HYDROLOGY OF THE NILE BASIN MAMDOUH SHAHIN International Institute for Hydraulic and Environmental Engineering Oude Del...
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HYDROLOGY OF THE NILE BASIN MAMDOUH SHAHIN International Institute for Hydraulic and Environmental Engineering Oude Delft 95, 2601 DA Delft, The Netherlands
ELSEVIER Amsterdam - Oxford
- New York - Tokyo
1985
E L S E V I E R SCIENCE P U B L I S H E R S B.V. Molenwerf 1 P.O. Box 21 1 , 1000 A E Amsterdam, The Netherlands
Distributors for the United States and Canada: E L S E V I E R SCIENCE P U B L I S H I N G C O M P A N Y I N C . 52, Vanderbilt Avenue N e w Y o r k , N Y 1001 7
I S B N 0-444-42433-4 (Val. 21) ISBN 0-444-41669-2 (Series)
0 Elsevier Science Publishers B.V., 1985 All rights reserved. N o p a r t o f this publication may be reproduced, stored in a retrieval system o r transmitted in any f o r m o r b y any means, electronic, mechanical, photocopying, recording o r otherwise, w i t h o u t the p r i o r w r i t t e n permission of the publisher, Elsevier Science Publishers B.V./Science & Technology Division, P.O. Box 330, 1000 A H Amsterdam, The Netherlands. Special regulations f o r readers in t h e USA - This publication has been registed w i t h the Copyright Clearance Center Inc. (CCC), Salem, Massachusetts. I n f o r m a t i o n can be obtained f r o m t h e CCC a b o u t conditions under which photocopies o f parts o f this publication m a y be made in the USA. A l l other copyright questions, including p h o t o c o p y i n g outside o f t h e USA, should be referred t o t h e publisher, Elsevier Science Publishers B.V., unless otherwise specified. Printed in The Netherlands
V
PREFACE T h i s book aims a t d e s c r i b i n g a number o f t h e h y d r o l o g i c a l a s p e c t s of t h e b a s i n o f t h e N i l e R i v e r and t h e d i f f e r e n t f a c t o r s a f f e c t i n g them. With t h i s aim i n mind i t d e a l s p r i m a r i l y w i t h t h e i n f l o w - o u t f l o w b a l a n c e of t h e N i l e s y s t e m from t h e s o u r c e up t o mouth s u b - b a s i n - w i s e . The components of t h e h y d r o l o g i c c y c l e which e n t e r i n t h e w a t e r b a l a n c e and which a r e c o n s i d e r e d h e r e a r e t h e r a i n f a l l , e v a p o r a t i o n , e v a p o t r a n s p i r a t i o n and t h e change of w a t e r i n s t o r a g e , b o t h i n volume and l e v e l . Each of t h e s e comp o n e n t s i s p r e s e n t e d a s o b s e r v e d i n n a t u r e , r e c o r d e d from e x p e r i m e n t s or found from c o m p u t a t i o n s , t o g e t h e r w i t h an e x p l a n a t i o n o f t h e p r o c e d u r e s used and t h e i n t e r p r e t a t i o n of t h e r e s u l t s o b t a i n e d . A t t e n t i o n i s p a i d t o t h e l o s s e s which t a k e p l a c e i n c e r t a i n p a r t s of t h e b a s i n . The m e t e o r o l o g i c and h y d r o l o g i c d a t a a t t h e key s t a t i o n s on t h e N i l e and i t s t r i b u t a r i e s a r e a n a l y z e d and t h e i r b a s i c s t a t i s t i c a l p r o p e r t i e s g i v e n . A s p e c i a l c h a p t e r i s d e v o t e d t o t h e geohydrology of t h e b a s i n and t o t h e groundwater s i t u a t i o n s and p o t e n t i a l i t i e s i n some of t h e c o u n t r i e s s h a r i n g t h e N i l e B a s i n . L a s t , b u t n o t l e a s t , a whole c h a p t e r h a s been l e f t t o t h e s t o r a g e , c o n t r o l and c o n s e r v a t i o n w o r k s , b o t h e x i s t i n g and p l a n n e d , and t o t h e i m p a c t s of s u c h works on t h e e n v i r o n m e n t . I n a t t e m p t i n g t o c o v e r h e r e as many of t h e h y d r o l o g i c a l a s p e c t s of t h e N i l e B a s i n a s p o s s i b l e , i t h a s n o t been i n t e n d e d t h a t t h i s book s h a l l compete w i t h e x i s t i n g l i t e r a t u r e on t h e same s u b j e c t biit r e t h e r c o m p l e t e i t . Moreover, t h i s book, when added t o t h o s e d e s c r i b i n g a s p e c t s o t h e r t h a n h y d r o l o g i c a l , s u c h a s b i o l o g i c a l and g e o l o g i c a l , s h a l l c e r t a i n l y h e l p t o p r o v i d e t h e r e a d e r w i t h a more c o m p l e t e p i c t u r e of t h i s r i v e r b a s i n . Almost t w o - t h i r d s of t h e s u r f a c e o f A f r i c a a r e d r a i n e d by s e v e n major r i v e r s , i n c l u d i n g t h e N i l e . So t h e knowledge g a i n e d o f t h e hydrology of any of them s h a l l , no d o u b t , c o n t r i b u t e t o a b e t t e r u n d e r s t a n d i n g o f t h e w a t e r r e s o u r c e s of a c o n t i n e n t w i t h an a c u t e w a t e r - s h o r t a g e problem. Such a n u n d e r s t a n d i n g i s needed by e v e r y h y d r a u l i c or w a t e r r e s o u r c e s e n g i n e e r aiming a t a more e f f i c i e n t u t i l i z a t i o n of t h e r e a d i l y a v a i l a b l e , a s w e l l a s t h e p o t e n t i a l , w a t e r r e s o u r c e s . The a u t h o r h a s depended i n some p a r t s of t h e book on h i s e x p e r i e n c e and viewp o i n t s , and on t h e e x i s t i n g l i t e r a t u r e i n t h e r e m a i n i n g p a r t s . A l i s t of t h e r e f e r e n c e s and d a t a s o u r c e s used a p p e a r s a t t h e end o f e v e r y c h a p t e r . A s e n s e of g r a t i t u d e must b e e x p r e s s e d h e r e f o r t h e many who d i r e c t l y and i n d i r e c t l y , by t h e i r c o n s t r u c t i v e c r i t i c i s m and a d v i c e , have h e l p e d i n t h e p r e p a r a t i o n of t h i s book. Most of t h e a p p r e c i a t i o n g o e s , i n f a c t , t o P r o f . i r L.J.
Mostertman, D i r e c t o r of t h e I n t e r n a t i o n a l I n s t i t u t e f o r H y d r a u l i c and
E n v i r o n m e n t a l E n g i n e e r i n g , D e l f t , The N e t h e r l a n d s , whose c o n t i n u o u s s u p p o r t and
vi
e n c o u r a g e m e n t d a t e s b a c k t o a s e a r l y a s 1962. G r a t i t u d e i s a l s o e x t e n d e d t o many o f t h e a u t h o r ' s f o r m e r c o l l e a g u e s a t t h e N i l e C o n t r o l D e p a r t m e n t ,
the
M i n i s t r y of I r r i g a t i o n , E g y p t , a n d a t t h e F a c u l t y o f E n g i n e e r i n g , C a i r o U n i v e r s i t y . S p e c i a l m e n t i o n must b e made o f M s P . E . R o e l l , L i b r a r i a n o f t h e International
I n s t i t u t e f o r H y d r a u l i c and E n v i ronment al E n g i n e e r i n g , D e l f t , f o r
p r o v i d i n g t h e a u t h o r w i t h a n enormous number o f r e f e r e n c e s a n d d o c u m e n t s , MS
P . S c h o t t - L e 6 n who u n d e r t o o k a l l t h e t y p i n g and M r W .
van N i e v e l t f o r p r e p a r -
i n g t h e g r a p h i c work i n t h i s b o o k . The a u t h o r i s g r e a t l y i n d e b t e d t o h i s f a m i l y , whose p a t i e n c e a n d t o l e r a n c e h a v e b e e n h i s s u p p o r t i n t h e many y e a r s s p e n t i n compiling t h e book.
M.M.A.
Shahin,
D e l f t , 1984
1
Chapter 1
HISTORICAL INTRODUCTION
1.1
SOURCE OF THE NILE
The known h i s t o r y o f t h e N i l e R i v e r d a t e s b a c k t o j u s t b e f o r e 5000 B . C . From t h e n t i l l r e c e n t l y v a r i o u s t h e o r i e s a b o u t t h e s o u r c e o f t h e N i l e and i t s
r i s e h a v e b e e n l a i d down. Some of t h e s e t h e o r i e s were s o c o n f l i c t i n g t h a t i t became c u s t o m a r y i n a n c i e n t Rome t o s a y : q u a e r e r e f o n t e s N i l e ( s e a r c h for t h e N i l e ) when someone t a l k e d a b o u t a m y s t e r i o u s or i m p o s s i b l e matter ( P i e r r e , 1974). The name N i l e i s s a i d t o b e d e r i v e d f r o m t h e G r e e k N e i l o s , whose o r i g i n i s unknown. B o t h N e i l o s and A i g u p t o s ( m a s c u l i n e ) w e r e u s e d i n G r e e k drama when r e f e r r i n g t o t h e N i l e , w h e r e a s A i g u p t o s ( f e m i n i n e ) a l o n e was u s e d when r e f e r r i n g t o Egypt ( E n c y c lo p a e d ia B r i t a n n i c a ,
1969).
I t i s o f t e n c l a i m e d t h a t t h e a n c i e n t E g y p t i a n s knew n o t h i n g o f t h e o r i g i n of t h e i r r i v e r . T h i s c l a i m i s s u p p o r t e d o n o n e h a n d by t h e n a i v e c o n v i c t i o n o f t h e common a n c i e n t E g y p t i a n t h a t t h e N i l e f l o w s o u t o f t h e f u l l b r e a s t s o f t h e N i l e God, H a p i . The p r i e s t s o f a n c i e n t E g y p t , o n t h e o t h e r h a n d , h a d t h e i r own t h e o r y , w h i c h t h e y f a i t h f u l l y f o u n d e d o n t h e o l o g i c a l g r o u n d s . T h o s e p r i e s t s were c o n v i n c e d t h a t somewhere t h e c o u r s e o f "The C e l e s t i a l N i l e " w a s b e s e t by m o n s t r o u s r o c k s and s t o n e s and t h a t b e l o w t h i s b a r r i e r rose E g y p t ' s N i l e or E g y p t ' s h e a v e n - d e s c e n d e d s t r e a m . A f t e r a l l , t h e N i l e p r i e s t s were one of t h e s o u r c e s t h a t provided Herodotus, t h e I o n i a n , w i t h m o s t o f t h e informat i o n t h a t a p p e a r e d i n h i s w r i t t e n a c c o u n t s on Egypt and t h e N i l e . The p h i l o s o p h e r s and s a v a n t s o f a n c i e n t Greece h a d t h e i r v i e w s o n , and o p i n i o n s o f , t h e s o u r c e o f t h e N i l e and o f i t s r h y t h m i c p a t t e r n o f f l o w . T h e s e views comprised t h e r o l e o f t h e E t e s i a n ( n o r t h e r l y ) winds i n t h e bui l d-up o f the N i l e ,
t h e o r i g i n o f t h e r i v e r from Oceanus ( t h e ocean s u r r o u n d i n g t h e
e a r t h ) , and t h e r i s e o f t h e N i l e f r o m t h e p e a k s o f t h e L y b i a n m o u n t a i n s and
i t s s u p p l y f r o m t h e m e l t e d snow t h e r e o n . H e r o d o t u s w r o t e i n h i s a c c o u n t s t h a t , d u r i n g h i s v i s i t t o Egypt i n 457 B . C . ,
one informant s a i d t h a t t h e Nile r o s e
from a p o w e r f u l s p r i n g f e e d i n g a d e e p l a k e s i t u a t e d b e t w e e n t h e h i l l s o f Mophi and C h r o p h i . T h e r e a r e two t r a i n s o f t h o u g h t :
t h e f i r s t is t h a t t h e
s a i d l a k e l i e s b e t w e e n t h e i s l a n d o f E l e p h a n t i n e and Aswan, w h i c h c o n f i n e s t h e w h o l e s t o r y t o t h e N i l e i n E g y p t , and t h e o t h e r i s t h a t t h e l a k e i s s i t u a t e d f a r more t o t h e s o u t h . I n c o m p a r i s o n w i t h r e c e n t d i s c o v e r i e s t h e l a t t e r i s s o m e t i m e s i n t e r p r e t e d as t h e l a k e s u p p l y i n g t h e S e m l i k i R i v e r which d i s c h a r g e s
i t s w a t e r i n t o Lake A l b e r t ( a l s o c a l l e d Mobutu-Sese S e k o ) . I f w e a r e prepared t o accept t h i s i n t e r p r e t a t i o n ,
i t is then f a i r t o conclude t h a t
Herodotus c a n b e complimented f o r t h r o w i n g some l i g h t on t h e w e s t e r n t r i b u t a r y o f t h e N i l e R i v e r . I n a p a p e r e n t i t l e d "The N i l e , i t s O r i g i n and R i s e "
it is
mentioned t h a t H e r o d o t u s b e l i e v e d t h a t t h e Upper N i l e flowed from w e s t t o e a s t , b u t h e c o n f u s e d t h e Niger w i t h t h e N i l e ( B i s w a s , A . , f o r q u i t e some t i m e . J u b a I 1 (20 A . D . ) ,
1966). T h i s i d e a p r e v a i l e d
t h e King o f M a u r i t a n i a , a f f i r m e d t h a t
t h e s o u r c e of t h e N i l e was i n w e s t e r n A f r i c a , t h e r e b y s u p p o r t i n g t h e c o n v i c t i o n t h a t t h e N i g e r i s a b r a n c h of t h e N i l e ( B i s w a s , A , , The Greek p h i l o s o p h e r A r i s t o t l e (384-322 B . C . )
1966, 1970).
thought t h a t t he r i v e r
descended from a mountain of s i l v e r (montagne d ' a r g e n t ) and t h a t heavy s p r i n g and summer r a i n s on t h e h i g h l a n d s of t h e catchment a r e a s (what we c a l l nowadays t h e B l u e and White N i l e s ) were r e s p o n s i b l e f o r t h e flow i n t h e N i l e . Almost two c e n t u r i e s a f t e r H e r o d o t u s , came t h e m a t h e m a t i c i a n and g e o g r a p h e r E r a t o s t h e n e s (276-194 B . C . ) who d e s c r i b e d t h e N i l e f a r b e t t e r t h a n any of h i s p r e d e c e s s o r s . H i s i d e a w a s t h a t two p r i n c i p a l streams s p r a n g o u t o f some l a k e s s i t u a t e d t o t h e e a s t and e n c i r c l e d Meroe, a c o n s i d e r a b l y l a r g e i s l a n d ( s e e map, F i g . 1 . 1 . ) . The e a s t e r n t r i b u t a r y was t h e A s t a b o r a s (now c a l l e d t h e A t b a r a ) and t h e w e s t e r n t h e A s t a s o b a s (now c a l l e d t h e B l u e N i l e ) . The Astapus
(now c a l l e d
t h e White N i l e ) was a d i f f e r e n t r i v e r e n t i r e l y , which r o s e from some l a k e s t o t h e s o u t h and c a r r i e d t h e summer r a i n s t o form t h e d i r e c t stream of t h e N i l e . P t o l e m y , t h e Roman, who l i v e d i n A l e x a n d r i a i n t h e s e c o n d c e n t u r y A . D . , t h o u g h t t h a t t h e main N i l e came from t h e Mountains of t h e Moon, which w e r e p e r m a n e n t l y c o v e r e d w i t h snow and p a s s e d t h r o u g h two l a k e s . I t i s p r o b a b l e t h a t h e meant by t h e Mountains of t h e Moon, t h e Ruwenzori r a n g e and by t h e two l a k e s , V i c t o r i a and A l b e r t Nyanza. The map of t h e N i l e a s d e v e l o p e d by Ptolemy
i s a s shown i n F i g . 1.2. From t h e s e c o n d h a l f o f t h e s e c o n d c e n t u r y o n w a r d s , f o r a t l e a s t t h i r t e e n c e n t u r i e s , t h e r e w e r e h a r d l y any d i s c o v e r i e s , w i t h t h e e x c e p t i o n of some d e s c r i p t i o n s o f t h e lower r e a c h e s o f t h e N i l e by t h e mosl m g e o g r a p h e r s i n E g y p t . Examples a r e Al-Khuwarazmi
i n 864 A . D .
and Al-Masoudy
n 957 A . D .
The t r i u m p h a n t wars of P o r t u g a l a g a i n s t t h e Moors i n n o r t h - w e s t
Africa i n
t h e f i f t e e n t h c e n t u r y h a d , no d o u b t , paved t h e r o a d t o t h e P o r t u g u e s e i n f i l t r a t i o n i n A f r i c a , b o t h e a s t and w e s t . Two P o r t u g u e s e m i s s i o n a r i e s , P e d r o Paez and Jerome Lobo, v i s i t e d E t h i o p i a i n t h e s e v e n t e e n t h c e n t u r y . F a t h e r Paez v i s i t e d Lake Tana ( 1 6 1 8 ) . H i s s u c c e s s o r , F a t h e r Lob0 w r o t e an a c c o u n t a b o u t h i s v i s i t t o T i s s i s a t F a l l s . About 150 y e a r s l a t e r , i n 1770 A . D . ,
a Scottish
e x p l o r e r named James B r u c e , a f t e r h a v i n g j o u r n i e d f i v e y e a r s i n E t h i o p i a , s u c c e e d e d i n d i s c o v e r i n g t h e s o u r c e of t h e l i t t l e Abbai ( s e e map, F i g . 1 . 1 . ) . A summary of B r u c e ' s views on Paez and Cheesman's on t h e e x p e d i t i o n of Bruce
c a n be found i n t h e Book on The B l u e N i l e (Moorehead, A , ,
1962).
