 2.5 –5 cm, respectively. Primerw604 (Aquatrols Corporation of America, Cherry Hill, New Jersey, USA) treatments at a rate of 1.85 ml/m2 (volume solution 70 ml/m2) were applied with a Mesto Pico backpack-type sprayer 12 times during the period of 22 April to 23 November 1999. 2.2. Soil sampling Between 22 April and 23 November 1999 the soil was sampled ten times in transects at depths of 0– 2.5 and 2.5 – 5 cm, using steel cylinders, with a diameter of 5 cm. In each transect and at both depths, 35 adjacent samples were taken over a distance of approximately 1.8 m. The cylinders were pressed vertically into the soil, emptied into plastic bags and re-used. Plastic bags were tightly sealed to minimize evaporation. The field-moist bagged soil was weighed. ‘Actual’ soil water repellency was measured (Dekker and Ritsema, 1994b), and after drying in a fan-oven during one week at 258C, potential water repellency was measured. Samples were further dried at a temperature of 1058C, and weighed again to calculate the water content and dry bulk density of each sample. 2.3. Water drop penetration time test The persistence or stability of water repellency of the soil samples was examined using the water drop

penetration time (WDPT) test (e.g. King, 1981). Using a standard medicine dropper, three drops of distilled water were placed on the smoothed surface of a soil sample, and the time that elapsed until the drops infiltrated was determined. Soil water repellency of all samples were measured under controlled conditions at a constant temperature of 208C and a relative air humidity of 50%. In general, a soil is considered to be water repellent if the WDPT exceeds 5 s (Dekker, 1998). We applied an index allowing a quantitative classification of the persistence of soil water repellency as described by Dekker and Jungerius (1990). Thus seven classes of repellency were distinguished based on the time for water drops to infiltrate the soil: class 0, wettable, non-water repellent (infiltration within 5 s); class 1, slightly water repellent (5 –60 s); class 2, strongly water repellent (60 – 600 s); class 3, severely water repellent (600 s– 1 h); and extremely water repellent (more than 1 h), further subdivided into class 4, 1 –3 h; class 5, 3 –6 h; and class 6, . 6 h. Water repellency was measured on the field-moist samples (actual water repellency), and again after drying at 258C. The severity of water repellency measured on dried soil samples, the so-called ‘potential’ water repellency, is considered to be the most appropriate parameter for comparing soils with respect to their sensivity to water repellency (Dekker and Ritsema, 1994b), because differences in water content are wiped out. We measured the ‘actual’ water repellency on the field-moist samples immediately after recording their wet weight. By measuring the water content of the samples, we could assess ‘critical soil water contents’ for both depths in the treated and untreated plot. The soil is wettable above, and water repellent below these values (Dekker and Ritsema, 1994b). 2.4. Wetting rate measurements Resistance to wetting was determined several times by measuring the wetting rate (Fig. 1) of fieldmoist samples collected at depths of 0 –2.5 cm in the treated and untreated plots prior to surfactant applications. The samples were collected in steel cylinders (50 cm3) with a height of 2.5 cm and a diameter of about 5 cm. To measure wetting rate, these samples, within their steel cylinders, were subjected to a constant

Treating water repellent surface layer with surfactant

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Fig. 1. Measurement of the wetting rate of eight soil samples, placed on ceramic filters and subjected to a pressure head of 22.5 cm water at the bottom of the samples. During the measurements the water level in the basin is kept constant. The water uptake of the samples is recorded by the balances and stored by the computer.

pressure head of 2 2.5 cm water applied at the bottom of the samples (Dekker et al., 1998). The experimental set-up (Fig. 1) was designed in such way that water content changes in 1.0 vol% increments were recorded automatically. All measurements were performed in a controlled environment laboratory with a constant temperature of 208C and a relative humidity of 50%.

3. Results 3.1. Actual water repellency The entire soil profile was wet at the beginning of the study on 22 April 1999 and all 70 samples taken at depths between 0 and 5 cm were wettable or non-water repellent, exhibiting WDPT values of less than 5 s. In the period that followed, the field-soil became drier, and as a consequence all samples taken on 17

May were water repellent. The repellency at depths of 0– 2.5 cm ranged between 60– 600 s and 1– 3 h and at depths of 2.5 – 5 cm between 600 – 3600 s and 3 –6 h (Fig. 2). The variability in actual water repellency was often high over short distances at both depths in the untreated and in the treated plot. WDPT values varied sometimes between less than 5 s and more than 6 h (Fig. 2). The thatch layer (0– 2.5 cm depth) was found to be wettable for both plots on 22 April, 12 August, 12 and 25 October, and 23 November. More wettable samples were recorded at depths of 0 – 2.5 and 2.5– 5 cm for the Primer treated plot in comparison with the untreated plot on 1 June and 8 July (Fig. 2). Significantly lower WDPT values were recorded for the samples taken at both depths in the treated plot when compared with the untreated plot on 2 and 21 September (Fig. 2). In conclusion, the application of Primerw604 resulted in a decrease of water repellency in the surface layer to a depth of 5 cm, due to adsorption of the surfactant in this zone.

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Fig. 2. Relative frequency of the persistence of actual water repellency of field-moist samples taken at depths of 0–2.5 cm and 2.5–5 cm in the untreated and treated dune sand on nine sampling days.

3.2. Critical soil water content Water content has a large effect on the actual water repellency of a soil (Letey et al., 1962c; Dekker, 1998). The concept of critical soil water content has been introduced by Dekker and Ritsema (1994b) as the soil water content below which the soil is water repellent and above which the soil is wettable. All samples taken in the thatch layer from the untreated plot between 17 May and 23 November 1999, and having a soil water content above 23 vol%, were determined as wettable (Fig. 3, upper left-hand diagram). All samples with a soil water content of

less than 18 vol% were slightly to extremely water repellent with WDPT values of 5– 60 s (class 1) up to 3– 6 h (class 5). Soil samples with a water content between 18 and 23 vol% (grey zone) were assessed as either wettable or water repellent, introduced here as the transition zone. This means that the critical soil water content of the thatch layer of the untreated plot is variable and ranges between 18 and 23 vol%, most likely depending on the wetting history of the soil, weather sequence, etc. The critical soil water content of the soil in the untreated plot at depths of 2.5– 5 cm was found to be between 14 and 20 vol% (Fig. 3, lower left-hand


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Fig. 3. Relationship between soil water content and persistence of actual water repellency at depths of 0– 2.5 cm and 2.5–5 cm in the untreated (left-hand diagrams) and treated (right-hand diagrams) dune sand. The transition zone, with critical soil water contents, is indicated with a grey tone.

diagram). Although there were large differences in severity of actual water repellency at specific soil moisture contents, there was a distinct increase in severity with decreasing soil water contents, as shown in the diagrams of Fig. 3. Treatment with Primerw604 caused a significant decrease in the critical soil water content of the thatch layer, as can be seen in the upper right-hand diagram of Fig. 3. The transition zone varied in this case between 12 and 16.5 vol%, in comparison of a variation between 18 and 23 vol% in the untreated plot. For instance, all soil samples with a water content of 17 vol% were determined as wettable in the Primerw604 treated plot, whereas all samples with

this water content in the untreated plot exhibited slight to extreme repellency. Also at depths of 2.5– 5 cm there was a slight shift of the critical soil water content. Soil samples with water contents of 8– 14 vol% were water repellent in the untreated plot, whereas in the Primerw604 treated plot a number of samples with these water contents were still wettable. In conclusion, the critical soil water content in the thatch layer (0 –2.5 cm) has been lowered significantly by the applications of Primerw604. This means that the surfactant treated soil dried to lower water contents than the untreated soil before water repellency was induced.

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3.3. Resistance to wetting of field-moist samples An instantly high wetting rate of the surface layer is important for the effective infiltration of rain and irrigation water as well as for prevention of erosion and runoff. During the first time of water application with the measuring device (Fig. 1) the wetting of the thatch layer (0– 2.5 cm) was generally more rapid for

samples from the plot treated with Primerw604, than for samples from the untreated plot. Differences in instant wetting rate were observed between thatch layer samples from the untreated and treated plot on five of the six sampling dates (Fig. 4). The uptake of water in mm gives an indication of the amount of rainwater, which can be absorbed readily. It is evident from the diagrams that water infiltrated more

Fig. 4. Increase in water content (vol%) of field-moist samples taken on six sampling days in the untreated and treated thatch layer (0–2.5 cm) during 1 h of wetting (Fig. 1). The water uptake in mm is also indicated.


effectively into the thatch layer of the treated plot than the control. Initial soil water content of the samples played an important role for the wetting rate during the first hour (Fig. 4). Especially, samples from the untreated plot with soil water contents below the critical soil water content exhibited less affinity for water absorption. Note that the wetting rate of the thatch layer did not increase after the first Primerw604 application, as shown by the diagram of 17 May, but a significant increase was determined after additional applications, as shown by the diagrams of 16 June to 21 September (Fig. 4). In conclusion, the application of Primerw604 increased the wetting rate of the thatch layer, which results in a more effective wetting of the root zone during rain events and/or irrigation event, thereby decreasing runoff.

