Design, Operation, Maintenance and Performance Evaluation of Surface Irrigation Methods

Surface and Subsurface Irrigation. IAMB-CIHEAM. E. Playán Design, Operation, Maintenance and Performance Evaluation of ...

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Surface and Subsurface Irrigation. IAMB-CIHEAM. E. Playán

Design, Operation, Maintenance and Performance Evaluation of Surface Irrigation Methods

Part of the International Course on:

Land and Water Resources Management: Irrigated Agriculture Istituto Agronomico Mediterraneo-CIHEAM Via Ceglie, 9. Valenzano, Bari, Italy

Enrique Playán Estación Experimental de Aula Dei, Consejo Superior de Investigaciones Científicas P. O. Box 202 50080 Zaragoza, Spain. [email protected]

Surface Irrigation. Enrique Playán

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Table of contents 0. Introduction............................................................................................................................4 1. Surface irrigation concepts ..................................................................................................10 1.1. Irrigation times and phases............................................................................................10 1.2. Infiltration and opportunity time ...................................................................................11 1.3. Irrigation performance...................................................................................................14 1.4. Surface irrigation performance compared to other irrigation systems ..........................18 1.5. Exercise: Advance, recession and performance indexes...............................................20 2. Types of surface irrigation systems .....................................................................................24 2.1. Level basins ...................................................................................................................24 2.2. Free draining borders.....................................................................................................30 2.3. Blocked-end borders......................................................................................................32 2.4. Furrows..........................................................................................................................34 2.5. Level furrows.................................................................................................................37 2.6. Other systems ................................................................................................................38 3. Characterisation of infiltration.............................................................................................39 3.1. The process of infiltration .............................................................................................39 3.2. Empirical equations of infiltration.................................................................................44 3.3. Measuring infiltration in basins and borders .................................................................48 3.4. Measuring infiltration in furrows ..................................................................................49 3.5. Estimating the basic infiltration rate in furrows ............................................................52 3.6. Exercise 1: ring infiltrometers .......................................................................................52 3.7. Exercise 2: Furrow infiltration / discharge in two points ..............................................55 3.8. Exercise 3: Furrow infiltration / blocked furrow...........................................................56 4. Measuring irrigation water...................................................................................................58 4.1. Overview .......................................................................................................................58 4.2. Broad crested weirs .......................................................................................................60 4.3. Exercise: design and construction of a broad crested weir ...........................................61 5. Surface irrigation models.....................................................................................................64 5.1. Models and reality .........................................................................................................64 5.2. Partial differential equations governing open channel flow..........................................65 5.3. Numerical models of surface irrigation .........................................................................67 5.4. Two-dimensional level basin models ............................................................................69 6. Surface irrigation evaluation................................................................................................71 6.1 The goals of evaluation ..................................................................................................71 6.2. Evaluation procedures ...................................................................................................71 6.3. Adjusted infiltration approach.......................................................................................72 6.4. Estimating infiltration and / or roughness from advance ..............................................73 6.5. Determining irrigation performance..............................................................................75 6.6. Exercise 1: Adjusted infiltration....................................................................................75 6.7. Exercise 2: Complete evaluation ...................................................................................77 6.8. Exercise 3: Infiltration and roughness from advance ....................................................81 7. Surface irrigation design......................................................................................................84 7.1. The goals of design........................................................................................................84 7.2. Basic design procedures for basins and borders............................................................85


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7.3. Basic design procedures for furrows .............................................................................85 7.4. Using models for design of irrigated areas....................................................................86 7.5. Design of the required capacity of the distribution network .........................................88 7.6. Design of in-line reservoirs ...........................................................................................89 8. Improving surface irrigation performance ...........................................................................92 8.1. Laser guided land leveling.............................................................................................92 8.2. Adjusting irrigation discharge .......................................................................................92 8.3. Cutback irrigation..........................................................................................................93 8.4. Surge irrigation..............................................................................................................94 8.5. Surface irrigation automation ........................................................................................95 8.6. Irrigation scheduling......................................................................................................96 9. Surface irrigation and the environment ...............................................................................97 10. Class project.......................................................................................................................99 11. Suggested readings ..........................................................................................................108 12. Self evaluation .................................................................................................................111 13. Index ................................................................................................................................121


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0. Introduction The term “surface irrigation” includes a large variety of irrigation systems sharing the common characteristic that water is applied on the soil surface and is distributed along the field by gravity. Therefore, the irrigation discharge decreases along the field due to the process of infiltration. Since gravity is in charge of water distribution, no other structures are required within the field (as would be the case for sprinkler or drip irrigation). At the same time, water does not have to be pressurised in order to attain the design uniformity and efficiency goals. These characteristics give surface irrigation two important advantages: 1. No on-farm equipment is necessary (no equipment costs); and 2. No pumping is required over field level (no extra energy costs).

When surface irrigation systems are properly designed and managed, surface irrigation can be very efficient and ensures uniform irrigation. If, on the contrary, operation or management fail, these advantages will be shadowed by other costs linked to this type of irrigation. Among them can be high labour costs, poor yields and / or low irrigation efficiency. The current challenge in irrigation engineering is to modernise and rehabilitate surface irrigation systems, so that deep percolation and surface runoff losses are minimised. This way, water will be optimally used and the irrigation projects will become environmentally sound.

From a historical perspective, irrigation systems have been part of the major societies of the world. Surface irrigation is thousands of years old, and has given place to much of the culture and wealth of ancient and modern societies. Up to the last quarter of the twentieth century, surface irrigation was the only irrigation system available, and therefore all irrigation projects included surface irrigation designs. This was not a problematic situation up to the sixties and seventies, when the availability of large scrapers resulted in the development of surface irrigation projects in areas where this irrigation system was not suited: with undulated topography and poor soils. These particular developments led to some good examples of poor water use in surface irrigation and of how an irrigation system can


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result in permanent damage to the soils and the environment. This situation happened in the past, but it does not have to happen necessarily in the future. Most of the soils adequate to surface irrigation have already been developed, and therefore in the future there will not be much new land set to surface irrigation. Instead, the focus will be on modernizing the existing surface irrigation systems and making sure that their water use standards are compatible with the excellence in water use that the society is now a days demanding. In the last decades pressurized irrigation systems have grown spectacularly, while surface irrigation systems have seen a continuous decrease in their acreage. As an example, figure 1 shows the results of a yearly survey on irrigation systems in the United States of America. This figure shows that, despite its progressive decrease in acreage, up to 1997 surface irrigation systems were still the most important in the United States. A similar study was performed in California from 1972 to 2001. Results for California are particularly relevant to this course, since this is an area characterized by a very high technological level, severe water scarcity and a very powerful agricultural sector. An additional detail on this area is that information on irrigation systems and water use is readily available. Unfortunately this is not the case in many areas of the world. The results of this study are presented in figure 2.

Irrigated Area (million ha)

30 25

Total

20

Surface

15

Sprinkler

10

Drip

5 0 1983 1985 1987 1989 1991 1993 1995 1997 1999 Year

Figure 1. Time evolution of the relative relevance of surface irrigation and pressurized irrigation systems in the United States of America (Source: Irrigation Journal, 19832000).


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California has also seen a decline in surface irrigation systems in the years of records. However, in 2001, half of the irrigated area was still surface irrigated. These data suggest that surface irrigation can be an adequate choice for particular crops and farming conditions even if technology is available and affordable and water is scarce. It is difficult to foresee the future trend, although it seems reasonable to expect a progressive decrease of surface irrigation acreage throughout the world. Only those areas where surface irrigation is the first technical, economical and social choice should retain this system. The balance among irrigation systems will be dictated in the future by the balance among the prices of water, labor and energy.

Figure 2. Time evolution of irrigation systems in California (USA) from 1972 to 2001. (Reproduced from Orang et al., 2008).

Table 1 shows the results of a detail survey on irrigation systems. In this case, data were collected in the State of California in 1991 (Snyder et al., 1996). The report elaborates on the relationship between types of crops and types of irrigation systems.


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Table 1. Irrigation systems by type of crops in California in 1991. (Snyder et al., 1996) Surface Irrigation

Sprinkler Irrigation

Drip Subsurface Irrigation Irrigation (%) (%)

(%)

(%)

Field crops

89

9

0

1

Vegetable

71

20

9

0

Orchard

32

32

36

0

Vineyard

45

13

42

0

Total

67

17

15

1

The study was repeated in California in 2001 (Orang et al., 2008). The following results were found (Table 2): Table 2. Irrigation systems by type of crops in California in 2001. (Orang et al., 2008) Surface Irrigation

Sprinkler Irrigation

Drip Subsurface Irrigation Irrigation (%) (%)

(%)

(%)

Field crops

84

12

0

4

Vegetable

43

36

21

0

Orchard

20

16

63

0

Vineyard

21

9

70

0

Total

50

16

33

2

These data reveal that in 2001 surface irrigation occupied a very relevant extension (50% of the total irrigated area), with a 16% for sprinkler irrigation, 33% for drip irrigation and 2% for subsurface irrigation. Then years earlier, in 1991, surface irrigation amounted to 67% of the area, with 17% for sprinkler and 15% for drip and 1% for subsurface. This relevant change in the irrigation systems is associated to changes in the availability of labor, water, irrigation technology and particularly to changes in the Californian cropping pattern.


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It is very interesting to note how crops and irrigation systems are related. For instance, when it comes to irrigate field crops (cereals and small grains), the performance of surface irrigation is unmatched, due to its low investment and energy costs. In the production of vegetables, furrow irrigation continues to be the leading irrigation system, since it is agronomically very well suited to these crops. Sprinkler irrigation is also well adapted to a number of vegetable crops, while drip irrigation (particularly underground drip irrigation) has gained acreage in the recent years. In orchards, however, drip irrigation is the current California leader, replacing sprinkler irrigation. Surface irrigation is still used in 20% of the fruit farms, showing a relevant decrease in acreage in the last decade. In vineyards, surface irrigation (leader in 1991) has quickly been replaced by drip irrigation (70% of the area). In Mediterranean climates, drip irrigation is the system of choice for fruit trees and vineyards. Since the evolution of irrigation systems seems to be so tightly linked to the evolution in the cropping alternative, it will be interesting to analyze the evolution of crops in California to gather insight on the possible future trends in the Mediterranean. Figure 3 confirms that field crops tend to decrease their importance in Mediterranean areas (due to water scarcity) and are replaced by orchards. Since vegetable crops are very labor intensive, their evolution is linked to labor availability. In 2007, the situation of irrigation systems in Spain was as follows: 37% for surface irrigation, 24% for sprinkler irrigation, 37% for drip irrigation and 2% for other systems.


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Figure 3. Time evolution of the relative relevance of different types of crops in California (Reproduced from Orang et al., 2008).


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1. Surface irrigation concepts The goal of irrigation is to supply water to the crop so that its growth is not limited by water availability. A proper salt balance can also be maintained in the soil if the leaching fraction is considered when deciding on the irrigation dose. In this chapter, the basic concepts on surface irrigation will be addressed.

1.1. Irrigation times and phases Surface irrigation is divided in phases that separate hydrological processes of different nature. These phases are useful to analyse water movement on the irrigated field. The phases are separated by characteristic times, in which certain singularities appear. These times are: 

Starting time (ts), when water first flows into the border, basin or furrow.



Time of advance (ta), when water completely covers the basin or border, or when water reaches the downstream end of a furrow.



Time of cut off (tc) when water stops flowing into the irrigated field.



Time of depletion (td), when a part of the basin, border or furrow becomes uncovered by water once the water has fully infiltrated or has moved to lower areas of the field.



Time of recession (tr), when water can no longer be seen over the field.

These characteristic times determine the length of the irrigation phases: 

Advance phase: between ta and ts. Water is covering the field.



Filling phase: between tc and ta. Water is building up in the field.



Depletion phase: between td and tc. Water level decreases in the field.



Recession phase: between tr and td. Water uncovers the field surface.


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Figure 4 represents the different times and phases characteristic of surface irrigation. Every surface irrigation system is characterised by a distribution of these times and phases, reflecting its design peculiarities.

Phases Advance

ts Starting time

Depletion Recession

Filling

ta Time of advance

tc

td

tr

Time of cut off

Time of depletion

Time of recession

Times Figure 4. Irrigation times and phases.

1.2. Infiltration and opportunity time Opportunity time () is the time water remains on top of a given point of the field. It is, therefore, the time between advance and recession on a point. During this time, there is a certain depth of water on top of the soil. This water has the opportunity to infiltrate in the soil at the speed dictated by the infiltration of the soil. Very often in surface irrigation  is measured in minutes. Advance and recession are frequently represented in the form of an advance-recession diagram (Figure 5). In this diagram, the length or percentage of the field covered by advance or uncovered by recession is presented in abscissae, while in ordinates the time since irrigation start is presented. In this chart, the points determining the trajectory of the advance and recession front form curves that are helpful to analyse the irrigation event. This diagram is very useful to determine the opportunity time and to estimate irrigation uniformity and efficiency. For an irrigation to be uniform, it is necessary that  is uniform along the irrigated field. In figure 6a, an example is used to illustrate the computation of  in two points of a field using an advance-recession diagram.


Time (min)

tr

12

Recession

td tc ta

 Advance

ts 0

Length (m)

L

tr td ta tc

a

B A

ts 0

L Length (m)

b

Infiltration (m)

Time (min)

Figure 5. Advance-recession diagram

ZB 0

B

ZA

A

Opportunity time (min)

Figure 6. Computation of opportunity time and infiltration in two points.

For points A and B, located along a free draining border, the opportunity time is computed as the vertical distance between the advance and recession curves. In this hypothetical case, A approximately doubles B. Figure 6b presents a typical infiltration curve. In this curve, the cumulative infiltration on a point is expressed as a function of the opportunity time. As is characteristic of the infiltration process, the infiltrated depth, Z, increases very fast at the beginning of the process and slowly towards the end. As a consequence, the difference between the infiltrated depths, ZA and ZB is much smaller than the difference between A and B. The infiltration curve can be obtained by a number of experimental procedures that will be discussed in chapter 3.


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During the advance phase, infiltration starts at each point when the advancing front reaches it. Figure 7 presents a typical overland flow – infiltration scheme on a field with a length L. The amount of water infiltrated at each point of the field can be measured or estimated. To measure infiltrated water, it is necessary to measure soil water before and after the irrigation event, and obtain soil water recharge as a difference.

Flow direction Flow depth Infiltrated depth

Overland flow Infiltration Advance L

Figure 7. Schematic representation of the irrigation advance front.

To estimate infiltrated water, usually infiltration curves are used in conjunction with opportunity times. The infiltrated water profile is often similar to the representation in Figure 8. The point located by the water inflow (always represented left in the pictures) is the one receiving more water. The point located at the downstream end is the one receiving less water.

L Infiltrated depth

Figure 8. Longitudinal profile of infiltrated depth.


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1.3. Irrigation performance The terminology used to describe irrigation performance usually includes “efficiency” and “uniformity” terms. These terms frequently mean different things to different authors. Unfortunately there is not a single performance term that can be used alone to describe irrigation performance. That is the reason why several performance indexes are often used to describe an irrigation event. Conceptually, irrigation performance depends on: 

The increment in soil water retained in the rootzone of a crop after an irrigation.



Deep percolation losses



Surface runoff losses.



The evenness (uniformity) of the infiltrated water along the field.



Soil water deficit after irrigation.

Irrigation uniformity is expressed by indexes. In all of them, a value of 100 indicates that all points in the filed receive the same amount of irrigation water. Figure 9 presents different cases of irrigation uniformity.

L Z

100% Uniformity

Acceptable uniformity

Low Uniformity Figure 9. Different cases of irrigation uniformity


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In surface irrigation, uniformity is often characterised using the Distribution Uniformity index (DU, %). This index can be computed as:

DU 

Average infiltrated depth in the 25% of the field less irrigated 100 Average infiltrated depth

[1]

The fate of the irrigation water applied to the field is described by three indexes that add up to 100%. These indexes are Application Efficiency (Ea), the Deep Percolation ratio (DPR) and the Surface Runoff ratio (SRO).

L

VSRO

VZR

ZR VDP

VD

Figure 10. Representation of the fate of irrigation water.