3
F i g . 1.1. L o c a t i o n o f t h e o l d i s l a n d o f Mero and t r i b u t a r i e s
The j o u r n e y s made by t h e Arab t r a d e r s on t h e e a s t c o a s t o f A f r i c a t o t h e i n t e r i o r , t h e r e l i g i o u s m i s s i o n s t o E a s t A f r i c a and t h e s t e a d i l y growing conn e c t i o n s between t h e C o p t i c c h u r c h e s o f A l e x a n d r i a and E t h i o p i a have a l l l e d t o more c o n c r e t e i n f o r m a t i o n on t h e snow-capped m o u n t a i n s ( t h e Mountains of t h e Moon) d e s c r i b e d by Ptolemy i n 150 A . D . The h i s t o r y o f e x p l o r i n g t h e N i l e r i v e r s y s t e m i n t h e n i n e t e e n t h c e n t u r y b e g i n s w i t h t h e i n v a s i o n of t h e Sudan by Mohammed A l i Pasha and h i s s o n s from 1821 onwards. A s a r e s u l t of t h i s , t h e B l u e N i l e was e x p l o r e d a s f a r a s i t s
e x i t from t h e E t h i o p i a n f o o t h i l l s , and t h e White N i l e a s f a r a s t h e S o b a t mouth. The Bahr e l Ghazal w a s e x p l o r e d by P e t h e r i c k i n t h e e i g h t e e n f i f t i e s . The c o n s i d e r a b l e i n t e r e s t o f t h e Europeans came s h o r t l y a f t e r t h e r e p o r t s o f Knoblecher (1850) a b o u t t h e e x i s t e n c e of some l a r g e l a k e s i n t h e s o u t h .
4
The M o u n t a i n s of the Moon
)
Second Lake
\
F i r s t Lake
) T h e holy Sprinq
v&&Donkola "
Aswan
Upper Land
"
K?
I
t
" M i d d i e Land
"
Asiut
A l - Ashmunein
A l - Muqattam
Mediterranean
Sea
I
F i g . 1 . 2 . Map of t h e N i l e from t h e s o u r c e a t t h e Mountains of t h e Moon t o t h e mouth i n t h e M e d i t e r r a n e a n S e a , a s p r e p a r e d by Ptolemy
From a b o u t 1857 onwards for a b o u t 30 y e a r s , t h e E q u a t o r i a l Lakes P l a t e a u and s u r r o u n d i n g s w e r e t r a v e r s e d by s e v e r a l e x p l o r e r s ,
a l l s e a r c h i n g for t h e
s o u r c e of t h e r i v e r t h a t h a d p u z z l e d t h e whole w o r l d for s e v e r a l c e n t u r i e s . The p r i n c i p a l e x p l o r e r s and t h e i r e x p e d i t i o n s w e r e as f o l l o w s : i)
B u r t o n and Speke went from Bagamoyo t o Tabora t o U j i j i on Lake Tanganyika (1857-59). I n 1858 Speke a l o n e went on n o r t h w a r d s and d i s c o v e r e d Lake V i c t o r i a .
5
S p e k e and G r a n t s e t o f f on an e x p e d i t i o n (1860-63)
ii)
around Lake V i c t o r i a
and r e a c h e d Ripon F a l l s . They t h o u g h t t h a t t h e stream f l o w i n g o u t of t h i s l a k e was t h e s o u r c e o f t h e N i l e . i i i ) Samuel B a k e r , a f t e r e x p l o r i n g t h e A t b a r a , went on t o d i s c o v e r Lake A l b e r t (1862-64)
a s w e l l a s t h e A l b e r t N i l e and t h e u p p e r r e a c h of
Bahr e l J e b e l . L i v i n g s t o n e , l i k e B u r t o n and S p e k e , s e t o f f on h i s r o u t e from Tabora
iv)
to U j i j i
( 1 8 7 2 ) . H e t h o u g h t t h e Lake Nyasa m i g h t b e d r a i n i n g i n t o Lake
T a n g a n y i k a , which c o u l d t h e n b e l i n k e d w i t h t h e A l b e r t Lake and t h e N i l e . v)
S t a n l e y t r a v e l l e d up from t h e e a s t c o a s t and c i r c u m n a v i g a t e d Lake V i c t o r i a ( 1 8 7 2 ) . H i s a t t e m p t t o g e t t o Lake A l b e r t was n o t s u c c e s s f u l , though h e r e a c h e d as f a r a s t h e e s c a r p m e n t a b o u t Lake George. I n a l a t e r j o u r n e y ( 1 8 8 9 ) h e r e a c h e d t h e S e m l i k i and Lake Edward. The r o u t e s o f t h e s e l e a d i n g e x p l o r e r s a r e i n d i c a t e d o n t h e map, F i g . 1 . 3 .
(Stamp and Morgan, 1 9 7 2 ) . I n s p i t e o f a l l t h e e x p e d i t i o n s a l r e a d y m e n t i o n e d , a number of t r i b u t a r i e s o f t h e N i l e remained u n d i s c o v e r e d . E x p l o r a t i o n m i s s i o n s s t o p p e d i n t h e p e r i o d 1881 up t o 1898 a s a r e s u l t o f t h e r e b e l l i o n s o f t h e Mahdis i n t h e S u d a n . A h i s t o r i c a l a c c o u n t o f t h i s p e r i o d c a n b e found i n t h e book e n t i t l e d "The White Nile"
(Moorehead, A , ,
1 9 6 0 ) . A f t e r t h e r e o p e n i n g of t h e Sudan i n 1898 and a t
a b o u t t h e same t i m e t h e o p e n i n g up o f Kenya and T a n z a n i a ( f o r m e r l y c a l l e d T a n g a n y i k a ) and Uganda, i r r i g a t i o n s e r v i c e s and s u r v e y and g e o l o g i c a l d e p a r t ments were e s t a b l i s h e d i n t h e r e s p e c t i v e c o u n t r i e s . S i n c e t h e n t h e s e d e p a r t ments h a v e t a k e n o v e r t h e e x p l o r a t o r y work a s w e l l a s t h e c o l l e c t i o n of h y d r o l o g i c and o t h e r r e l e v a n t d a t a ( H u r s t , H . ,
1952).
A t a b o u t t h e end o f 1902 t h e I r r i g a t i o n Department o f Egypt s t a r t e d two e x p e d i t i o n s ; t h e one u n d e r M r C . E .
Dupuis t o v i s i t Lake Tana and t h e o t h e r
u n d e r S i r W . G a r s t i n t o v i s i t t h e Lakes V i c t o r i a and t h e t h e n A l b e r t and Edward. The e x p e d i t i o n s which f o l l o w e d w e r e s e n t t o c o l l e c t , or r e p o r t o n , h y d r o l o g i c a l d a t a needed f o r t h e N i l e p r o j e c t s ( H u r s t , H . E . ,
1 9 3 1 ) . Unfortun-
a t e l y , t h e M a c m i l l a n - J e s s e n e x p e d i t i o n s i n 1902 and 1905 f a i l e d i n e x p l o r i n g Lake Tana o r f i n d i n g t h e s o u r c e o f t h e B l u e N i l e . A f t e r t h i s t h e r e was a l o n g p e r i o d o f i n a c t i v i t y on t h e r i v e r u n t i l R . E .
Cheesman a r r i v e d i n 1925 i n
E t h i o p i a . F o r t h e n e x t e i g h t y e a r s Cheesman d e v o t e d h i m s e l f t o t h e e x p l o r a t i o n o f t h e g o r g e o f t h e B l u e N i l e and t o c i r c u m n a v i g a t e Lake Tana. T h i s accomplishment was t e r m i n a t e d by w r i t i n g an a c c o u n t on Lake Tana and t h e B l u e N i l e (Cheesman, R . ,
1936).
More r e c e n t e x p e d i t i o n s h a v e b e e n s e n t , e s p e c i a l l y by t h e E g y p t i a n Governm e n t , t o e x p l o r e more o f t h e t r i b u t a r i e s i n t h e c a t c h m e n t s of t h e E q u a t o r i a l L a k e s , t h e White N i l e and t h e S o b a t . However, i t was n o t u n t i l 1937 t h a t t h e
6
southernmost s o u r c e of t h e N i l e i n t h e headstreams of t h e Kagera, l a r g e s t trib u t a r y o f Lake V i c t o r i a , was l o c a t e d . F u r t h e r e x p l o r a t i o n o f t h e N i l e t r i b u t a -
r i e s i n t h e E t h i o p i a n P l a t e a u h a s b e e n f o r q u i t e some t i m e l e s s f o r t u n a t e t h a n t o o t h e r p a r t s o f t h e N i l e B a s i n . The g o r g e o f t h e B l u e N i l e ( G r e a t A b b a i ) was n o t f u l l y t r a v e r s e d u n t i l a B r i t i s h m i l i t a r y and s c i e n t i f i c e x p e d i t i o n c o n quered i t d u r i n g t h e f l o o d o f 1968 ( B l a s h f o r d - S n e l l , J . N . ,
- - -Burton - - -Speke +++++
N
8, Speke 1857-'59
1858
Speke & Grant 1860-'63
.. ....... -
----
1970).
Baker Stanley
1862-'64 1871-'72, 1874-'77 8, 1887 - '8 9 L i v i n g s t o n e 1872
b Bagamoyo Tabora r o u t e s ~ m ~ l afor r Speke & Grant a n d S t a n l e y
F i g . 1 . 3 . The r o u t e s o f some l e a d i n g e x p l o r e r s o f t h e N i l e S o u r c e ( S t a m p , L . D . and Morgan, W.T., 1 9 7 2 )
1.2 1.2.1
HISTORY O F HYDROLOGY OF THE NILE BASIN
From 3200 B . C . t o 1900 A . D .
W h e t h e r t h e a n c i e n t E g y p t i a n s knew t h e o r i g i n o f t h e i r r i v e r or n o t ,
( s e e 1.1) w h a t c a n n o t b e d e n i e d i s t h a t t h e y were d e e p l y k e e n t o o b s e r v e a l l a s t r o n o m i c a l phenomena and t e r r e s t r i a l e v e n t s a s s o c i a t e d w i t h , o r r e l a t e d t o , the N i l e floods,
7
One of t h e most a n c i e n t r e c o r d s i s a drawing of an i m p e r i a l macehead h e l d by t h e p r o t o d y n a s t i c k i n g S c o r p i o n when c e l e b r a t i n g t h e o c c a s i o n of c u t t i n g an i r r i g a t i o n d i t c h some 3200 y e a r s B . C .
(Biswas, A . ,
1 9 7 0 ) . The same r e f e r -
e n c e m e n t i o n s , among o t h e r s : damming o f f of t h e N i l e and d i v e r t i n g i t s c o u r s e by King Menes i n 3000 B . C .
A t t h e same t i m e t h e a n c i e n t E g y p t i a n s began t o u s e
t h e N i l o m e t e r s t o r e c o r d t h e f l u c t u a t i o n of
t h e const r uct i on of
( P a g a n t s dam) some 30 k i l o m e t r e s s o u t h of C a i r o and i t s f a i l u r e
Sadd e l - K a f a r a i n 2850 B . C . ,
the N i l e ,
t h e c o n n e c t i o n o f t h e N i l e and t h e Red S e a by a n a v i g a t i o n a l
c a n a l d u r i n g t h e r e i g n of S e o s t r i s I i n a b o u t 1950 B . C . ,
t h e h y d r a u l i c works
of Amenemhet I 1 1 ( i n c l u d i n g Lake M o e r i s ) i n a b o u t 1850, t h e w a t e r codes of King Hammurabi i n 1750 B . C . , d e e p ) i n a b o u t 1700, e t c .
J o s e p h ' s w e l l n e a r C a i r o (more t h a n 100 metres
(Biswas, A , ,
1 9 7 0 ) . Another r e f e r e n c e s t a t e s t h a t
" h y d r a u l i c e n g i n e e r i n g " a t t h a t t i m e r e a c h e d a h i g h d e g r e e of accomplishment; r e c l a m a t i o n schemes on t h e l e f t bank of t h e N i l e were i n i t i a t e d d u r i n g t h e f l o o d s ; dams, d i k e s and c a n a l s w e r e c o n s t r u c t e d and l a t e r w a t e r - l i f t i n g machinery was i n v e n t e d ( T e c l a f f and T e c l a f f , 1 9 7 3 ) . Probably t h e f i r s t n o n - r e l i g i o u s
t h e o r y e x p l a i n i n g t h e y e a r l y f l o o d i n g was
t h a t d e r i v e d by t h e Greek T h a l e s (600 B . C . ) ,
t h e f o u n d e r of d e d u c t i v e geo-
m e t r y . H i s t h e o r y was t h a t t h e E t e s i a n winds d r o v e t h e s e a h i g h a g a i n s t t h e mouths of t h e r i v e r and t h e r e u p o n p r e v e n t e d them from d i s c h a r g i n g t h e i r w a t e r . The r i v e r t h e r e f o r e r e t u r n e d upon i t s e l f and, whenever i t c o u l d , i t b u r s t o u t i n t o f o r b i d d e n g r o u n d . A c o u p l e of c e n t u r i e s l a t e r a n o t h e r Greek g e o m e t e r , D e m o c r i t u s , came up w i t h a somewhat d i f f e r e n t t h e o r y . H e t h o u g h t t h a t when snow i n t h e n o r t h e r n p a r t s o f t h e w o r l d was m e l t e d a t t h e t i m e of t h e s u m m e r s o l s t i c e and flowed away, c l o u d s w e r e formed by t h e v a p o u r . When t h e c l o u d s
w e r e d r i v e n towards t h e s o u t h and towards Egypt by t h e E t e s i a n w i n d s , v i o l e n t s t o r m s a r o s e and c a u s e d t h e l a k e s f e e d i n g t h e R i v e r N i l e t o be f i l l e d (Frisinger, H . ,
1959).
S i n c e t h e N i l e u s e d t o r i s e i n f l o o d a t a b o u t t h e same t i m e e v e r y y e a r , i t s b e h a v i o u r had b e e n d e s c r i b e d a s r e g u l a r . The B i b l e t e l l s u s , however,
"
....
t h e r e came s e v e n y e a r s o f g r e a t p l e n t y t h r o u g h o u t t h e l a n d of E g y p t . And t h e r e s h a l l r i s e a f t e r them s e v e n y e a r s o f f a m i n e
.."
( G e n e s i s 4 1 , 29-30).
The i n t e r -
p r e t a t i o n by J o s e p h t o t h i s dream o f t h e P h a r a o of Egypt was p r o b a b l y t h e f i r s t indication of the persistence i n the hydrologic t i m e s e r i e s . The Roman s a v a n t , P l i n y (23-79 A . D . ) ,
t h o u g h t t h e two p r o b a b l e t h e o r i e s
a b o u t t h e f l o o d i n g of t h e N i l e w e r e t h o s e o f T h a l e s and Democritus.
After a l l ,
i n b o t h t h e o r i e s t h e a c t i o n of t h e E t e s i a n winds was t h e c u l p r i t . A f t e r P l i n y , t h e E n g l i s h h i s t o r i a n and t h e o l o g i a n , Beda (674-735 A . D . ) ,
compiled and sum-
m a r i z e d t h e knowledge t h e n a v a i l a b l e a b o u t t h e N i l e f l o o d . The t h e o r y h e p r o p o s e d was v e r y s i m i l a r t o t h a t of T h a l e s . H e c l a i m e d t h a t t h e n o r t h e r l y
8
winds f o r c e d t h e s e a waves t o p i l e up s a n d a t t h e N i l e mouths, t h u s c a u s i n g t h e r i v e r t o back up upon i t s e l f and f l o o d ( F r i s i n g e r , H . ,
1 9 5 9 ) . From t h a t
t i m e onwards u n t i l t h e n i n e t e e n t h c e n t u r y t h e r e was v e r y l i t t l e done t o e x p l o r e t h e s o u r c e s o f t h e N i l e and i t s t r i b u t a r i e s , e x c e p t f o r t h e r e c o r d i n g s which
w e r e done whenever p o s s i b l e . The n i n e t e e n t h c e n t u r y was t h e c e n t u r y of d i s c o v e r i n g t h e N i l e s o u r c e and t r i b u t a r i e s r a t h e r t h a n c o l l e c t i n g a n d / o r i n t e r preting its hydrologic data. The r e c o r d of t h e N i l e l e v e l s d a t e s back t o a b o u t 3000 t o 3500 y e a r s B . C . The r i v e r gauge is. c a l l e d N i l o m e t e r ( i n A r a b i c Miqyas An-Nil).
Three t y p e s of
N i l o m e t e r s were u s e d . The f i r s t t y p e c o n s i s t e d s i m p l y o f marking t h e w a t e r l e v e l s on c l i f f s on t h e banks of t h e r i v e r , e . g . t h e s e c o n d c a t a r a c t a t Semna. The s e c o n d t y p e c o n s i s t e d e s s e n t i a l l y of a s c a l e , u s u a l l y o f m a r b l e , on which t h e w a t e r l e v e l was o b s e r v e d . The s t a n d a r d gauge c o n s i s t e d o f a s e r i e s o f s t e p s
or p i l l a r s b u i l t i n t o t h e r i v e r bank t o e a c h of which a s e c t i o n of t h e s c a l e was f i x e d . I t i s c l a i m e d t h a t t h e r e a r e 140 o f t h e s e gauges s c a t t e r e d o v e r t h e b a s i n o u t s i d e E g y p t , and many more on t h e N i l e i n E g y p t . Most o f them a r e o b s e r v e d d a i l y , and t h e r e a d i n g s o f t h e more i m p o r t a n t a r e t e l e g r a p h e d or t e l e phoned t o C a i r o ( H u r s t , H . E . ,
1 9 5 2 ) . The c a t a s t r o p h i c f l o o d s o f 1954 and 1958
were measured a t 9 3 g a u g i n g p o i n t s i n Egypt o n l y , 36 gauges i n upper Egypt and t h e r e m a i n d e r i n l o w e r Egypt (Hashem and E l - S h e r b i n i ,
1 9 6 1 ) . The t h i r d and most
a c c u r a t e N i l o m e t e r used t o b r i n g w a t e r o f t h e N i l e t o a w e l l and t h e w a t e r l e v e l was marked e i t h e r on t h e w a l l s o f t h e w e l l or on a c e n t r a l p i l l a r . The most n o t a b l e N i l o m e t e r i s s i t u a t e d a t Roda n e a r C a i r o . The r e c o r d e d w a t e r l e v e l t h e r e d a t e s back t o 6 4 1 A . D . t h e r e i n 715 A . D .