3.4. Persistence of potential water repellency The persistence of potential water repellency of samples taken at depths of 0– 2.5 and 2.5– 5 cm in the untreated and treated plots was measured with the WDPT test after drying at 258C. All field-moist samples from the 22 April transect were wettable, but the WDPT varied between 60– 600 s (class 2) and 3 –6 h (class 5) after drying at 258C (data not shown). Differences in potential water repellency (samples dried at 258C prior to WDPT) occurred between samples taken at the same depths but also between samples taken on different sampling dates (Fig. 5). Notable differences in persistence of water repellency were found for both the 0 –2.5 and 2.5– 5 cm depths in the untreated plot between the 2 September and 12 October transect. All samples from 2 September exhibited extreme water repellency with WDPTs between 1– 3 and . 6 h, whereas the 12 October samples showed strong to severe water repellency with WDPTs varying between 60 – 600 and 600 –3600 s (Fig. 5). Highly spatial and temporal variability in potential water repellency were also determined for the samples taken at depths of 2.5 –5 cm in the treated plot between 17 May and 23 November, as demonstrated in Fig. 5. Because all samples were dry, the differences in water repellency

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must be due to differences in initial water content of the samples and a process of initiating water repellency in the field. The persistence of potential water repellency of the samples dried at 258C is clearly negatively related to the initial soil water content and positively to the persistence of the actual water repellency of the samples. For example, the relatively dry and severe to extreme actual water repellency of the 17 May and 2 and 21 September samples (Fig. 2) resulted in locally extreme potential water repellency (Fig. 5). Significantly lower WDPT values after drying at 258C were detected for the samples taken at both depths in the Primerw604 treated plot between 1 June and 23 November 1999, when compared with the untreated plot (Fig. 5). A majority of the samples taken from the thatch layer (0 –2.5 cm) of the treated plot between 8 July and 25 October exhibited only slight water repellency after drying at 258C, whereas most samples from the untreated plot exhibited severe to extreme water repellency (Fig. 5). In most cases the persistence of potential water repellency of the samples was significantly higher than the actual water repellency, after the samples had been dried at 258C. However, it is worthy of note that the potential water repellency of samples on some days was less severe than the actual water repellency of field-moist samples on other days, thus underestimating the maximal persistence of water repellency that can occur in the field (see Figs. 2 and 5). This indicates that processes which are taking place in the field during dry weather cannot be artificially generated during drying in a laboratory oven over a time span of several days. Regardless of how water repellency was measured in soil samples, surfactant treatment generally shifted water repellency classes (actual or potential) to more wettable classes. Surfactant induced shifts in water repellency classes were most evident in potential water repellency results. In conclusion, the spatial and temporal variability in persistence of water repellency after drying at 258C was high at both depths for the treated and untreated plot. But more important, significantly lower WDPT values were detected for samples at depths of 0 –2.5 and 2.5 –5 cm from the Primerw604 treated plot, when compared with the untreated plot.

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Fig. 5. Relative frequency of the persistence of potential water repellency of samples dried at 258C, and taken at depths of 0–2.5 cm and 2.5–5 cm in the untreated and treated dune sand on nine sampling days.

4. Discussion and conclusions Soil wetting agents have been developed as a possible means for overcoming the problems caused by water repellency (e.g. Letey et al., 1962c; Kostka et al., 1997; Cisar et al., 2000). Applications of Primerw604 resulted into less persistent water repellency in the thatch layer and surface layer to a depth of 5 cm (Fig. 2). Water content has a large effect on the actual water repellency of a soil. The critical soil water content introduced by Dekker and Ritsema (1994b), appears not to be a sharp static threshold above which a soil is

wettable and below which a soil is water repellent, but rather a transitional range value. This range of critical soil water contents for a certain depth is introduced here as the ‘transition zone’. Soil samples can be either wettable or water repellent within the transition zone, depending on the wetting history, sequence of weather conditions, etc. In the untreated plot of the dune sand studied the transition zone was assessed at depths of 0– 2.5 and 2.5– 5 cm, as being between soil water contents of 18– 23 and 14– 20 vol%, respectively. Applications of Primerw604 lowered these transition zones to 12 –16.5 and 8 –20 vol%, respectively (Fig. 3). This implicates that the surface layer in


the treated soil can dry to lower water contents than in the untreated soil before water repellency is induced. Primerw604 applications increased also significantly the wetting rate of the field-moist samples from the thatch layer (Fig. 4). This may result in a more effective supply of water (irrigation), thereby inducing a better grass growth and a reduction in runoff.

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The spatial and temporal variability in persistence of potential water repellency after drying at 258C was high at depths of 0 –2.5 and 2.5– 5 cm in the treated and untreated plot (Fig. 5). More important, significantly lower WDPT values were detected for the samples from the Primer treated plot, when compared with the untreated plot.


Chapter 27

Management of water repellency in Australia P.S. Blackwell Soil Management Group, Agriculture Western Australia, 283 Marine Terrace, Geraldton, WA 6530, Australia

Abstract The three most westerly states of southern Australia have the largest area of water repellent soils, which limit agricultural production, of any country in the world. Simplified principles of the problems caused by repellency and the principles of soil management solutions are considered and related to experimental evidence. The phenomena of diverted soil water flow and isolated dry soil can explain most of the problems caused by repellency. Plant adaptation, soil or hydrophobic removal, reduced soil drying, reduced surface tension, water harvesting, avoidance, masking and, perhaps, water movement along dead root systems are the main soil management principles. Dead roots may play a role in zero till cropping systems, allowing more uniform wetting of dry hydrophobic soil at the base of a dead plant and along the dendritic pattern of the dead root system. Application of these management principles, especially water harvesting, avoidance and masking (by the use of deep trenching, furrow sowing methods or claying), have made a considerable improvement to sustainability and productivity of farming systems on the water repellent soils of Australia. Evidence is selected to assess risks of preferential flow, pesticide concentration and leaching for different agricultural soil management methods. All management methods can have some risks, but claying seems to have the least risk and furrowing the highest risk of encouraging preferential flow, pesticide concentration and leaching. It is suggested we have insufficient information and understanding to quantify the risks of groundwater contamination for different environments, farming systems and soil management methods to control repellency. There is an urgent need to develop quantified guidelines to minimise any possible groundwater contamination hazard for the extensive areas using farming systems with furrows and increasing amounts of pesticide and fertiliser.

1. Introduction The three most westerly states of southern Australia have the largest area of water repellent soils, with limitation to agricultural animal and grain production, of any country in the world. About 2 Mha in Western Australia, 2 Mha in South Australia and 1 Mha in Victoria are known to be affected and a further large area has the potential to be repellent, due to soil textures associated with low surface area (i.e. a further 3 Mha in WA; Blackwell, 1993; Moore and Blackwell, 1998). Thus in Australia an area of at q 2003 Elsevier Science B.V. All rights reserved.

least 5 Mha, equivalent to a larger area than the Netherlands (4.8 Mha), has agricultural production affected by water repellence. Australia is a very dry continent with peripheral farming areas dependent on intercepted precipitation and groundwater for potable water supply. In WA surface water dams and bores from perched groundwater are the main source of potable water supply in rural areas. Farming practices can influence the quality of groundwater. Hirshberg and Appleyard (1996) reported an increase in ammonium and nitrate concentrations and high sulphate/chloride ratios as