Where ZR is the required irrigation depth. Application efficiency can be described as:

Ea 

Volume of water retained in the roozone V 100  ZR 100 Volume of applied irrigation water VT

[2]

The applied water that is not retained in the rootzone has been lost to one of two processes: surface runoff or deep percolation. These two terms can be computed as:


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DPR 

V Volume of water lost to deep percolation 100  DP 100 VT Volume of applied irrigation water

[3]

SRO 

V Volume of water lost to surface runoff 100  SRO 100 VT Volume of applied irrigation water

[4]

Finally, the last aspect of irrigation to be considered in the characterisation of performance is adequacy: the percent of the pre-irrigation soil water deficit that is covered by irrigation. For this matter, the Water Requirement Efficiency is presented.

Er 

VZR Volume of water stored in the rootzone 100  100 Volume of water deficit in the rootzone prior to irrigation ZR L W

[5]

where W is the field width. Some useful relationships can be derived from Figure 10. For instance, the total volume of irrigation water VT can be computed as: VT  Q t c

[6]

The volume of irrigation water is either infiltrated or ran off: VT  VZ  VSRO

[7]

where VZ is the total volume of water infiltrated in the soil. This volume of water is either retained in the root zone or deep percolated: VZ  VZR  VDP

[8]

These last three relationships express simple concepts of mass conservation that are very useful in the analysis of surface irrigation.


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Proper use of irrigation water implies that the plants receiving less water have enough to complete their evapotranspirative processes. At the same time, the differences in water availability between parts of the field must be small. These criteria imply high efficiency and uniformity. A high level of uniformity is mandatory if a high efficiency must be attained. At the same time, cases where uniformity is high but efficiency is low are not infrequent. Different combinations of the parameters are presented in Figure 11.

a)

DU = 80% Ea = 40% Er = 100%

b)

DU = 80% Ea = 80% Er = 90%

c)

DU = 80% Ea = 100% Er = 60%

Figure 11. Different cases of irrigation uniformity and efficiency.

The three cases considered differ in the value of the required irrigation depth, ZR. The relative size of ZR versus the irrigation water application determines the values of the efficiency indexes. The value of DU remains constant while Ea ranges from 40 to 100%.


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1.4. Surface irrigation performance compared to other irrigation systems It is generally accepted that surface irrigation performance is lower that that of pressurised systems. To illustrate the discussion in irrigation system performance, data from California will again be presented. Table 3 presents the results of a survey on irrigation performance based on the field evaluation of almost a thousand on-farm Californian irrigation systems. The main conclusion that can be drawn from this table is that (in the California conditions) the performance of all irrigation systems is very similar. If the reader want to extract more detailed information, surface irrigation systems are among the most efficient systems. If a surface irrigation system is well adapted to the soil and crop conditions, and it has been properly designed and managed, its performance can match that of any other irrigation system. Table 3. Irrigation system performance in California in 1995. Standard deviation is provided

in parenthesis. Letters indicate statistical differences at the 95% probability level (According to Hanson et al., 1995). Irrigation system Sprinklers Pivots and Rangers Under tree Sprinkler Drip (permanent crops) Drip (annual crops) Furrows Borders and basins

Sample size 164 57 28 458 23 157 72

DU (%) 62 (15) c 75 (10) a 79 (16) ab 73 (15) a 63 (16) c 81 (14) b 81 (14) b

Ea (%) 69 (13) ac 81 (11) ab 81 (18) ab 76 (18) a 66 (14) c 80 (14) ab

The following table presents similar data from a different source. In this case, potential application efficiency (the efficiency that could be attained with optimal design and management) is estimated for different irrigation systems.


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Table 4. Potential application efficiency for different irrigation systems. These data are

representative of properly designed and managed systems. (According to Clemmens and Dedrick, 1994). Irrigation system Furrow Furrow, modern Borders Basins Paddy rice Micro sprinkler LEPA Ranger Pivot Boom Hand-move sprinkler Solid-set sprinkler

Ea (%) 50-70 60-80 55-80 65-90 40-60 85-90 80-90 75-90 75-90 60-75 65-85

70-85

Finally, Table 5 presents data of estimated actual and potential irrigation efficiency for different irrigation systems in California, 1991. Table 5. Application efficiency of different irrigation systems in California (According to Keller, 1991). Irrigation system

Surface Pivots and Rangers Solid-set sprinkler Under tree sprinklers Drip or microjet, permanent Drip, portable

Average efficiency (%) 70 75 65 75 70 65

Ideal Efficiency (%) 85 85 80 90 85 80

The differences revealed by these authors between ideal and current irrigation performance evidence that a lot is still to be done in the areas of irrigation design and management to improve the current standards of water use all over the world.


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1.5. Exercise: Advance, recession and performance indexes An irrigation evaluation was performed on a border. The border was 140 m long and 30 m wide. Infiltration was measured in situ and a Kostiakov infiltration equation was adjusted. The resulting equation was: Z = 0.010 0.410 Where the opportunity time is expressed in minutes and the infiltration in meters. Using this equation, it is possible to obtain the infiltrated depth from observations of the opportunity time. The irrigation discharge was 60 l s-1, and the irrigation event lasted 190 min. Stations were marked along the border every 20 m to record the advance and recession phases. The target irrigation depth (ZR) was computed from soil depth and soil water retention. The resulting value for ZR was 95 mm. The following table presents the advance and recession times recorded at the observation stations:

Station number 1 2 3 4 5 6 7 8

Distance to origin (m) 0 20 40 60 80 100 120 140

Advance time (min) 10:30 10:45 11:03 11:23 11:48 12:19 12:55 13:40

Recession time (min) 16:21 16:30 16:35 16:40 16:50 16:55 17:00 17:15

These data were elaborated in the following table. The times of advance and recession were computed taking differences to the time of advance to a distance of 0 m (the time at which irrigation starts). The opportunity time is obtained at each station by difference between the time of recession and the time of advance. The infiltration equation was used to compute Z from . The two remaining columns, deep percolation and soil water storage, were computed comparing Z and ZR. If Z exceeds the target irrigation depth, the soil stores the maximum value of ZR and water in excess is lost to deep percolation. If Z is less than or equal to ZR, there is no deep percolation and all water infiltrated is stored in the soil profile.


Station number 1 2 3 4 5 6 7 8

Distance ta (m) (min) 0 0 20 15 40 33 60 53 80 78 100 109 120 145 140 190

tr (min) 351 360 365 370 380 385 390 405

 (min) 351 345 332 317 302 276 245 215

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Z Percolation Stored in root (m) (mm) zone (mm) 0.111 16 95 0.110 15 95 0.108 13 95 0.106 11 95 0.104 9 95 0.100 5 95 0.095 0 95 0.090 0 90

Using the data from the following table, the total volume infiltrated, percolated and stored in the root zone can be computed. These computations are performed for each reach between two stations. The volume in a reach is obtained multiplying the average depth by the distance between stations and the field width.

Stations

1-2 2-3 3-4 4-5 5-6 6-7 7-8 Total

Average Infiltration (mm) 110.5 109.0 107.0 105.0 102.0 97.5 92.5

Average Percolation (mm) 15.5 14.0 12.0 10.0 7.0 2.5 0.0

Average Storage (mm) 95.0 95.0 95.0 95.0 95.0 95.0 92.5

Volume Volume Infiltration Percolation (m3) (m3) 66.3 9.3 65.4 8.4 64.2 7.2 63.0 6.0 61.2 4.2 58.5 1.5 55.5 0.0 434.1 36.6

Volume Stored (m3) 57.0 57.0 57.0 57.0 57.0 57.0 55.5 397.5

From the infiltrated volume, the average irrigation depth is 103 mm. The total irrigation volume was 684 m3, computed from irrigation discharge and time. The volume of runoff can be computed as 250 m3. Threfore, SRO = 37 %. DP can be computed from the volume of deep percolation (36.6 m3), and its value is 5 %. Ea can be computed as 100 - SRO - DP. Its value is 58 %. Ea can also be computed as 397.5 / 684 * 10, to obtain the same result. Between stations 7 and 8 some water deficit occurs, because in station 8, only 90 mm are infiltrated. The deficit is therefore 5 mm in station 8 and 2.5 mm on the average for reach


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7-8. The volume of deficit can be computed to be 1.5 m3. The volume of water that could be stored in the root zone is 95 * 140 * 30 /1000 = 399 m3. therefore, Er can be computed as 99.5 %. The distribution uniformity can be computed as the percent of the average infiltrated depth in stations 7 and 8 over the average infiltrated depth in stations 1 to 8. The resulting value of DU is 90%. As a conclusion, the irrigation event analysed in this exercise is very uniform, but very inefficient. Ea is low because the surface runoff is very important. Agronomically the irrigation event is very satisfactory, since Er is very high and uniformity is very good, too. The following figures present the advance-recession diagram and a plot of infiltration depth and ZR. 450 400 350 300 Time 250 (min) 200 150 100 50 0 0

20

40

60

80

Field length (m)

100

120

140


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Field length (m) 0 0,00 0,02 0,04 Z and ZR 0,06 (m) 0,08 0,10 0,12

20

40

60

80

100

120

140


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2. Types of surface irrigation systems It is not an easy task to establish types of surface irrigation systems, for often the differences between them are not clear. The categories are frequently bound to management practices more than to design criteria. The following types comprise most of the surface irrigation systems in place today.

2.1. Level basins In this case, the irrigated field is surrounded by a dike that prevents runoff. The main characteristics of this irrigation system are that the field is levelled to zero slope and that there is no provision for runoff. The basin shape is often close to being square. The size is very variable, but generally fits in the range 0.3 to 3.0 ha. Since there is no gravitational slope, water advance in the field is exclusively due to the slope of the water profile. Figure 12 presents two typical configurations of basin irrigation. In the first case (a), a basin is irrigated from a corner. This set-up is advantageous since the gate does not interfere with farm machinery operations. In the second case (b) a different basin is simultaneously irrigated from several water sources. This disposition is frequent when the distribution ditch runs parallel to the basin and when the basin is particularly wide. In this way a better water coverage of the basin is ensured.

a)

b)

Gate

Gates

Ditch

t1

Ditch

t1 t2

t2 t3

t3

Figure 12. Two typical configurations of level basin irrigation.


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Level basin irrigation is very important, since it is a very common system in traditional surface irrigation systems. In our days, this irrigation systems is enjoying a great interest, since the introduction in the seventies of Laser guided land scrapers has allowed farmers to obtain high quality levelling in large basins. This technique has enabled large level basins with very precise levelling. In these conditions this irrigation system can attain high uniformity and efficiency at very low labour cost. Figure 13 presents the schemes corresponding to the two phases theoretically present in level-basin irrigation: advance and depletion.

Advance

Depletion Figure 13. Theoretical phases of level-basin irrigation.

Usually, water is cut off before completion of advance. Therefore, the filling phase is omitted. This peculiarity in the time of cut off is due to the fact that the field is levelled to no slope. Often, basin irrigation applies large irrigation depths. In these conditions, the goal is to irrigate using the smallest volume of water that allows full coverage of the field. The skill of the irrigator determining the time of cut off permits the advance phase to be completed. Once water is cut off, the water profile becomes horizontal, water stagnates and the depletion phase takes place until recession occurs simultaneously at all points in the field. The concavity of the advance curve corresponds to the fact that advance is fastest at the beginning of the irrigation event (Figure 14). As the advance phase proceeds, the surface of the field infiltrating water increases. Therefore, the discharge remaining for advance diminishes. In these circumstances, for the irrigation to attain high uniformity, advance must be as fast as possible. This is often accomplished by using the largest discharge possible. The upper bounds for irrigation discharge are the capacity of the distribution system and the


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erodibility of the soil. Erosion control structures are often built just downstream from the irrigation gate to prevent soil degradation.

Recession

Time (min)

td= tr ta

Advance

tc ts 0

87

Area (%)

100

Figure 14. Theoretical advance-recession diagram for level basin irrigation.

Now that the general case of level basin irrigation has been presented, two deviations from the rule will be presented that will bring the analysis closer to the real basin irrigation systems. The first deviation focuses on the assumption that the slope is zero. The second objection is related to the definition of level basin as a system with zero surface runoff losses. Even if Laser guided land scrapers are used, the field surface is never plane. The general slope can be zero, but the soil surface always shows small scale undulations that result in perturbations of the analysis just presented. The magnitude of the soil surface undulations can be measured with the standard deviation of soil surface elevation (SDe), obtained from a set of survey points. A typical value of SDe for recently Laser levelled fields is 10 mm. For conventionally levelled fields, values up to 40 mm are not uncommon. Figure 15 illustrates how, in fact, the number of phases present in level basin irrigation is three: advance, depletion and recession. The recession phase appears due to the fact that at high spots recession happens shortly after cut off. At low spots, recession is delayed by the accumulation of stagnant water.


27

Advance Depletion Recession Figure 15. Real phases of level-basin irrigation.

When the irrigation event is expressed in the form of an advance-recession diagram, the effect of non-levelness is apparent. In figure 16, the diagram is built using advance distance in abscissae. In figure 17, percent area is used instead of advance length. In both cases, the recession phase is apparent. Very often the recession phase lasts longer then the advance phase. For this reason, the classical approach to the analysis of level basin irrigation (neglecting recession) does not seem to be suited to analyse irrigation performance.

Low spot High Spot

Time (min)

tr Recession td ta tc



Advance

ts 0

Length (m)

L

Figure 16. Real advance-recession diagram of level-basin irrigation (in length).


28

Time (min)

tr t2 t1

Recession

td ta tc

Advance

ts 0

20

80

Area (%)

100

Figure 17. Real advance-recession diagram of level-basin irrigation (in percent area).

Figure 18 presents two common configurations of level basins in which water is allowed to runoff and the runoff water is reused at the downstream basin. Scheme b) is known in the Southwest of the US as a “drain back” system. Both schemes form a category of runoff reuse (RR) level-basin systems. As discussed early in the text, one of the problems with level basin is that the irrigation application depth can be too large when compared to the soil water deficit. As a result, the water retained in the root zone can be small in comparison with the total applied water. As a consequence, the DPR will be large and Ea will be small. The key to this problem is that at completion of advance a large volume of overland water inundates the basin. If a substantial part of this water can be removed from this basin the irrigation event will be uniform and efficient. The operation will not be practical unless this water is beneficially used. This is accomplished reusing the water in the following basin.

a)

b)

Ditch

Ditch Outlet Outlet

Irrigating

Irrigated

Irrigating

Irrigated

Figure 18. Two runoff and reuse (RR) schemes in level basin irrigation


29

Scheme a) creates an additional irrigation point at the next basin. Often the outlet is located at the downstream end of the first basin, and the second basin is irrigated from its two extremes. Scheme b) is more elaborated, and requires a special set-up of the supply system which is presented in Figure 19. In this case, low pressure buried pipes are used for water supply. A sliding gate is introduced in the concrete water check to cover the exit pipe and divert water into the first basin. When irrigation is completed, the gate is lifted and the water level in the field provokes runoff into the check. The runoff discharge is added to the original discharge and the next basin is irrigated with a larger discharge. The benefit from this scheme is twofold: 1. It is an RR scheme: it has potential to increase irrigation efficiency. 2. Advance will be faster in all basins (except for the first). A faster advance will result in a more uniform irrigation.

b) Dewatering

a) Irrigating Sliding gate Water level

Irrigation turnout

Irrigation in course

Water level

Buried pipe

Increased discharge downstream Outlet in course

Figure 19. Field implementation of an RR scheme.

RR systems are good options when a level basin irrigation system is set up in soils with poor water retention. Under these circumstances the increase in Ea can be substantial.


30

2.2. Free draining borders Borders are strips of land, usually rectangular in shape and showing a longitudinal slope. Borders are often long and narrow. If the cross slope is uniform, borders will have straight boundaries, as shown in Figure 20a. In other cases, the relief is dominated by little hills and depressions, and the border boundaries are coincident with contour lines. In these cases, the borders are often referred to as “contours” (figure 20b). Water is applied at the high, narrow border boundary. In this irrigation system all phases described in Figure 4 are present, as depicted in Figure 21.

a)

b)

Irrigation

Irrigation

Runoff

Runoff

Figure 20.Borders and contours.

Advance Filling Depletion Recession Figure 21. Schematic representation of the phases in free draining border irrigation.


31

Water advance is facilitated by the positional energy supplied by the longitudinal slope. After completion of advance, water starts to run off the field. This runoff adds an additional problem to the design of these irrigation systems: the disposal of runoff water. A surface runoff conveyance network will be required to convey this amount of water to a disposal point or to a reuse point. Most of the runoff water will be reused in the same irrigation system, although often this water reuse is not evident because it is performed far downstream from the point where runoff was generated. If runoff is to be used on farm, a reservoir and a pumping station will have to be added to the runoff disposal system, as indicated in the hypothetical case depicted in figure 22.