The Arab c h a l i p h s (Kings) b u i l t a new N i l o m e t e r
T h i s was r e b u i l t i n 8 6 1 A . D .
I t c o n s i s t s of a squar e w e l l
c o n n e c t e d t o t h e N i l e by means o f t h r e e c o n d u i t s . A t t h e c e n t r e o f t h e w e l l i s a g r a d e d o c t a g o n a l p i l l a r of w h i t e m a r b l e d i v i d e d i n t o 19 c u b i t s (see F i g . 1 . 4 ) . I t was r e p o r t e d t h a t a s a r e s u l t o f p o o r j o i n i n g o f t h e lower b r o k e n p a r t o f t h e p i l l a r , t h e c o r r e s p o n d i n g c u b i t now measures 31 c m o n l y i n s t e a d o f t h e o r i g i n a l 54 c m ( G h a l e b , K., 1 9 3 5 ) . T h i s example and many o t h e r s shows t h a t t h e records a v a i l a b l e s i n c e 641 A . D .
need much a d j u s t m e n t b e f o r e h a v i n g them
a n a l y z e d . The s u i t a b i l i t y o f t h e r e c o r d f o r s t a t i s t i c a l a n a l y s i s h a s b e e n a r g u e d r e c e n t l y : "Simply,
t h e r e l i a b i l i t y of f l o o d d a t a f o r t h e assessment of
w a t e r a v e r a g e s , t h e r e l i a b i l i t y o f p r e s e r v a t i o n o f a l o n g - r a n g e c o n s t a n t gauge datum i n t h e p a s t , and a c c u r a c i e s i n o b s e r v a t i o n s ( a l l k i n d s o f c h a n g e s ) , unf o r t u n a t e l y do n o t p e r m i t one t o draw d e p e n d a b l e c o n c l u s i o n s " .
(Yevjecvich, V . ,
1983). 1.2.2
From 1900 A . D .
t i l l now
The t u r n of t h e l a s t c e n t u r y and t h e b e g i n n i n g o f t h e t w e n t i e t h c e n t u r y w i t n e s s e d a number of n o t a b l e a c c o m p l i s h m e n t s . When t h e t h e n Anglo-Egyptian
9
F i g . 1 . 4 . The N i l e gauge a t Roda, C a i r o , Egypt
10
S u d a n w a s r e o c c u p i e d i n 1 8 9 8 a l l swamp r i v e r s were f o u n d b l o c k e d u p . I t w a s n o t b e f o r e 1 9 0 5 t h a t a c h a n n e l t h r o u g h t h e B a h r e l J e b e l h a d b e e n made c l e a r . The G h a z a l was n o t made n a v i g a b l e t o t h e p r i n c i p a l c a p i t a l Wau, u n t i l 1 9 0 4 . I n 1 9 0 2 t h e w o r k s i n t h e f i r s t Aswan dam and some o f t h e B a r r a g e s o n t h e N i l e i n E g y p t were c o m p l e t e d . The h i s t o r y o f s c i e n t i f i c s t u d y o f t h e h y d r o l o g y
o f t h e N i l e b e g i n s w i t h t h e i n t r o d u c t i o n o f c u r r e n t metres by S i r H . Lyons i n about 1902. Previous t o t h i s ,
f l o w m e a s u r e m e n t s h a d b e e n made by f l o a t s . The
S u r v e y D e p a r t m e n t o f E g y p t became c h a r g e d w i t h t h e s u r v e y o f a l l r i v e r g a u g e r e c o r d s s o u t h o f Aswan i n 1 9 0 2 and 1 9 0 3 . T h i s w a s t h e o r i g i n o f t h e H y d r o l o g y S e r v i c e w h i c h formed p a r t o f t h e l a t e r P h y s i c a l D e p a r t m e n t o f E g y p t ( H u r s t and P h i l i p s , 1931) . The r e s u l t s o f t h e e x p e d i t i o n s t o L a k e s T a n a , V i c t o r i a , A l b e r t and Edward c o n d u c t e d by D u p u i s and G a r s t i n i n 1901-1904
( s e e 1 . 1 ) were c o n c l u d e d i n t h e
1904 r e p o r t by S i r W. G a r s t i n . S h o r t l y a f t e r t h a t , i n 1 9 0 6 , S i r H . Lyons publ i s h e d h i s book "The P h y s i o g r a p h y o f t h e N i l e " .
T h i s book c o n t a i n e d t h e i n f o r -
m a t i o n g a t h e r e d f r o m t r a v e l l e r s and s c i e n t i f i c e x p l o r e r s a v a i l a b l e a t t h a t
time.
I n 1905 S i r M .
MacDonald i n t r o d u c e d a new method o f r i v e r m e a s u r e m e n t a t
Aswan u s i n g t h e f l o w t h r o u g h t h e s l u i c e s o f t h e dam. A l a r g e masonary t a n k w a s used t o measure t h e d i s c h a r g e of one type o f s l u i c e under a l l c o n d i t i o n s of h e a d and s l u i c e o p e n i n g . The r e s u l t s were t h e n a p p l i e d t o f l o w from o t h e r sluices. The S u d a n b r a n c h o f t h e E g y p t i a n I r r i g a t i o n S e r v i c e w a s formed i n 1 9 0 5 w i t h t h e O b j e c t o f p e r fo r m in g a l l h y d r o l o g i c and h y d r o g r a p h i c works f o r t h e d i f f e r e n t p r o j e c t s a i m i n g a t t h e i m p r o v e m e n t o f t h e w a t e r s u p p l y o f E g y p t and t h e d e v e l o p m e n t o f p e r e n n i a l i r r i g a t i o n i n t h e S u d a n . The d a t a c o l l e c t e d i n t h e p e r i o d 1906 t o 1 9 1 3 w e r e u s e f u l i n t h e d e s i g n o f t h e J e b e l A u l i a dam on t h e White N i l e ,
a b o u t 4 5 km a b o v e t h e j u n c t i o n o f t h e W h i t e a n d t h e B l u e N i l e s ,
and Makwar dam o n t h e B l u e N i l e , some 360 km a b o v e t h e same j u n c t i o n . The f o u n d a t i o n s o f t h e p r e s e n t M e t e o r o l o g i c a l S e r v i c e o f E g y p t w e r e l a i d i n 1 9 0 0 . S i n c e t h e n many s t a t i o n s w e r e e s t a b l i s h e d and more o b s e r v a t i o n s were t a k e n . I n 1 9 1 5 a l l t h e work o f a p h y s i c a l n a t u r e d o n e by t h e S u r v e y D e p a r t m e n t , and t h e h y d r o l o g i c a l work d o n e by t h e I r r i g a t i o n D e p a r t m e n t , w e r e combined t o form t h e P h y s i c a l D e p a r t m e n t o f t h e M i n i s t r y o f P u b l i c W o r k s , E g y p t . The work o n t h e N i l e p r o j e c t s s t o p p e d d u r i n g t h e F i r s t World War (1914-1918), t h o u g h r o u t i n e o b s e r v a t i o n s w e r e c a r r i e d o n . From 1 9 1 2 up t o 1 9 2 3 , e s p e c i a l l y i n t h e post-war
p e r i o d , most o f t h e p r o g r e s s w a s d i r e c t e d a t e s t a b l i s h i n g p e r -
manent d i s c h a r g e s i t e s a t a number o f i m p o r t a n t s t a t i o n s w h e r e r e g u l a r o b s e r v a t i o n s h a d b e e n t a k e n . Advances i n m e a s u r i n g d e v i c e s and t e c h n i q u e s f o l l o w e d . D a t a c o l l e c t i o n and a n a l y s i s w e n t o n and t h a t w a s a g r e a t h e l p i n t h e d e s i g n a n d c o n s t r u c t i o n o f t h e Makwar dam, w h i c h w a s f i n a l l y b u i l t i n 1 9 2 5 on t h e
11
Blue N i l e f o r t h e b e n e f i t of t h e Sudan. I n 1923 t h e M i n i s t r y o f P u b l i c Works, E g y p t , s e n t a m i s s i o n t o t h e E q u a t o r i a l L a k e s w i t h t h e aim o f p r e p a r i n g a programme f o r i n v e s t i g a t i o n s i n c o n n e c t i o n w i t h t h e p o s s i b l e Upper N i l e p r o j e c t s . T h a t m i s s i o n was f o l l o w e d by o t h e r s i n 1 9 2 4 , 1 9 2 6 , 1930 and 1931 t o i n v e s t i g a t e t h e h y d r o l o g y o f t h e Lake P l a t e a u , Bahr e l Ghazal and t h e White N i l e b a s i n s . The E a s t A f r i c a n M e t e o r o l o g i c a l S e r v i c e , l a t e r D e p a r t m e n t , EAMD, was formed i n 1 9 2 7 . T h i s d e p a r t m e n t w a s p a r t l y f i n a n c e d by t h e E g y p t i a n Government.
It
o p e r a t e s o v e r a l l o f E a s t C e n t r a l A f r i c a and t h e d a t a i t c o l l e c t s are u n d o u b t e d l y v a l u a b l e f o r t h e h y d r o l o g y of t h e N i l e B a s i n . The o r i g i n a l N i l e w a t e r s a g r e e m e n t w a s l a i d down i n 1929 and h a d , f o r some
t i m e , b e e n t h e b a s i s o f t h e w a t e r a l l o c a t i o n b e t w e e n E g y p t and t h e S u d a n . The most i m p o r t a n t i t e m i n t h e a g r e e m e n t was
". . .
no works s h o u l d b e c o n s t r u c t e d
o r m e a s u r e s t a k e n , on t h e N i l e or i t s b r a n c h e s or on t h e l a k e s from which i t flows,
i n t h e S u d a n or i n t h e t e r r i t o r i e s u n d e r B r i t i s h a d m i n i s t r a t i o n , which
w o u l d a f f e c t t h e f l o w o f t h e r i v e r i n s u c h a way as t o c a u s e p r e j u d i c e t o t h e i n t e r e s t s of Egypt". To e n s u r e t h e c o n t i n u i t y o f d i s s e m i n a t i o n o f t h e k n o w l e d g e o n t h e h y d r o l o g y o f t h e N i l e B a s i n and t o p r e s e n t t h e e v e r - i n c r e a s i n g d a t a i n a s y s t e m a t i c way, f o r b o t h s c i e n t i f i c and p r a c t i c a l p u r p o s e s ,
i t w a s d e c i d e d t o i s s u e t h e volumes
a n d s u p p l e m e n t s o f "The N i l e B a s i n " s u c c e s s i v e l y . The b a s i c d a t a a b o u t t h e s e r e f e r e n c e s are as f o l l o w s :
Volume No.
Subject matter
Author ( s)
Year of pub1 i c a t i o n
I
General d e s c r i p t i o n of t h e b a s i n ; meteorology, topography of t h e White N i l e
H . E . H u r s t and P. P h i l i p s
11
D i s c h a r g e and s t a g e m e a s u r e m e n t s o f t h e N i l e and i t s t r i b u t a r i e s (with 9 supplements: 1928-32, 33-37, 38-42, 43-47, 48-52, 53-57, 58-62, 6 3 - 6 7 and 1968-72)
H.E. Hurst, P. P h i l i p s , Y .M. Simaika, R . B l a c k and N i l e Control S t a f f
from 1932 onward
Ten-day mean and m o n t h l y mean g a u g e r e a d i n g s o f t h e N i l e and its t r i b u t a r i e s (with 9 supplements: 1928-32, 33-37, 38-42, 43-47, 48-52, 53-57, 58-62, 6 3 - 6 7 and 1968-72)
H.E. Hurst, P. P h i l i p s , Y . M . Simaika, R . P . B l a c k and N i l e Control S t a f f
from 1932 onward
I11
1931
12
Subject matter
Volume No.
Year of publication
Author ( s )
IV
Ten-day mean and monthly mean d i s c h a r g e s o f t h e N i l e and i t s t r i b u t a r i e s (with 9 supplements: 1928-32, 33-37, 38-42, 43-47, 48-52, 53-57, 58-62, 63-67, 1968-72)
H . E . Hurst, P. P h i l i p s , Y . M . Simaika, R . P . B l a c k and N i l e Control Staff
V
The h y d r o l o g y of t h e Lake P l a t e a u and Bahr e l J e b e l
H . E . H u r s t and P. Philips
VI
Monthly and a n n u a l r a i n f a l l t o t a l s and number o f r a i n y days a t s t a t i o n s i n and n e a r t h e N i l e B a s i n f o r p e r i o d s : 1938-42, 43-47, 48-52, 53-57, 58-62, 63-67 and 1968-72 ( 7 s u p p l e ments)
H.E. R.P. Y.M. Nile
VI I
The f u t u r e c o n s e r v a t i o n of t h e Nile
H.E. Hurst, R.P. Black and Y . M . Simaika
1946
VIII
The h y d r o l o g y of t h e S o b a t and White N i l e and t h e t o p o g r a p h y of t h e B l u e N i l e and A t b a r a
H.E.
Hurst
1950
IX
The h y d r o l o g y o f t h e B l u e N i l e and A t b a r a and t h e Main N i l e t o Aswan w i t h some r e f e r e n c e t o projects
H.E. Hurst, R.P.. Black and Y . M . Simaika
1959
X
The m a j o r N i l e p r o j e c t s
H.E. Hurst, R.P. B l a c k and Y . M . Simaika
1966
from 1933 onward
1938
Hurst; Black, S i m a i k a and Control S t a f f .
from 1950 onward
The d a t a and i n f o r m a t i o n c o n t a i n e d i n t h e a b o v e - l i s t e d volumes o f t h e N i l e Basin, t o g e t h e r with those appearing i n t h e o t h e r papers of t h e Physical D e p a r t m e n t , l a t e r t h e N i l e C o n t r o l D e p a r t m e n t , have b e e n employed i n t h e d e s i g n and c o n s t r u c t i o n o f t h e major h y d r a u l i c works on t h e N i l e and i t s b r a n c h e s and t r i b u t a r i e s . Examples o f t h e s e a r e : t h e h e i g h t e n i n g of t h e o r i g i n a l Aswan dam i n 1912 and i n 1 9 3 7 , t h e J e b e l A u l i a dam on t h e White N i l e i n 1934, t h e Owen F a l l s dam a t t h e e x i t o f Lake V i c t o r i a i n 1 9 5 0 ,
...
etc.
The i d e a o f c o n s t r u c t i n g a h i g h dam a t Aswan l e d t o a n o t h e r agreement between Egypt and t h e Sudan i n 1959 f o r t h e f u l l u t i l i z a t i o n of t h e N i l e w a t e r s . I t i s w o r t h w h i l e m e n t i o n i n g h e r e t h a t t h e d e s i g n o f t h i s dam was b a s e d on t h e t h e o r y o f o v e r - y e a r s t o r a g e . The e a r l i e s t t h o u g h t o f t h i s t h e o r y goes back t o b e f o r e 1946 ( H u r s t e t a l , 1 9 4 6 ) . The development o f t h e t h e o r y marked t h e b i r t h o f modern h y d r o l o g y , e s p e c i a l l y t h e s t o c h a s t i c p a r t of i t ( M a n d e l b r o t and W a l l i s , 1 9 6 8 ) . The y e a r 1959 a l s o w i t n e s s e d t h e f i r s t a t t e m p t t o p l a n f o r t h e u l t i m a t e h y d r a u l i c development of t h e N i l e V a l l e y u s i n g an e l e c t r o n i c d i g i t a l computer
13
( M o r r i c e and A l l a n , 1 9 5 9 ) . Although t h e N i l e h a s been t h e b e s t - s t u d i e d
river i n
t h e w o r l d f o r a g e n e r a t i o n , t h e need f o r many more i n v e s t i g a t i o n s and r e s e a r c h work i s t h e r e . The c o m p l e t i o n of t h e f i r s t p h a s e of t h e R o s e i r e s dam on t h e B l u e N i l e , t h e Khashm e l - G i r b a dam on t h e A t b a r a and t h e High dam on t h e Main N i l e a t Aswan a r e h y d r o l o g i c h i g h l i g h t s i n t h e n i n e t e e n hundred and s i x t i e s . A
very i m p o r t a n t s t e p which began i n 1 9 6 7 , and h a s c o n t i n u e d f o r q u i t e some y e a r s , i s t h e c o l l a b o r a t i o n between Kenya, T a n z a n i a , Uganda, t h e Sudan and Egypt i n a h y d r o m e t e o r o l o g i c a l s u r v e y of t h e c a t c h m e n t s of Lakes V i c t o r i a , Kyoga and A l b e r t . T h i s p r o j e c t i n c l u d e d t h e u p g r a d i n g of some of t h e e x i s t i n g h y d r o m e t r i c a l s t a t i o n s and t h e e s t a b l i s h m e n t of new h y d r o m e t r i c a l s t a t i o n s and r i v e r d i s c h a r g e measurement s i t e s . A l l t h e s e s t a t i o n s have been equipped w i t h modern i n s t r u m e n t s . The r a i n f a l l - r u n o f f
r e l a t i o n s h i p s were s t u d i e d i n a
number of e x p e r i m e n t a l b a s i n s and t h e r e l e v a n t p a r a m e t e r s e s t i m a t e d (WMO, 1974). The e a r l y s i x t i e s and t h e l a t e s e v e n t i e s of t h i s c e n t u r y w i t n e s s e d an unu s u a l r i s e i n t h e s u r f a c e w a t e r l e v e l s o f t h e E q u a t o r i a l Lakes and of o t h e r A f r i c a n l a k e s a s w e l l . The l e v e l o f Lake V i c t o r i a r o s e by o v e r 2 . 5 m e t r e s between 1959 and 1964 ( K i t e , G . ,
1 9 8 1 ) . For t h e same p e r i o d , t h e r i s e r e a c h e d
3 . 3 metres f o r Lake A l b e r t , 2 . 6 f o r Lake Tanganyika and 1 . 5 m e t r e s f o r Lake Malawi. The s e c o n d s u b s t a n t i a l r i s e began i n 1978 and by mid 1979 r e a c h e d about 1 . 8 metres f o r Lake V i c t o r i a , 3 . 0 f o r Lake Malawi and 1 . 0 metre f o r Lake
T a n g a n y i k a . The c o n s i d e r a b l e r i s e i n t h e Lake V i c t o r i a w a t e r l e v e l i n 1964 l e d t o an e x c e s s i v e flow i n t h e N i l e t o s u c h an e x t e n t t h a t i t f l o o d e d some p a r t s of C a i r o a t t h a t t i m e . Two p r i n c i p a l p r o j e c t s have been t a k i n g p l a c e d u r i n g t h e l a s t few y e a r s and a r e p r o b a b l y w o r t h r e c o r d i n g h e r e . One i s t h e f i r s t p h a s e of t h e d i v e r s i o n scheme ( c a l l e d J o n g l e i c a n a l ) . The c a n a l c o n n e c t s t h e B a h r e l J e b e l a t Bor s t r a i g h t t o a b o u t Malakal on t h e White N i l e and conveys 20 m i l l i o n m 3 p e r day a t maximum. The a n n u a l volume o f w a t e r s a v e d by t h i s scheme i s 3 . 8 m i l l i a r d m 3 e s t i m a t e d a t Aswan. Half o f t h i s amount w i l l b e t a k e n by t h e Sudan and t h e o t h e r h a l f by Egypt ( E x e c u t i v e Organ f o r t h e Development P r o j e c t s i n J o n g l e i Area, 1 9 7 5 ) . The s e c o n d e v e n t , s i n c e 1 9 7 8 , is t h e j o i n t work o f t h e M i n i s t r y of I r r i g a t i o n , E g y p t , r e p r e s e n t e d by i t s o r g a n s ( m a i n l y t h e Master Water P l a n and t h e R e s e a r c h I n s t i t u t e f o r Water R e s o u r c e s Development), t h e U n i v e r s i t y o f C a i r o , E g y p t , and t h e M a s s a c h u s e t t e s I n s t i t u t e of Technology, U.S.A. i n t h e a n a l y s i s of t h e h y d r o l o g i c d a t a of t h e N i l e B a s i n . The r e s u l t s s o f a r o b t a i n e d a r e a v a i l a b l e i n a s e r i e s of t e c h n i c a l r e p o r t s . A d d i t i o n a l l y , once e v e r y two y e a r s , t h e y o r g a n i z e a s o r t of c o n f e r e n c e where problems r e l a t e d t o w a t e r r e s o u r c e s p l a n n i n g , management and development a r e d i s c u s s e d , t o g e t h e r w i t h the possible solutions.