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a result of widespread fertiliser use in farming areas north of Perth. Application of soil management principles to overcome or use repellency, especially water harvesting, avoidance and masking (by the use of furrow sowing methods, deep trenches, or claying) have made a considerable improvement to sustainability and productivity of farming systems on the water repellent soils of Australia in recent years. However, there are still concerns about accelerated groundwater contamination from preferential flow and leaching of pesticides and nutrients through water repellent soils. A current cause for concern is the increasing use of the pesticide atrazine in the cultivation of triazine tolerant (TT) Canola. Western Australia’s 1998 TT canola crop is estimated at 350,000 ha. A large proportion of this area is on water repellent soil and much of this is sown dry in northern areas, due to the long growing season needed for the varieties used. The Australian National Registration Authority for Agriculture and Veterinary Chemicals (NRA) has only allowed a temporary permit for atrazine use in WA for 1998; limited to two applications per season and a maximum of 4 L/ha for soils with pH , 6:5 and 2 L/ha for soils with pH . 6:5: A monitoring program is also being conducted by Agriculture Western Australia to comply with the temporary permit. The growing of high yielding wheats with more resistance to leaf diseases on water repellent soils is also encouraging more use of aphicides and fungicides in some higher rainfall areas. This may also be a cause for concern. Management of water repellency in Australia is very challenging if groundwater contamination risks are to be reduced. The extensive occurrence of the problem and small financial margins of many agricultural enterprises often constrict management methods to low cost and simplicity; furrow sowing methods are in this category. The most effective solution to repellency, and the management method with potentially the lowest risk of groundwater contamination is probably claying; because it can remove the water-repellent behaviour of the soil. Where appropriate clay is available and the financial return over a few years is encouraging, the process of claying is attractive; but it is still relatively slow, expressed as area treated per year. Therefore, it is not advisable to rely on claying as the only solution to

groundwater contamination by crop husbandry practices. It would be better to devise management guidelines and simplified decision aids which are based on knowledge of the principles involved and evaluation of the risks by quantitative modelling of leaching. This paper outlines the principles involved in repellency problems and solutions as well as presenting some evidence for specific processes contributing to preferential flow, pesticide concentration and leaching. Suggestions are then made for further research and development for new management tools to minimise groundwater contamination risk due to cropping water repellent soils.

2. Principles of water repellency problems Simplified principles of the problems caused by repellency and the principles of soil management solutions can be considered to provide a conceptual framework for discussion of groundwater contamination problems within management solutions. Water repellency of dry soil can lead to the following problems for agricultural production: (letters relate to Fig. 1). † More water erosion and wind erosion, due to increased lateral flow and poor cover (E). † Isolated dry soil and poor topsoil wetting, due to preferential vertical infiltration (I). † Accelerated leaching of solutes due to preferential water movement (L). † Delayed emergence of weed and crop plants from the initially unwetted zones (D). † Concentration of some solutes (e.g. pesticides) by lateral flow and splash (CO). † Carry-over of seeds, unactivated pesticides and fertilisers in isolate dry soil (CA). Staggered germination of weeds can create complications for weed control by selective pesticides which are more efficient when the weed and crop are at predictable and uniform growth stages. An extreme example of preferential flow is the observation of water moving ‘through’ dry soil zones along the dry remains of plant roots on excavation of soil beneath infiltrometers. Emmerson (1954) reported laboratory and field evidence of preferential

Management of water repellency in Australia

293

Fig. 1. A schemetic outline of processes encouraged in cropping water-repellent soils without improved management; the letters are explained in the text.

water movement through severed roots of grasses below the depth of autumn cultivation of an English clay soil. He partially attributed this to an assumed hydrophilic nature of the central meta-xylem cavity of species such as Lolium perenium, along which the water could move as a syphon in laboratory experiments. A consequence of such preferential movement of water along dead roots may be the observation of early wetting of old plant rows by the first rains of the following season on water repellent sands in Western Australia. Dekker and Ritsema (1997) have made similar observations of preferential soil wetting along dead maize roots which grew in water repellent soil. In extreme cases of isolated dry soil volumes, crop seed planted in one year has been observed to germinate in the following year, after another crop has been sown. In contrast to agricultural production problems, it is possible that preferential flow to established plants and seeds of competitive plants isolated in dry zones may be an advantage to the survival of native vegetation. Such vegetation often grows on deepwater repellent siliceous sands with poor water holding capacity and suffers much competition for water and nutrients. The phenomena of diverted soil water flow, both laterally and vertically, and isolated dry soil can explain most of the problems listed above caused by repellency. Accelerated leaching, erosion and concentration of pesticides is a consequence of diverted

flow. Poor plant establishment and cover, as well as carry-over of pesticides and fertilisers is a consequence of isolated volumes of dry soil.

3. Principles of water repellency solutions Management options for water repellent soils used for agriculture are quite numerous, compared to many other soil problems, such as acidity. The various options can be explained in terms of simplified principles (Blackwell, 1993). These are explained below, the letters relate to the schematic diagram in Fig. 2. 3.1. Adaptation Perhaps the oldest solution in Western Australia was to select pasture species which were naturally adapted to water repellent sands (AD); Blue Lupins (Lupinus cosentinii) are used as animal pasture and are able to germinate and establish on the soil surface, thus avoiding any need for burial and being isolated in dry water repellent soil. Fodder shrubs and trees, once established, rely more on the subsoil than topsoil for water supply and nutrition, thus they have also been adapted to avoid some repellency problems and provide animal feed and timber for agricultural enterprises.

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P.S. Blackwell

Fig. 2. A schematic outline of processes used in management of water repellency, letters are explained in the text. The relative scale is shown by bars, furrow sowing, claying and zero till share the same scale.

3.2. Avoidance

3.5. Reduced drying

Avoidance of water repellent topsoil (AV) is also a well established practice in some areas, especially for establishment of trees and fodder shrubs. Avoidance involves removal the topsoil by lateral grading of a deep trench. Seeds or seedlings are then planted in the non-repellent subsoil. Some forms of furrow sowing allow this for crop establishment, by sowing the crop seed onto shallow moist soil below dry surface soil.

Zero till methods in general can reduce soil drying by accumulation of crop residues to minimise soil drying and the development of repellency (Z) and may employ dead root systems from previous crops and pastures to assist redistribution of water during infiltration.

3.3. Water harvesting Furrow sowing of various kinds employs water harvesting (H) to divert the preferential flow to the seed. Various combinations of points, discs and presswheels are used to make furrows.

3.6. Masking and degradation Claying initially employs soil particle masking (M) to cover the hydrophobic surfaces, and may encourage degradation of hydrophobic material (D) by improved microbiological activity. Recent experiments show relatively small amounts of clay and lime added to water repellent soil can encourage microbial degradation of hydrophobic material. 3.7. Export

3.4. Reduced surface tension Surfactant bands in furrows, to make more economical use of surfactants, employ reduced surface tension (T) to enhance soil wetting.

In extreme cases, repellent soil can be removed and exported (X) to other locations in the profile or local area. Such practice is not common in Australia due to the high erosion risks created by such soil exposure. This happens on a small scale during soil trenching.

295


matter can generally increase repellency, in a frequently cultivated system, zero till offers the benefits of reduced soil drying by surface residues restricting evaporation. Such conditions may improve the microbial environment, with amelioration of nutritional limitations. More organic matter and microbial activity may increase adsorption and degradation of pesticides. Water movement along dead root systems may enhance such interaction. † Reduced surface tension from surfactants may increase adsorption of some nutrients and pesticides by surface organic matter, due to the greater wetted volume of soil available for adsorption when surfactant is used; see Fig. 3. † Isolated dry soil can hold some nutrients and pesticides which may be released when and if the soil is eventually wetted. Patches of unwetted soil occur in unameliorated conditions and after some forms of furrow sowing. Delayed release of solutes from initially dry zones may reduce the concentration in a ‘slug’ of leached pesticide and nutrient.

4. Common principles affecting the risk of groundwater contamination from repellency problems and management solutions The most obvious principle or process increasing risk of groundwater contamination from cropping water repellent soil is preferential flow, either across the surface, through hollows or furrows and along unstable wetting fronts. This occurs both in unimproved soil (CO and L in Fig. 1), as well as in improved systems using water harvesting (H in Fig. 2). Principles and processes which may reduce the risk of groundwater contamination are: † Masking from claying, which can reduce repellence and increase adsorption of solutes. Dellar et al. (1994) showed that phosphorous retention of water repellent sand was increased by incorporation of clay. Despite this increase in the ability to retain nutrients by claying, the measured values by Dellar et al. (1994) are still in the range of values of Phosphorous Retention Index (PRI) of , 2. Weaver and Summers (1998) classified such values of PRI as still having the greatest risk of losing phosphorous by leaching. † Increased macro-organic matter accumulation by zero-tillage practices. Although increased organic

5. Relative occurrence of management solutions The most extensive management solutions for water repellency currently used in Australia are for

120.0

F F

100.0

8 4 2

Wet soil (%)

80.0

F

60.0

1 40.0

F

0 20.0

L F

F 0.0 0.0

2.0

4.0

6.0

8.0

10.0

12.0

14.0

Rainfall, (mm) Fig. 3. Soil wetting 40 mm below the bottom of furrows (F) or a level surface (L). Wetting after addition of banded wetting agent previous to the rain is also shown (B) and the rate of application, on a total area basis, indicated in L/ha.