Diversion U1

B1 B2 B3

Irrigation

B4 B5 P

Central Pivot

R

B6 U2

P U3

Figure 22. A reuse scheme for free draining borders.

In this case study, water from a series of borders (B1 to B6) is collected by the runoff conveyance system depicted by the dashed line. Runoff water is collected in the reservoir (R), and can be used in several ways: First, to irrigate the same borders (U1), after passing through a pumping station. Second, to supply the centre pivot (U2), after passing again through a pumping station. Finally, water can be released into a general runoff outlet where it can be reused downstream (U3).


32

In those cases when free draining borders constitute the optimum design solution, the complexity derived from managing the runoff water will be compensated by its adaptation to the conditions of the farm and its efficient use of water. The advance-recession diagram characteristic of this irrigation system was presented in figure 5. In borders (and furrows), the diagram uses distance rather than area, for the distance is readily measurable in the field. This is not the case for level basins. Since the longitudinal slope accelerates advance, often water is cut off a certain time after completion of advance. As a consequence, the irrigation is more uniform and the average infiltrated depth is larger. The longer the duration of the filling phase, the larger the volume of runoff will be.

2.3. Blocked-end borders This type of irrigation system is actually a blend of level-basin and free draining border irrigation. The irrigated fields are borders: rectangular and with longitudinal slope, but no runoff outlet is available. This irrigation system has therefore an important management problem: is water is cut off too late, water logging will be an important issue at the downstream end of the field. A prolonged water logging can endanger the yield and even the survival of sensible crops, such as alfalfa. On the contrary, if water is cut off too early, the irrigation could be incomplete, because the advancing front would not reach the downstream end of the field. All four irrigation phases are present in blocked-end borders, as can be seen in Figure 23. Nevertheless, to avoid water logging in mild slopes with low infiltration rates, irrigation water is often cut off before completion of advance, therefore eliminating the filling phase.


33

Advance Filling Depletion Recession Figure 23. Schematic representation of the phases in a blocked-end border irrigation.

In this type of irrigation system, errors in the determination of the proper time of cutoff are accumulated at the downstream end. These errors show up as under- or overirrigation. In the case of level basins, these errors are spread all over the field, since there is not a general slope. The advantage of blocked-end borders is that the slope accelerates advance (thus providing uniformity) and that a runoff conveyance network is not needed. The success of this system often depends on the skills of the irrigator, mainly on his wisdom determining the proper time of cut off. Management problems are often proportional to the longitudinal slope. For this reason it is a good idea to use moderate slopes, below 1‰. Figure 24 presents the advance-recession diagrams corresponding to the two management problems associated to blocked-end borders: under- and over irrigation. The correspondence of these diagrams with the infiltrated depth is shown in figure 25. Proper management of the time of cut-off can result in uniformities similar to those attained with other surface irrigation systems.


a)

34

b)

tr td tc ts 0

Time (min)

Time (min)

tr

Length (m)

L

td tc ta ts 0

Length (m)

L

Figure 24. Advance-recession diagrams for blocked-end borders:

a) incomplete advance; b) overirrigation.

a)

b) L

L

Figure 25. Infiltrated profile for blocked-end borders:

a) incomplete advance; b) overirrigation.

2.4. Furrows The main difference between furrows and borders/basins is that in the case of the furrows, irrigation water does not cover the surface of the field in full. Rather, water flows along the concavities created by the mechanical undulation of the soil surface. An important characteristic of this irrigation system is that the field can have a transversal slope (figure 26).


35

Irrigation Furrow flow Runoff collector

Longitudinal slope Side slope

Figure 26. Scheme of a furrow irrigated field.

Furrow length can attain more than 300 m. Usually furrows have a longitudinal slope, although furrows levelled to zero slope are frequent and will be discussed in the following entry. Disposal of runoff will require a surface drainage conveyance system similar to those discussed for free-draining borders. Irrigation water is applied to each furrow individually, using different procedures, like siphons or gated pipes. The siphon system is illustrated in figure 27. The irrigation introduces the whole siphon in the conveyance ditch, filling it with water. Once full, the siphon is placed between the ditch and the furrow. The difference in elevation between the water surface at the ditch and at the furrow will determine the discharge, which usually falls in the range of 0.5 and 3.0 Ls-1 per furrow. The siphons are made of rigid plastic or metal pipes 30 to 50 mm in diameter and 1 to 1.5 m in length.

Irrigation ditch

Siphon Furrow

Furrow cross-section

Figure 27. A siphon system for furrow irrigation


36

Lately, gated pipes are becoming more popular than siphons, because they require less labour and less pressure head. A scheme of a gated pipe set-up is presented in figure 28. The system in based on a distribution pipe 100 to 250 mm in diameter made of plastic or aluminium. Plastic pipes can be either rigid or flexible. The pressure inside the pipe should not exceed 1 m of water. In one side of the pipe, sliding gates are located at a spacing similar to furrow spacing, allowing for water regulation into the furrows.

Gated pipe Sliding gate

A dv

ance

Figure 28. A gated pipe system for furrow irrigation.

Furrow irrigation is agronomically very well suited for row crops sensible to water logging. If crops are shown on top of the furrow the plant will not be covered during irrigation and the root system will never be fully under water. This will guarantee the aeration of roots even during very long irrigation events. Furrow irrigation is also very well suited for soils with bad structure, where the contact between the soil surface and the irrigation water can produce crusts that induce compaction and reduce soil aeration. The hydraulic characteristics of this irrigation system are similar to those of free draining borders. Nevertheless, often the irrigation phase is much shorter than the filling phase. The depletion and recession phases are also very short, since in furrow irrigation the volume of overland water per unit area is always very small in comparison with basins and borders.


37

2.5. Level furrows Level furrows try to combine the best features of the level basin and the furrow irrigation systems. Irrigation is performed with only one check per field: water is not distributed individually to each furrow. A usually large field with furrows inside is irrigated at a time. The field does not show slope in any direction. When entering the field, the water first flood an embankment area that acts as a distribution canal, allocating water to the individual furrows (figure 29).

Non-furrowed area

Ditch

Furrows

Figure 29. Scheme of a level-furrow irrigation system.

The furrows are transversally connected at the upstream and downstream ends. If water flows fast in any furrow and reaches the tail end of the field very early in the irrigation process, water will flow to the neighbouring furrows and start its way back to the inlet. This process avoids long flooding of areas of the field and guarantees high irrigation uniformity. Level furrows offer two important benefits. First, the labour required to conduct the irrigation is significantly reduced from that needed for a furrow system. Second, the irrigation depth applied can be significantly lower than that resulting from a level basin irrigation event. This last characteristic can be very important if the water holding capability of the soil is small.


38

The phases of this type of irrigation are common to those characterising level basin irrigation: advance and depletion. Advance is completed when all furrows in the field are fully covered by water.

2.6. Other systems In many areas of the world, surface irrigation systems have evolved during hundreds or thousands of years with the crops, techniques and technologies locally available. This evolution has resulted in local variations of the systems described that respond to very concrete needs. Some systems are characterised by having large water allocations and a very small technological level. Such is the case of wild flood irrigation systems, typical of pastures in mountainous areas. In this system, a stream of water is diverted into a pasture area with little or no levelling. The resulting irrigation practice will have very low uniformity and efficiency, but can effectively increase the forage yield without other investment than local labour. In other areas, the overflow irrigation is practised. A ditch runs parallel to a contour line and overflows to irrigate areas of a hill with a slightly systematised slope. In this way, areas with an important slope can be irrigated at the cost of very intense labour. A completely different case is constituted by the variations of furrow irrigation used to irrigate sparse fruit trees or young plantations. In these cases, furrow irrigation can be effectively used to save water, since only a small area of the field is covered by water. One or two furrows per tree row are used in this kind of applications.


39

3. Characterisation of infiltration 3.1. The process of infiltration Infiltration can be defined as the vertical, one-dimensional flow of water into the soil. In some cases, however, water flow into the soil is clearly two-dimensional (as is furrow and drip irrigation). In irrigation engineering the term infiltration or cumulative infiltration (Z) is used as the volume of water that has entered a unit area of soil at a given point since the beginning of irrigation. Z is usually expressed in m or mm (m3 m-2 or mm3 mm-2) when referring to basins and borders. Common units for Z in furrows are m3 m-1, indicating the volume of water infiltrated per unit of furrow length. When the furrow spacing is taken into consideration, these units can be converted to m or mm. The time derivative of Z is the infiltration rate or instantaneous infiltration, and follows the expression:

i

Z t

[9]

The customary units for i are m min-1 or mm min-1 for borders and basins. For furrows, common units are m3 m-1 min-1. Infiltration is one of the most relevant factors in the design and management of surface irrigation systems. The main characteristic of this family of irrigation systems is that water is conveyed on the soil surface while it infiltrates into the soil. The infiltration rate is one of the factors responsible for the balance between advance and infiltration. Figure 30 presents a schematic representation of the volumetric water content in depth during an infiltration event. Soil water content fluctuates in depth between the initial water content (i) and the saturation water content (s). Five zones can be defined in this soil water profile. The saturated zone is often very limited in depth, with no more than 1.5 cm. The transition zone is about 5 cm deep, and in it the water content is reduced to that characteristic of the transmission zone, which is the


40

largest of the five zones. In it, water content remains constant in depth and in time, at about 80 to 90% of the saturation water content. The humectation zone shows a drastic reduction of soil water content. The humectation front can be visually determined, and is coincident with the limit of the infiltration process.

0

0

i

s

Saturated zone Transition zone

Soil Depth (m)

Transmission zone

Humectation zone Humectation front Figure 30. Schematic representation of infiltration in depth.

Infiltration depends on a large number of factors, among which soil texture, soil structure, tillage operations, compaction, soil water content, air entrapment and soil and water salinity are relevant. Nevertheless, the most relevant variable is the time since the onset of infiltration at a given point: the opportunity time (). Figure 31 presents the typical curves for Z and i as a function of The terminal value of i (when it is apparent) is called the basic infiltration rate, and can be assimilated to the value of the vertical, saturated hydraulic conductivity. The continuous decrease of i in time can be theoretically explained using Darcy’s law, expressed as:

v   K H

[10]


41

where v is the average flow velocity, k is the hydraulic conductivity, and  H is the hydraulic gradient, computed in this one-dimensional case as:

H 

H l

[11]

where H is the hydraulic head, expressing the energy per unit mass of water, and l is the distance separating the two points between which the gradient is computed. Equation [11] can be interpreted to say that the driving force of soil water flow is the gradient of the hydraulic head. The negative sign in equation [10] indicates that the flow is directed towards low potentials.

Z i (m min-1)

Z (m) i  (min)

Figure 31. Typical curves of cumulative infiltration (Z) and infiltration rate (i).

Figure 32 presents the application of these equations to two points in a soil profile infiltrating water from a water surface with a depth d. The two points are b (located at the soil surface), and a (located below b, at the infiltration front). The zero elevation datum is located at point a. The hydraulic head (or potential) can be computed as:

H zh

[12]


42

where z is the head due to elevation, h is the head due to water pressure, and  is the matric head (due to the attraction of the water particles by the micropores created between the soil particles).

d

Water surface b

Soil surface

L a

Infiltration front

Figure 32. The process of infiltration from a depth of surface water.

In the two pints considered, the hydraulic head takes the following values: Ha  0  0  

[13]

At point a, elevation is zero, the pressure head is zero, and the matric head takes a certain value  , negative by definition and presumably large, since the soil water content is low prior to an irrigation. Hb  L  d  0

[14]

At point b, elevation is L, the water pressure head is d (the depth of pounding water), and the matric head is zero, because the soil is at saturation. In these conditions, the hydraulic head can be computed as:

H 

Ld L

[15]


43

Assuming that the soil between the surface and the infiltration front is at saturation, the hydraulic conductivity between a and b can be taken as the saturated hydraulic conductivity, K0. Also, the flow velocity within the soil is equal to the infiltration rate, i. Therefore, Darcy’s equation can be expressed as:

i  K 0

Ld L

[16]

If the value of d is neglected when compared with L and  , the resulting equation can be written as:   i   K 0 1   L 

[17]

From this equation, it can be concluded that: 

Since  is always negative, the infiltration rate will always be larger than the saturated hydraulic conductivity.



As the infiltration process advances, and L grows, i tends to K0 asymptotically.



When irrigating a saturated soil (  = 0), i = K0.



The depth of the pounding water has little effect on the resulting infiltration curve. Infiltration decreases during the irrigation season due to soil compaction and the

formation of a surface crust (Figure 33). Infiltration is often very large in the first irrigation of the season and decreases drastically at the second irrigation. It is a good procedure when designing irrigation systems to analyse system performance in the first irrigation of the season and in the rest of the irritations separately.


44

First irrigation Z Subsequent irrigations Time (min)

Figure 33. Infiltration curves for the first irrigation

of the season and subsequent irrigations.

3.2. Empirical equations of infiltration Darcy´s equation is a theoretical approximation to the process of infiltration. This approach has given birth to a number of analytical infiltration equations, such as the Green and Ampt or the Parlange equations. These equations have seldom been used for irrigation purposes, because of their intense data requirements. Moreover, the spatial variability of most of the infiltration related properties makes infiltration estimation a difficult task. Irrigation engineers resort to infiltration measurements and fit empirical relationships to the field observations. To cope with the problem of spatial variability a large number of infiltration observations is often required. The parameters of empirical infiltration equations are obtained by regression, and do not have physical meaning. The main drawback of such equations is that the parameters are characteristic of the location and time of the experiment. No extrapolation is possible to other location or soil conditions. Using an empirical equation for other conditions is always a risky practice. Among the different empirical equations that have been used historically, three are worth a bit of detail. The Kostiakov equation is one of the first empirical approximations to the infiltration process: Z  k a

[18]


45

From this equation, an expression for i can be derived:

i  ka a 1

[19]

Use of this equation requires estimation of two parameters. The parameters can be obtained by non-linear regression. If a logarithmic transformation is applied to equation [15], linear regression can also be used. The coefficient a is always smaller than 1, and often ranges between 0.25 and 0.50.This is the reason why i decreases in time. Following equation [16], for    , i  0 . This is not a good approximation to the physical process of infiltration, unless the saturated hydraulic conductivity of the soil is negligible. The equation is also appropriate if used for relatively small values of  . This is the case for some basin and border experiments. For the same reason, the Kostiakov equation is often not appropriate to describe infiltration under furrow irrigation. This is actually the reason why a modification was introduced in the Kostiakov model. The Modified equation is often referred to as the Kostiakov-Lewis equation, and is characterised by the following expressions:

Z  k a  f 0 

[20]

i  ka a 1  f 0

[21]

In this case, the terminal value of i for large enough values of  is the basic infiltration rate, f0. This infiltration model is often used for long irrigation events or if data inspection reveals a linear trend in Z for large values of  . The price to pay for a better representation of the process is an extra empirical parameter, that can be obtained together with a and k by nonlinear regression. Finally, in the last years a new infiltration model has been described: the branch infiltration function:


Z  k a

  f a

Z  k f  f 0 

  f

46

[22]

The first branch of the infiltration equation is a Kostiakov function. The second branch is a straight line with a slope f0, experimentally determined. The parameters of this function are k, a, f0 and  f . A condition of continuity for    f yields an additional equation:

f  1 f   0   ak  a  1

[23]

Consideration of equation [23] reduces the number of parameters to three. The advantage of this equation is that the basin infiltration rate is reached instantaneously instead of asymptotically, as was the case with the Kostiakov-Lewis equation. The parameters of the empirical infiltration equations can be loosely related to soil texture. Table 6 presents average values of the Kostiakov-Lewis parameters for different Infiltration families as defined by the Soil Conservation Service of the U. S. Department of Agriculture. These values should be used cautiously to obtain rough estimations of infiltration.


Table 6. Average values of the Kostiakov-Lewis parameters

for the different infiltration families and textural classes. Family k (m min-a) 0.05 0.00426 0.10 0.00383 0.15 0.00360 0.20 0.00346 0.25 0.00337 0.30 0.00330 0.35 0.00326 0.40 0.00323 0.45 0.00321 0.50 0.00320 0.60 0.00320 0.70 0.00321 0.80 0.00324 0.90 0.00328 1.00 0.00332 1.50 0.00361 2.00 0.00393

a f0 (m min-1) Texture 0.258 0.000022 0.317 0.000035 clay 0.357 0.000046 0.388 0.000057 0.415 0.000068 0.437 0.000078 clay loam 0.457 0.000088 0.474 0.000098 0.490 0.000107 0.504 0.000117 0.529 0.000136 silty loam 0.550 0.000155 0.568 0.000174 0.584 0.000193 sandy loam 0.598 0.000212 0.642 0.000280 0.672 0.000337 sandy

47


48

3.3. Measuring infiltration in basins and borders The simplest procedure to measure infiltration in basins and borders is to use a simple ring (Figure 34). A metal cylinder opened in both ends is inserted into the soil to isolate a

volume of soil and prevent horizontal infiltration. The cylinder dimensions should exceed 30 cm in diameter and 40 cm in height. The metal thickness should not exceed 2 mm unless the lower circle is sharpened as indicated in the figure. The ring is inserted in the soil to a depth of about 15 cm. It is important that the ring does not compact the interior soil and that it seals tightly so that water does not find preferential flow paths.