14
L a s t b u t n o t l e a s t , b o t h Egypt and t h e Sudan a r e working j o i n t l y t o e s t a b l i s h a commission o f a l l c o u n t r i e s s h a r i n g t h e N i l e w a t e r s . The r o a d t o r e a l i z i n g t h i s s t e p i s , no d o u b t , rough and f u l l of d i f f i c u l t i e s . N e v e r t h e l e s s , such a s t e p i s , i n t h e a u t h o r ' s o p i n i o n , unavoidable i f t h e s e c o u n t r i e s a r e keen on h a v i n g a more e f f i c i e n t u t i l i z a t i o n of t h e w a t e r r e s o u r c e s i n t h e N i l e Basin. REFERENCES Bixwas, A . K . , 1966. The N i l e , i t s o r i g i n and r i s e . Water and Sewage Works, 1 1 3 : 283-292 B i s w a s , A . K . , 1970. H i s t o r y of h y d r o l o g y . North-Holland, Amsterdam, 336 p p . B l a s h f o r d - S n e l l , J . N . , 1970. Conquest of t h e Blue N i l e . Geogr. J o u r n . 1 3 6 : 42-51. Cheesman, R . E . , 1 9 3 6 . Lake Tana and t h e B l u e N i l e . Macmillan, London, 400 p p . E n c y c l o p a e d i a B r i t a n n i c a , 1 9 6 9 . N i l e , Vol. 1 6 : 516-523. E x e c u t i v e Organ f o r t h e Development P r o j e c t s i n J o n g l e i A r e a , 1975. J o n g l e i P r o j e c t ( P h a s e O n e ) . Tamaddon P . P r e s s , Khartoum, 99 p p . F r i s i n g e r , H . H . , 1959. E a r l y t h e o r i e s on t h e N i l e f l o o d s . Weather, V o l . 2 0 : 206-207. G h a l e b , K . O . , 1935. D i s c u s s i o n o f : F l o o d - s t a g e r e c o r d s of t h e R i v e r N i l e , by C . S . J a r v i s . T r a n s . ASCE, P a p e r No. 1944: 1063-1067 ( d i s c u s s i o n : 1063-1067). Hashem, A . and E l - S h e r b i n i , H . , 1961. The h y d r o l o g i c f e a t u r e s of t h e 1954 and 1958 f l o o d s ( i n A r a b i c ) . The Government P r i n t e r , C a i r o , 98 p p . H u r s t , H . E . and P h i l i p s , P . , 1931. The N i l e B a s i n , Vol. I , G e n e r a l d e s c r i p t i o n o f t h e b a s i n , m e t e o r o l o g y and topography of t h e White N i l e B a s i n . P h y s i c a l Department P a p e r 2 6 , Government P r e s s , C a i r o , 128 p p . H u r s t , H . E . , B l a c k , R . P . and S i m a i k a , Y . M . , 1 9 4 6 . The N i l e B a s i n , Vol. V I I , The f u t u r e c o n s e r v a t i o n o f t h e N i l e , P h y s i c a l Department P a p e r 5 1 , E a s t e r n P r e s s , C a i r o , 159 p p . H u r s t , H . E . , 1 9 5 2 . The N i l e , a g e n e r a l a c c o u n t of t h e r i v e r and t h e u t i l i z a t i o n o f i t s w a t e r s . C o n s t a b l e , London, 326 p p . K i t e , G . W . , 1981. Recent c h a n g e s i n t h e l e v e l of Lake V i c t o r i a . B u l l e t i n o f H y d r o l o g i c a l S c i e n c e s , No. 2 6 , 3: 233-243. M a n d e l b r o t , B . B . and W a l l i s , J . R . , 1968. Noah, J o s e p h , and O p e r a t i o n a l Hydrology. Water R e s o u r c e s R e s e a r c h , V o l . 4 , N o . 5 : 909-918. Moorehead, A , , 1 9 6 0 . The White N i l e . Hamish H a m i l t o n , London. 385 pp. M o o r e h e a d , . A . , 1 9 6 2 . The B l u e N i l e . Hamish H a m i l t o n , London. 308 p p . M o r r i c e , A . W . and A l l a n , W . M . , 1 9 5 9 . P l a n n i n g f o r t h e u l t i m a t e development o f t h e N i l e V a l l e y . P r o c . I n s t . C i v i l Eng. 1 4 , P a p e r 6372: 101-155. P i e r r e , B . , 1974. Le Roman du N i l . L i b r a r i e P l o n , P a r i s , 480 p p . Stamp, D . L . and Morgan, W . T . , 1972. A f r i c a : A s t u d y i n t r o p i c a l d e v e l o p m e n t . J o h n Wiley and S o n s , I n c . , N e w York, 520 p p . T e c l a f f , L . A . and T e c l a f f , E . , 1973. A h i s t o r y of w a t e r development and w a t e r q u a l i t y . I n : Environment Q u a l i t y and Water Development ( E d i t o r s : Goldman, C . R . , McEvoy 111, J . and R i c h e r s o n , P . M . ) . W . H . Freeman and Company, San F r a n c i s c o : 26-77. World M e t e o r o l o g i c a l O r g a n i z a t i o n , 1974. H y d r o m e t e o r o l o g i c a l s u r v e y of t h e c a t c h m e n t s o f Lakes V i c t o r i a , Kyoga and A l b e r t , RAF 66-025, Tech. R e p o r t 1, Vols I , 1 1 , I 1 1 and IV. Y e v j e v i c h , V . , 1 9 8 3 . The N i l e R i v e r B a s i n : h a r d c o r e and s o f t c o r e w a t e r p r o j e c t s . Water I n t e r n a t i o n a l V o l . 8 , N o . 1: 23-34
15
Chapter 2
PHYSIOGRAPHY O F THE NILE BASIN
2.1
INTRODUCTION
The N i l e B a s i n c o v e r s a s u r f a c e o f a b o u t 2 . 9 m i l l i o n s q u a r e k i l o m e t r e s , approximately one-tenth
to
o f t h e s u r f a c e a r e a o f A f r i c a . I t e x t e n d s from 4’s
31°N l a t i t u d e and from a b o u t 21°
30’E t o 40’
30’E l o n g i t u d e . The h y d r o g r a p h i c
b o u n d a r i e s o f t h e N i l e s y s t e m a r e as shown on t h e map, F i g . 2 . 1 . The h i g h e s t and t h e l o w e s t p o i n t s i n t h e b a s i n a r e r e p r e s e n t e d by t h e t o p o f t h e Ruwenzori Range and t h e t r o u g h o f E l - Q u a t t a r a d e p r e s s i o n r e s p e c t i v e l y .
They are a t e l e v a -
t i o n s o f a b o u t 5 1 2 0 metres a b o v e mean sea l e v e l ( a . m . s . 1 . )
and a b o u t 160 metres
b e l o w mean s e a l e v e l ( b . m . s . l . ) ,
respectively.
T h e l e n g t h o f t h e R i v e r N i l e f r o m i t s most r e m o t e s o u r c e , a t t h e h e a d o f R i v e r L u v i r o n z a , n e a r L a k e T a n g a n y i k a , t o i t s mouth on t h e M e d i t e r r a n e a n S e a , i s a b o u t 6 500 k i l o m e t r e s . The r i v e r c o u r s e and i t s t r i b u t a r i e s t r a v e r s e t h e t e r r i t o r i e s o f T a n z a n i a , Uganda, Rwanda, B u r u n d i , The Congo ( Z a i r e ) , Kenya, E t h i o p i a , t h e S u d a n and t h e Arab r e p u b l i c o f E g y p t . T h i s s t a t e o f a f f a i r s h a s made an i n t e r n a t i o n a l r i v e r o f t h e N i l e , whose w a t e r i s s h a r e d by a number of c o u n t r i e s . A l t h o u g h t h e N i l e i s an a n c i e n t r i v e r ,
t h e e x i s t i n g h y d r o l o g i c p a t t e r n may b e
a s y o u n g as 10 000 y e a r s . I f w e e x c l u d e t h e d r a s t i c c h a n g e s i n t h e b a s i c n a t u r e o f t h e r i v e r , t h e u n i n t e r r u p t e d l i f e o f t h e modern N i l e c o n f i g u r a t i o n h a s b e e n a s h o r t o n e . A d d i t i o n a l l y , a wide v a r i e t y o f topographic f e a t u r e s
- climate,
l o g y , s o i l , p l a n t and v e g e t a l c o v e r and o t h e r h y d r o l o g y - a f f e c t i n g
factors
geo-
-
c a n b e f o u n d i n t h e N i l e B a s i n . The i n t e g r a t e d e f i e c t o f s u c h c a u s a t i v e f a c t o r s on t h e r u n - o f f discharge,
f r o m a r i v e r b a s i n c a n b e r e p r e s e n t e d by t h e s o - c a l l e d s p e c i f i c
i.S p e c i f i c
d i s c h a r g e v a l u e s o f some o f t h e w o r l d r i v e r s a r e l i s t e d
i n Table 2 . 1 . These values have been c a l c u l a t e d u s i n g t h e r e l a t i o n s h i p where
a
= &A,
i s t h e l o n g - t e r m mean d i s c h a r g e and A = s u r f a c e area o f t h e r i v e r c a t c h -
ment. I t i s v e r y c l e a r from T a b l e 2 . 1 t h a t o f a l l w o r l d r i v e r s w i t h d r a i n a g e b a s i n
a r e a s , e a c h l a r g e r t h a n 1 m i l l i o n km’, discharge.
I f the estimated
t h e River N i l e has t h e lowest s p e c i f i c
o f t h e Congo B a s i n , w h i c h i s g e o g r a p h i c a l l y t h e
c l o s e s t to t h e N i l e Basin, is f a i r l y c o r r e c t , t h e s p e c i f i c discharge of the
l a t t e r would t h e n b e j u s t o n e t e n t h t h e s p e c i f i c d i s c h a r g e o f t h e Congo. The two c a u s a t i v e f a c t o r s w h i c h p r o b a b l y h a v e t h e b i g g e s t e f f e c t o n t h e r u n o f f f r o m a d r a i n a g e b a s i n a r e t h e c l i m a t e and t h e t o p o g r a p h y . The f o r m e r w i l l b e d i s c u s s e d i n Chapter 3 , w h i l e t h e topography of t h e N i l e Basin i s d e a l t with i n this chapter.
16
2 5O
3 0"
3 5O
4FO
F i g . 2 . 1 . The h y d r o g r a p h i c b a s i n of t h e N i l e s y s t e m ( t h e b o u n d a r i e s of t h e d r a i n a g e b a s i n a r e i n d i c a t e d by a d a s h - d o t l i n e )
17
TABLE 2.1
S p e c i f i c d i s c h a r g e s of r i v e r s w i t h c a t c h m e n t a r e a s each l a r g e r t h a n 1 million square kilometres (Kalinin, G . ,
River
Site
Catchment a r e a , A , kmz
1971)
Long-term
-
d i s c h a r g e , 0, m3/sec ~~
Specific d i s c h a r g e , 9, l i t/sec/km2
~
Nile
Aswan
2.880 .OOO
Missouri
Hermann
1.369 .OOO
2 187
1.69
Mississippi
S t . Louis
1.817.000
4 900'
2.70 4.51
2 830
0.98
Amur
Khabarovsk
1.620.000
7 300
Ob
Salekhard
2.450.000
12 460
5.09
Volga
Kuibyshev
1.220.000
7 480
6.13
Lena
Kyusyur
2.430 .OOO
15 900
6.54
Yenisei
Ingarka
2.470.000
18 100
7.33
Congo+
R i v e r mouth
3.700.000
36 000
9.73
Yangtze
Hankow
1.490.000
23 700
15.91
+estimated
The g e n e r a l t o p o g r a p h i c map, F i g . 2.2., shows t h a t t h e b a s i n of t h e N i l e i s c h a r a c t e r i z e d by t h e e x i s t e n c e of two mountainous p l a t e a u s r i s i n g some thousands of metres above mean s e a l e v e l . The Lake P l a t e a u i n t h e s o u t h e r n p a r t of t h e N i l e B a s i n i s g e n e r a l l y a t a l e v e l of 1 0 0 0 t o 2 000 metres. The Ruwenzori mountainous r a n g e e x t e n d i n g between Lakes Edward and A l b e r t (Mobutu-Sese Seko) a t t h e w e s t o f t h e Lake P l a t e a u h a s a peak r i s i n g more t h a n 5 100 m e t r e s whereas t h e peak o f M t . Elgon n o r t h - e a s t
o f Lake V i c t o r i a is a t a l a t i t u d e of 4 300
metres. A l l t h e l a k e s i n t h i s p l a t e a u , e x c e p t Lakes V i c t o r i a and Kyoga a r e a t l e v e l s below 1 0 0 0 metres a . s . 1 . The o t h e r mountainous p l a t e a u i n t h e b a s i n a f t h e N i l e i s t h e E t h i o p i a n or A b y s s i n i a n P l a t e a u , bThich forms t h e e a s t e r n p a r t of t h e b a s i n . The p e a k s o f t h i s p l a t e a u r i s e t o more t h a n 3 500 m e t r e s a . m . s . 1 . North of t h e Lake P l a t e a u t h e b a s i n d e s c e n d s g r a d u a l l y t o t h e Sudan p l a i n s where t h e N i l e r u n s a t a l t i t u d e s lower t h a n 500 metres i n i t s n o r t h e r l y d i r e c t i o n . A t a b o u t 200 k i l o m e t r e s s o u t h of t h e s o u t h e r n f r o n t i e r o f E g y p t , t h e r i v e r c u t s i t s c h a n n e l i n a narrow t r o u g h bounded from each s i d e by t h e c o n t o u r l i n e o f 200 metres ground s u r f a c e l e v e l . I n g e n e r a l , t h e w i d t h of t h i s t r o u g h i n c r e a s e s a s t h e r i v e r p r o c e e d s n o r t h w a r d s . Almost two hundred k i l o m e t r e s b e f o r e d i s c h a r g i n g i n t o t h e s e a , t h e r i v e r b i f u r c a t e s and i t s two b r a n c h e s encompass t h e N i l e D e l t a . A f a i r l y d e t a i l e d d e s c r i p t i o n of t h e v a r i o u s p a r t s o f t h e N i l e Basin i s p r e s e n t e d i n t h e f o l l o w i n g s e c t i o n s .
18
Fig. 2.2.
Topographic map of t h e N i l e B a s i n
19
2.2 2.2.1
THE EQUATORIAL LAKES PLATEAU Lake V i c t o r i a
The G r e a t R i f t V a l l e y which r u n s w i t h some i n t e r r u p t i o n s from Zimbabwe t o t h e Jordan V a l l e y , i n c l u d i n g t h e Red S e a , i s d i v i d e d i n t o two b r a n c h e s i n t h e s o u t h -
e r n p a r t of t h e N i l e B a s i n . The e a s t e r n b r a n c h of t h e R i f t V a l l e y r u n s through Kenya and i s n o t i n c l u d e d i n t h e N i l e B a s i n . The w e s t e r n b r a n c h , however, cont a i n s Lakes T a n g a n y i k a , Kivu, Edward, George and A l b e r t .