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P.S. Blackwell

improved cropping. Furrow sowing, usually as part of a no-till system, using narrow points and presswheels is the currently most extensive method used in cropping. Claying is becoming more common, especially in the south coast of Western Australia, South Australia and Victoria. Large plantations of fodder shrubs can be found in the water repellent soils north of Perth. Surfactants are used commonly in the establishment of fodder from seed and are becoming more common in some parts of Western Australia and South Australia for crop establishment, in conjunction with furrows. Zero till methods, using full stubble retention and crop seeding by precision disc machines, are growing in use. Thus, the most extensively used management principle for management of water repellency in Australia is water harvesting, which is essentially the control of a process which is a common problem feature of water repellent soils, preferential or diverted flow.

6. Choice of management solutions Current advice for choices of soil management methods to reduce repellence problems are based on the current repellency status of the soil, the risk of the soil developing repellency, the type of farming system being used as well as the availability of clay and the ability to finance a more expensive solution from the short term profits of a less expensive one (Moore and Blackwell, 1998). Abadi (1994) has outlined a method for choosing the relative investment in different repellency management solutions, depending on the amount of repellent soil and the availability of different solutions. The addition of management guidelines to minimise groundwater contamination risks may also be able to be added to such decision aids in the future.

7. Risks of preferential flow Water harvesting into furrows can be an advantage for small rainfall events (. about 4 mm) and in dryer seasons (Blackwell, 1997b). As the rainfall events become larger (approx. . 50 mm), of increased intensity and the season becomes wetter, the advantages of water harvesting can become a disadvantage

of preferential water flow, leading to furrow filling and leaching. In this section some experimental evidence of preferential flow through water repellent soils in Western Australia is shown.

8. Preferential surface flow of water Field experiments were done on a dry water repellent sand near Geraldton, MED of 2.8, coarse sand 88%, fine sand 8.5%, silt 1.5%, clay 2% and total carbon content of 0.91%. A rainfall simulator and natural rain were used for precipitation on initially airdry sand. The precipitation was on an artificial and simplified furrow of two flat sides, with different slopes and surfaces shedding water into a collection trough with instrumentation to measure runoff. Measurements were also made of soil wetting, by excavation and point intercept methods, on real furrows with or without narrow bands of surfactant in the furrow base. Further details are provided in Blackwell et al. (1994b). The results of soil wetting below real furrows and the effect of different amounts of simulated rain (50 mm/h) and presence of different amounts of surfactant are shown in Fig. 3. The percentage interception of wet soil at seed depth (40 mm below the furrow base or 40 mm below a level surface) increased as the amount of rain applied increased. A natural rainfall of 12 mm at 67 mm/h on the same furrows and adjacent level dry soil showed a very large increase in amount of wetted soil at seed depth, about 70% more wetted area. Addition of banded surfactant for the same amount of artificial rain increased the amount of wetted soil. Application of surfactant equivalent to 1 L/ha, uniform application, resulted in as much soil wetting as more than twice as much rainfall (2.5 mm compared to about 6 mm). Measurements of runoff from the simplified artificial furrow showed slope has little effect on the amount of harvested water compared to furrow width and the presence or absence of prostrate or erect stubble. Fig. 4 shows the output from a simple model derived from the results. No water is harvested until the surface layer and residue has been wetted, approximately 0.5 –1 mm of rain. Water harvesting


297

Fig. 4. Results of a simple model of water harvesting#. The influence of a bare surface (X) prostrate stubble (þ ) or erect stubble (A) is shown for a spacing of 500 mm, as well as erect stubble for a spacing of 200 mm (B). 50 mm/h is the assumed rainfall rate. #H ¼ ððP 2 IÞðS 2 CÞRÞ=C; where H ¼ harvested water (mm); P ¼ rainfall (mm); I ¼ rain to initiate runoff (mm); S ¼ spacing or furrow width (mm); C ¼ collection width in base of furrow (mm); R ¼ runoff ratio.

is more efficient from a bare surface than with surface residues and an erect stubble restricts harvesting less than prostrate stubble. This is a convenient synergism of stubble with agronomic needs. Water repellent soils cause more splash than non-repellent soils (Terry and Shakesby, 1993). Splashed soil can carry fungal spores and pesticides onto the plant, but erect stubble reduces the splash and makes water harvesting more efficient if there is surface residue. Larger catchments from wider furrows in Fig. 4 result in more water harvesting.

9. Preferential flow of soil Furrow filling can result in seed deeper than ideal, but is considerably reduced by retained cover and reduced disturbance. Evidence of this is shown from measurements at Badgingarra in 1994. Repeated measurements of furrow depth were made on dry furrow sown water repellent sand at Badgingarra before and after the strong wind

and autumn rain in May 1994. Surface cover (vegetation and clods) at the same locations as measurements of furrow depth, was measured by point intercept methods. Rain and wind from initial fronts in May 1994 caused more furrow filling as surface cover of pasture residues and soil clods declined (Fig. 5). These results emphasise the need for adequate cover to minimise furrow filling. Furrow filling can induce seed being buried too deep for successful emergence. Management methods need to conserve adequate surface covers, especially of pastures, and use sowing along contours where the risk of water erosion is increased by surface slope. Soil clods strengthened by dead plant roots are very valuable to provide protection in pastures, yet are easily lost by cultivation. Soil carried into the bottom of a furrow can also concentrate associated pesticides in the crop row, leading to possible overdose of the crop and facilitating possible leaching through the bottom of the furrow.

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P.S. Blackwell

Fig. 5. Changes of furrow depth for different amounts of surface cover following the passage of two frontal systems at Badgingarra, 1994. Lines have been drawn through the data by eye. Loss of furrow depth is similar to the increased depth of seed. From Blackwell et al. (1994f).

10. Risks of pesticide concentration Evidence of problems from pesticide concentration in furrows is shown from an experiment in 1993. Lupins were sown dry on 300 mm spaced furrows, 50 mm deep either in a pasture area or in stubble from a crop of wheat which yielded 2 t/ha the previous year. The sowing operation resulted in bare ridges from the pasture and ridges covered by 30% crop residue in the stubble. The crop was germinated by water harvested by a 9 mm shower in late April. The pesticide simazine was then sprayed on dry soil at different rates. A further shower of 9 mm occurred when the crop was emerging. Mortality of the Lupins (Fig. 6) increased rapidly for the bare ridges, killing almost half the plants at the higher rate. The stubble residue reduced the mortality by more than 40%, because less soil and pesticide was moved into the bottom of the furrow, avoiding as much pesticide overdose of the crop.

This is an extreme example of pesticide toxicity due to increased dosage by diverted flow during water harvesting. Current practices minimise the problem by later application of broadleaf pesticides and formulations more easily metabolised by the lupin crop. Smaller furrows also help to minimise the risk.

11. Risks of leaching A preliminary study of deep-water movement from furrows was made at Geraldton in 1992. Changes of water content beneath level or furrow sown crop row was evaluated with four paired sites, 3 m apart. At each paired site water content was measured with a neutron moisture meter through a 3 m deep pvc access tube. One tube was installed in a level crop row and one tube in a wide furrow sown crop row (355 mm spacing, 50 mm deep). The tubes were installed directly into the duplex profile using an internal


299

Fig. 6. Lupin seedling mortality from different rates of pesticide with (B) or without (O) the protection of a wheat stubble.

auger, with suction to remove the debris, and the bottoms sealed by underwater cement. Regular measurements were made through the 1992-growing season. The most information (Fig. 7) came from measurements in July before or after a 58 mm rain, the subsoil was significantly more wet beneath the furrows than beneath the level surface. The simulator applied 100 mm of rain and was controlled to avoid surface ponding in the furrow to minimise the risk of preferential flow along the interface between the

outside of the access tube and the soil. The water content profiles after artificial rain was only successfully measured on two paired plots. The resulting changes of water content were so dissimilar between the pairs that only one profile is shown in Fig. 7. If this is still representative of the process, it shows a large influx of water into the B horizon by preferential water movement through the furrow. The pattern of wet soil below wide furrows revealed by excavation after rain has often shown the soil beneath the row to

Fig. 7. The effect of wide furrows on profile water content in a duplex soil after 58 mm of natural rain (X) or 100 mm of artificial rain (O) The texture change from sand to sandy clay is at 110 cm. The water content difference between paired sites with or without furrows, in percentage by volume, is shown at different depths. For the natural rain it is a mean of changes at four sites; the data between 150 and 240 cm depth shows significant changes at P , 0:05:

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P.S. Blackwell

be much wetter than the soil beneath the ridge at the same depth. Thus wide furrows may be contributing to significantly more leaching of nutrients than level sowings and narrower furrows during heavy rains of about 100 mm or more. This hypothesis is supported by observations of wide furrow sown cereals at some farms in 1993 after 80 mm opening rains. The early growth of these cereals appeared less and the plants looked more nitrogen deficient than adjacent level sown crops. Thus there is a reasonable case for more detailed studies of leaching risk and effects on the water balance by wide furrows. This may be more of an advantage in lower rainfall areas and more of a problem in high rainfall areas. Management strategies, such as tactical applications of fertiliser to cereals, may be needed to correct any effects of severe nutrient leaching on crops sown in wide furrows. These results of measurements of preferential flow of water, soil and pesticides in furrow systems emphasize the importance of surface cover, especially erect stubble and intact clods of pasture, and furrow width. Narrower furrows with most retention of cover will offer the least risk from problems caused by

preferential flow. Such small furrows can be made more efficient for water harvesting by the addition of a surfactant band.