Measuring scale Hook

Water surface Soil surface

Figure 34. Schematic representation of a single ring infiltrometer.

Infiltrometer rings should be installed in places representative of the field being characterised. An earthen ditch should be built around the ring and water should be added to the soil into and out of the ring at the same time. The water level inside the ring should be monitored frequently using a metal hook and a measuring scale. Water level records should be spaced so that the difference in level is between 1 and 3 mm. As discussed previously, the water level inside the ring should not influence the infiltration measurements much, but it is a good procedure to keep the water level close to the expected water level during the irrigation of the field, refilling the ring as needed. The experiment should last at least for the expected average opportunity time. Instead of constructing an earthen ditch around the ring, a double ring configuration is often used. This set-up is easier and faster to prepare. The only drawback is that often the diameter of the outer ring is not as large as it would be desirable. The best way to assure one


49

dimensional infiltration flow is to perform single ring infiltrometer tests at the same time of an irrigation event, adding water to the ring when the advancing front reaches it. Infiltrometer rings are low cost, easy to read devices. These characteristics make them a good choice for infiltration characterisation in many surface irrigation situations.

3.4. Measuring infiltration in furrows Before getting into the practical aspects of infiltration measurement, one peculiarity of infiltration in furrows should be discussed: the dependence of infiltration on the wetted perimeter. This aspect is of mayor importance and should be taken into account when planning the experiments. In sloping furrows the wetted perimeter in normal conditions corresponds to a discharge. This is the reason why often when describing an infiltration experiment the irrigation discharge rather than the wetted perimeter is used. An infiltration experiment should be performed in the usual irrigation conditions. When extrapolation is required, the following experimental expression has often been used:

 WP     WP0 



Z   Z 0 

[24]

[25]

Where WP is the wetted perimeter, the subindex 0 indicates measurement conditions,  is an empirical coefficient, and  is the adjustment coefficient for Z. While the procedure is simple, the obtention of a reliable value for  is not an easy task and requires a large number of experiments. At this point, it will not be a surprise that measurement of infiltration in furrows is much more complicated than in borders or basins. Three methods will be discussed for infiltration measurements, differing basically in technology requirements.


50

The simplest way it to measure discharge in two points. This method, however, is complicated to implement correctly, and the data are not easy to process. Under field conditions, the errors committed by the flow measurement devices used in furrows are usually in the range of 5 – 10%. It is therefore very important that the distance separating the two measurement points is large. Otherwise, the differences in discharge could be in the order of the accuracy of the flumes and the error could be very large. Measuring infiltration in long furrow reaches has the added benefit of averaging the spatial variability of infiltration. It is very important that the flumes do not interfere with the flow, increasing the water level upstream and therefore the infiltration rate. Parshall and RBC flumes have often been used for this purpose. It is important to verify during the experiment that the flumes remain horizontal and that there is no significant leakage around them. Pairs of upstream and downstream discharges (Qu and Qd, respectively) are measured at different opportunity times. The difference between them will be the infiltration discharge, QZ. This discharge must be transformed into an instantaneous infiltration rate, so that it can be expressed in m3 min-1 m-1. A non-linear regression can be performed to estimate the infiltration parameters. A more technified solution to furrow infiltration measurement is the blocked furrow. A section of about 1 m of furrow is isolated from the rest using metal plates. The furrow is irrigated in full and so are the bordering furrows. The furrow is treated as an infiltrometer ring, with the only exception that the wetted perimeter needs to be held constant in time. An automated system, like the one presented in Figure 35 is frequently needed for this purpose. The buoyancy system ensures a constant water level, and the decrease in the level of the supply tank indicates the volume of infiltration, that needs to be expressed as a volume per unit time and furrow length.


51

Several authors have pointed out that water velocity affects infiltration in furrows. In fact, experiments performed in stagnant conditions, such as the one just described, often yield low estimates of infiltration. It seems reasonable that sedimentation of soil materials can result in a sealing effect that reduces infiltration. In actual flow conditions this sedimentation would not occur and therefore infiltration would be larger. This is the principle that inspired the furrow recycle infiltrometer.

Plan view

Metallic plate Water level recording system 0.3-1.2 m Side view

Water reservoir

Measured furrow Buoyancy system Figure 35. Schematic representation of a blocked furrow infiltrometer.

A scheme of this device is presented in Figure 36. A furrow stretch (in the order of 5 m) is bounded by two pits. Runoff is collected at the downstream pit and pumped to a water reservoir, where it is used to irrigate the furrow from the upstream pit with the desired, constant discharge. At given time intervals, the volume in the reservoir tank is recorded and the differences in volume are attributed to infiltration, following the procedure common to the other two methods.


52

Water reservoir with level measuring scale

Measured furrow Pump 5-6 m Figure 36. Schematic representation of a recycling furrow infiltrometer.

3.5. Estimating the basic infiltration rate in furrows One of the characteristics of furrow irrigation is that the opportunity times are often large and the basic infiltration rate is often attained along the furrow. Under these conditions, the basic infiltration rate can easily be determined following a hydrological approach. If the irrigation discharge (Qi) and the runoff discharge (Q0) are measured in time during the irrigation event, a time will arrive when their difference becomes constant. At this time, f0 can be estimated as:

f0 

Qi  Q0 L

[26]

where L is the furrow length. Estimation of the basic infiltration rate reduces the infiltration parameters to two, and therefore reduces the uncertainty in their estimation.

3.6. Exercise 1: ring infiltrometers During a blocked-end border irrigation evaluation, infiltration measurements were performed using three ring infiltrometers. Three representative points were chosen in the border and rings were carefully installed. The evaluation revealed that the average opportunity time was 1528 min, the irrigation discharge was 125 l s-1, the irrigation time was


53

200 min, and the field area was 9,800 m2. The data obtained in each ring are presented in the table that follows. Tasks: 1. Present in a plot the infiltration rate, i, vs. time for ring # 1. 2. Compute the Kostiakov infiltration coefficients for ring # 1. 3. Compute a Kostiakov infiltration equation representative of the three rings.

Ring 1 t (min) 1 2 3 4 10 20 30 43 58 63 93 123 183 243 303 403 463 523 583

Z (m) 0.011 0.013 0.015 0.016 0.020 0.029 0.033 0.038 0.046 0.048 0.058 0.065 0.075 0.082 0.090 0.097 0.103 0.108 0.114

Ring 2 t (min) 1 2 3 4 5 6 11 16 26 41 56 86 116 146 206 266 326 386 446 506 566

Z (m) 0.007 0.011 0.014 0.016 0.018 0.019 0.024 0.029 0.034 0.041 0.047 0.055 0.065 0.072 0.082 0.093 0.104 0.110 0.117 0.124 0.130

Ring 3 t (min) 1 2 3 4 5 12 17 22 32 92 152 212 272 332 392

Z (m) 0.005 0.007 0.008 0.010 0.011 0.017 0.022 0.026 0.029 0.050 0.063 0.075 0.084 0.093 0.102


54

Results: 1) Infiltration rate vs.  for ring #1: 0,012000 i (m min -1)

0,010000 0,008000 0,006000 0,004000 0,002000 0,000000 0,0

200,0

400,0

600,0

 (min)

2) Cumulative infiltration and Kostiakov equation for ring #1 (k = 0.0096; a = 0.39). 0,12 0,10

Z (m)

0,08 0,06 y = 0,0096x 0,3859 R2 = 0,9937

0,04 0,02 0,00 0

100

200

300

400

500

600

t (min)

3) Kostiakov equation for the three rings (k = 0.0073; a = 0.44) 0,14 0,12

Z (m)

0,10 0,08 0,06 y = 0,0073x0,4447 R2 = 0,9698

0,04 0,02 0,00 0

100

200

300 t (min)

400

500

600


55

3.7. Exercise 2: Furrow infiltration / discharge in two points During a furrow irrigation evaluation, discharge was measured in two points of a furrow separated by a distance of 50 m. Discharge measurements at the upstream (Qu) and downstream (Qd) points are presented in the following table: 







(min) 50 100 150 200 400 600 900 1200 1500

Qi

Qo

(l/s) 1.35 1.40 1.40 1.60 0.63 0.63 0.50 0.55 0.54

(l/s) 1.10 1.25 1.22 1.50 0.50 0.50 0.45 0.50 0.50

Determine the coefficients of the Kostiakov equation corresponding to this irrigation evaluation. Use plots as needed. Solution: First, the infiltrated discharge must be converted to m3 min-1 m-1. At each time, a value is obtained for the infiltration rate. 0,00035

i (m 3 min -1 m -1)

0,00030

y = 0,0021x -0,4861 R2 = 0,8063

0,00025 0,00020 0,00015 0,00010 0,00005 0,00000 0

500

1000

1500

2000

 (m in)

In the regression equation, (k a = 0,0021), and (a-1 = -0,4861). therefore:

Z  0,00409 0,5139


56

3.8. Exercise 3: Furrow infiltration / blocked furrow The blocked furrow method was used to estimate the coefficients of the Kostiakov equation in a silty loam soil. A length of furrow of 1.5 m was blocked and water was supplied to the system using a buoyancy tank 0.7 m in diameter with an initial water level of 1 m. The following table presents the data obtained during the experiment. 

Water level

(min)

(m)

5 15 25 50 75 100 150 200 400 600 800 1000 1300 1600 1900

0.98 0.97 0.96 0.94 0.92 0.90 0.88 0.85 0.76 0.68 0.60 0.53 0.45 0.37 0.29

Determine the coefficients of the Kostiakov equation. Use plots as needed. Results The volume of water infiltrated at each time is computed as the difference with the volume in the tank at time = 0. If this volume is expressed in units of cubic meters per meter of furrow length, data are ready to compute the Kostiakov parameters by regression.


57



Level

Volume

(min)

(m)

in tank

Volume

Infiltration

(m3)

m3

(m3 m-1)

0,385 0,377 0,373 0,369 0,362 0,354 0,346 0,339 0,327 0,292 0,262 0,231 0,204 0,173 0,142 0,112

0,000 0,008 0,012 0,015 0,023 0,031 0,038 0,046 0,058 0,092 0,123 0,154 0,181 0,212 0,242 0,273

0,000 0,005 0,008 0,010 0,015 0,021 0,026 0,031 0,038 0,062 0,082 0,103 0,121 0,141 0,162 0,182

3

-1

Z (m m )

0 5 15 25 50 75 100 150 200 400 600 800 1000 1300 1600 1900

1,00 0,98 0,97 0,96 0,94 0,92 0,90 0,88 0,85 0,76 0,68 0,60 0,53 0,45 0,37 0,29

0,200 0,180 0,160 0,140 0,120 0,100 0,080 0,060 0,040 0,020 0,000

Infiltrated Cumulative

y = 0,0014x0,63 R2 = 0,99

0

500

1000

 (min)

1500

2000


58

4. Measuring irrigation water 4.1. Overview Measuring irrigation water is one of the keys to the success of any surface irrigation system. Although there is no strict linkage between the irrigation system and the conveyance system, in practice surface irrigation systems are fed by canals and pressurised irrigation systems are fed by pipes. While it is common knowledge that pressurised irrigation systems can be monitored using water meters, it is generally accepted that in surface irrigation systems water can not me measured. It is the responsibility of irrigation technicians to implement surface irrigation systems with the measuring devices required to effectively control water. When water records are of good quality and regularly kept, transparency is added to water management, and conflicts in water allocation dim out. Measuring water is allowing many surface irrigation districts to use proportional billing. Contrary to general belief, a large variety of methods are available for water measurement in open channels. A few of them will be discussed in order to promote their use. Discussion will be restricted to the devices that can be used in conveyance canals or onfarm ditches: furrow meters will not be discussed. A simple way to establish a stage-discharge relationship in a water course is to use a propeller meter. This is a good method if a long stretch of canal with uniform slope and

section is available. It is important that the flow conditions within the reach do not change in time, i.e., no back flow can occur from downstream structures. The method is based on the capability of the propeller to measure flow velocity. When velocity is measured in a series of points and the flow area is accurately measured, the discharge can be estimated as the product of area and average velocity. When computing average velocity, it should be considered that, by definition, velocity at the canal banks is zero.


59

A higher stage of measurement quality is obtained when methods using critical sections are used. These methods are discharge measurement independent from the downstream conditions. The peculiarities of critical flow ensure that a function exists relating flow depth and discharge at this section. Therefore:

Q  Q( h )

[27]

where h is the flow depth. Many designs are available to produce critical conditions in a canal or ditch. All of them fall in the categories of flumes or weirs, although some designs look more like hybrids between them both. The design by Parshall is a classic between flumes. Parshall flumes are available in a series of sizes and their stage-discharge relationships are tabulated. These flumes are not very easy to build in situ, for they are composed of nine surfaces with different sizes, orientations and angles. This is the reason why Parshalls are often built in mechanical shops out of steel plates. An additional problem with these structures is that construction errors can not be calibrated, for the tables are provided for perfectly built flumes. Among the advantages of using Parshall flumes, the first is that they perform great even if the available head loss is very limited. This makes them a good choice when discharge needs to be measured in open channels with little freeboard. Another advantage is that they can be chain manufactured, thus reducing the construction cost.


60

4.2. Broad crested weirs A recently developed alternative to flumes is the Broad Crested Weir. A scheme of the device is presented in figure 37. The only construction required is a sill and a ramp. These two can be built without need to modify the original canal section. Concrete can be used for this purpose, although other materials, such as wood or metal would be appropriate. The head loss requirements are larger than those for Parshall flumes, but they are moderate. Two advantages of this weir are very relevant. First, the simplicity of its construction, with only two control surfaces. Second, a complete hydraulic analysis of this structure is available in the form of equations. This second advantage is more important than it seems, for software programs exist that incorporate these equations and permit iterative design of the structure considering any size of each piece of the structure. Besides allowing detail design, probably the best feature is that the designer can obtain the final measuring scale using the real measurements instead of the projected ones. Therefore, calibration is implicit in the design process.

Measuring gage

Top of canal

Water level

Ramp

Hydraulic jump

Sill

Canal bottom

Figure 37. Schematic representation of a broad crested weir.

The characteristics of a broad crested weir are presented in detail in figure 38:


Roughness factor: Smooth Metal...... 0.020 to 0.100 mm Smooth Wood....... 0.200 to 1.000 mm Smooth Concrete... 0.100 to 2.000 mm Rough Concrete.... 0.500 to 5.000 mm

61

Side slope = x/y y

x

Base width

Staff gauge Upstream section

Ending

Troat section

Downstream section

Beginning

Sill height

distance

Sill height

Ramp length

Throat length

Ramp length

Figure 38. Constructive characteristics of broad crested weirs.