I t continues north
along t h e Bahr e l J e b e l . The r a n g e of Mufumbiro m o u n t a i n s , w i t h peaks r e a c h i n g 4 500 m e t r e s a . m . s . l . ,
e x t e n d s between Lakes Edward, Kivu and Tanganyika and
s e p a r a t e s t h e l a t t e r two l a k e s from t h e N i l e B a s i n ( s e e map, F i g . 2.3.: Stamp and Morgan, 1 9 7 2 ) . The most u p s t r e a m t r i b u t a r y of t h e N i l e , a l s o t h e most i m p o r t a n t f e e d e r o f Lake V i c t o r i a , i s t h e R i v e r Kagera. T h i s t r i b u t a r y h a s a d r a i n a g e b a s i n of 6 3 000 km2 i n an a r e a s i t u a t e d between '1 between 29O 30'and
31° 40'E
and 4's
l a t i t u d e and
l o n g i t u d e a s shown on t h e map, F i g . 2 . 4 . P r a c t l c a l l y
t h e whole of t h e Kagera B a s i n i s mountainous c o u n t r y and t h e g r e a t e r p a r t
it
Of
i s s i t u a t e d between t h e 1 200 and 1 600 metre l e v e l s . I n t h e extrer.ie w e s t , t h e c o u n t r y l e v e l i s a t 2 500 metres n.m.s.1.
and r i s e s t o about 4 500 m e t r e s t o
form t h e peaks o f t h e Mufumbiro Range. The Kagera B a s i n i s a complex of s t r e a m s o f v a r y i n g o r d e r which a r e i n t e r c e p t e d and i n t e r c o n n e c t e d by l a k e s and swamps. T h i s complex b e g i n s w i t h t h e R i v e r L u v i r o n z a i n t h e s o u t h - w e s t of t h e Kagera Basin a b o u t 40 km from t h e e a s t e r n s h o r e of Lake T a n g a n y i k a . A f t e r f l o w i n g i n a
v e r y w i n d i n g c o u r s e for a b o u t 100 km a t l e v e l s h i g h e r t h a n l 6 0 0 m a . m . s . l . ,
it
c o n t i n u e s f o r some 180 km i n a r e l a t i v e l y s t r a i g h t c h a n n e l t r a v e r s i n g a lowerl y i n g c o u n t r y . T h e r e t h e r i v e r name changes t o Ruvuvu and i t j o i n s t h e Kagera downstream o f t h e Bugufi F a l l s . The Ruvuvu draws i t s s u p p l i e s from t h e h i g h l a n d i n B u r u n d i . Moreover, t h i s r i v e r i s j o i n e d by a number of s e a s o n a l - f l o w i n g
streams a l l coming from t h e e a s t and by t h e Nyavarongo from t h e w e s t , a few k i l o m e t r e s u p s t r e a m of t h e B u g u f i F a l l s . The R i v e r Nyavarongo f l o w s from t h e high l a n d e a s t o f Lake Kivu and r e c e i v e s w a t e r from t h e R i v e r Akanyaru i n t h e s o u t h and t h e R i v e r Nyaranda i n t h e n o r t h - e a s t
( s e e map, F i g . 2 . 4 . ) . Below t h e
j u n c t i o n of t h e s e r i v e r s t h e main stream t r a v e r s e s an a r e a s u r r o u n d e d by l a k e s and swamps up t o t h e c o n f l u e n c e w i t h t h e Ruvuvu. Downstream o f t h e Bugufi F a l l s t h e Kogera r u n s t o t h e n o r t h t h e n t o t h e n o r t h - w e s t
i n a less w i n d i n g c o u r s e f o r
a b o u t 170 km, where i t i s j o i n e d by t h e R i v e r K a l a n g a s s a from t h e south-west
and
by t h e R i v e r Kakitumba from t h e w e s t . The Kagera t h e n c o n t i n u e s i t s c o u r s e a l o n g t h e s o - c a l l e d b i g e a s t w a r d bend t o t h e v i l l a g e of B i b a t u r a where i t e n t e r s a r e l a t i v e l y l o w - l y i n g c o u n t r y . About 70 km f u r t h e r t o t h e s o u t h - e a s t
t h e Kagera
r e c e i v e s some w a t e r b r o u g h t by a s t r e a m f l o w i n g o u t of t h e Muisha swamp i n a n o r t h e r l y d i r e c t i o n . A few k i l o m e t r e s below t h e c o n f l u e n c e of t h i s s t r e a m w i t h t h e Kagera t h e l a t t e r i s j o i n e d by t h e R i v e r Ngono which r u n s w e s t o f t h e c o a s t
20
o f L a k e V i c t o r i a . The K a g e r a c o n t i n u e s i t s c o u r s e a l o n g t h e e a s t w a r d b e n d for a b o u t 20 k m b e f o r e i t f i n a l l y e n t e r s L a k e V i c t o r i a ( H u r s t , H . E .
1927).
Fig. 2 . 3 . R i f t V a l l e y s and v o l c a n i c a r e a s o f e a s t e r n A f r i c a ( S t a m p , L . D . and Morgan, W . T . , 1 4 7 2 )
21
D rainage b a s i n s : .
1. R . K a g e r a , 2 . V i c ? c r l a N W . , 3. V i c t o r i a N i l e & L . K y o g a , 4. L George & E d w a r d , 5 . L. A l b e r t & R. S e r n l i k i , 6. Victoria
7.
S.E.,
Victoria N.E. a n d 8 R. A s s u a
Fig. 2.4.
Map s h o w i n g t h e d r a i n a g e b a s i n s i n t h e E q u a t o r i a l L a k e s P l a t e a u
22
The L a k e s P l a t e a u i s s i t u a t e d between t h e two b r a n c h e s of t h e G r e a t R i f t V a l l e y . The a v e r a g e e l e v a t i o n o f t h i s p l a t e a u is a b o u t 1 300 m e t r e s a . m . s . l . T h e p l a t e a u c o n t a i n s Lakes V i c t o r i a , George, Edward and A l b e r t . Lake V i c t o r i a i s a d e p r e s s i o n whose s u r f a c e h a s an a r e a of a b o u t 69 000 km‘, w a t e r l e v e l of 1 134 m e t r e s a . m . s . 1 .
corresponding t o a
The n e t w a t e r a r e a i s a b o u t 4% less t h a n
t h e t o t a l a r e a , t h e d i f f e r e n c e is o c c u p i e d by t h e S e s e i s l a n d s i n t h e n o r t h - w e s t and t h e Ukenve i s l a n d i n t h e s o u t h - e a s t
and many o t h e r less i m p o r t a n t i s l a n d s .
The water s u r f a c e i s d i v i d e d between Kenya, a b o u t 5%, T a n z a n i a , a b o u t 51%, and Uganda, 44%. The l a k e h a s an 0-shaped Oo 30”
s u r f a c e which e x t e n d s from a b o u t 3OS t o
l a t i t u d e and from a b o u t 31° 40’E t o 34O 5 0 ’ E
l o n g i t u d e . The a v e r a g e
d e p t h of t h e l a k e i s 40 m and t h e maximum d e p t h as f a r a s i t h a s been sounded
i s 79 m . The b a t h y m e t r i c map o f t h e l a k e i s shown i n F i g . 2 . 5 .
(Talling, J . F . ,
1 9 6 9 ) . The s h a l l o w d e p t h o f t h i s l a k e i s why t h e r e i s no s t r a t i f i c a t i o n i n t h e
w a t e r t e m p e r a t u r e . I n s t e a d , c o m p l e t e m i x i n g o c c u r s and t h e water t e m p e r a t u r e v a r i e s between 2 3 . 8 O C and 2 6 . 0 ° C , d e p e n d i n g on t h e t i m e of t h e y e a r (Beauchamp, R . A .
Fig. 2.5.
,
1964)
.
B a t h y m e t r i c map of Lake V i c t o r i a ( T a l l i n g , J . F . , 1966)
23
The l a n d p o r t i o n o f t h e Lake V i c t o r i a c a t c h m e n t i s a b o u t 1 9 3 000 km2. T h i s a r e a i s d i v i d e d b e t w e e n Kenya, 44 0 0 0 , T a n z a n i a , 8 4 2 0 0 , Uganda, 3 2 100 and Rwanda-Burundi,
33 600 km2 ( Z a g h l o u l , S . S . , 1 9 8 2 ) . An i n s i g h t i n t o t h e t o p o -
g r a p h y o f t h e c a t c h m e n t s u r f a c e c a n b e s e e n from t h e c r o s s - s e c t i o n s which a r e presented i n Fig. 2 . 6 . Three s o u r c e s c o n t r i b u t e t o t h e n e t s u p p l y t o Lake V i c t o r i a . These a r e : t h e o u t f l o w o f t h e R i v e r K a g e r a , t h e d i r e c t p r e c i p i t a t i o n o n t h e l a k e s u r f a c e and t h e run-off
f r o m t h e l a n d p o r t i o n o f t h e c a t c h m e n t . The K a g e r a B a s i n h a s
a l r e a d y b e e n d e s c r i b e d . I n s p i t e o f t h e f a c t t h a t i t r e c e i v e s more r a i n f a l l t h a n t h e o t h e r two s o u r c e s and t h e s l o p e o f t h e streams d i s c h a r g i n g i n t o t h e Kagera i s , g e n e r a l l y , n o t s m a l l , t h e d i s c h a r g e o f t h e Kagera is r a t h e r low. The r e a s o n s b e h i n d i t a r e t h e swamps and l a k e s w h i c h e x i s t i n t h e b a s i n and t h e c o n s i d e r a b l e l e n g t h o f streams f l o w i n g i n i t . The d i r e c t p r e c i p i t a t i o n on t h e Lake V i c t o r i a is a l m o s t l o s t by t h e e v a p o r a t i o n from i t s s u r f a c e . A l t h o u g h t h e d i f f e r e n c e b e t w e e n t h e a v e r a g e d e p t h s o f p r e c i p i t a t i o n and e v a p o r a t i o n i n a y e a r i s t o o s m a l l , t h e c o r r e s p o n d i n g volume
i s q u i t e b i g . A y e a r l y e x c e s s o f t h e p r e c i p i t a t i o n o n t h e l a k e s u r f a c e o f 1 5 mm o v e r t h e e v a p o r a t i o n f r o m t h e l a k e s u r f a c e means a g a i n t o t h e volume o f l a k e w a t e r c o n t e n t of 1 m i l l i a r d m 3 .
This t h e r e f o r e c o n s t i t u t e s an important source
o f s u p p l y t o t h e l a k e . The t h i r d s o u r c e o f s u p p l y t o Lake V i c t o r i a i s formed by t h e p e r e n n i a l streams i n t h e e a s t e r n s i d e o f t h e l a k e . Of t h e s e may b e m e n t i o n e d t h e S i m i y u and t h e Ruwand w h i c h f l o w i n t o S p e k e G u l f , t h e Mara R i v e r which e n t e r s t h e l a k e somewhere a b o u t t h e m i d d l e p o i n t o f t h e e a s t e r n s h o r e , and t h e Nzoya, Y a l a and S i o w h i c h e n t e r t h e l a k e i n i t s n o r t h - e a s t e r n c o r n e r .
2.2.2
The Upper V i c t o r i a N i l e
The Upper V i c t o r i a N i l e is t h e o n l y o u t l e t o f Lake V i c t o r i a and i t c o n n e c t s t h e l a t t e r w i t h Lake Kyoga. The r i v e r i s a b o u t 130 km l o n g and t h e d i f f e r e n c e i n l e v e l b e t w e e n i t s h e a d and i t s t a i l i s a b o u t 1 0 5 m . T h i s d i f f e r e n c e h a s b e e n b r o u g h t by t h e Owen a n d t h e R i p o n R a l l s .
T h e s e f a l l s a r e formed by a r e e f of
r o c k c r o s s i n g t h e stream d i a g o n a l l y . S i n c e 1 9 5 2 t h e N i l e l e a v e s Lake V i c t o r i a t h r o u g h t h e t u r b i n e s o f t h e power p l a n t a n n e x e d t o t h e Owen D a m which i s b u i l t
a t t h e f o o t o f t h e Owen F a l l s . The w i d t h o f t h e w a t e r s u r f a c e i n t h e Upper V i c t o r i a N i l e v a r i e s b e t w e e n 300 and 6 0 0 m . The r e g i o n n o r t h o f L a k e V i c t o r i a h a s b e e n t i l t e d i n s u c h a way as t o r e v e r s e t h e f l o w i n t h e u p p e r p a r t o f t h e Kafu R i v e r . The v a l l e y s a t t h e h e a d o f t h e r i v e r t h e r e f o r e became f l o o d e d , t o f o r m t h e p r a c t i c a l l y c o n t i n u o u s p a i r o f L a k e s Kyoga and Kwania (Wayland, E . J . ,
1934).
24
Luera
1000 -
800
2200
-
a
-
Section
through
the
equator
-
Scales
~
Hor. : 1 / 4,000,000
2000
Ver. : 1 /
20,000
1800
-
1600
ui
g 1400 0
v1
2
1200
i
I
;1000
L
- Section
b
;2400 r--L.
through
1"s.
latitude
Kivu
.-
-;2200 1
R Nyavarongo
w
2000
1800
1600 1400 1200
F
L
Y
(1135)
loo0 800
31
30' c -
Fig. 2.6.
Victoria
Section
through
I
Is I Ukerewe
32 " 2 " S.
33"
latitude
S e c t i o n s a c r o s s Lake V i c t o r i a and a d j a c e n t c o u n t r y
34" E
25
2 . 2. 3
Lakq-K,yoga
Lake Kyoga h a s undergone some c h a n g e s i n i t s o l d p a t t e r n . The o l d e r Kyoga was a l a r g e r l a k e t h a n t h e p r e s e n t body o f w a t e r . I t i s a s h a l l o w d e p r e s s i o n c o n s i s t i n g o f a number o f a r m s , many o f which a r e f i l l e d w i t h swamp v e g e t a t i o n . The l a k e h a s a b a s i n 75 000 km2 i n a r e a i n c l u d i n g 6 270 km2 which form t h e a r e a s o c c u p i e d by t h e l a k e arms and e n c l o s i n g h i g h l a n d up t o an e l e v a t i o n o f 1 0 3 0 m e t r e s a . m . s . 1 . The d e p t h o f t h e l a k e a t i t s w e s t e r n end i s from 3 t o 5 m , t h e maximum r e c o r d e d d e p t h i s 7 m. The d r a i n a g e b a s i n o f Kyoga, w i t h t h e excepof t h e D e b a s i e n Mountain and t h e w e s t e r n h a l f o f t h e Elgon Mountain, i s c h a r a c t e r i z e d by a s e r i e s o f low h i l l s and f l a t v a l l e y s w i t h impeded d r a i n a g e ( s e e c r o s s - s e c t i o n s i n F i g . 2 . 7 ) . I n s p i t e o f t h e a l m o s t 1 300 mm y e a r l y r a i n f a l l , t h e e x c e s s i v e e v a p o t r a n s p i r a t i o n from t h e swamps c o v e r e d w i t h c y p e r u s p a p y r u s and w a t e r l i l i e s and t h e i n s i g n i f i c a n t s u p p l y b r o u g h t by many o f t h e r i v e r s d r a i n i n g i n t o t h e l a k e make Lake Kyoga a s o u r c e o f l o s s . Heavy r a i n s i n t h e l a k e b a s i n a r e l i k e l y t o s e t l o o s e l a r g e masses o f v e g e t a t i o n which b l o c k t h e o u t l e t o f t h e N i l e from t h e l a k e .
2.2.4
The Lower V i c t o r i a N i l e
The Lower V i c t o r i a N i l e l e a v e s Kyoga a t P o r t Masindi and r u n s a s a s l u g g i s h swampy r i v e r t o t h e n o r t h f o r a d i s t a n c e o f a b o u t 75 km. Here i t b e n d s w e s t wards and a f t e r a s u c c e s s i o n o f r o c k s and r a p i d s d e s c e n d s t h e Marchison F a l l s and s h o r t l y a f t e r w a r d s e n t e r s Lake A l b e r t t h r o u g h a swampy d e l t a . On t h e west o f t h e V i c t o r i a N i l e B a s i n t h e r e i s a l a r g e s y s t e m o f swamps whose d r a i n a g e e n t e r s t h e N i l e by t h e Kafu R i v e r ( s e e F i g . 2 . 4 . ) . The c o n t r i b u t i o n o f t h i s r i v e r , e x c e p t a f t e r heavy r a i n s , may b e c o n s i d e r e d n e g l i g i b l e . The d i f f e r e n c e i n t h e w a t e r l e v e l between t h e two e n d s of t h e Lower V i c t o r i a N i l e , i . e . b e t ween Lake Kyoga and A l b e r t i s a l m o s t 410 metres.
2.2.5
Lake A l b e r t (Mobutu-Sese Seko)
A s m e n t i o n e d e a r l i e r , t h e c h a i n formed by t h e l a k e s A l b e r t , Edward, and George, t o g e t h e r w i t h t h e i r r e s p e c t i v e d r a i n a g e b a s i n s , Great R i f t Valley.
forms a p a r t o f t h e
I n some p l a c e s t h e e s c a r p m e n t s o f t h e v a l l e y r i s e d i r e c t l y
from t h e w a t e r s u r f a c e o f Lake A l b e r t , which i s a t a n a l t i t u d e o f a b o u t 617 metres a . m . s . l . ,
r e a c h i n g an e l e v a t i o n of 2 000 metres or h i g h e r a s h o r t d i s -
t a n c e i n l a n d from t h e l a k e s ( s e e t h e s e c t i o n s i n F i g . 2 . 7 . ) . Lake A l b e r t h a s a s u r f a c e a r e a of 5 300 km2 c o r r e s p o n d i n g t o an e l e v a t i o n of 617 m e t r e s a . m . s . 1 . The B a t h y m e t r i c map ( F i g . 2 . 8 . ) shows t h a t t h e d e p t h o f w a t e r r e a c h e s 50 m e t r e s a t some p l a c e s i n t h e l a k e . The r u n - o f f
from t h e d r a i n a g e b a s i n o f Lake A l b e r t ,
1 7 000 km2 i n a r e a , p l u s t h e d i r e c t p r e c i p i t a t i o n on t h e l a k e i t s e l f , a r e a l l l o s t b y e v a p o r a t i o n from t h e l a k e s u r f a c e . The n e t g a i n by A l b e r t comes from
26
1800
-
1600
-
1400
-
1200
-
1000
-
800
-
600
a -
Section
through
2 O N
latitude
1600
I
1400 1200 1000
Kyoga
L
A
v
S w a m p s of the
Kafu basin
1 3000
LJ
800
2800
(620)
;600 i
2600 b -
0
Section
through
1
30' N latitude
ul
2400
L
u r
E
2200
C
2 000
.-0
I
-
1800
W
a,
1600
1400
1
1800
1
1200
yk k -
1000
- I
"
j
Scales
c
- Section I
31 Fig. 2.7.
through
l0N
latitude
Hor Ver
1 / 4,000,000 1 / 20,000
I
I
I
I
32'
33O
34O
35O
Sections across Lake Kyoga and adjacent country
27
t h e o u t f l o w of t h e R i v e r S e m l i k i , which e n t e r s t h e l a k e from t h e s o u t h - w e s t . The S e m l i k i c o n n e c t s Lake Edward t o Lake A l b e r t , a f t e r f l o w i n g a d i s t a n c e of about 250 km down t h e R i f t V a l l e y t o t h e w e s t o f t h e Ruwenzori Mountain. The d r a i n a g e b a s i n o f t h e S e m l i k i i s 8 000 km2 i n a r e a . I t c o v e r s t h e w e s t e r n s l o p e s o f t h e Ruwenzori Range and is t r a v e r s e d b y many s t r e a m s . The d i f f e r e n c e i n w a t e r l e v e l between t h e two e n d s o f t h e S e m l i k i i s 295 metres. Most o f t h e drop t a k e s p l a c e o v e r t h e r a p i d s which e x i s t i n t h e u p p e r o f t h e r i v e r ' s c o u r s e . I n t h e l o w e r p a r t , t h e r i v e r h a s a w i d t h o f 150 m i n f l o o d r e d u c e d t o 50 m a t low s t a g e . The a v e r a g e d e p t h of w a t e r i n t h e s e two s e a s o n s i s 5 m and
3 m respectively.