12. Relative risk and hazard of increased groundwater contamination An estimate of risks and hazards of increased groundwater contamination, from different systems for managing repellency, can be made from existing information, experience and some assumptions. In Table 1, the relative risk is scored by the possible occurrence of preferential flow, thus furrow sowing is high and claying is low. Surfactants reduce the risk by extra soil wetting. Zero tilled soil still has risk, because it is still repellent, despite extra cover to reduce drying. Removal of repellency by masking (claying) or degradation has the most effect. The relative use of pesticides and fertilisers combines with the risk and area in use to calculate a relative current hazard. A relative potential hazard assumes the systems could be used anywhere.

Table 1 An approximation of relative risk and hazard for increased groundwater contamination, from different systems System

Relative risk

Relative area in use (WA 1998)

Relative use of pesticides and fertilisers

Actions to reduce risk

Relative current hazard (risk £ area £ pesticide use)

Relative potential hazard (risk £ pesticide use)

Furrow sowing

Very high 4

Large 4

High 5

Very high 80

20

FS with banded surfactant Zero till

High 3

Small 1

High 5

Small 15

15

Medium 2

Small 2

High 5

Moderate 20

10

Fodder shrubs

Medium 2

Moderate 3

Small 3

Moderate 18

6

Tree plantations

Medium 2

Moderate 3

Small-high 3-5

Moderate-high 18-30

6–10

Blue Lupins

Very high 4

Small 2

Very small 1

Very low 4

4

Claying

Low 1

Small 2

High 5

Small 10

5

Microbial remediation

Low 1

Nil

High 5

Smaller furrows more residue isolation in dry core Low persistence surfactants Maximise stubble retention Fill trenches after establishment Reduce pesticide use Top dress fertilisers in winter Deeper incorporation and higher quantities Nil

Nil

5


Thus, furrow sowing has the greatest current and potential hazard and blue lupin pasture the least potential hazard, because no pesticides are used. Possible actions to reduce risk are indicated in Table 1. Low persistence surfactants minimise the risk of reduced crop yield, despite improved establishment (Blackwell, 1997b). Deeper incorporation of larger amounts of clay may improve solute adsorption and retention. Filling deep trenches after fodder trees are established may reduce the amount of excess water harvesting. Reduction of pesticide use in tree plantations may reduce the overall hazard. Top-dressing fertiliser in winter, instead of summer or autumn, may reduce the hazard of nutrient leaching. A better quantification of these risks and hazards needs further research using quantitative modelling. Quantitative modelling solutions for water repellent soils have already been useful to predict soil temperature and evaporation for different designs of furrow systems (Yang et al., 1996) and for changes in soil water content (Ritsema et al., 1998b).

13. Conclusions Management systems to reduce problems from water repellent soils in Australia have a wide range of relative risk of preferential flow, pesticide concentration and leaching of pesticide and nutrient; resulting in possible increased groundwater contamination. To help assess this risk, this analysis has used principles and processes involved in repellent soil and improved agricultural methods. Repellency

301

problems can be simplified as either preferential flow or isolated volumes of dry soil. Repellency solutions can be classified as adaptation, avoidance, water harvesting, reduced surface tension, reduced drying, masking, degradation or export. Masking by claying and degradation offer the lowest increase of risk of contamination, because repellency is removed. Preferential flow through furrows offer the highest risks of increased contamination, and is mainly controlled by furrow width and soil cover. The narrowest furrows with the most cover offer the least risk; this approximates to a zero till system. Banded surfactant can make water harvesting in zero till more efficient. Banded surfactant can also allow more soil to be wetted, to adsorb more solute. Dead root systems in zero till may play a useful role and more research is needed on the effect of dead root systems on water redistribution and interaction between solutes, organic matter and microbial activity. The hazard depends on the amount of pesticide and fertiliser used, thus cropping systems are generally more at risk than grazing systems. Claying in combination with zero till offer potential long term benefits from increased adsorption and organic matter, however claying is a slow process and not feasible for all areas. Therefore there is an urgent need for new research to enable good quantification of the risks. This could assist the formulation of management guidelines, tools and policies to identify the situations of greatest risk and reduce any possible hazards. Best current advice may be to minimise application of atrazine to dry water repellent soil in cropping systems using furrows.


Chapter 28

Water repellency: a whole-farm bio-economic perspective A.K. Abadi Ghadim* Agriculture and Resource Economics Group, Faculty of Agriculture, University of Western Australia, Perth 6907, Australia

Abstract A whole-farm bio-economic model was used to assess the profitability of innovations aimed at improving agricultural production on the non-wetting sands of wheatbelt farms of Western Australia. It was found that amelioration of water repellency might be an economical option for some farms in the northern wheatbelt of Western Australia. It was shown that a minimum of 30% increase in lupin yields and a 10% increase in wheat yields would be required before expenditure on innovations aimed at improving production on non-wetting soils could be justified. However, due to costs of amelioration of repellency much higher crop yield responses may be required for economical adoption of such innovations on most farms. The decision to ameliorate water repellency depends not only on the consideration of direct benefits and costs per hectare of ameliorated sand but also on other whole-farm factors. One such factor found to be important was the scale of relevance or the soil mix of the farm. It was found that farms with proportionately large areas of non-wetting sands were more likely to benefit from adoption of innovations aimed at amelioration of repellency. Another important factor in the decision to adopt innovations for amelioration of water repellency is availability of alternative enterprises on the non-wetting soils. In particular, whether or not sandplain lupins (Lupinus cosentinii ) and tagasaste (Chamaecystisus proliferus ) were options that a farmer could consider determined the profitability of taking remedial measures against water repellency. This study identified, through a series of sensitivity analyses, the magnitude of production responses that may be required for profitable amelioration of water repellency. Some gaps in our knowledge of biological and economic parameters related to costs and benefits of various innovations have also been highlighted and discussed.

1. Introduction Water repellency, or non-wetting, a widespread condition of sandy soils in Australia, causes reduced and uneven infiltration of water into soils, resulting in poor crop and pasture germination and yield. Poor crop and pasture establishment makes these sandy soils susceptible to wind and water erosion (Harper and Gilkes, 1994). It is estimated that around 5.75 million hectares of agricultural soils of the states of South Australia and Western Australia are non-wetting sands (Carter et al., 1994b; Cann and Lewis, 1994). * Tel.: þ61-8-9380-7869; fax: þ 61-8-9347-4021. E-mail address: [email protected] (A.K. Abadi Ghadim). q 2003 Elsevier Science B.V. All rights reserved.

Blackwell et al. (1994e) suggested that water repellency is a cause of an annual average loss of 40% in crop production in the Australian farming systems. A number of different strategies have been proposed for improving agricultural production on these soils. Some strategies are aimed at overcoming water repellence while others utilise the water repellence characteristics of non-wetting sands to achieve the objective of enhancing water infiltration. These strategies include: application of clay, furrow sowing, band spraying of wetting agents, cultivation during rain, deep cultivation (subsoiling), bioremediation (improving wax degradation by micro-organisms) (Blackwell, 1993; Michelsen and Franco, 1994; Roper, 1994).