4.3. Exercise: design and construction of a broad crested weir Use the computer program for broad crested weir design to dimension a measuring device with the following characteristics:  Trapezoidal section  Canal lined with rough concrete  Canal slope: 0.0005  The structure will be built in rough concrete  Base width: 0.50 m  Side slope: 1:1 (horizontal to vertical)  Required accuracy: 10 l/s  Measurement range: 30-150 l/s. Describe your final structure, detailing the required design parameters. Assume you send the staff to build it and when you visit the place you find out that:


62

 The base width at the construction area is actually 0.51 m  They came up with a throat section 1 cm longer than it should  The concrete they used was very well finished, very smooth. Design the final staff gauge to be placed in location. Plot the values. Results: The constructive parameters of the broad crested weir are presented in the following lines for the design and the calibration with actual field measurements: Design

Upstream Section

Throat Section

Downstream Section

Base Width = 0.500 m

Base Width

Side Slope = 1.000 -

Side Slope

Sill Height = 0.250 m

Throat Length = 0.400 m

Ramp Length = 1.000 m

Roughness Factor= 2.000 mm Ramp Length = 0.000 m

= 1.000 m = 1.000 -

Base Width = 0.500 m Side Slope = 1.000 Sill Height = 0.250 m

Staff Gauge Location: 1.000 m upstream of the ramp base Calibration

Upstream Section

Throat Section

Downstream Section








Throat Length = 0.410 m


Ramp Length = 1.000 m

Roughness Factor= 0.150 mm Ramp Length = 0.000 m

Staff Gauge Location: 1.000 m upstream of the ramp base. The resulting discharge rating is:


63

Discharge (m 3 s-1)

0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 0.05

0.1

0.15

Upstream Depth (m)

0.2


64

5. Surface irrigation models 5.1. Models and reality Models are common tools these days to analyse many complex problems. Models are simplifications of reality, and can only reproduce the real world within the scope of these simplifications. Even if this is kept in mind, the user should be aware that the outcome of a model will probably not be more reliable than the least reliable of the input data. During the preceding chapters, many references have been made to the spatial and time variability of properties such as infiltration or soil surface elevation. Most of the currently available surface irrigation models do not consider spatial variability at all. Accordingly, the accuracy of the results will need to be contrasted with field experiments, and the results themselves should be looked at with some degree of scepticism. When models are used with poor data, the results may not be realistic. Nevertheless, if a model has been properly formulated, its use will always be a source of insight on the processes being simulated. Spatial variability has been shown to have a large influence on performance indexes, but it affects the times of cut-off and advance only marginally. Surface irrigation models are therefore very indicated for planning purposes (water allocation in time). As for the performance indexes, they will be lower than model predictions, but the trends indicated by the models will surely reflect what would happen in the real world. This is the reason why models find great application in the evaluation of design alternatives. Surface irrigation design is applied to a border or basin (dimensions L x W) or to an individual furrow (characterised by a length L and a furrow spacing W) (Figure 39).


65

L

L W Wm 1W

W

Figure 39. Design units for borders / basins and furrows. The flow cross sectional area

is shown hatched and the wetted perimeter is shown as a thick line.

5.2. Partial differential equations governing open channel flow These equations have been known for centuries. The fact that a closed, analytical solution is not available even today has limited their application until the generalisation of the use of digital computers. The equations can be expressed as: A Q i0  x t y 1 Q 2Q Q  S0  Sf  1  F2  2 x Ag t A g x





(a ) [28] ( b)

Where: 

A is the flow cross-sectional area (see figure 39);



Q is the discharge at a given point (x) along the border, basin or furrow and at a given time (t);



i is the infiltration rate;



g is the acceleration of gravity



F is the Froude number;



y is the flow depth;



S0 is the field slope; and



Sf is the friction slope. This set of hyperbolic partial differential equations accounts for the conservation of

mass (first equation) and momentum (second equation). These equations express all phases and peculiarities of any surface irrigation event. Sf can be computed using Manning equation:


2

Q

1 A R3 Sf n

1 2

66

[29]

Where R is the hydraulic radius (area divided by wetted perimeter) and n is known as “Manning n”. Common values for Manning n are: 0.04 for bare soil, 0.06 for row crops, and between 0.15 and 0.30 for dense crops fully covering the soil, like wheat or alfalfa. The Froude number, F, can be expressed as:

Q h F gh

[30]

The mass conservation equation [28 a] can easily be interpreted: the increment (or decrement) of discharge within a flow section must be due to a build-up of water in time within the section or to the infiltration process. The momentum conservation equation [28 b], also known as the energy equation, is not as easily interpreted. It expresses that the energy in the flow is due to its depth (weight and pressure), the slope and the friction generated by roughness.


67

5.3. Numerical models of surface irrigation The solution of set of equations [28] is the basics of any surface irrigation model. The volume balance approach constitutes a simplified solution of the governing equations,

solving only equation [28 a]. Other simplified approaches to the solution of the equations are the kinematic wave and the zero inertia models. In the kinematic wave approach, the mass conservation equation is solved together with an approximation for the energy equation:

S0  Sf

[31]

Equation [31] implies that the energy obtained from the field slope is used to overcome the friction losses. This equation is only valid for sloping fields. Strictly, this assumption is only valid for uniform flows (any derivative in time or space equals zero). Physically, the model is interpreted as a succession of reaches of uniform flow. The next level of simplification is the zero inertia model. This approximation is based on the hypothesis that the Froude number is low. When the inertial terms are ignored, the resulting governing equations have mathematical properties that simplify the solution of the problem. The Froude number is indeed low in many surface irrigation applications. This is the reason why this approach is still used in many surface irrigation models. Solution of the full set of equations constitutes a hydrodynamic approach. This is the most complicated case and traditionally has been avoided because of computational speed and memory requirements. Current personal computers solve any of the mentioned approaches in seconds. Currently the problem is not in the computational time but in the accuracy and reliability of the computations. The numerical solution of the governing equations and their simplifications can be accomplished using a wide variety of numerical methods. Numerical schemes are based on


68

the discretization of the governing equations so that all variables are known at the nodes of a network established along the field or furrow. Discretization in time results in a series of time steps. The solution procedure is based on advancing the solution by one time step. At the starting time (t = 0), an initial condition is required. The values of all dependent variables must be known. A common initial condition implies zero depth and discharge at all nodes. At t = t all variables are unknown. Their values will only be known when the solution procedure has advanced one time step. In order to do so, an additional piece of information is needed: the boundary conditions. At the upstream boundary, the usual condition is an inflow hydrograph: a value of the irrigation discharge at each time. At the downstream boundary, two conditions are possible: no flow and runoff. The no flow condition is characteristic of blocked-en irrigation systems, such as basins and certain types of borders and furrows. Numerical methods can belong to a series of categories: 1) Implicit or explicit. Explicit models permit computation of the values of the dependent variables at each node independently from the rest of the nodes. The main characteristic of implicit models is that the solution for all nodes is known simultaneously for all nodes. Explicit models take less computational time to advance the solution one time step, but are required to take shorter time steps than the implicit models. 2) Finite differences or finite elements. These are variants due to the way in which the discretization of the governing equations is performed. In the finite differences method, numerical approximations are introduced to evaluate the partial derivatives at each node. In finite elements, a linear portion of the field is considered as an element (a flow reach) for which equations are built that minimise the error at each of the two nodes forming the element. Numerical solutions are never guaranteed: computation can fail due to a number of causes. These failures are inherent to the nature of the equations. The frequency of numerical


69

failures is also related to the numerical approach, to the above mentioned categories and to the quality of the software in which the model is implemented.

5.4. Two-dimensional level basin models Most surface irrigation events can be successfully simulated using one-dimensional approaches such as those based on the solution of equations [28]. In other cases, the onedimensional hypothesis fails and a two-dimensional model is required. This is the case for level basins, where the fields often are not long and narrow, but squarish in shape. Also, the irrigation discharge is often not evenly distributed along one of the sides. Figure 40 presents a few cases of clearly non one-dimensional flow.

Figure 40. Some irrigation cases not suited for one-dimensional modelling.

In these cases, a two-dimensional model offers better perspectives. These models are based on the two-dimensional equivalent to equations:


70

h  uh    vh   i  0  y t x  2  uh    u 2 h   uvh   g   h x  2 y x t

   gh S 0 x  S fx   D lx  0  2    vh    v 2 h   uvh   g   h   gh S 0 y  S fy   D ly  0 y  2  x y t

 

[32]

 

The equation of conservation of momentum is now expressed in two equations accounting for conservation along the x and y axes. Solution of the equations [32] has been accomplished using different numerical methods. Explicit finite differences offer better perspectives than other approaches. Computational speed is still an issue in 2D modelling, since the computational effort is much more important than for 1D simulations. The simulation of microtopography in level basins has also been accomplished using 2D models.


71

6. Surface irrigation evaluation Irrigation evaluation consists on a series of procedures determined to obtain information from an irrigation event. These procedures are performed both at the field and at the office, and involve measurements and computations.

6.1 The goals of evaluation Irrigation evaluation permits assessment of irrigation performance and often allows determination of parameters such as roughness, infiltration, field size, slope and levelling accuracy. The primary goals of evaluation are therefore irrigation uniformity and efficiency. Parameter estimation is very important, since the evaluation process allows estimation of spatially averaged values of infiltration and roughness. Measurement techniques permit to obtain local values of these variables.

6.2. Evaluation procedures The evaluation process involves the following procedures: 

At the field: 1. Measuring the field size 2. Determining the average longitudinal slope and the accuracy of land levelling 3. Characterising the flow section 4. Measuring irrigation water 5. Determining the characteristic times of the irrigation event. 6. Determining the runoff hydrograph. 7. Measuring the soil water deficit prior to irrigation: this will be often equal to the target irrigation dose (ZR). 8. Measuring infiltration. 9. Determining the advance and recession times at a network of points along the irrigated field.




72

At the office: 1. Constructing the advance-recession diagram. 2. Determining the irrigation volume. 3. Constructing the opportunity time diagram. 4. Determining the average infiltration equation. 5. Estimating the infiltrated depth at the nodes of the network. 6. Estimating ZR. 7. Determining performance indexes; uniformity and efficiency. There are many variants from this approach. Each evaluation usually focuses on a

particular variable, and procedures must be adapted to reveal this variable in detail. Two important variants from this scheme are described in the following sections. Both are related to parameter estimation.

6.3. Adjusted infiltration approach This is the first approach used to estimate infiltration from advance. The rationale of this method is that the whole irrigated field can be used as a ring infiltrometer to obtain one point of the infiltration equation: This point is formed by the average opportunity time (  ) and the average infiltrated depth ( Z ).

Z

VZ Area

[33]

The total infiltrated volume (VZ) can be computed as the difference between inflow and surface runoff.


73

Using any of the methods previously discussed for irrigation measurement, and aggregating several infiltration measurement points, an expression for Z has been obtained following any of the proposed infiltration equations. The Kostiakov equation will be used here for clarity: Z  k a

[34]

Most likely, this infiltration curve will not contain the point (  , Z ). To improve the significance of the infiltration equation, the value of k is adjusted so that the equation contains the desired point. The new, adjusted, value of k is denoted as k’. Z  k ' a Z k' a

[35]



Using the adjusted infiltration approach the average of the infiltration estimates based on  and the adjusted infiltration equation is approximately coincident with Z . On the other hand, simulations based on an adjusted infiltration equation yield realistic advance-recession trajectories.

6.4. Estimating infiltration and / or roughness from advance Once an evaluation has been performed and a simulation model is being implemented to reproduce the observed irrigation event, the observed and model simulated advancerecession trajectories may still be not coincident. This can happen even if an adjusted infiltration approach has been used. Under these circumstances, it is a common practice to modify the value of the infiltration and roughness parameters to obtain agreement between the reality and the model. It should be noted that in order to do so, the user should be very confident on the value of the rest of the parameters used in the model.


74

If no infiltration measurements were performed during the evaluation process, using a model can still yield reasonable estimates of infiltration and roughness. In this case, the advance recession trajectory and the overland profiles will need to be used to estimate the required parameters. Here is an outline of the procedure: At the field: 1. Do not measure infiltration 2. Concentrate on advance-recession: determine it accurately. 3. Measure flow depth at a location close to the inlet (10 m downstream from it). To avoid interference with the differences in soil surface elevation, take a number of measurements across the border/basin or along the furrow in a short stretch. In sloping fields flow depth should be measured just before cut off, when normal flow conditions have been attained at the upstream end of the field. In level fields flow depth should also be measured just prior to cut off (flow depth does not stabilise in time). In both cases the measurement time should be recorded. 

At the office: 1. Prepare a simulation of the irrigation event. Use guesses for infiltration and roughness. Base your guess for roughness on the state of the soil surface and the crop. Base you guess for infiltration on previous experience on the area or on soil texture. 2. Adjust Manning n. Compute the average flow depth and check it against the results of the model for this same location and time. If the model shows a smaller depth, increase roughness, and vice versa. 3. Adjust the infiltration parameters. This is an iterative, trial and error process. A few hints will be helpful to find the solution faster: 

The a parameter introduces curvature in the advance curve. The higher the value of a, the more curved advance will be.



The k parameter scales up and down the advance curve. The larger the value of k the higher (lower) the advance curve will be.




75

The f0 parameter is more apparent at the recession curve. High values of f0 pull down the recession curve.

4.

Repeat steps 2 and 3 to verify that all parameters match.

Though it is very attractive to save the field effort to measure infiltration properly, estimating infiltration from advance can be a difficult task, particularly if no previous experience on the procedures and on the area is available. Computer programs have been developed to assist the office procedures, but their use is not still simple and reliable enough.

6.5. Determining irrigation performance The procedures for measuring and estimating irrigation performance have already been described. At this point, only one consideration is necessary. Each performance index should be accompanied by a description of how it was obtained. Measurements of performance based on soil water measurements usually result lower than performance estimates based on advance-recession diagrams. Between performance estimates, the differences can be important depending on how the evaluation was performed. For instance, in level basins, one of the most important points is the consideration of microtopography. Finally, performance estimates derived from models often result higher than estimates based on irrigation evaluations.

6.6. Exercise 1: Adjusted infiltration Using the data from exercise 3.6 in the infiltration chapter, adjust the equation obtained for the three rings to the results of the irrigation evaluation. Plot the average equation, the adjusted average equation and the three original Kostiakov equations.


76

Results: The infiltration parameters obtained from regression in section 3.6 are: k = 0.0073 m min-a and a = 0.44. The co-ordinates of the adjustment point are (1528 min, 0.153 m). The adjusted value of the k coefficient can be computed as: k'

0.153  0.0061 m min  a 0.44 1528

The adjustment figure is: 0,18 0,16 0,14 Z (m)

0,12 0,10 0,08 0,06 0,04 0,02 0,00 0

500

1000 t (min)

1500


77

6.7. Exercise 2: Complete evaluation An irrigation evaluation was performed on an experimental blocked end border. The border length was 275 m, and the width was 3.84 m. The average soil water deficit before irrigation was 35 mm. The border was irrigated with a discharge of 17.40 l s-1 for a time of 44 min. Stations were marked along the border at 25 m intervals. The following table summarises the advance and recession data: Station

0 1 2 3 4 5 6 7 8 9 10 11

Distance (m) 0 25 50 75 100 125 150 175 200 225 250 275

Advance time 9:06 9:10 9:14 9:18 9:24 9:29 9:34 9:39 9:45 9:51 10:05 10:08

Recession time 10:20 10:20 10:20 10:30 10:30 10:40 10:50 11:00 12:00 13:15 14:45 16:30

Three infiltrometer rings were installed in the border. The infiltration experiment was performed at the same time of the irrigation evaluation. The data obtained in the rings are presented in the following table:


Time 9:11 9:12 9:13 9:14 9:15 9:20 9:25 9:30 9:35 9:40 9:45 9:50 10:00 10:10 11:03

Ring 1 Reading 21.50 21.20 21.10 21.00 21.00 20.80 20.70 20.70 20.50 20.40 20.40 20.20 20.20 19.90 19.10

Time 9:19 9:20 9:21 9:22 9:23 9:24 9:29 9:34 9:44 9:54 10:04 10:14 11:04

Ring 2 Reading 26.20 25.70 25.60 25.50 25.40 25.40 25.10 25.10 24.95 24.90 24.60 24.60 24.20

78

Time 9:40 9:41 9:42 9:43 9:44 9:50 9:55 10:05 10:15 11:05

Ring 3 Reading 24.40 24.30 24.20 24.20 24.20 23.90 23.80 23.70 23.50 23.00

Determine the following: 1. Border area 2. Volume of water applied 3. Average infiltrated depth 4. Kostiakov Infiltration parameters for each of the three rings and for all three at a time. 5. Adjusted infiltration parameters. 6. Irrigation uniformity 7. Irrigation efficiency 8. Deep percolation ratio 9. Surface runoff 10. Water requirement efficiency


79

Results: 1. Border area: 1056 m2. 2. Volume of water applied: 46 m3 3. Average infiltrated depth: 43.6 mm 4. Kostiakov Infiltration parameters for each of the three rings and for all three at a time. k

a

Ring

m min-a

-

1

0.0029

0.40

2

0.0051

0.29

3

0.0011

0.59

All

0.0026

0.43

5. Adjusted infiltration parameters. The co-ordinates of the adjustment point are the average opportunity time and the average infiltrated depth. The following table analyses the advance and recession times: Advance time (min) 0 4 8 12 18 23 28 34 39 45 59 62

Recession time Opportunity time (min) (min) 74 74 74 70 74 66 84 72 84 66 94 71 104 76 114 80 174 135 249 204 339 280 444 382


80

The average opportunity time is 131.3 min. The coordinates of the adjustment point are (131.3 min, 0.0436 m). The adjusted k can be computed as: 0.0436  0.0054 m min a , and the adjustment figure follows: 131.30.43

Z (m)

k'

0.050 0.045 0.040 0.035 0.030 0.025 0.020 0.015 0.010 0.005 0.000

0.43 Z = 0.00535 

0.4313

y = 0.0026x 2

R = 0.7174 0

50

100

150

t (min)

The infiltration parameters are used to estimate infiltration at each station. Using these estimates, the uniformity and efficiency terms can be computed:

Distance (m) 0 25 50 75 100 125 150 175 200 225 250 275

Z, Estimated Infiltration mm 34.37 33.56 32.72 33.97 32.72 33.76 34.77 35.57 44.51 53.15 60.91 69.61

Water stored in Deep the percolation rootzone (mm) (mm) 34.37 0.00 33.56 0.00 32.72 0.00 33.97 0.00 32.72 0.00 33.76 0.00 34.77 0.00 35.00 0.57 35.00 9.51 35.00 18.15 35.00 25.91 35.00 34.61

Length (m) 25 25 25 25 25 25 25 25 25 25 25

Average water storage

Volume of water storage

(mm) 33.96 33.14 33.34 33.34 33.24 34.26 34.88 35.00 35.00 35.00 35.00

(m3) 3.26 3.18 3.20 3.20 3.19 3.29 3.35 3.36 3.36 3.36 3.36


81

The total volume of soil water storage is 36.1 m3 The total volume of water lost to deep percolation is 46.0 - 36.1 = 6.9 m3 6. Irrigation uniformity: 79 % 7. Application efficiency: 78 % 8. Deep percolation ratio: 22 % 9. Surface runoff: 0 % 10. Water requirement efficiency. The volume of water that could be stored in the rootzone was 275 m * 3.84 m * 0.035 m = 37.0 m3. Therefore, the water requirement efficiency is 98 %.