Fig. 2.8.
2.2.6
B a t h y m e t r i c map o f Lake A l b e r t ( f r o m Rzbska, J . , 1977)
Lakes Edward and George
Lake Edward i s c o n n e c t e d t o Lake George by t h e Kazinga C h a n n e l . Lake George
i s s i t u a t e d on t h e e q u a t o r and i t s s u r f a c e a r e a a t an e l e v a t i o n o f 915 m e t r e s a.m.s.l.,
i s 300 km2. T h i s l a k e h a s a d r a i n a g e b a s i n 8 000 km2 s u r f a c e a r e a .
28
I t i s d r a i n e d by a number of s t r e a m s f l o w i n g down from t h e Ruwenzori i n t o t h e swamps a t t h e n o r t h e r n end of t h e l a k e . The p r i n c i p a l t r i b u t a r y , t h e Mbuku, c a r r i e s a c o n s i d e r a b l e f l o w d u r i n g t h e f l o o d . The o u t f l o w from Lake George r u n s t h r o u g h t h e Kazinga C h a n n e l , which i s p r a c t i c a l l y n o t h i n g b u t a c a r r i e r . Lake Edward l i e s i n t h e w e s t e r n R i f t V a l l e y and a t an a l t i t u d e of a b o u t 915 metres a . m . s . 1 . h a s a s u r f a c e a r e a of a b o u t 2 200 km2. The c r o s s - s e c t i o n
(Fig. 2.9.)
shows how t h e e s c a r p m e n t o f t h e R i f t V a l l e y rises s t e e p l y from t h e w a t e r surf a c e l e v e l of a b o u t 915 m t o more t h a n 2 500 m on t h e w e s t e r n s i d e of Lake Edward. T h i s i s , however, n o t t h e c a s e f o r t h e o t h e r s i d e s of t h e l a k e , though a t the north-east
c o r n e r t h e o u t l y i n g h i l l s of t h e Ruwenzori Range come down
w i t h i n a few k i l o m e t r e s of t h e l a k e . The l a k e h a s a b a s i n 12 000 km2 i n a r e a , which i s t r a v e r s e d by a number of s t r e a m s o f t e n f r i n g e d by t h i c k f o r e s t a t t h e i r low e n d s . The p r i n c i p a l s t r e a m s debouching t h e i r w a t e r s i n t o Lake Edward a r e : Nyamgasani f l o w i n g down t h e Ruwenzori Range n o r t h - e a s t
of t h e l a k e , t h e
R i v e r s B e r a r a r a and I s h a s h a f l o w i n g from t h e e a s t i n a n o r t h e r l y d i r e c t i o n towards t h e l a k e , a s y s t e m of r i v e r s p o u r i n g i n t o t h e main s t r e a m , t h e Ruchuru, r u n n i n g down t h e Mufumbiro m o u n t a i n s towards t h e l a k e i n a n o r t h e r l y d i r e c t i o n and t h e R i v e r R u i n d i r e a c h i n g Lake Edward a t i t s s o u t h - w e s t
corner.
From t h e above d e s c r i p t i o n , i t i s c l e a r t h a t t h e R i v e r S e m l i k i s u p p l i e s Lake A l b e r t with t h e run-off
from a t o t a l catchment o f a b o u t 30 500 km2 i n a r e a ,
i n c l u d i n g t h e s u r f a c e s c o v e r e d by Lakes George and Edward. The N i l e f l o w s o u t of Lake A l b e r t a t t h e e x t r e m e n o r t h c o r n e r of t h e l a k e under t h e name of t h e Upper White N i l e or Bahr e l - J e b e l .
22 00
m
E 2000 ti In 1800 L
aJ
2
E
1600
c‘ 1400 ._
:1200
I
aJ -
LLJ
1000 800
Fig. 2.9. latitude
Scales H or . : 1
/ 1,000,000
Ver. i
/
1
20,000
S e c t i o n a c r o s s Lake Edward and a d j a c e n t c o u n t r y t h r o u g h Oo 3 0 ’ s
29
THE BAHR EL JEBEL BASIN
2.3
The m a j o r a f f l u e n t s o f t h e Bahr e l J e b e l or t h e Upper White N i l e and t h e swamps and l a k e s a l r e a d y d i s c u s s e d i n t h e p r e c e d i n g s e c t i o n , a r e summarized by t h e d r a w i n g p r e s e n t e d i n F i g . Z.lO.(Thompson,
K.,
1975).
From t h e o u t l e t o f Lake A l b e r t down t o Nimule, 225 km downstream, t h e r i v e r i s a r a t h e r b r o a d , s l u g g i s h , s t r e a m f r i n g e d w i t h swamps and l a g o o n s . I t meanders e a s t and w e s t t h r o u g h a narrow f l o o d p l a i n b e t w e e n h i l l y c o u n t r y on e i t h e r s i d e so t h a t t h e a r e a o f t h e swamp i s w e l l d e f i n e d . The a r e a o c c u p i e d by swamps and
open w a t e r i s e s t i m a t e d a t a b o u t 380 km2. The Bahr e l J e b e l from t h e o u t l e t o f Lake A l b e r t t o Nimule i s a p l a c i d stream w i t h a n a v e r a g e s l o p e o f o n l y a b o u t I t i s n o t a d e e p r i v e r and i t s w i d t h v a r i e s from 100
2 . 2 cm/km o r 2 . 2 x t o 300 m .
A number of s m a l l s t r e a m s j o i n t h e Bahr e l J e b e l from b o t h s i d e s i n
t h i s reach ( s e e F i g . 2 . 4 . ) .
'
B u f u m b i r a Mts.
2 50C
M t Elgon
aJ
-2 2000 0
In aJ
;
Central
C
Uganda
1500
aJ
> 0
n 0
1000 aJ c L
E
aJ
500
Murchison
Falls/
J
Upper N i l e swamps (Sudan)
0
I
6500
6000 Distance
,
i 500 from
1
5000
4500
sea. k m .
Fig. 2.10. A l t i t u d i n a l map o f m a j o r a f f l u e n t s o f t h e Upper White N i l e and t h e o c c u r r e n c e o f swamps (Thompson, K . , 1975)
30
A t Nimule t h e r i v e r c o u r s e i s t w i s t e d i n a s h a r p b e n d and i t s d i r e c t i o n c h a n g e s s u d d e n l y t o t h e w e s t t h e n t o t h e n o r t h and n o r t h - e a s t
up t o M o n g a l l a .
I n t h e r e a c h b e t w e e n Nimule and R e j a f , a d i s t a n c e o f a b o u t 1 5 6 km, t h e r i v e r
i s a n a r r o w and f a s t stream i n t e r r u p t e d by s u c h r o c k y r a p i d s as t h e F o l a and B e d a n . The r i v e r f l o w s i n t o a n a r r o w v a l l e y c u t t h r o u g h h i l l y c o u n t r y and d e s c e n d s a b o u t 150 m .
The a v e r a g e s l o p e i s n e a r l y 1 m/km or 1 x
I n t h e r e a c h from Nimule t o M o n g a l l a t h e B a h r e l J e b e l r e c e i v e s a number o f
s m a l l b u t t o r r e n t i a l streams w h i c h r u n f u l l a f t e r h e a v y r a i n s . Of t h e s e s t r e a m s t h e A s s u a , t h e Kaia and t h e K i t a r e t h e l a r g e s t . They c a r r y some f l o w e v e n i n t h e d r y s e a s o n . The R i v e r A s s u a j o i n s t h e B a h r e l J e b e l a t i t s r i g h t bank a l m o s t 20 km b e l o w N i m u l e . The f o r m e r rises i n t h e v i c i n i t y o f t h e Moroto M o u n t a i n and i s j o i n e d by some s t r e a m s d e s c e n d i n g f r o m t h e n o r t h a n d f r o m t h e e a s t n o t f a r f r o m t h e M o r o n g o l e M o u n t a i n . The A s s u a t h u s d r a i n s a v a s t a r e a e a s t o f t h e B a h r e l J e b e l , w h e r e a s t h e R i v e r Kaia d r a i n s p a r t o f t h e c o u n t r y
w e s t o f i t . The K a i a i s j o i n e d by t h e R i v e r K i j o a few k i l o m e t r e s b e f o r e i t s j u n c t i o n w i t h t h e B a h r e l J e b e l . The R i v e r K i t r i s e s from t h e h i g h l a n d b e t w e e n t h e T e r e t e i n i a and I m a t o n g M o u n t a i n s and a f t e r f l o w i n g a d i s t a n c e o f a b o u t 160 km, i t j o i n s t h e B a h r e l J e b e l a t t h e r i g h t b a n k a few k i l o m e t r e s u p s t r e a m
o f R e j a f , where i t e n t e r s t h e Sudan P l a i n s . N e v e r t h e l e s s , t h e s l o p e i n t h e 5 7 km from R e j a f t o M o n g a l l a i s r a t h e r s t e e p , o n a v e r a g e 0 . 3 m/km,
falling off
g r a d u a l l y a s t h e M o n g a l l a i s a p p r o a c h e d . From R e j a f t o some d i s t a n c e n o r t h w a r d s the valley is well-defined,
t h o u g h s h a l l o w , and t h e r i v e r w i n d s a b o u t i n t h e
p l a i n f o r m i n g t h e v a l l e y f l o o r ( H u r s t , H.E., 1 9 3 1 ) . I n t h e r e a c h from R e j a f t o M a l a k a l o n t h e W h i t e N i l e ,
t h e r i v e r i s n o t con-
f i n e d t o a s i n g l e c h a n n e l e x c e p t a t Mongalla where i t is i n one c h a n n e l a t l o w s t a g e . Between R e j a f and B o r , a d i s t a n c e o f a b o u t 180 km, t h e v a l l e y i s w i d e and f l a t and t h e r e i s u s u a l l y a c h a n n e l o n e i t h e r s i d e a l o n g t h e h i g h e r g r o u n d w h i l e o c c a s i o n a l c h a n n e l s cross t h e swampy v a l l e y f l o o r ( s e e F i g . 2 . 1 1 . ) . The d i s t r i b u t i o n o f t h e swamp v e g e t a t i o n on t h e f l o o d p l a i n i n t h e r e a c h b e t w e e n J u b a and B o r was i n v e s t i g a t e d .
I t w a s found t h a t t h i s d i s t r i b u t i o n i s
c o n t r o l l e d by s e a s o n a l f l o o d i n g and t h e f o r m o f c o n t r o l c a n b e d e d u c e d by r e l a t i n g t h e d i s t r i b u t i o n t o e l e v a t i o n and t h u s t o h y d r o l o g i c a l c o n d i t i o n s (Sutcliffe, J.V.,
1 9 7 4 ) . F u r t h e r m o r e , i t seems t h a t t h e r e s u l t s o f t h a t i n v e s -
t i g a t i o n apply to t h e r i v e r reach north of B o r . N o r t h o f Bor t h e V a l l e y w i d e n s and becomes more swampy, w h i l e t h e s i d e s a r e
less d e f i n e d . E x t e n s i v e swamps s p r e a d o u t o n e i t h e r s i d e o f t h e r i v e r and cont i n u e down t o L a k e No. T h i s r e g i o n i s known as t h e S u d d . N o r t h o f K e n i s a , a b o u t
85 km
d o w n s t r e a m o f B o r , t h e d r y l a n d c a n h a r d l y b e s e e n , e x c e p t i n a few
p l a c e s . The r i v e r f l o w s n o r t h w a r d s b e t w e e n w a l l s o f p a p y r u s and t a l l g r a s s e s r e a c h i n g 4 o r 5 m i n h e i g h t . These p l a n t s h a v e t h e i r r o o t s i n w a t e r and t h e
31
ro I
.d
m N d .
d
d
c
0
k
a,
4
c a,
m
9
a
m a,
r(
e
5
a,
4
k
c a,
m
9
R
0
+I
2
4
rl
M
N .d
h
32
r i v e r bank i s p a r t l y formed of masses of r o o t s o f f o r m e r v e g e t a t i o n . F u r t h e r more, t h e r e a r e many p a t c h e s of open w a t e r a l o n g s i d e t h e r i v e r n o r t h o f B o r , many o f which a r e c o n n e c t e d d i r e c t l y w i t h t h e r i v e r or w i t h t h e s i d e c h a n n e l s . Of t h e l a t t e r t h e Awai and A t e m R i v e r s and G a g e ' s and P e a k e ' s C h a n n e l s may b e mentioned ( s e e map, F i g . 2 . 1 1 . ) . North of Ghaba Shambe, some 140 km from B o r , t h e swamps a r e wide and t h e p l a i n i s f u l l of v e g e t a t i o n and l a g o o n s . A p l a n and c r o s s - s e c t i o n of a t y p i c a l l a g o o n i n t h i s a r e a i s shown i n F i g . 1 . 1 2 . (Hurst, H . E .
1 9 3 1 ) . Because o f t h e h i g h r a t e of loss o f w a t e r from t h e Sudd
r e g i o n and t h e v a s t a r e a from which t h i s l o s s t a k e s p l a c e ,
the t o t a l loss i n
a n a v e r a g e y e a r amounts t o a p p r o x i m a t e l y one h a l f of t h e t o t a l f l o w a t M o n g a l l a . I n a n a t t e m p t t o t r a n s p o r t t h e w a t e r i n t h i s r e g i o n w i t h less l o s s , t h e Bahr e l J e b e l was j o i n e d t o t h e Bahr e l Z a r a f by two c u t s a t d i s t a n c e s of
106 and 112 km from Shambe. U n f o r t u n a t e l y
t h e s e two c u t s and t h e c h a n n e l which
r u n s between them a r e s o h e a v i l y b l o c k e d w i t h v e g e t a t i o n t h a t t h e i r e f f i c i e n c y i n r e d u c i n g t h e t r a n s m i s s i o n l o s s e s i n t h e swamps i s q u e s t i o n a b l e . Between t h e two c u t s and Lake No i n t h e n o r t h t h e r e a r e o c c a s i o n a l i s o l a t e d s p o t s o f h i g h ground compared t o t h e s u r r o u n d i n g swamps. A t Lake No t h e Bahr e l J e b e l i s j o i n e d by t h e Bahr e l Ghazal and t h e combined s t r e a m t u r n s a b r u p t l y t o t h e e a s t , b e a r i n g t h e name "The White N i l e " . Here t h e swamps end and t h e White N i l e flows northwards i n a f a i r l y w e l l - d e f i n e d
v a l l e y of moderate w i d t h .
The Bahr e l Z a r a f s t a r t s somewhere a b o u t l a t i t u d e 7O 20"
i n t h e swamps
e a s t o f t h e mouth o f t h e R i v e r Awai. I t i s p r o b a b l e t h a t t h e r e a r e some c h a n n e l s c o n n e c t i n g t h e J e b e l , t h e Awai, t h e A t e m and t h e Z a r a f t h r o u g h which t h e l a t t e r d e r i v e s i t s s u p p l y o f w a t e r . The Bahr e l Z a r a f h a s a w i n d i n g c o u r s e o f a b o u t 280 km i n l e n g t h t o i t s mouth o n t h e White N i l e some 80 km from Lake No. Along t h i s c o u r s e t h e r e i s n o t s o much p a p y r u s a l o n g t h e J e b e l and t h e p r i n c i p a l swamp p l a n t i s um s o o f , w i t h r e e d s and b u l r u s h e s . H i g h e r ground e x i s t s n o t v e r y f a r t o t h e e a s t o f t h e Upper Z a r a f s o t h a t swamps r e a c h t h e i r d e f i n i t e l i m i t i n t h e e a s t . Some o f t h i s h i g h g r o u n d , howe v e r , becomes swampy a f t e r heavy r a i n .
I n t h e neighbourhood of t h e J e b e l - Z a r a f
c u t s and f o r a l o n g way n o r t h , t h e Z a r a f f l o w s t h r o u g h swamp, w i n d i n g a b o u t forming lagoons i n
its
bends l i k e t h e Bahr e l J e b e l .
The e d g e s o f t h e Bahr e l Z a r a f a r e swampy i n p l a c e s a s f a r n o r t h as k i l o -
metre 100 (measured from t h e mouth) and t h e r e i s always a f r i n g e of um s o o f . The banks g r a d u a l l y become h i g h a s one g o e s n o r t h w a r d s u n t i l t h e y form d e f i n i t e b o u n d a r i e s l i m i t i n g t h e Z a r a f t o a narrow c h a n n e l .