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A.K. Abadi Ghadim

The objective of this study was to show, through a series of whole-farm scenario analyses, the conditions that make amelioration of water repellency a viable option. A review of past research in this field showed that firstly, the costs and benefits vary between different amelioration techniques and secondly, long-term experimental field data for the impact of different strategies is scarce. This meant that attempts to conduct a benefit –cost analysis of amelioration of water repellency had to deal with the costs and benefits of various amelioration techniques at a generic level. A farming system model was used to identify the circumstances that would warrant farmers adopting innovations for amelioration of water repellency, across their entire farm (e.g. crop yield response). Existing farm models (Morrison et al., 1986; Abadi Ghadim et al., 1991) were modified to include nonwetting soils and innovations aimed at ameliorating water repellency. This paper, first, briefly describes these farming system models and their modification to represent agricultural production under water repellency. Secondly, this paper describes various sensitivity analyses that were conducted to identify the minimum benefits required from innovations aimed at improving agricultural production on non-wetting sands of Western Australian farms.

2. Overview of the farming system The analyses reported in this paper are for farming systems in the north and mid-western regions of the medium rainfall wheatbelt of Western Australia. Table 1 shows the three typical farm types that have been investigated in this study. They are

Table 1 Area of the main soil types of the three farming systems that are representative of the modelled region Soil type

Farm 1 (ha)

Farm 2 (ha)

Farm 3 (ha)

Non-wetting sands Deep yellow-brown loamy sands Gravely sands Red loams

100 1500 1000 400

500 200 1300 1000

1500 500 1000 0

representative of the farming systems of this region with different proportions of non-wetting sands. The sandy soils are weathered and many are infertile requiring yearly application of phosphate and nitrogen. These regions experience a Mediterranean climate with mild wet winters and hot dry summers. Most farms have a mix of crop and livestock enterprises. Most of the annual rainfall, of around 450 mm, falls from May to October, and is usually followed by a summer drought from December to March. Crops are sown in May – July and harvested in November and December. The main crops are wheat and other cereals. Wheat yields average around 2 t/ha. Table 2 shows the crop yields used in the analysis. These yields reflect an expected (long-term average) season and a farm with average management knowledge and input. Small proportion of the farm area may be sown to crops like canola, or legume crops such as lupins and peas. Crops and pastures are commonly grown in rotation. Farm operations are highly mechanised and most farms are owner-operated with not more than one

Table 2 Some typical crop yields and pasture carrying capacities observed in rotations common in the farming systems under investigation in this study Soil type

Lupina (kg/ha)

Wheata (kg/ha)

Field peasb (kg/ha)

Winter pasturec (DSE/ha)

Non-wetting sands Deep yellow-brown loamy sands Gravely sands Red loams

600 1100 1100 1000

800 1400 1400 1600

N/A N/A 600 800

2 3 4 5

a b c

Wheat and lupin yields (kg/ha) in a rotation of 2 years of cereals followed by 1 year of lupin. Field pea yield (kg/ha) in a rotation of 3 years of cereals followed by 1 year of field peas. Winter grazing capacity of a continuous pasture in dry sheep equivalents per hectare.

Water repellency: a whole-farm bio-economic perspective

other permanent labourer. Casual labour is hired for only a few months of the year to assist in the main tasks such as seeding, harvesting and shearing sheep. Livestock consists mainly of Merino sheep for wool and meat production. Lambing is in late autumn or early winter and shearing is in spring and autumn. The sheep feed consists of annual pastures during winter and a combination of crop residues and dry annual pastures in summer. The pastures are based on subterranean clover, medics, annual ryegrass and volunteer grasses (e.g. barley grass, brome grass) and herbs (capeweed, geranium). Sheep carrying capacities of pastures in these soils ranges from 2 to 5 ha21. Sandplain lupins (Lupinus cosentinii ) is a volunteer annual pasture species characteristic of this region. It is a valuable source of summer and autumn feed for sheep. Tagasaste (Chamaecytisus palmensis), is a perennial fodder shrub that has become well adapted to the sandy soils of this region (Lefroy et al., 1992). This shrub has a life expectancy of 10 – 15 years. Tagasaste requires an initial establishment cost as well as some ongoing maintenance costs associated with fencing, water points and grazing management. When tagasaste is available on a farm, it is a valuable livestock feed source during autumn and summer. Sandplain lupins and tagasaste, when available, are valuable feeds for livestock during the autumn feed gap. These two sources of sheep feed reduce the need for supplementary grain feeding of livestock substantially. Filling the autumn feed gap increases the sheep carrying capacity of the farm and thus increases the profitability of the sheep enterprise (Abadi Ghadim and Morrison, 1992).

3. Model of the farming system The model of the farming systems of this region of Western Australia is the medium rainfall northern wheatbelt version of a model known as Model of Integrated Dryland Agricultural System (MIDAS). MIDAS models represent the biology and economics of the farming systems of several regions in Western Australia. Early versions of MIDAS are described in detail by Morrison et al. (1986) and Kingwell

305

and Pannell (1987). Abadi Ghadim et al. (1991) and Kingwell et al. (1995) describe a revised northern wheatbelt version of MIDAS. The version of MIDAS that was revised for use in this study differs from earlier versions because: † Specific soil classes are included to simulate representative farming systems of the region. † Tagasaste and sandplain lupins are included. † Three new rotations are added to rotation options on the non-wetting sands to allow for whole-farm bio-economic analysis of water repellency. These include: 2 years of cereal crops followed by a lupin crop, 1 year of a lupin crop followed by 1 year of a cereal crop and finally, 2 years of pasture followed by 1 year of a cereal crop. † The feed budget of the farm has been adjusted to allow for combined grazing management of perennial fodder as well as annual pastures and crop residues. † Fixed and variable costs and use-pattern of labour, machinery and finance have been adjusted to simulate a medium rainfall farming system with higher crop yields and much higher sheep carrying capacity during winter, than in previous version of the model. † Utilisation of stubble and pasture residues by livestock has been constrained to reflect changing management approach to prevention of soil erosion from wind and water. † Input costs (e.g. herbicides and fuel) and output prices (e.g. wheat and wool prices) have been revised to account for medium term outlook for these business decision variables. † When wheat is grown within a rotation, the effects of the rotation phases, such as the yield boost from a previous legume crop, are represented. The model is based solely on expected values and therefore assumes a certainty of knowledge about prices, costs and input –output relationships. MIDAS is a steady state model based on the expected weatheryear (i.e. long-term average) and assumes the goal of the farmer to be maximisation of farm profit. However, other managerial goals and behaviour are implicitly accounted for in the structure of activities included in the model.

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Output from the model is a set of profit-maximising enterprise and rotational activities as well as shadow price information about the marginal value of farm resources and alternative activities. In the model farm profit is calculated as a net return to capital and management. This return equates to monies left over from production receipts after deducting all fixed and variable costs, depreciation and opportunity costs associated with farm assets, excluding land. The model was developed through a consultative process involving farmers, researchers and advisers. This model emphasises the interdependencies between activities within farming systems, as do other versions of MIDAS. The model simulates a farming system with a choice of around 500 activities that can be selected in such a combination as to meet around 300 constraints acting on this system. The model’s framework is a single period equilibrium structure, inclusive of inter-year effects. This allows inter-relationships between phases of rotations to be represented within the planning horizon of a single production year. The model describes the production alternatives on up to seven soil types. For the purpose of this analysis, the model was restricted to soil classes and mix of soil types specified in Table 1. Up to 20 rotation options are described for each soil class. The crop options include wheat, barley, oats, white lupins, triticale, and field peas. Production of over 25 classes of sheep based on a self-replacing flock is depicted to allow for a myriad of flock sizes and structures. The pasture production in each rotation phase is also described for each soil class. The non-linear yield responses of cereals to nitrogen are described for each soil class (Morrison et al., 1986). Enterprise interdependencies are a feature of the MIDAS models. The effects on cereal yields of previous leguminous pasture or legume crops are depicted. The increased weed burden in crops due to previous pastures is described. The deleterious effect of cropping on subsequent pasture production is also represented. Many sources of feed for livestock are described in the model: green and dry pastures including blue lupins), tagasaste grown in plantation, grain stored onfarm or bought in and crop residues including spilt grain. Restrictions on the use of crop lupin stubble as well as dry blue lupin in pastures, as a source of sheep

feed during summer and autumn, are also represented to account for risk of lupinosis. The model also represents on a monthly basis the energy requirements and appetite of each sheep class and the energy sources and feed qualities within the farming system. The model represents current farm management technology insofar as the types of tillage practices, machinery complements, herbicides used and rates applied, tasks contracted and crop livestock options considered are consistent with those used or being canvassed by leading farmers of the region. Finally, the model describes the major constraints on farm operations. These constraints include the physical limits imposed by: † † † † †

farm size; areas of different soil classes; supply of labour; working capital or seasonal finance; and capacity of farm machinery for sowing and harvesting.