6.8. Exercise 3: Infiltration and roughness from advance

An irrigation evaluation was performed on a rectangular alfalfa field. The field is a blocked end border with the following characteristics: 105 m long and 20.5 m wide. The irrigation time was 94 min. Discharge was measured twice during the experiments, and the values obtained were 64.74 and 63.89 l s-1. Flow depth was measured in 20 points across the border width at 10 m down the upstream end. Just before cut-off, the flow depth was 10.5 cm. A land levelling survey was performed on the border, with the following results: Distance 10 20 30 40 50 60 70 80 90 100

Elevation -0.060 -0.072 -0.090 -0.101 -0.167 -0.177 -0.177 -0.137 -0.132 -0.117

Advance was recorded at a number of stations along the border. The registered advance times were:


Station 0 1 2 3 4 5

Distance 0 20 40 60 80 100

82

Advance 10:33 10:42 10:57 11:16 11:33 12:07

Determine the following: 1. Border area: 2. Unit discharge 3. Volume of water applied 4. Average infiltrated depth 5. Manning n and the parameters of an infiltration equation obtained from advance using a surface irrigation model.

Results: 1. Border area: 2153 m2 2. Unit discharge: 3.14 l s-1 (for an average discharge of 64.3 l s-1) 3. Volume of water applied: 362.7 m3 4. Average infiltrated depth: 0.169 m 5. Manning n and the parameters of an infiltration equation obtained from advance using a surface irrigation model. As a first step, the elevation data are processed:


83

0.000 -0.020 -0.040

y = -0.00085x - 0.07613 2 R = 0.36923

Elevation (m)

-0.060 -0.080 -0.100 -0.120 -0.140

Slope: 0.085 % SDe: 39 mm

-0.160 -0.180 -0.200 0

10

20

30

40

50

60

70

80

90

100 110

Distance (m)

The field slope and the rest of the variables were introduced in the model. First, a value of n = 0.25 was obtained. The value is reasonable for the crop. Then, different simulations were performed with combinations of values for k and a. Finally, the following values were adopted: k = 0.0126 m min-a and a = 0.50. The following plot presents the observed and simulated advance curve:

120

Tiempo (min)

100 80 60 40 20 0 0

20

40

60

80

Area cubierta (m)

100

120


84

7. Surface irrigation design The design process can be viewed as the inverse of the evaluation process. It would be desirable that the design phase followed and irrigation evaluation, but this is only possible when rehabilitating a system already in place. Although graphical procedures for design have been designed and used since the seventies, this text will focus on computer assisted design based on the use of surface irrigation models. Models are only marginally more accurate than other design procedures, but they offer an additional advantage: insight into the relationships between the variables involved in surface irrigation. Use of the models for design teaches the user on the relevance of the different variables and on their impact on irrigation performance.

7.1. The goals of design The infiltration, roughness and required irrigation depth are fixed, and the designer is looking for the type of surface irrigation system, the geometry of the field, the slope, the discharge and the time of cut off that guarantee that a certain performance level is attained. Application efficiency is often used as the performance index for design purposes. A design criterion could be for instance “Ea > 80%”. When rehabilitating a surface irrigation system, most of the variables will be fixed, and only the time of cut off and occasionally the discharge can be modified. Often rehabilitation includes modifications of slope and the inclusion or removal of runoff disposal systems.


85

7.2. Basic design procedures for basins and borders If large discharges (>100 ls-1) are used to irrigate a basin or border, erosion at the inlet can be an important issue. In these cases, an energy dissipation structure should be designed for each field. The need for these structures also depends on the erodibility of the soil. Equations have been proposed to compute the maximum non-erosive discharge, but local experience will often be more suited than these equations. Energy dissipation structures are systematically used in the Southwest of the U.S., where basins can attain 4 ha and the discharge is often beyond 300 ls-1. The dike surrounding the fields should be built according to the maximum flow depth expected for the worst design conditions (large discharge and roughness). A freeboard should always be considered.

7.3. Basic design procedures for furrows Use of furrow irrigation is somehow more complicated than basins and borders. The design furrow discharge should also be aggregated so that the number of furrows simultaneously irrigated add up to the total available discharge. Erosion can be a very important problem in furrow irrigation, affecting even the durability and stability of the furrows themselves. The maximum non-erosive flow velocities for furrows should be 8-10 m min-1 for erosive soils and 13-15 m min-1 for non-erosive soils. These velocities should be compared with those predicted by the model at the upstream end of the furrows.


86

Additional checks should be performed to ensure that the furrows are not flooded and that the runoff conveyance system can dispose of the runoff water properly. The field should be divided in sets (composed by a number of furrows irrigated simultaneously) and all the sets in the field should have the same number of furrows. Care should be applied when establishing the irrigation dose in furrows. The dose per unit furrow length is expressed in m3m-1. If this dose is to be compared with evapotranspiration data, referred to the whole field, the dose should be divided by the furrow spacing to yield a dose per square meter of the field.

7.4. Using models for design of irrigated areas Simulation models are applied to the irrigation unit: the basin, border or furrow. How should the problem of designing a farm or district (composed of hundreds of basins/borders or thousands of furrows) be faced? How should the alternatives be evaluated and selected? There are a number of answers to these questions, involving many theories on design of irrigation systems. The method presented in this text is just an approximation to the problem, and is characterised by a focus on balancing investments and irrigation efficiency. The method is presented in reference to an irrigation rehabilitation project, but could just as well be applied to a new development. The first step is to divide the area into groups. Groups are defined in this context as sets of fields with homogeneity in the irrigation related properties. These groups are supplied by the same canals, have similar soils (infiltration and water holding properties) and enclose a number of irrigation units with the same geometry. The following step is to define the properties of an irrigation unit characteristic of each group. All the variables required for a simulation model should be estimated. Simulations should be performed for each group to identify different alternatives. These alternatives can be based on different rehabilitation strategies, such as changes in the


87

discharge, changes in the irrigation method, or changes in the slope. Once all simulations are finished, the average performance indexes can be computed for each alternative. Besides application efficiency, an interesting index is the time required to irrigate a hectare. This index indicates the speed at which irrigation progresses, and can be used to characterise the adequacy of the water distribution system. In fact, the irrigation must be fast enough to cover the area irrigated from a canal turnout before the soil water is depleted by the evapotranspiration process. The last step is to estimate the investment costs required to implement each of the alternatives and to compare investments and efficiencies. Investment costs are the sum of: 1. On-farm investments (levelling and ditches), 2. Construction of the distribution network, 3. Construction of in-line reservoirs. The drainage system must also be considered, but these considerations exceed the purposes of this course. Not always the most expensive alternatives will yield the highest efficiencies.


88

7.5. Design of the required capacity of the distribution network When designing a new surface irrigated area or the rehabilitation of an existing one, it is a common practice to assign a uniform irrigation discharge to one or several groups irrigated from the same distribution ditch. When the ditches are considered on their way from an irrigation unit to the supply canal, there is often a point in which the capacity of the canal must be doubled in order to convey enough water to irrigate the farms located downstream. This is the origin of the "module" concept. A few decades ago, the module was defined as "the discharge that an irrigator can handle". This concept is radically obsolete now, since it was defined for a time when ditches were not lined and instead of using gates farmers constructed mud dams to divert water to their irrigation units. The module used to be about 30 l s-1, while today it is not rare to find areas where farmers receive a discharge ten times higher. In our days, the module is defined as the ratio between the design capacity of a ditch and the irrigation discharge in the area. If a ditch conveys three modules and the irrigation discharge is 100 l s-1, the discharge flowing through the ditch is 300 l s-1. This means that downstream from this point three farmers will be irrigating their farms at the same time. The physical design of the ditches that constitute the distribution network exceeds the objectives of this course. Our purpose, however, is to design the required capacity of a given ditch. The number of modules will be obtained dividing the time required to irrigate the area downstream a point in the irrigation ditch by the critical time between two irrigations. The number of modules must be rounded by excess to yield a feasible design.


89

The number of days required to irrigate a given area can be computed multiplying the time it takes to irrigate a hectare, the number of hectares and the ratio of 24 to the number of daily hours of irrigation operation. Currently it is recommended to operate irrigation networks 12 hours a day. The time required to irrigate an area is independent of the climatic conditions and to some extent of the crop or crops planted in the area. The only effect of the crop would be through the Manning n: a dense crop would delay advance and more time would be required for irrigation. The effect of Manning n on the time of advance rarely exceeds 10%. The time between two irrigation events depends on the climatic conditions and on the crop or crops being produced. The design condition often implies correct system operation in a given percentage of the irrigation seasons. It is generally accepted that a system is properly designed if it can ensure water delivery 80% of the years. For this matter, designs are often performed using the climatic data corresponding to the 20% return probability year. This is often referred to as a "dry year". The design procedure involves computing the monthly net irrigation requirements of the relevant crops, and obtaining a weighted average. The resulting figure, expressed in mm day-1 constitutes the critical net irrigation requirements. On the other hand, the target irrigation depth (ZR, mm) is computed from soil physical data. The ratio between the target irrigation depth and the critical net irrigation requirements yields the critical number of days between two irrigations.

7.6. Design of in-line reservoirs In-line reservoirs are a must in any irrigation project. We use this term to mane reservoirs located at locations inside the irrigated perimeter. These reservoirs are often constructed in by-pass of the main distribution canals and ditches. As a rule of thumb, it is often said that an irrigation project must be able to store as much water in the main reservoir system than in the in-line reservoirs. These reservoirs are often smaller than 1 hm3, although in occasions they reach over 10 Hm3.


90

The design criteria for in-line reservoirs are very varied. Each irrigation project solves their water regulation problems in different ways. As a reference for reservoir sizing, two limits will be discussed. The lower limit is the night reservoir. The upper limit is the irrigation period reservoir. The night reservoir is connected to the idea expressed in the previous paragraph of irrigating only 12 hours a day. The advantages of such a scheme are evident: no labour required to irrigate at night, when surface irrigation management is often careless, and the resulting efficiency is much lower than during daytime. The problem is that even if the distribution network does not distribute water during the night, the main canal requires water delivery to irrigated areas 24 hours a day. This is a typical operating rule of canals, where often a steady state operation is sought. To solve this problem, a night reservoir is built within the main line of the distribution network. Imagine that the required discharge of the distribution network (during the 12 daytime hours) is Q. The water distribution office should file a water order with the canal for a discharge of Q/2. During the night, this discharge will be stored in the night reservoir, and no water distribution will be performed. During daytime, Q/2 from the canal will be supplemented by the Q/2 from the night reservoir, adding to the desired discharge. Therefore, the night reservoir capacity must be equal to half of the volume required to irrigate the area supplied by the distribution ditch served by the reservoir. To ensure proper functioning of the reservoir, the irrigation volume is computed again for the critical month of the 20% return probability year. Do not forget to divide the daily water requirements by two (for the night period) and by the average application efficiency. This last division is required because the water stored in the reservoir covers the gross application depth. If an irrigation period reservoir is built instead of a night reservoir, the farmers will be benefited by a much larger regulation capacity. The design criteria of this reservoir guarantees that if the reservoir is full, farmers will be able to perform an irrigation event on all the area supplied by the distribution ditch. A detailed study is required to determine the type of reservoir best suited for each situation. Several factors should be considered when


91

taking such a decision. Among these factors, the most relevant would be the cost, the economic value of the crops and the reliability of the canal system.


92

8. Improving surface irrigation performance 8.1. Laser guided land leveling The relevance of land levelling in level basin irrigation has already been discussed. To evaluate its incidence on the distribution uniformity of an irrigation event, the following approximate formula has been proposed: 2 2  SD e  DU     DU'  100 1  1.27   1    Z  100       

[36]

where DU is the “flat soil” uniformity, and DU’ is the microtopography corrected. Application of this equation to a few case studies will reveal the relevance of Laser guided land levelling to keep acceptable levels of DU. An interesting result is that there is a significant difference in DU between perfect levelling (SDe = 0 mm) and Laser levelling DU (SDe  10 mm). The incidence of Laser levelling on other types of surface irrigation has not yet been properly quantified, but it is surely relevant. Laser levelling should be regarded as a necessary tillage operation to be performed every few years. If performing laser levelling is important, preserving the quality of levelling should be regarded as a priority: tillage operations creating undulations on the soil should be avoided.

8.2. Adjusting irrigation discharge Increasing the irrigation discharge results in a faster advance and therefore reduces the differences in opportunity time. In level basin irrigation, increasing the discharge will always result in improved uniformity. When discharges are too large, the final average infiltrated depth can be larger than the required depth and as a consequence the resulting efficiency will be low.


93

In blocked-end borders and furrows, increasing the discharge will potentially increase irrigation uniformity, but the risk to under- or overirrigate will also increase. In fact, using high discharges, the performance of these systems become very dependent upon the time of cut off. This is the reason why very high discharges do no result practical. In free draining borders and furrows, using high discharges increases irrigation uniformity and has a high potential to diminish deep percolation losses. The bad news is that runoff losses will increase. In these systems irrigation performance always shows an optimum for intermediate values of discharge.

8.3. Cutback irrigation In free draining systems, high discharges diminish deep percolation losses due to lack of uniformity during the advance phase. Low discharges have the advantage of diminishing runoff after completion of advance. Cutback gets the best of both worlds: large discharge during advance and a decreased discharge later on. In siphon tube systems for instance, this is accomplished by reducing the number of siphons from say three to two or one. The reduced discharge usually represents 50-65 % of the original discharge. The reduced discharge must be high enough to maintain water flowing all along the furrow. The reduction in surface runoff can be spectacular.


94

8.4. Surge irrigation Surge irrigation, together with laser levelling, are the two contributions of the XX century to surface irrigation. Surge irrigation was started by farmers as a practice to complete advance when infiltration was extremely large for the inflow discharge, as happens usually at the first irrigation of the season. What farmers used to do was to quit irrigation, let the overland water infiltrate, and then irrigate again. In some soils, soil sealing due to wetting and drying would reduce infiltration in such a way that the following irrigation “surge” would advance extremely fast and permit completion of advance. When researchers analysed this practice, a high potential was determined. Surge irrigation as we know it today is based on a pulsing valve that alternates irrigation between two sets of furrows. One set is on, the other is off. In a given set, there is a time on and a time off, usually of the same length. The result is the advance-recession diagram shown in Figure 41:

No

Irrigatio 5 4

Time

3 2 1 0

Length (m)

Advanc Recessio L

Figure 41. Advance-recession diagram of a surge irrigation system.