33
Hor.: Ver. :
t> 24.0 22.0
600
0 Distance
500
in m e t r e s
1000
1 / 20,000 1 / 200
1500
Fig. 2 . 1 2 . P l a n and c r o s s - s e c t i o n o f a l a g o o n i n t h e Bahr el J e b e l B a s i n ( r e p r o d u c e d from t h e N i l e B a s i n Vol. I : H u r s t , H . E . and P h i l i p s , P . , 1931)
34
THE BAHR EL GHAZAL BASIN
2.4
The Bahr e l Ghazal i s t h e name g i v e n t o t h e waterway from Meshra e l Req t o Lake N o ( s e e map, F i g . 2 . 1 1 . ) . Though t h e l e n g t h o f t h i s stream does n o t e x c e e d 160 km, t h e s i z e o f i t s b a s i n i s a p p r o x i m a t e l y 526 000 km2 which i s by f a r t h e l a r g e s t o f any o f t h e s u b - b a s i n s o f t h e t r i b u t a r i e s of t h e N i l e R i v e r . The a n n u a l r a i n f a l l on t h e b a s i n i s e s t i m a t e d a t 500 x lo9 m 3 . Of t h i s amount o n l y 0.6 x
lo9
m3/year r e a c h e s t h e b a s i n o u t l e t a t Lake No.
A l l a l o n g t h e Bahr e l G h a z a l and t o t h e s o u t h and e a s t o f i t a r e l a r g e a r e a s o f swamp which a r e f e d by a number of streams. The c o u n t r y where t h e u p p e r c o u r s e s o f t h e s e streams flow i s e n t i r e l y c o v e r e d by a s o r t o f s a v a n n a h f o r e s t . Nevertheless,
i n t h e r a v i n e s formed by t h e streams, t h e r e i s a t h i c k f o r e s t
s i m i l a r t o t h e t r o p i c a l r a i n f o r e s t o f p a r t s o f t h e Lake P l a t e a u and t h e Congo B a s i n . On t h e l o w e r c o u r s e s o f a l l t h e t r i b u t a r i e s o f t h e Bahr e l Ghazal and a l o n g t h e Ghazal i t s e l f a r e l a r g e a r e a s o f swamps. U n f o r t u n a t e l y , most o f t h e flow c a r r i e d by t h e t r i b u t a r i e s i s l o s t i n t h e swamps. Near t h e Bahr e l Arab, t h e f o r e s t i s o f t h e t h o r n y savannah t y p e and t h i s g r a d u a l l y c h a n g e s t o s h r u b s t e p p e a s one g o e s n o r t h w a r d s . On e i t h e r s i d e o f t h e Uganda-Sudan boundary t h a t c o i n c i d e s w i t h t h e d i v i d e between t h e N i l e and t h e Congo B a s i n s , numerous s t r e a m s a r i s e . Most o f them d e s c e n d t o a l a r g e swampy p l a i n i n which t h e y wind and f i n a l l y s p r e a d and c e a s e t o e x i s t as streams w i t h d e f i n i t e c o u r s e s , e x c e p t f o r t h e J u r , which p r e s e r v e s i t s c h a n n e l and j o i n s t h e Bahr e l G h a z a l . The t r i b u t a r i e s of t h e Bahr e l Ghazal from e a s t t o w e s t a r e : t h e G e l or T a p a r i , t h e Y e i or Lau, t h e Naam, t h e M e r i d i o r G e l , t h e I b b a o r T o n j , t h e J u r , t h e Lo1 and t h e Bahr e l Arab. Some o f t h e d a t a b e l o n g i n g t o t h e s e r i v e r s a r e i n c l u d e d i n T a b l e 2 . 2 . A map i l l u s t r a t i n g t h e d r a i n a g e b a s i n of t h e Bahr e l Arab and s u r r o u n d i n g s i s shown i n F i g . 2 . 1 3 . The d a t a p r e s e n t e d i n T a b l e 2 . 2 may h e l p t o show t h a t t h e J u r i s t h e most i m p o r t a n t t r i b u t a r y o f t h e G h a z a l . The f o r m e r h a s two main t r i b u t a r i e s , namely, t h e Sueh and t h e B u s s e r i . Both a r e r e l a t i v e l y l a r g e s t r e a m s . Lake Ambadi i s a b o u t 10 km l o n g by 1 km wide and m o s t l y l e s s t h a n 3 m d e e p . From t h i s l a k e down t h e Ghazal t o t h e mouth o f t h e Bahr e l Arab, t h e c o u n t r y r e m a i n s swampy and t h e r i v e r d o e s n o t h a v e a d e f i n e d b a n k . The v e g e t a t i o n b o r d e r i n g t h e r i v e r i s um s o o f w i t h l i t t l e p a p y r u s . The lower Ghazal i s f r i n g e d by p a p y r u s , though i t s growth i s s t u n t e d and l e s s l u x u r i a n t t h a n on t h e J e b e l . The many t e m p o r a r y s t r e a m s which j o i n t h e Ghazal o n b o t h s i d e s a r e u s u a l l y b l o c k e d and t h e r e f o r e c a n n o t c o n t r i b u t e much w a t e r . A s Lake No i s a p p r o a c h e d , t h e d r y l a n d n e a r s t h e r i v e r on t h e n o r t h and t h e r i v e r l o s e s i t s d e f i n e d banks. Lake No i s n o t h i n g b u t a l a r g e s h a l l o w l a g o o n . Here t h e s l u g g i s h Bahr el Ghazal
35
TABLE 2 . 2
Some d a t a o f t h e t r i b u t a r i e s o f t h e Bahr e l G h a z a l (Hurst, H . E . and P h i l i p s , P . , 1938) T r i b u t a r y of t h e Bahr e l Ghazal
I tern
River+ Tapari
River+ Yei
River Naam
River Meridi
Fiver Tonj
River
Basin a r e a , km
1 2 800
25 000
16 000
22 000
27 000
Mean r a i n f a l l , mm/yr
1 050
1 250
1 200
1 200
Trough width, m
50
100
90
Max. depth, m
4
5 400
Max. d i s charge, m 3 / s Place of observation
a
near Amadi
River Lo1
Bahr e l Arab
64 000
8 2 000
209 000++
1 220
1 200
1 100
700
-
70
130
270
35
4
-
3
6
3
5
160
-
110
600
500
near Rumbek
-
Jur
Tonj
Wau
b
C
+ ++
u s u a l l y c o n s i d e r e d as a t r i b u t a r y o f t h e B a h r e l J e b e l and n o t o f t h e Bahr e l Ghazal E x c l u d i n g t h e swamps
a A t t h e r o a d c r o s s i n g b e t w e e n Amadi and T e r r a k e k k a b I n t e r s e c t i o n w i t h r o a d from N y a m l e l l t o B a h r e l Arab ‘Safaha
due n o r t h o f Nyamlell
j o i n s t h e B a h r e l J e b e l a f t e r h a v i n g a t r e m e n d o u s volume o f w a t e r w a s t e d i n t h e v a s t swamps. A s c h e m a t i c c r o s s - s e c t i o n
t h r o u g h t h e Upper N i l e swamps from Bahr
e l Ghazal t o t h e B a h r e l J e b e l c a n b e s e e n i n F i g . 2 . 1 4 .
(Jonglei Report,
1954).
2.5
THE SOBAT BASIN The S o b a t B a s i n , a p p r o x i m a t e l y 225 000 km2 i n a r e a , i n c l u d e s most o f t h e
p l a i n e a s t o f t h e B a h r e l J e b e l and B a h r e l Z a r a f and p a r t s o f t h e A b y s s i n i a n Mountains and t h e L a k e s P l a t e a u ( s e e F i g . 2 . 1 5 . ) .
I n view of t h e l a r g e s i z e of
t h e b a s i n a r e a and t h e d i v e r s i t y i n i t s t o p o g r a p h y , t h e a n n u a l r a i n f a l l v a r i e s
from a b o u t 6 5 0 mm n e a r t h e mouth o f t h e S o b a t , t o a b o u t 2 000 mm i n t h e most e l e v a t e d p a r t s o f t h e b a s i n e a s t w a r d s . The S o b a t i s formed by t h e j u n c t i o n o f
i t s two main t r i b u t a r i e s , t h e B a r o and t h e P i b o r . T h e B a r o i s c l a i m e d t o b e t h e p r i n c i p a l f e e d e r of t h e S o b a t , though i t s b a s i n h a s a s u r f a c e a r e a o f 41 400 k m 2 , w h e r e a s t h e s u r f a c e a r e a o f t h e P i b o r i s 10 900 km2.
36
37 E&O]Tl
IT]
Ech & Ory
I
;o
1 2
1
Bahr e l Ghazal
388
I
I
E c h & Ory
3
4
I [
P
I
I
5
6
I
Echinochloa
T
1
Typhia
P
I
Papyrus
pyramidalis
&
I 7
I
Bahr e l Jebel
D i s t a n c e - km
E c h 8, O r y
P
Oryza
barthii
Fig. 2.14. S c h e m a t i c c r o s s - s e c t i o n t h r o u g h Upper N i l e swamps from Bahr e l Ghazal t o t h e Bahr e l J e b e l ( J o n g l e i R e p o r t , 1954) The Baro i s formed by a number of streams which i n some p l a c e s flow through deep g o r g e s i n t h e i r d e s c e n t from t h e p l a t e a u . A good d e a l o f t h e mountainous p a r t of t h e b a s i n l i e s above 1 5 0 0 m w i t h p o r t i o n s even h i g h e r t h a n 2 000 metres a . m . s . 1 . Soon a f t e r l e a v i n g t h e m o u n t a i n s , t h e Baro r e a c h e s Gambeila, which i s on t h e p l a i n , a l m o s t 520 metres a . m . s . 1 . H e r e t h e Baro does n o t r e c e i v e t r i b u t a r i e s , b u t r a t h e r s h a l l o w swampy khors*, a r e Khor Jokau coming from t h e n o r t h , t h e A t u r a b r a n c h ,
t h e p r i n c i p a l s o f which and t h e Mokwai.
Down of Gambeila up t o t h e j u n c t i o n o f t h e Baro w i t h t h e P i b o r , i s t h e B a r o , with an a v e r a g e w i d t h o f 100 metres, i n c r e a s i n g t o 250 metres i n some p l a c e s and an a v e r a g e d e p t h o f more t h a n 6 metres d u r i n g t h e f l o o d . U n f o r t u n a t e l y , some 40 km u p s t r e a m o f t h e j u n c t i o n some w a t e r l e a v e s t h e Baro through Khor Machar t o f e e d a l a r g e swampy a r e a n o r t h and e a s t o f E l - N a s i r .
T h i s swamp i s
f e d by o t h e r streams f l o w i n g from t h e A b y s s i n i a n P l a t e a u ( s e e map, F i g . 2 . 1 5 . ) . The p a r t f l o w i n g t o Khor Machar c o n s t i t u t e s , however, a permanent s o u r c e o f
l o s s i r o n t h e Bar0 ( H u r s t e t a l , 1 9 6 6 ) . The R i v e r P i b o r r u n s i n a n o r t h e r l y d i r e c t i o n . I t draws t h e g r e a t e r p a r t o f
i t s s u p p l y from A b y 6 s i n i a and t h e rest comes from t h e n o r t h e r n s l o p e s o f t h e l a k e p l a t e a u and from t h e Sudan p l a i n s . The b a s i n a r e a of t h e P i b o r h a s a l r e a d y been mentioned a s b e i n g l a r g e r t h a n t h a t of t h e Bar0 and t h e a n n u a l r a i n f a l l i s probably more, s i n c e t h e mountalnous p o r t i o n i n A b y s s i n i a is l a r g e r t h a n t h e c o r r e s p o n d i n g p o r t i o n o f t h e B a r 0 B a s i n . I n s p i t e o f t h e s e two f a c t o r s , t h e flow of t h e P i b o r i s q u i t e i n f e r i o r t o t h a t o f t h e B a r o . T h i s i s b e c a u s e t h e
*A khor i s a temporary stream which r u n s f u l l d u r i n g and a f t e r r a i n f a l l . I t could be o f a t o r r e n t i a l n a t u r e
38
Fig. 2.15. Map s h o w i n g t h e a p p r o x i m a t e b o u n d a r i e s o f t h e d r a i n a g e b a s i n o f t h e River Sobat
39
s l o p e of t h e P i b o r i s v e r y f l a t compared t o t h e Baro and h a s c o n s e q u e n t l y more chance of forming l a r g e swamps and e v a p o r a t i n g t h e w a t e r t h e r e o f . The P i b o r is formed by t h e j u n c t i o n o f t h e Veveno, L o t i l l a and t h e Kangen
(see map, F i g . 2 . 1 5 . ) . None of t h e s e streams c a r r i e s much w a t e r and a l l a r e reduced t o p o o l s i n t h e d r y s e a s o n . C r o s s - s e c t i o n s o f t h e s e s t r e a m s c a n be found i n Vol. V I I I of t h e N i l e B a s i n ( H u r s t e t a l , 1 9 5 0 ) . Some of t h e s e c r o s s s e c t i o n s a r e r e p r o d u c e d i n F i g . 2 . 1 6 . The P i b o r , l i k e many of i t s t r i b u t a r i e s , becomes n a r r o w e r and d e e p e r i n s e c t i o n a s i t goes down. Near i t s mouth, t h e c l e a r w i d t h o f t h e r i v e r c h a n n e l d r o p s from 150 t o 60 m , whereas t h e d e p t h i n c r e a s e s from a b o u t 4 m i n t h e upper r e a c h e s t o a b o u t 6 . 5 m . Downstream o f t h e j u n c t i o n of t h e Kangen and L o t i l l a t h e P i b o r r u n s n o r t h wards i n a w i n d i n g c o u r s e and r e c e i v e s t h e A g w e i , Akobo, G i l a and Khor Makwai on t h e e a s t b a n k . On t h e w e s t b a n k , i t r e c e i v e s t h e Khor A d e i t and s e v e r a l smaller khors.
.’ 421
E
’
5
420
-
419:
-G aJ
-aJ w
0 880
Fig. 2.21.
660 Distance
440 from
head
in
380
300
16
kilornetres
L o n g i t u d i n a l p r o f i l e of t h e R i v e r Atbara
THE MAIN NILE
2.8
A d e s c r i p t i o n o f t h e p h y s i o g r a p h y and t h e t o p o g r a p h y o f t h e b a s i n of t h e
Main N i l e from Khartoum up t o t h e M e d i t e r r a n e a n S e a is g i v e n i n V o l . I X o f t h e N i l e B a s i n ( H u r s t e t a l , 1 9 5 9 ) . A summary o f t h e marked f e a t u r e s o f t h e N i l e Basin i n t h i s s t r e t c h is a s f o l l o w s :
2.8.1
From Khartoum t o Aswan
A t Khartoum t h e B l u e N i l e j o i n s t h e White N i l e and t h e combined w a t e r s flow f o r 1 8 8 5 km t o Aswan t h r o u g h a r e g i o n o f Nubian s a n d s t o n e o v e r l y i n g an o l d e r o d e d l a n d s u r f a c e of c r y s t a l l i n e r o c k s which h a s b e e n l a i d b a r e a t p l a c e s i n t h e c o u r s e of t h e s t i l l incomplete d e g r a d a t i o n o f t h e r i v e r bed. These
LEGEN9
++ +
International boundary
-..-
B o u n d a r y of river b a s i n
-----* Unsurveyed s t r e a m
Fig. 2 . 2 2 .
Map s h o w i n g t h e d r a i n a g e h a s i n of t h e A t b a r a and t h e Main N i l e below Khartoum
4 4
48
c r y s t a l l i n e r o c k s o f f e r a much g r e a t e r r e s i s t a n c e t o t h e r i v e r ' s a c t i o n t h a n does t h e s o f t e r Nubian s a n d s t o n e . Upstream, t h e r e f o r e , i n t h e p l a c e where t h e former r o c k s are e x p o s e d , d e g r a d a t i o n c e a s e s for a w h i l e , w h i l e t h e r i v e r c u t s
i t s way t h r o u g h t h e rocky o b s t a c l e . The r i v e r ' s c o u r s e t h u s c o n s i s t s o f a s e r i e s of p l a c i d r e a c h e s o f mild s l o p e s e p a r a t e d by rocky r a p i d s , c a l l e d t h e C a t a r a c t s , where t h e s l o p e i s g r e a t e r and t h e f l o w more t u r b u l e n t . The r a p i d s t h e m s e l v e s a r e c a u s e d by b a r s of h a r d r o c k c r o s s i n g t h e c o u r s e o f t h e r i v e r . These r o c k s a r e more s l o w l y e r o d e d t h a n t h e n e i g h b o u r i n g r o c k s and s o form a r e a o f t h e Main N i l e a t t h e
s i l l s o r s t e p s . The a p p r o x i m a t e c r o s s - s e c t i o n a l
l o c a t i o n s o f t h e C a t a r a c t s and i n between them i s i n c l u d e d i n T a b l e 2 . 3 . F i g . 2.23.
a l s o shows t h e l o n g i t u d i n a l p r o f i l e of t h e w a t e r s u r f a c e t o g e t h e r w i t h
t h e w a t e r s u r f a c e width a t t h e d i f f e r e n t s t r e t c h e s of t h e r i v e r . Approximate s e c t i o n a l a r e a s , i n s q u a r e metres of some s t r e t c h e s
TABLE 2 . 3
of t h e Main N i l e between A t b a r a and H a l f a S e c t i o n a l area, m2,
Location
J a n . F e b . Mar. Apr. May
f o r months o f t h e y e a r
J u n . J u l . Aug. S e p . O c t . Nov. Dec.