4. The analysis As discussed earlier the information on costs and benefits of each of the techniques aimed at improving crop and pasture performance on non-wetting sands is far from complete. For this reason analytical approach adopted for weighing up the costs and benefits of amelioration of water repellency was a generic scenario analysis at the whole-farm level. Using this technique innovations for amelioration of repellency are not evaluated on an individual basis. Instead, the analysis is based around various scenarios each with different sets of assumptions relating to the scale and intensity of water repellency and the costs and benefits associated with adoption and non-adoption of innovations aimed at improving production on the nonwetting soils. The results of this type of analysis provide an indication of the size of the net difference in whole-farm profitability between adoption and nonadoption of innovations under different scenarios. This approach can provide valuable information about the direction of research and extension programs when it is combined with sensitivity analysis for selected parameters.

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Sensitivity analysis (sometimes also referred to as what-if analysis) is recommended when there are variables for which field data is missing or inconclusive and also in situations where input parameters are subject to variability. Such parameters include prices, costs, plant and livestock production as well as the change in the farming system associated with adoption of an innovation (Pannell, 1997; Abadi Ghadim and Pannell, 1998). This analysis assesses the impact of only those parameters that were considered to be major determinants of the economic feasibility of amelioration of water repellency. In considering adoption of innovations aimed at increasing production on the nonwetting sands the parameters that a Western Australia farmer would most likely be concerned about are: † crop yields and livestock carrying capacity of pastures associated with adoption and non-adoption; and † costs associated with adoption across the wholefarm. The costs and production responses included in the sensitivity analysis were relative to the costs and production of crops and livestock in the absence of amelioration of repellency. Every possible permutation and combination of the following parameters were included in this analysis: † Three farm types with soil type mixes specified in Table 2. † Nine levels of crop yield responses to amelioration of water repellency as shown in Table 3. † Five levels of costs associated with amelioration of water repellency. These costs are shown in Table 4 as percentage of costs of a rotation excluding

Table 3 Range of crop yield responses to amelioration of repellency used in the analysis Response

Increase in wheat yielda (%)

Increase in lupin yielda (%)

1 2 3 4 5 6 7 8 9

0 0 10 10 10 10 20 20 20

0 10 20 30 40 50 60 80 100

a

Percent of yield in rotations specified in Table 2.

treatment for repellency. † Sandplain lupins were an option on the nonwetting soil type. When this option was made available, the model could select any area of sandplain lupins, if it was profitable. † Tagasaste was an option on the sandy soil types. It was available in the form of a plantation. When this option was made available, the model could also select any area of tagasaste, if it was profitable to do so.

5. Results and discussion 5.1. Is amelioration of repellency a viable option? Yes and No! Improving crop yields by amelioration of water repellency and crop rotations are amongst a range of activities that a farmer can choose for these soils. Just because water repellency results in poor crop yields, and a simple gross margin analysis

Table 4 Extra costs of amelioration of repellency used in the analysis Rotation

CCL CL PPC

Base cost of rotation ($/ha)

64 104 105

Extra costs of amelioration of repellency (as % of base cost) 0

5

10

20

30

0 0 0

3.2 5.2 5.3

6.4 10.4 10.5

12.8 20.8 21

19.2 31.2 31.5

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A.K. Abadi Ghadim

shows that the benefits outweigh the costs, does not mean it is a viable option for all farmers who have repellent sands. The issue of amelioration of water repellency needs to be looked at from a whole-farm perspective. The “Yes” answer is for farms with large areas of repellent sands growing annual pastures year-in yearout and/or poor yielding crops. Given adequate cropping gear, and under the relevant wool price scenarios, it is more profitable to be able to sow larger areas of higher yielding crops. Under these circumstances it would be economical to ameliorate water repellency and crop 55% of this soil type in a cereal – lupin rotation if:

the repellent soils and growing more cereals and lupins. The profitability of the soil treated for repellency would rise by 3$/ha. This would allow a 17% increase in the cropping program of the farm. Results shown in Table 5 indicate that if amelioration of repellency improved lupin yields by 60%, the net increase in profit could be as high as 20$/ha. Results also show that at higher yield responses it would be worth treating larger areas of repellent soils to bring more of this soil type into crop production. However, if the costs of amelioration of water repellency were to be 20% higher than the conventional rotational costs, then the break-even lupin yield response would need to be as high as 60% of the usual lupin yields. Results shown in Table 5 are a subset of results generated from a sensitivity analysis using MIDAS. This large data set was managed using a MIDAS Interactive Database (MID). MID is a tool that can be easily developed for storage and interrogation of data from any sensitivity analysis with models like MIDAS (Abadi Ghadim and Pannell, 1998). A MID was developed for analysis of the issue of water repellency in the medium rainfall northern wheatbelt farming systems. This database summarises the results of around 500 sensitivity analysis runs where every possible permutation of the factors discussed above was put through the model. MID provides an easy to use interactive method of viewing MIDAS results

† treatment cost were only 10% more than the usual costs of the cereal –lupin rotation (i.e. 10$/ha); † if it was possible to achieve a 30 –40% increase in lupin yields (i.e. 180– 240 kg/ha) and 10% increase in wheat yields (i.e. 80 kg/ha) (Table 5). Table 5 shows that under the conventional management of these water repellent sands the most economical thing to do is to leave 80% of the soil in continuous pasture and about 20% in cereal – lupin rotation. However, if crop and pasture production were to respond to amelioration of repellency (e.g. 10% increase in wheat yield and 30% increase in lupin yields) then it would be worth treating 55% of

Table 5 The minimum yield required for profitable amelioration of repellency on Farm 3 where 50% of the soils are non-wetting. The sandplain lupin and tagasaste options are not made available in this scenario Crop yield response to amelioration of repellency

Model results showing optimal levels of treatment of water repellency and other key indicators under different scenarios

Selected activities on the non-wetting soils

Wheat (%)a

Lupin (%)a

Treated area (ha)

Extra profit ($/ha)

Farm in crop (%)

Cost ($/ha)

Pasture (ha)

Wheat (ha)

Lupin (ha)

0 0 10 10 10 10 20 20 20

0 10 20 30 40 50 60 80 100

0 0 0 833 833 833 833 1416 1500

0 0 0 3 6 9 19 19 23

61 61 61 78 78 78 78 76 76

0 0 0 10 10 10 10 10 10

1174 1174 1174 667 667 667 667 84 0

217 217 217 555 555 555 555 944 1000

108 108 108 275 275 275 275 467 495

a



where the user specifies the input parameters and the optimised farm plan and profit are displayed in a graphical or tabular format. No knowledge of MIDAS, modelling or economics is required to operate MID. This version of MID runs on EXCELe (version 4) for Windowse (version 3.1) or later versions. It is available from Amir Abadi by mail or by email. 5.2. Does the soil mix of the farm matter? Yes. An important finding from this analysis is that soil endowment of the farm is a large determinant of economic impact of water repellency. Table 6 shows that on a farm where proportion of non-wetting sands is relatively small (i.e. 100 ha out of a total farm area of 3000 ha), it is not economical to attempt to improve these soils unless lupin yields can be improved by as much as 80%. This is because such farms have other more fertile soils suitable for cropping. Soils like fertile sandplain soils and red loams grow the highest yielding crops without the costs of amelioration incurred on repellent sands. The stubble from these crops provides good sheep carrying capacity in summer. So from a whole-farm perspective it is best to leave the repellent sands in continuous pasture because it provides the required winter green feed. This type of finding, showing the economic significance of scale of relevance (proportion of non-wetting Table 6 The minimum yield responses required for profitable adoption of amelioration innovations on three different farms Wheat crop yield Lupin crop yield Selected area of non-wetting response (%)a sands in three different response (%)a farm types (ha) Farm 1b Farm 2b Farm 3b 0 10 10 10 10 20 20 20 a b

10 20 30 40 50 60 80 100

0 0 0 0 0 0 100 100

0 0 0 0 0 0 500 500

Percent of yield in rotations specified in Table 2. Soil mix as specified in Table 1.