95

In this case, five surges were required to complete advance. In satisfactory cases, advance can be completed in the two sets using the same water and time that it would take to irrigate one set continuously. This is not the general case, particularly when the soil has been compacted by previous irrigation events. This is the reason why surge irrigation should be regarded as an irrigation practice rather than an irrigation system. Nevertheless, thousands of hectares are currently surge irrigated.

8.5. Surface irrigation automation Surface irrigation is often regarded as a labour intensive system, as opposed to sprinkler or drip irrigation. In many cases this is true, but this does not mean that it has to be true. Properly engineered and managed irrigation systems are proving economically sound in economies with very high labour costs. Several factors should be considered as keys to reduce labor in surface irrigation: 

Use large discharges: Doubling discharge will often reduce irrigation time to one third. Increasing discharge is a sound investment.



Use this reduction in the required irrigation time to reduce the daily irrigation time. Use of 12 hours per day will increase the social acceptation of irrigation and increase irrigation efficiency (night water allocations are often poorly used).



Introduce reservoirs in the system: store canal diversions in the reservoirs at night.



Enlarge the irrigation units as possible: irrigating large units fast is the most reliable

automation system. If all these measures were not enough, the conventional automation of surface irrigation has been studied for decades. A wide variety of automation systems based on electronics, pneumatics and timers have been developed.


96

8.6. Irrigation scheduling Scheduling is one of the keys to attain high irrigation efficiencies. This practice is often left in the hands of the farmers, who take their decisions based on the available information.

Automated

irrigation

scheduling

systems,

and

public

broadcast

of

evapotranspiration data can effectively help reduce water use. In surface irrigation, one extra irrigation can mean 25 % of the seasonal water use. This large volume of water justifies extra care in decision making.


97

9. Surface irrigation and the environment The development of a surface irrigation project requires a construction of a number of infrastructures. Among them: 1) Diversion works; 2) Reservoirs; 3) Network of canals and ditches; 4) Communications network; and 5) Drainage and runoff conveyance network. The incidence on the landscape of these elements can be analysed from a variety of viewpoints. As a general rule, it can only be concluded that surface irrigation projects are characterised by an impact of low intensity (as compared to other land uses such as industrial or urban), but large geographical extent. The project itself can induce environmental problems such as: 1) Alteration of the hydrological functioning of rivers and riparian areas; 2) Aquifer overdraft; 3) Alteration of wetlands; 4) Development of shallow water tables; 5) Build-up of soil salinity / sodicity; 6) Build-up of water salinity and general degradation of water quality via return flows; 7) Soil erosion; and 8) Land abandonment. Irrigation management must be optimised to avoid or minimise these environmental hazards. The environmental problems related to surface irrigation are mainly due to


98

overirrigation, a problem common to many surface irrigated fields, that results in deep percolation and surface runoff losses. Deep percolation mobilises soluble salts within the soil, increasing the salinity of return flows. When these flows mix with good quality water, the quality of the whole water resource is degraded, and its usability limited. Deep percolation also results in nutrient losses, mainly nitrates. Nutrients and pesticides can also induce water pollution problems that can even affect human health. Surface runoff, on its side, can produce erosion that damages the top soil, reducing its fertility.


99

10. Class project In this practical exercise, the modernisation of the Almudévar irrigation district will be analysed using the tools discussed in the lectures. This exercise comprises most of the topics covered in the course and will serve as a comprehensive review. The district is located in the Huesca province of Aragón, in the North East of Spain. The irrigated area is close to 3,000 ha. The district is part of the Monegros project, one of the emblematic water supply projects of the Government of Spain in the XX century. The district is almost exclusively irrigated using blocked end borders, constructed between 1920 and 1950. The design criteria used at that time suppose strong limitations to the development of a competitive agriculture now a days. The weak capacity of the distribution network, and the lack of internal regulation oblige to irrigate 24 hours a day during most of the season. The small discharges used make on farm irrigation slow and inefficient. The following figure presents the location of the district. The irrigated area is surrounded by three canals that supply water towards the interior of the district. Two large creeks merge into a unique return flow collector that receives all water losses.


100

Almudévar Irrigation District

Ebro River

4660000 s ro eg on M

4659000 Valsalada

4656000

ot irr ig at ed

Violada creek

Violada canal

4657000

Valsalada road

Almudévar

N

4658000

l na ca

Spain

ayNot irrigated hw ig h a

sc ue Not irrigated -H d a e t z ga go i a r A r r rtas ti ona Za cree No k

4655000 4654000

Artasona

Not irrigated

4653000

canal Santa Quiteria

4652000 694000

696000

698000

700000

702000

The following is a map of crop soil depth (cm) in the district. Deep soils are located along the creeks. Shallow soils are located in residual plains.

4660000 4659000 4658000 4657000 4656000 4655000 4654000 4653000 4652000 694000

696000

20

35

698000

50

65

700000

80

95

110

702000


101

The following is a map of crop extractable water in the district (mm). Deep soils are often associated with fine textures, and therefore retain a lot of water. The areas with very weak water retention are located in the residual plains. The shallow soils often have sandy textures.

4660000 4659000 4658000 4657000 4656000 4655000 4654000 4653000 4652000 694000

696000 25

35

698000 45

55

700000 65

75

702000

85

Land tenure is poorly distributed, as shown in the following plot, where the percent area in a farm, property of irrigation management is presented as a function of the area. For instance, the average (50% area) farm has 3 ha, the average property has 10 ha, and a person dedicated to irrigate manages 15 ha on the average.


102

100

Percent Area (%)

90 80 70 60 50 40 30

Farm Holding Irrigation management

20 10 0 0

10

20

30

40

50

60

70

80

Area (ha)

The following table presents the crop water requirements for the most popular crops in the area (in mm/month). Data are presented for 1994, the average year and the year with a 20% return probability (a dry year). Alfalfa 1994

Corn Ave.

20%

1994

Wheat Ave.

20%

1994

Sunflower Ave.

20%

Jan.

7.2

8.0

18.5

6.9

7.3

17.5

Feb.

14.2

11.3

26.3

20.5

18.1

29.3

Mar.

30.5

41.3

59.2

46.5

55.1

75.8

Apr.

42.5

55.6

77.5

65.9

75.0

99.4

May.

69.2

88.5

113.5

50.1

44.9

64.3

Jun.

130.1

118.7

148.7

110.2

102.8

131.9

Jul.

161.2

150.9

166.0

187.6

191.8

Aug.

95.4

125.7

145.4

153.2

Sept.

64.3

73.0

95.0

Oct.

45.1

35.9

54.7

Nov.

2.4

7.6

17.7

Dec.

3.1

6.8

15.3

723

938

Total

1994

Ave.

20%

95-2 113.3 140.9

47.6

43.2

62.4

76.2

99.3

92.7

121.3

210.2

165.4 193.6

212.4

162.5

184.7

146.3 159.3

181.3

79.6

85.9

109.4

36.5

28.4

51.2

752

90.2

43.7

4.1 616

65.9

8.1

19.2

343

472

50.1

68.3

539

646


103

A total of 15 irrigation evaluations were performed in the district. Infiltration parameters were estimated from advance, and four equations were obtained that were associated to soil units. This process is illustrated in the following plot:

0.25

ZII = 0.0120  0.450

0.20 Infiltration (m)

1 2 3 5 6 8 9 10 12 13 14 15 I II III IV

ZI = 0.0144 0.475 ZIII = 0.0147 0.393 ZIV = 0.0145  0.351

0.15 0.10 0.05 0.00 0

100

200 300 Time (min)

400

The area was divided in groups using information from a soil map, a water infrastructure map and a topographical map. The resulting groups are presented in the following map: 4660000

1

2

4

4659000

3

27 28

5 6 7

4658000

33

30

8

9

32

26 29

4657000

y (UTM)

11 13

4656000

15

10

35

34

31

39

36

45

38 37

12

42

14

54 53

41

44

40

52

16 18 19

17

21 22

51 50

64

49

63

48

4653000 25

62 59

24

23

57 58

60

76

68

67

47 20

4654000

73

72 69 70 75

46

4655000

Dry farming

71

55

43

74 81

78

77

80

82

79

66

83

65

85

84 90

Dry farming 61

86 88

89

92

87

91

56

4652000 694000

695000

696000

697000

698000

x (UTM)

699000

700000

701000

702000


104

The characteristics of each group are presented at the end of this document. Two properties were assumed uniform in the district: the border slope of 0.05% and the Manning n of 0.12. The future distribution of crops was assumed to involve 50% corn, 25% alfalfa and 25% wheat. Your duty will be to design the modernisation of a part of the district. For this matter, you will be required to:  Analyse the current performance of each group: Application efficiency and irrigation time (hours ha-1). You will have to use a surface irrigation model.  Come up with an improved irrigation performance. This improvement can come from a series of options, like:  Increasing the irrigation discharge  Modifying the slope  Changing the type of surface irrigation system.  Allowing runoff… whatever.  Estimate the future performance of each group.  Design the required system capacity at the main irrigation ditch.  Design a night reservoir for all your groups. Your design criterion is that you want to have enough capacity to irrigate your area the driest month of a dry year (return probability 20%).  Design a Broad Crested Weir for your main irrigation ditch.  Be ready to present your designs to the class and defend them before your colleagues.


Group

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

Soil Depth (cm) 90 80 110 70 94 95 105 100 105 105 60 120 105 110 100 120 90 85 90 110 90 110 100 110 105 50 100 100 85 115 95 55 60 60 50 115 120 70 60 60 60 115 115

Area (ha) 35.0 51.7 47.4 60.4 48.4 23.5 18.1 40.8 24.1 42.1 42.0 19.5 21.8 34.2 26.8 23.7 40.2 19.9 47.7 24.5 59.1 35.9 24.1 51.6 23.1 74.3 54.1 31.4 46.6 17.6 24.1 26.0 31.1 63.2 92.3 41.5 25.5 30.9 27.5 35.9 23.3 36.6 39.8

ZR (mm) 50 47 78 44 73 85 71 79 74 75 48 90 55 111 52 98 69 56 53 92 52 50 88 85 74 40 64 64 61 64 66 20 28 49 25 91 127 46 32 51 44 116 94

Length (m) 183 155 205 265 250 190 140 227 260 260 210 180 160 360 220 165 197 163 187 240 250 220 263 308 185 145 167 215 170 220 290 133 120 150 208 340 380 380 270 140 323 380 275

105

Width (m) 50 50 28 33 34 43 20 47 50 33 23 40 40 35 40 40 42 27 40 40 39 30 43 42 71 40 27 35 43 20 38 43 60 38 53 45 35 35 35 55 35 35 33

Discharge (l s-1) 69 84 35 69 69 52 23 64 64 81 81 70 67 52 58 35 58 47 35 35 35 58 58 58 58 58 81 69 139 81 139 75 112 139 81 81 116 139 139 81 116 116 139

Infiltration Equation 1 1 1 2 4 4 1 4 1 4 1 4 3 4 3 3 4 3 3 3 1 1 1 4 3 1 4 2 2 4 4 2 2 2 2 4 4 4 2 2 2 4 4


Group

44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85

Soil Depth (cm) 105 85 90 105 105 80 115 110 110 60 100 120 110 55 100 110 105 100 105 120 100 110 90 55 100 55 60 55 90 100 55 95 110 115 110 90 100 60 50 105 110 115

Area (ha) 62.2 37.4 59.3 20.2 26.0 28.3 30.8 56.5 22.3 37.7 42.0 28.7 11.9 34.0 46.3 14.7 22.7 11.5 14.9 11.8 30.7 25.4 63.6 23.5 23.1 41.4 27.1 53.6 27.9 39.1 45.3 49.7 33.0 72.5 11.9 26.9 50.0 41.0 47.3 44.4 37.6 47.4

ZR (mm) 51 47 53 64 84 79 74 78 70 29 52 62 96 45 74 74 98 94 98 98 82 94 100 43 118 23 26 26 40 55 36 99 116 104 71 99 114 74 43 102 94 96

Length (m) 300 225 233 245 280 273 190 266 315 193 140 280 240 161 215 140 172 220 220 220 150 183 195 273 380 195 210 180 260 130 130 333 340 290 120 205 200 243 195 215 252 246

106

Width (m) 43 44 40 40 38 35 44 41 30 50 45 30 65 59 41 50 41 70 50 35 45 53 38 47 35 55 38 42 25 43 50 43 30 50 30 40 33 49 45 35 42 39

Discharge (l s-1) 69 69 81 81 81 81 81 81 81 139 81 81 93 93 93 81 139 93 110 110 81 81 139 81 81 81 81 64 64 139 139 70 58 112 104 104 104 139 69 46 87 69

Infiltration Equation 2 2 3 3 4 4 2 2 2 2 2 2 3 3 3 4 4 3 4 4 3 3 3 4 4 2 2 2 2 2 4 4 4 4 4 4 4 3 3 3 4 3


Group

86 87 88 89 90 91 92

Soil Depth (cm) 105 110 110 110 75 90 60

Area (ha) 37.7 31.3 58.4 55.3 29.8 25.9 22.4

ZR (mm) 101 51 90 88 49 68 49

Length (m) 223 267 256 194 160 166 160

107

Width (m) 48 33 43 48 60 45 38

Discharge (l s-1) 93 93 81 70 81 93 64

Infiltration Equation 4 4 3 3 3 4 3


108

11. Suggested readings Clemmens, A. J., Bos, M. G., Groenestein, J. M. and Relogle, J. A. (1993) "Flume 3.0: a computer program for designing flumes and weirs," In: Management of irrigation and drainage systems, Park City, Utah. 867-874. Clemmens, A. J. and Dedrick, A. R. (1981). "Estimating distribution uniformity in level basins." Trans. ASAE, 24(5), 1177-1180. Clemmens, A. J., Dedrick, A. R. and Strand, R. J. (1993) "Basin 2.0 for the design of levelbasin irrigation systems," In: Management of irrigation and drainage systems, Park City, Utah. 875-882. Clemmens, A. J., El-Haddad, Z., Fangmeier, D. D. and Osman, H. E. B., 1999. Statistical approach to incorporating the influence of land-grading precision on level-basin performance. Trans. ASAE, 42(4):1009-1017. Clemmens, A. J., El-Haddad, Z. and Strelkoff, T. S., 1999. Assessing the potential for modern surface irrigation in Egypt. Trans. ASAE, 42(4):995-1008. Cuenca, R. H. (1989) Irrigation system design: an engineering approach. Prentice-Hall, Inc., Englewood Cliffs, New Jersey. 552 pp. Dedrick, A. (1979). "Land leveling. precision attained with laser controlled drag scrapers."

American Society of Agricultural Engineers, Dedrick, A. R. and Zimbelman, D. D. (1981) "Automatic control of irrigation water delivery to and on-farm in open channels," In: Symposium of the International Commission on Irrigation and Drainage, Grenoble, France. 113-128. García-Navarro, P., Playán, E. and Zapata, N., 2000. Solute transport simulation in overland flow: application to fertigation. J. Irrig. and Drain. Engrg. ASCE, 126(1):33-40. Hagan, R. M., Haise, H. R. and Edminster, T. W. (1967). Irrigation of agricultural lands. Agronomy series, 11. American Society of Agronomy. Madison, Wisconsin, USA. Hart, W. E., Collins, H. G., Woodward, G. and Humperys, A. S. (1980) "Design and operation of surface systems." Design and operation of farm irrigation systems. M. E. Jensen, Ed. ASAE Monograph, St. Joseph, Michigan. 501-582. Jaynes, D. B. and Clemmens, A. J. (1986). "Accounting for spatially varied infiltration in border irrigation models." Water. Resour. Res., 22(8), 1257-1262.


109

Katopodes, N. D. and Strelkoff, T. (1977). "Hydrodynamics of border irrigation - complete model." ASCE J. Irrig. Drain. Div., 103(IR3), 309-323. Lecina, S., Playan, E., Isidoro, D., Dechmi, F., Causape, J. and Faci, J. M., 2005. Irrigation evaluation and simulation at the irrigation District V of Bardenas (Spain). Agric. Wat.

Manage., 73(3):223-245. Mailhol, J. C. (1992). "Un modèle pour ameliorer la conduite de l'irrigation à la raie." ICID

Bull., 41(1), 43-60. Mailhol, J. C., Zairi, A., Slatni, A., Ben Nouma, B. and El Amani, H., 2004. Analysis of irrigation systems and irrigation strategies for durum wheat in Tunisia. Agric. Wat.