0 0 833 833 833 833 1416 1500

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sands), highlights the importance of the whole-farm, bio-economic approach in evaluating various innovations aimed at amelioration of repellency. 5.3. The sandplain lupin (Lupinus cosentinii ) option Sandplain lupins (Lupinus cosentinii ) or WA blue lupins are a self-regenerating pasture species that when grown in continuous annual pasture can provide valuable stock feed in early winter and provide an excellent dry summer/autumn feed. Sandplain lupins grow very well on the infertile repellent soils of the northern wheatbelt. Where sandplain lupins are an option, their grazing value increases the value of water repellent sands in the absence of amelioration. This increase in grazing value of non-wetting soils means that any amelioration of repellency has to result in at least 60% or more lupin crop yields before it would be worth the treatment. This finding applies even on a farm with high proportion of non-wetting sands (see Table 7). 5.4. The tagasaste (Chamaecystisus proliferus ) option Tagasaste (Chamaecystisus proliferus ) is a leguminous shrub suited to the non-wetting sands. It can substantially increase the livestock carrying capacity of the farm by providing valuable green feed in autumn and early winter. It requires annual cutting to promote prostrate growth and allow easy access for livestock. When tagasaste is not an option and there are no sandplain lupins, it is worth ameliorating repellency on around 55% of the sands to allow cropping lupins in rotation with cereals (Table 5). However, where tagasaste is an option, it is best to plant around 12% of the repellent soils to this shrub. Tagasaste improves the carrying capacity of the farm during the feed gap period (autumn and early winter) and makes it profitable to leave around 74% of the repellent sands in continuous annual pastures. This means that, where tagasaste option is practised on such a farm, it is only worth ameliorating 13% of the repellent soils to allow cereal –lupin cropping (Table 8). Sandplain lupins and tagasaste, when available, are valuable feeds for livestock during the autumn feed gap. These two sources of sheep feed reduce the need for supplementary grain feeding of livestock substantially. Filling the autumn feed gap increases

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A.K. Abadi Ghadim

Table 7 The minimum yield required for profitable amelioration of repellency on Farm 3 where 50% of the soils are non-wetting. The sandplain lupin are an option on this farm. Tagasaste is not made available in this scenario Crop yield response to amelioration of repellency



Wheat (%)a

Lupin (%)a

Treated area (ha)

Extra profit ($/ha)

Farm in crop (%)

Cost ($/ha)

Pasture (ha)

Wheat (ha)

Lupin (ha)

Sandplain lupins (ha)

0 0 10 10 10 20 20 20

0 10 20 30 50 60 80 100

0 0 0 0 0 833 1443 1443

0 0 0 0 0 5 9 15

50 50 50 50 50 78 78 78

0 0 0 0 0 10 10 10

1378 1378 1378 1378 1378 430 0 0

0 0 0 0 0 555 962 962

0 0 0 0 0 275 476 476

122 122 122 122 122 237 57 57

a


the sheep carrying capacity of the farm and thus increases the profitability of the sheep enterprise. It was found that unless any treatment to ameliorate repellency can result in wheat yield increases of around 40% and lupin crop yield increases of 60% then the farmer is better off with sheep grazing on a

combination of sandplain lupin pastures and tagasaste on the non-wetting sands. Where sandplain lupins and tagasaste are both available on the repellent sands it is best to have a combination of tagasaste, sandplain lupins and annual pastures rather than cropping (Table 9).

Table 8 The minimum yield required for profitable amelioration of repellency on Farm 3 where 50% of the soils are non-wetting. Tagasaste is an option on this farm. Sandplain lupins are not made available in this scenario Crop yield response to amelioration of repellency


Wheat (%)a

Lupin (%)a

Treated area (ha)

Extra profit ($/ha)

Farm in crop (%)

Cost ($/ha)

Pasture (ha)

Wheat (ha)

Lupin (ha)

SP lupinb (ha)

Tagc (ha)

0 0 10 10 10 10 20 20 20

0 10 20 30 40 50 60 80 100

0 0 0 199 326 326 910 1399 1399

0 0 0 0 0 2 5 9 15

50 50 50 57 61 61 77 76 76

0 0 0 10 10 10 10 10 10

1283 1280 1283 1119 1002 1009 477 0 0

0 0 0 133 218 218 606 932 932

0 0 0 66 108 108 300 462 462

0 0 0 0 0 0 0 0 0

217 220 217 182 172 164 114 101 101

a b c

Percent of yield in rotations specified in Table 2. Sandplain lupins. Tagasaste.


311


Table 9 The minimum yield required for profitable amelioration of repellency on Farm 3 where 50% of the soils are non-wetting. Tagasaste and sandplain lupins are options that are both made available on this farm Crop yield response to amelioration of repellency



Wheat (%)a

Lupin (%)a

Treated area (ha)

Extra profit ($/ha)

Farm in crop (%)

Cost ($/ha)

Pasture (ha)

Wheat (ha)

Lupin (ha)

SP lupinb (ha)

Tagc (ha)

0 10 10 10 10 20 20 20

10 20 30 40 50 60 80 100

0 0 0 0 0 1235 1381 1410

0 0 0 0 0 2 5 11

50 50 50 50 50 70 76 76

0 0 0 0 0 10 10 10

1230 1227 1230 1227 1230 0 0 0

0 0 0 0 0 824 920 940

0 0 0 0 0 408 456 465

145 148 145 148 145 118 66 61

125 125 125 125 125 147 53 29

a b c

Percent of yield in rotations specified in Table 2. Sandplain lupins. Tagasaste.

6. Conclusions and recommendations This analysis has shown that a minimum of 30% increase in lupin yields and a 10% increase in wheat yields is probably the minimum requirement before any expenditure on innovations aimed at improving production on non-wetting soils could be justified. However, due to costs of amelioration of repellency much higher crop yield responses may be required for economical adoption of such innovations on most farms in Western Australia. The decision to ameliorate water repellency depends not only on the consideration of direct benefits and costs per hectare of ameliorated sand but also on other whole-farm factors. Some of the factors found to be important were the scale of relevance or the soil mix of the farm and whether or not sandplain lupins (Lupinus cosentinii ) and tagasaste (Chamaecystisus proliferus ) are options available to the farmer. It is important to point out that this study is not meant to cover all aspects and complexities of the

issue of amelioration of water repellency. Rather, it is a study aimed at illustrating some of the important principles involved in the economic evaluation of complex issues such as repellency. Future bio-economic evaluation of water repellency would need to have access to a larger range of experimental and field data regarding the benefits and costs of various amelioration techniques. This study also aims to stimulate discussion on the gaps in our knowledge and where more biological and economic information is required for a more rigorous analysis. The data used in this analysis was a combination of subjective and trial data. Therefore, it was necessary to show through “what-if” analysis which parameters are likely to influence the decision to ameliorate repellency. In fact, this is the best use of this type of economic evaluation (Pannell, 1997). Rather than provide hard and fast rules about the economics of repellency this study aims to show the direction for further research and development by highlighting areas where data is either scarce or missing.


BIBLIOGRAPHY AND REFERENCES


Chapter 29 More than one thousand references related to soil water repellency Louis W. Dekkera,*, Leonard F. DeBanob, Klaas Oostindiea and Erik van den Elsena b

a Alterra, Green World Research, P.O. Box 47, 6700 AA Wageningen, The Netherlands Watershed Management, School of Renewable Natural Resources, University of Arizona, Tucson, AZ 85721, USA

1. Introduction Soil water repellency is much more wide-spread than formerly thought. DeBano gives a historical overview of soil water repellency, covering the period 1920 until 2000, in chapter 2 of this book. All references cited in the chapters 2 to 28 and related to soil water repellency have been included in the present chapter, whereas the references not related to water repellency are given in chapter 30. Chapter 29 presents an improved and extended version of the water repellency bibliography published by DeBano and Dekker (2000) in Journal of Hydrology 231 – 232. At this moment more than one thousand references have been documented. Although several publications were published between 1883 and 1961, an average of 50 to 100 papers per 5 years was reached in the period 1962 to 1986 (Fig. 1). From that time on an enormous increase took place, resulting in more than 200 publications in the last 5 years. More than two hundred of these publications have been written by only 18 senior authors: DeBano (31), Dekker (27), Ritsema (19), Blackwell (14), Bond (13), Doerr (13), Scott (12), Letey (10), Giovannini (10), Karnok (9), McGhie (9), Fink (9), Bachmann (8), Carter (8), Robichaud (7), Roy (7), Valoras (7), and Wallis (7). But also many junior authors had a worthy input in the production of publications on water repellency. For instance, Oostindie was 12 times * Corresponding author. Tel.: þ31-317-474267; fax: þ 31-317419000. E-mail address: [email protected] (L.W. Dekker). q 2003 Elsevier Science B.V. All rights reserved.

Fig. 1. Number of publications related to soil water repellency during the period 1883 to 2002.

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co-author of papers written by Dekker et al. and Ritsema et al., and Coelho was 7 times junior author of papers published by Ferreira et al. and Shakesby et al. This list of publications on water repellency will also become available on internet. The authors intend to update the list on internet regularly with new and missing publications. They are grateful for anyones help in getting the information concerning these publications.

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