Manage., 70(1):19-37. Merriam, J. L. and

Keller, J. (1978) Farm irrigation system evaluation: a guide for

management. Utah State University, Logan, Utah. 271 pp. Oyonarte, N. A. and Mateos, L., 2003. Accounting for soil variability in the evaluation of furrow irrigation. Trans. ASAE, 46(1):85-94. Orang, M.N., Matyac, J.S. and Snyder, R.L., 2008. Survey of irrigation methods in California in 2001. Journal of irrigation and drainage engineering, 134(1), 96-100. Playán, E., Faci, J. M. and

Serreta, A. (1996). "Modeling microtopography in basin

irrigation." J. Irrig. and Drain. Engrg., ASCE, 122(6), 339-347. Playán, E., Merkley, G. P. and Walker, W. R. (1992) B2D, Two-dimensional basin irrigation

simulation model: User's guide. Department of Biological and Irrigation Engineering, Utah State University. Logan, Utah. Playán, E. and Faci, J. M., 1997. Border fertigation: field experiments and a simple model.

Irrig. Sci., 17:163-171. Replogle, J. A., Clemmens, A. J. and Bos, M. G. (1990) "Measuring irrigation water."

Management of farm irrigation systems. G. J. Hoffman, Howell, T. A. and Solomon, K. H., Ed. ASAE, St. Joseph, Michigan. 315-370. Rhoades, J. D. (1989). "Intercepting, isolating and reusing drainage waters for irrigation to conserve water an protect water quality." Agric. Wat. Manage,, 16, 37-52. Skogerboe, G. V. and Merkley, G. P. (1996) Irrigation maintenance and operation learning

process. Water resources publications, Highlands Ranch, Colorado. 358 pp. Snyder, R.L., Plas, M.A. and Grieshop, J.L., 1996. Irrigation methods used in California: grower survey. J. Irrig. Drain. Div., ASCE, 122(4), 259-262.


110

Strelkoff, T. (1970). "Numerical solution of the Saint-Venant equations." ASCE J. Hydr.

Div., 96(HY1), 223-252. Strelkoff, T. S. (1990). "A computer program for simulating flow in surface irrigation.

Furrows-basins-borders.". USWCL, USDA-ARS. Phoenix, Arizona. USA Strelkoff, T. S., Clemmens, A. J., El-Ansary, M. and Awad, M., 1999. Surface irrigation evaluation models: application to level basins in Egypt. J. Irrig. and Drain. Engrg. ASCE, 42(4):1027-1036. Valiantzas, J. D. (1994). "Simple method for identification of border infiltration and roughness characteristics." J. Irrig. and Drain. Engng., ASCE, 120(2), 233-249. Walker, W. R. (1993) SIRMOD, Surface irrigation simulation software. Utah State University. Logan, Utah. Walker, W. R. and

Skogerboe, G. V. (1987) Surface irrigation. Theory and practice.

Prentice-Hall, Inc., Englewood Cliffs, New Jersey. 386 pp. Walker, W. R. (1989). Guidelines for designing and evaluating surface irrigation systems. FAO irrigation and drainage paper 45. Food and Agriculture Organization of the United Nations. Roma, Italia. Willardson, L. S. (1972). "Attainable irrigation efficiencies." ASCE J. Irrig. Drain. Div., 98(IR2), 239-246. Zapata, N. and

Playán, E., 2000. Simulating elevation and infiltration in level-basin

irrigation. J. Irrig. and Drain. Engrg., ASCE, 126(2):78-84. Zapata, N., Playán, E. and Faci, J. M., 2000. Water reuse in sequential basin irrigation. J.

Irrig. and Drain. Engrg. ASCE, 126(6):362-370.


111

12. Self evaluation 1. An irrigation event was evaluated. It was found that DU = 90 %, Ea = 50 %, Er = 90%, DPR = 40 % and SRO = 10%. a) This is a level furrow system b) It seems like the soil is not very well suited for surface irrigation c) This is not a bad situation if surface runoff can be reused. 2. Identify which of the following combinations is possible for an irrigation event: a) DU = 40%, Er = 90%; SRO = 0%, Ea = 50% b) Ea = 45%, DPR = 30%, DU = 50%, SRO = 10% c) Er = 90%, Ea = 30%, DU = 90%, DPR = 70% 3. Now that we are about to build a night reservoir for our ditch, it is a good time to think about measuring irrigation water. Only one of the following sentences is correct: a) The night reservoir will not permit water measurements during the daytime. b) Use of a broad crested weir will require calibration of the measurement device in the laboratory. c) Water measurement will add transparency to water management and will make our irrigation district more peaceful. 4.

In the following figure the numbers indicate volumes of water in m3: L Zr

312 12

72

a) The application efficiency is 64 % b) The water requirement efficiency is 100 % c) The distribution uniformity is 72 % 5. In the figure from question 4, a) The deep percolation ratio is 23 % b) The surface runoff ratio is 21 % c) The total volume of water applied is 499 m3

Runoff: 103


112

6. The figure from question 4: a) Can only correspond to the irrigation of a level basin b) Could correspond to the irrigation of a furrow c) Was obtained using a ring infiltrometer 7. An irrigation event was performed on a blocked end border with a length of 150 m and a width of 30 m. The irrigation discharge was 100 L/s and the time of cutoff was 60 min. An experiment was performed to measure infiltration with three ring infiltrometers. The Kostiakov equation obtained from the ring infiltrometers was: Z = 0.0075 t 0.5 (in meters and minutes). Distance (m) 0 50 100 150

ta (min) 0 30 70 120

tr (min) 300 270 290 500

a) The target irrigation depth (ZR) is 80 mm. b) In this irrigation event the distribution uniformity would have resulted higher if the time of cutoff had been a bit shorter. c) In this type of situations cutback is a good strategy to improve the application efficiency.

8. Same problem as in question 7. Adjust the infiltration equation obtained with the infiltrometers. a) The adjusted “a” parameter will be derived from the average opportunity time and the average infiltrated depth. b) The adjusted “k” parameter will be approximately equal to 0.005. c) The adjusted infiltration parameter depends on the value of the Manning coefficient.


113

9. Same problem as in question 7. a) The resulting distribution uniformity is about 88 % b) The resulting distribution uniformity is about 115 % c) The resulting distribution uniformity can not be computed.

10. Measuring infiltration in furrows: a) When using the blocked furrow method, the downstream end of the furrow is blocked to avoid runoff. Infiltration is determined with a propeller meter. b) It is important to irrigate at least three furrows and measure infiltration in the central furrow. c) If we want to use the method of “discharge in two points”, it is important that we set the two measurement devices close to each other so that we can verify that they are properly installed. 11. The following graph presents the dependence of the application efficiency, the deep percolation ratio and the surface runoff on the irrigation discharge of a furrow. Who is who? 1 2

Ea, DPR and SRO (%)

3

Discharge

a) Curve 1 is Ea, curve 2 is DPR and curve 3 is SRO b) Curve 1 is DPR, curve 2 is Ea and curve 3 is SRO c) Curve 1 is DPR, curve 2 is SRO and curve 3 is Ea 12. I performed an infiltration ring experiment on a level-basin, and obtained the following results :


t (min) 1 2 5 10 20 30 50 120 200

114

Z (m) 0.01 0.02 0.04 0.06 0.07 0.09 0.10 0.16 0.22

I transferred the data to a spreadsheet and produced the following chart: 0.25

Z = 0.0136 t 0.5376

Z (m )

0.20 0.15 0.10 0.05 0.00 0

100

200

300

t (m in)

The field length (L) is 300 m and the field width (W) is 40 m. After the experiment, the field was irrigated for 3 hours with a discharge of 200 L/s. According to the advanceregression diagram, the average opportunity time (  ) was 220 min. The experiment… a) indicates that the field should be laser levelled b) would be more representative if I had used more infiltration rings c) indicates that the soil could belong to the infiltration families 0.10 or 0.15 13. In the experiment of question 12, according to the ring infiltrometer data, between the times 120 and 200 min, the infiltration rate is… a) i = 0.19 m min-1 b) at its maximum value c) i = 0.00075 m min-1


115

14. in the experiment of question 12: a) The experimental results have value, but should be corrected to consider the influence of the wetted perimeter on infiltration. b) It would have been a good idea to run the infiltration experiment during the irrigation event. c) The infiltration measurement would be more adequate if a recycle furrow infiltrometer had been used. 15. Suppose you go to the field and perform a few surface irrigation evaluations from which three advance-recession diagrams are obtained: b)

c)

Time (min)

a)

Distance (m)

a) Diagram a) could be a blocked-end border b) Diagram b) could be a free-draining border c) Diagram c) could be a level basin 16. The following is a printout of the hydraulic summary of SRFR (version 4.06) for a particular case:


116

It corresponds to the simulation of irrigation in a… a) Level basin b) Free draining border c) Level furrow 17. The irrigation simulation in question 16: a) Has been obtained using cutback b) Could benefit from cutback c) Could not use cutback 18. In the irrigation simulation in question 16: a) The Distribution Uniformity is close to 100% b) The Water Requirement Efficiency is about 50% c) The Deep percolation ratio is about 50%

19. This is the advance-recession diagram from a blocked furrow irrigation evaluation: Time (min)

25 20 15 10 5 0 0

5

10 15 Distance (m)

20

a) The concavity of advance indicates that infiltration was not well characterized by the rings b) In furrow irrigation the filling phase is usually much longer than in blocked-furrow irrigation c) Since the opportunity time shows large differences along the furrow, efficiency can not be high.


117

20. In question 19, we measured infiltration with a number of blocked furrow infiltrometers, and obtained the following equation: Z = 0.0066 0.98 (in m and min). Discharge was estimated as 0.83 L/s, and the time of cut off was 22.35 min. x (m) 0 5 10 15 20

ta (min) 0.0 0.8 3.2 8.8 21.5

tr (min) 23.0 23.4 23.6 23.8 23.9

a) The adjusted k was 365 mm/hra b) The adjusted k was determined using the model c) There is not enough information in this text to obtain the adjusted k

21. In question 19, the advance-recession diagram suggests that uniformity is low. How could it be improved in this particular case? a) Increasing the time of cut off. b) Decreasing the slope c) Using the adjusted infiltration approach 22. The following is a printout from the WinSRFR Hydraulic summary. Only one of the following options is possible:


118

a) DU = 98%, Ea = 32%, SRO = 51 %, DPR = 17 % b) DU = 41%, Ea = 32%, SRO = 12%, DPR = 15 % c) DU = 95 %, Ea = 32%, SRO = 17 %, DPR = 51% 23. The case in question 22 could correspond to a: a) Level basin b) Blocked-end border c) furrow 24. In the following figure the numbers indicate volumes of water in m3:

L Zr

400 20

100

Runoff: 250

a) The application efficiency is 53 % b) The water requirement efficiency is 100 % c) The distribution uniformity is 95 % 25. In the figure from question 24, a) The deep percolation ratio is 53 % b) The surface runoff ratio is 33 % c) The total volume of water applied is 770 m3 26. An irrigation event was evaluated. It was found that DU = 40 %, Ea = 50 %, Er = 97%, DPR = 45 % and SRO = 5%. a) This is a level furrow system b) It seems like the soil is not very well suited for surface irrigation c) Increasing discharge would improve DU and reduce DPR. 27. Identify which of the following combinations is possible for an irrigation event: a) DU = 50%, SRO = 10%, Ea = 45%, DPR = 30% b) DU = 40%, Er = 100%; SRO = 0%, Ea = 90% c) DPR = 70%, Er = 100%, Ea = 30%, DU = 90%


119

28. During the evaluation of irrigation in a furrow, we find out that VT = 27 m3. The furrow length is 300 m. The furrow width is 0.75 m. We estimated SRO to be 40%. Determine the average infiltrated depth in mm. a) 72 mm b) 48 mm c) 120 mm 29. Measuring infiltration in furrows: a) When using the blocked furrow method, the water level must be kept constant during the experiment. b) It is a good idea to use the infiltrometer rings during the advance phase. c) If we want to use the method of “discharge in two points”, it is important that we set the two measurement devices close to each other so that we can verify that they are properly installed. 30. We are managers of a tomato farm. The crop evapotranspiration for the past week is 40 mm. We plan to re-establish this water with irrigation. The furrow length is 100 m, the furrow spacing is 1.5 m, application efficiency is 50%, the longitudinal slope is 0.0001 and the furrows have a semicircular section. How much water should we apply to each furrow? a) 4 m3 b) 8 m3 c) 12 m3


120

Solutions to the self evaluation test

Question # 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

a

b X

c

X X X X X X X X X X X X X X X X X X X X X X X X X X X X X


121

13. Index A adequacy ................................... 16 adjusted infiltration................... 72 advance ............................... 10, 13 advance curve ........................... 25 advance-recession diagram ....... 72 air entrapment ........................... 40 alfalfa........................................ 32 application Efficiency............... 15 aquifer....................................... 97 automation ................................ 95

B basic infiltration rate ........... 40, 52 basins ........................................ 85 blocked furrow.......................... 50 blocked-end borders.................. 32 borders ...................................... 85 boundary conditions ................. 68 branch infiltration function ....... 45 broad crested weir..................... 60 buoyancy system ...................... 50

C calibration ................................. 60 compaction ............................... 40 contours .................................... 30 costs .......................................... 87 critical section........................... 59 crust .......................................... 43 cumulative infiltration .............. 39 cut off ....................................... 10 cutback...................................... 93

D Darcy ........................................ 40 deep percolation........................ 98 deep Percolation ....................... 15 depletion ................................... 10 design........................................ 84 design units ............................... 65 diagram ..................................... 11 discharge................................... 92 discharge in two points ............. 50 discretization............................. 68 distribution.................................. 4 distribution Uniformity ............. 15

double ring.................................48 downstream boundary ...............68 drain back ..................................28 drip ............................................95

E efficiency.............................14, 71 elevation ....................................42 embankment ..............................37 empirical....................................44 environment...............................97 erosion ...........................26, 85, 97 evaluation ..................................71 evapotranspiration .....................17 explicit.......................................68

F family ........................................47 field experiments .......................64 field size ....................................71 filling .........................................10 finite differences........................68 finite elements ...........................68 flow section ...............................71 flume .........................................59 flumes ........................................50 free draining borders..................30 fruit trees ...................................38 furrow irrigated depth................86 furrow recycle infiltrometer ......51 furrows ................................34, 85

G gated pipes.................................36 Green and Ampt ........................44 groups ........................................86

H head loss ....................................59 homogeneity ..............................86 hook...........................................48 humectation front ......................40 hydraulic conductivity.........40, 43 hydraulic gradient......................41 hydrodynamic............................67 hyperbolic..................................65

I Implicit ......................................68 infiltration...................... 39, 71, 73 infiltration curve........................12 infiltration rate...........................39 Infiltration table.........................47 infiltrometer ring .......................48 initial condition .........................68 input data...................................64 instantaneous infiltration ...........39 irrigation sets.............................86 irrigation volume.......................72

K kinematic wave..........................67 Kostiakov ..................................44 Kostiakov-Lewis .......................45

L labour ........................ 4, 36, 37, 95 landscape...................................97 laser ...........................................92 laser levelling ............................25 level basins................................24 level furrows .............................37

M management ..............................97 mass conservation .....................66 matric ........................................42 microtopography .................26, 70 models .......................................64 modernisation..............................4 modified Kostiakov ...................45 momentum conservation ...........66

N nodes .........................................68 non-erosive velocity ..................85 numerical models ......................67 numerical schemes ....................67


O one-dimensional........................ 39 opportunity time ................. 11, 40 overflow.................................... 38

P Parshall ............................... 50, 59 partial differential equations ..... 65 performance .............................. 75 phases ....................................... 10 potential .................................... 41 propeller meter.......................... 58 proportional billing ................... 58

R RBC flumes .............................. 50 recession ................................... 10 regression.................................. 44 rehabilitation......................... 4, 84 required irrigation depth ........... 15 ring infiltrometer....................... 72 roughness .................................. 73

runoff...................................31, 93 runoff and reuse.........................28 runoff hydrograph......................71

S salinity ...........................10, 40, 97 scheduling..................................96 simple ring.................................48 siphon ........................................35 sliding gates...............................36 slope ..........................................31 Soil Conservation Service .........46 soil structure ..............................40 soil texture .................................40 soil water content.......................40 spatial variability .................44, 64 sprinkler.....................................95 surface Runoff ...........................15 surge ..........................................94

122 two-dimensional ........................69

U uniform flow..............................67 uniformity............................14, 71 upstream boundary ....................68

W water logging.......................32, 36 water records .............................58 water Requirement Efficiency...16 wetted perimeter........................49 wild flood ..................................38

Z zero inertia.................................67

T tillage.........................................40 transition zone ...........................39 transmission zone ......................39

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