Atbara to 5th Cataract
2600 1900 1500 1250 1160 1800 2800 4500 6750 6100 4600 3300
5th Cataract
1800 1400 1100
900
800 1000 2000 4000 4850 4500 3500 2500
5th Cataract t o 2500 1800 1300 1100 1000 1300 2500 5700 6950 6400 5000 3600 4th Cataract 4th Cataract
1800 1400 1100
900
800 1000 2000 4000 4850 4500 3500 2500
4th Cataract t o 2300 1700 1300 1100 1000 1300 2400 5400 6550 6100 4700 3300 3rd C a t a r a c t 2600 2000 1500 1100
860
900 1900 4100 5450 5000 3900 3300
3rd C a t a r a c t t o 2600 2000 1500 1100 2nd C a t a r a c t
860
900 1900 4100 5450 5000 3900 3300
3rd C a t a r a c t
2nd C a t a r a c t
1800 1400 1100
900
860 1000 2000 4000 4850 4500 3500 2500
For t h e f i r s t 80 km n o r t h of Khartoum, t h e r i v e r f l o w s n o r t h w a r d s , t h e n c e t o B e r b e r (km 387 from Khartoum). The c o u r s e o f t h e r i v e r r u n s s u c c e s s i v e l y e a s t , north-east,
and n o r t h . North o f B e r b e r t h e r i v e r t u r n s n o r t h - w e s t
t o Abu-Hamed
(km 5781, where i t a b r u p t l y t u r n s s o u t h - w e s t t o K o r t i (km 8 7 2 ) . From K o r t i t h e c o u r s e s w i n g s around a bend back t o t h e n o r t h a t Kerma (km 1 1 4 5 ) , where i t p r o c e e d s n o r t h and n o r t h - e a s t From E l - D e r r
p a s t Wadi-Halfa
(km 1 435) t o E l - D e r r
t h e r i v e r , a f t e r a s h o r t right-hand
n o r t h e r l y d i r e c t i o n t o Aswan ( s e e F i g s . 2 . 2 2 . ,
(km 1 6 7 1 ) .
loop t o t h e s o u t h , flows i n a
2.24.,
and 2 . 2 5 . ) .
LTY~~, Khartoum
400
375
vi
i d
At baro
'""\ , Shirri
325300
Island
End of 4 t h .
-
C ._ Q,
U
225-
0
"-
5
c
200175-
a 3
-
0
-
Cataract
!
275 250-
Scales
rapids
Abu F a t m a ( Hennek, Simit and Shaban rapids ) K a i b a r rapids
I
7
Tongur
S h a b l u k a or 6 th. C a t a r a c t
I
,
rapids
Amuaol
raoids
'
I
I
I
5 th. Cataract 4 t h ~ a t a r a ct
I
I
I
I
I
I
I
i
80
41 2 I
A s w a n or lSt. C a t a r a c t
I
580 683 I
I
7
822 I
1168 I
1301 1
1438 1526 I
I
1885
50
A s d e g r a d a t i o n is s t i l l i n p r o g r e s s t h r o u g h o u t t h e r e a c h d e s c r i b e d , t h e r i v e r d e p o s i t s no f l o o d p l a i n s . C u l t i v a t i o n ,
therefore,
is c o n f i n e d t o t h o s e
few s t r e t c h e s w h e r e n a t u r a l c o n d i t i o n s p e r m i t i r r i g a t i o n . The f i r s t s t o r a g e work i n t h e N i l e Valley, t h e o l d Aswan D a m , w a s b u i l t i n
1902 a t t h e f o o t o f t h e Aswan C a t a r a c t .
2.8.2
From Aswan t o t h e M e d i t e r r a n e a n S e a
The o l d Aswan D a m h a s b e e n h e i g h t e n e d t w i c e ; o n c e i n 1912 and t h e s e c o n d
t i m e i n 1934. T h i s dam, t o g e t h e r w i t h t h e o t h e r s t o r a g e works on t h e B l u e and W h i t e N i l e s h a v e c h a n g e d t h e N i l e from Aswan t o t h e sea i n t o a p a r t i a l l y r e g u l a t e d r i v e r i n s t e a d o f a n a t u r a l l y f l o w i n g o n e . F u l l r e g u l a t i o n h a s a l m o s t been a c h i e v e d as a r e s u l t o f t h e f o r m a t i o n o f t h e Nasser L a k e u p s t r e a m o f t h e h i g h dam a t Aswan i n 1965. T h i s h u g e a r t i f i c i a l impoundment o f t h e N i l e w a t e r e x t e n d s f r o m Aswan t o a l i t t l e s o u t h o f t h e D a l C a t a r a c t b e t w e e n t h e 23O 5 8 ” and 20°
27” l a t i t u d e and 30° 35’E and 33O 1 5 ’ E l o n g i t u d e ( s e e F i g . 2 . 2 5 . ) .
T a b l e 2.4 shows t h e d e v e l o p m e n t o f t h e r e s e r v o i r l e v e l and c a p a c i t y i n t h e p e r i o d from 1964-65 t i l l 1975-76.
TABLE 2.4
The g r a d u a l f i l l i n g o f t h e r e s e r v o i r formed by t h e High Dam a t Aswan (Abu e l - A t a a ,
Highest level, m a.m.s.1.
Year
A,,
Date of occurrence
1978) Maximum storage
10’ m 3
Lowest level, m a.m.s.1.
Date Of
occurrence
Minimum storage 10’ m 3 -
9.620
111.89
1.08.1964
4.11.1966
13.590
119.02
29.07.1966
4.650
4.02.1967
24.320
113.48
26.07.1967
14.130
151.21
10.10.1967
39.640
145.27
21.07.1968
28.516
156.55
21.11.1968
39,005
150.85
22.07.1969
39.005
1964/65
127.60
18.01.1965
1965/66
132.86
1966/67
142.48
1967/68 1968/69 1969/70
161.30
25.10.1969
62,400
153.81
3.08.1970
42.280
1970/71
164.88
26.11.1970
77.468
159.65
23.08.1971
60.450
1971/72
167.64
4.12.1971
87.757
162.49
28.07.1972
68.774
1972/73
167.52
1.01.1973
87,320
158.20
8.07.1973
56.960
1973/74
166.32
9.11.1973
82.776
161.00
16.07.1974
64.500
1974/75
170.63
5.11.1974
100.309
165.60
30.07.1975
80.060
1975/76
175.71
10.12.1975
124 ,990
172.42
26.07.1976
108.370
T h i s t a b l e shows c l e a r l y t h a t t h e w a t e r l e v e l u p s t r e a m o f Aswan h a s r i s e n i n t h e l a s t t e n y e a r s by a minimum o f 40 metres compared t o t h e f l o o d l e v e l i n t h e pre-High
D a m p e r i o d ( 1 2 1 . 0 metres a . s . 1 . ) .
T h i s c o n s i d e r a b l e r i s e i n t h e water
Fig. 2.24.
M a p showing t h e Main N i l e i n t h e reach from t h e Atbara . j u n c t i o n t o Wadi-Halfa
52
l e v e l h a s r e s u l t e d i n t h e i n u n d a t i o n o f some p a r t s o f N u b i a . I n i t h n a t u r a l c o n d i t i o n , t h e l e n g t h o f t h e r i v e r from Aswan t o t h e D e l t a B a r r s g e s was 9 6 8 km i n t h e low-flow t h e s l o p e was 7 . 7 x lo-'
and 8 . 5 x
s e a s o n and 9 2 3 km i n t h e f l o o d s e a s o n and
lo-'
d u r i n g t h e f l o o d w a s a b o u t 7 500 m 2 ,
.Jelocity between 1.0 m / s
respectively.
The c r o s s - s e c t i o n a l
area
t h e mean w i d t h a b o u t 900 m and t h e mean
and 2 . 0 m / s .
From C a i r o t o a l i t t l e s o u t h o f L u x o r t h e c u l t i v a t e d l a n d i s u s u a l l y s e v e r a l k i l o m e t r e s w i d e b u t t o w a r d s Aswan i t n a r r o w s t o a b o u t o n e k i l o m e t r e , a n d i n u l a c e s t h e d e s e r t h i J l s a r e c l o s e t o t h e r i v e r . These c o n d i t i o n s p e r s i s t f o r a l o n g way s o u t h o f Wadi-Halfa. P e r e n n i a l i r r i g a t i o n i n E g y p t h a s become p o s s i b l e o n l y a f t e r t h e c o n s t r u c LioIl o f a number o f b a r r a g e s o n t h e N i l e and i t s b r a n c h e s . A b a r r a g e
time:;
-
some-
c a l l e d a n o p e n - t y p e w e i r - i s d i f f e r e n t from a dam as i t s f u n c t i o n i s n o t
t o form a s t o r a g e r e s e r v o i r , b u t m e r e l y t o r a i s e t h e l e v e l o f t h e w a t e r ups t r e a m of
i t so a s t o d i v e r t s o m e o f i t i n t o t h e c a n a l s whose e n t r a n c e s a r e
a b o v e t h e b a r r a g e . The o l d D e l t a b a r r a g e s w e r e c o m p l e t e d i n 1 8 6 1 and t h e new Ones i n 1 9 3 9 . O t h e r b a r r a g e s were b u i l t a t E s n a , Nag-Hammadi,
A s s i u t and Z i f t a .
The s u r f a c e of t h e c u l t i v a t e d a r e a i n b o t h t h e N i l e V a l l e y and t h e N i l e D e l t a amounts t o o n l y 3% o f t h e t o t a l s u r f a c e a r e a o f E g y p t . The e a s t e r n and ' w e s t e r n d e s e r t s o c c u p y 2 3 and 74'1, o f t h e s u r f a c e a r e a o f E g y p t , r e s p e c t i v e l y . The e a s t e r n d e s e r t i s r u g g e d and m o u n t a i n o u s and is much c u t up by d e e p v a l l e y s ( W a d i s ) , down w h i c h o c c a s i o n a l h e a v y r a i n s c a u s e t o r r e n t s t o f l o w . T h e r e a r e no wadi:^
i n t h e w e s t e r n d e s e r t . T h i s d e s e r t is l o w e r and more u n d u l a t i n g , b u t is
n e v e r t h e l e s s s h a r p l y d i v i d e d from t h e N i l e V a l l e y , b e c a u s e c u l t i v a t i o n c e a s e s its s o o n a s t h e g r o u n d b e g i n s t o r i s e a b o v e t h e l e v e l which c a n b e f l o o d e d by
t h e Yilo water. T h e r t ? art' a number o f o a s e s i n t h e w e s t e r n d e s e r t . T h e s e a r e s i m p l y d e p r e s -
s i o n s w h e r e t h e g r o u n d l e v e l i s n e a r t h e water l e v e l , w h i c h i s e a s i l y r e a c h e d by w e l l s .
A cross-section
e x t e n d i n g from t h e c o a s t o f t h e Red S e a i n t h e e a s t
t o t h e w e s t e r n b o u n d a r y of E g y p t p a s s i n g t h r o u g h t h e El-Khargah
O a s i s is shown
i n Fig. 2.26. The Fayum i s a d e p r e s s i o n s i t u a t e d a b o u t 70 k i l o m e t r e s s o u t h o f C a i r o and s e p a r a t e d f r o m t h e N i l e V a l l e y by a n a r r o w s t r i p o f d e s e r t . T h i s d e p r e s s i o n i s q u i t e d i f f e r e n t b o t h from t h e o a s e s i n t h e d e s e r t and t h e c u l t i v a t e d a r e a i n t h e N i l e V a l l e y o r t h e D e l t a . On one h a n d t h e Fayum g e t s i t s w a t e r v i a a c a n a l f r o m t h e N i l e , w h e r e a s t h e oases a r e s u p p l i e d by g r o u n d w a t e r . On t h e o t h e r hand t h e l a n d i n El-Fayum h a s a c o n s i d e r a b l e s l o p e compared t o t h e l a n d i n t h e Valley o r i n t h e Delta. The b o t t o m o f t h e El-Fayum
d e p r e s s i o n i s f i l l e d by L a k e Q a r u n ( i n a n c i e n t
t i m e s c a l l e d L a k e M o e r i s ) and most o f t h e r e e a i n d e r i s c u l t i v a t e d . The l a k e h a s
: t
r 4 0 E
30
,100.i
El
Farafrah
/
Oasis
El
!'
-.
i?
-,
v
L
- Dakhlah Oasis
G
Y
c
1
t t
D E S E R T
+
I I I t
I+' + + + +
I
j
+-+++++
t
I
t t
S
t
0
t
+ t Fig. 2 . 2 5 .
I
U
Scale
so 1
100 1'0 I
++++
I
200 i
++ + + + +
D km
Map showing the N i l < ? R i v e r f r o m s o u t h u f Wadi-Halfa
to a little n o r t h of A s s i u t
cn
w
54
no o u t l e t and r e c e i v e s t h e d r a i n a g e w a t e r from t h e c u l t i v a t e d l a n d . I t s l e v e l i s k e p t f a i r l y c o n s t a n t by e v a p o r a t i o n b a l a n c i n g t h e i n f l o w , s o t h a t t h e l a k e
w a t e r s t e a d i l y becomes more s a l i n e . About 50 k i l o m e t r e s s o u t h - w e s t of El-Fayum town and 80 k i l o m e t r e s w e s t of Beni-Suef
l i e s a n o t h e r d e p r e s s i o n , known a s
Wadi e l Rayyan. T h i s Wadi had o f t e n been c o n s i d e r e d a s a p o s s i b l e s o l u t i o n f o r s i d e s t o r a g e of t h e f l o o d w a t e r . The Wadi i s a d e p r e s s i o n whose maximum d e p t h
is 50 metres b . s . 1 .
a s compared w i t h t h e 45 metres o f t h e p r e s e n t Lake Qarum
level. The l a r g e s t , and a t p r e s e n t t h e d e e p e s t , d e p r e s s i o n i n t h e E g y p t i a n p a r t of t h e western d e s e r t , is t h e Qattarah depression. This depression has a s u r f a c e a r e a c o r r e s p o n d i n g t o t h e mean sea l e v e l of a b o u t 4 m i l l i o n f e d d a n s or a b o u t 50% l a r g e r t h a n t h e a r e a c o n t a i n e d between t h e two b r a n c h e s of t h e N i l e ( s e e Fig. 2.27.).
The d e e p e s t p o i n t i n t h e d e p r e s s i o n i s a t a l e v e l o f 159 m e t r e s
b . m . s . 1 . The Q a t t a r a h d e p r e s s i o n h a s , f o r t h e l a s t t h i r t y y e a r s or more, been c o n s i d e r e d a s a p o s s i b l e scheme f o r g e n e r a t i n g e l e c t r i c power. T h i s c a n b e a c h i e v e d by c o n n e c t i n g t h e d e p r e s s i o n w i t h t h e M e d i t e r r a n e a n S e a by an i n t a k e a t , or a b o u t , El-Alamein. The y e a r l y i n f l o w t o t h e d e p r e s s i o n w i l l b e b a l a n c e d by t h e y e a r l y e v a p o r a t i o n , s o t h e s u r f a c e w a t e r l e v e l i n t h e d e p r e s s i o n remains constant. The N i l e n o r t h of C a i r o b i f u r c a t e s i n t o t h e R o s e t t a and D a m i e t t a b r a n c h e s . Very c l o s e t o t h e mouth of each b r a n c h i s t h e s i t e where an e a r t h bank used t o b e c o n s t r u c t e d each y e a r . T h i s was completed when t h e flow i n t o t h e r i v e r was s h u t o f f a t t h e D e l t a B a r r a g e and a l l t h e w a t e r d i v e r t e d t o t h e c a n a l s b e c a u s e
i t w a s needed f o r c u l t i v a t i o n . The banks p r e v e n t e d s a l t w a t e r from p e n e t r a t i n g i n l a n d , and a l s o e n a b l e d s e e p a g e and d r a i n a g e back i n t o t h e r i v e r t r o u g h from t h e D e l t a B a r r a g e n o r t h w a r d s t o b e c o l l e c t e d and used for i r r i g a t i o n o f s m a l l a r e a s f u r t h e r n o r t h a l o n g t h e r i v e r . The e a r t h bank on t h e R o s e t t a b r a n c h was r e p l a c e d i n 1951 by t h e E d f i n a B a r r a g e , whereas t h e p l a n n e d b a r r a g e a t F a r a s k o u r f o r t h e D a m i e t t a b r a n c h h a s n e v e r been e x e c u t e d . A f t e r t h e c o n s t r u c t i o n of t h e High Dam a t Aswan i n 1964, t h e flow o f t h e N i l e from Aswan t o t h e M e d i t e r r a n e a n S e a h a s been under d i f f e r e n t r e g u l a t i o n . I n t h e pre-High Dam t i m e ,
i t w a s c l a i m e d t h a t t h e D a m i e t t a b r a n c h was
g r a d u a l l y s i l t i n g up and t h e r e f o r e d e c r e a s i n g i n s i z e , whereas t h e R o s e t t a b r a n c h was s c o u r i n g i n h i g h f l o o d s . The mean w i d t h of t h e R o s e t t a b r a n c h i s 500 m and t h e mean s e c t i o n a l a r e a d u r i n g t h e f l o o d was 4 000 m 2 .
The maximum,
minimum, and mean d i s c h a r g e s were a b o u t 6 500, 2 600 and 4 000 m3/sec., r e s p e c t i v e l y . The mean w i d t h o f t h e D a m i e t t a b r a n c h i s 270 m and i t s mean s e c t i o n a l a r e a d u r i n g t h e f l o o d was 2 700 m 2 .
The maximum, minimum, and mean
d i s c h a r g e s w e r e a b o u t 4 600, 1 300, and 2 300 m 3 / s e c . ,
respectively.
0 0
(
D
'
0 0
D
'
0 0
r
"
0 0
g
0 0
m
0 0
a
.
0 0
.
r
0 0
P
k
-
0 0
0
0
r 0
0
0 0 N
0 0 c?
111
c
g x cn
aJ
0
u "
0
m 0
f
55
56
The two b r a n c h e s o f t h e N i l e h a v e t h e i r mouths s i t u a t e d a t t h e c o a s t o f t h e M e d i t e r r a n e a n S e a . The c o a s t l i n e from A l e x a n d r i a t o P o r t S a i d i s an u n d u l a t i n g l i n e t h a t b e a r s t h e f e a t u r e s o f an a d v a n c i n g d e l t a ( F i g . 2 . 2 7 . ) . T h r e e s h a l l o w l a k e s occupy a g r e a t p a r t o f t h e n o r t h e r n s e c t i o n of t h e D e l t a . T h e s e a r e : Lake Idku i n t h e w e s t , L a k e B u r u l l u s i n t h e m i d d l e and Lake Manzala i n t h e east. These l a k e s r e c e i v e a c o n s i d e r a b l e amount o f t h e d r a i n a g e wager from t h e D e l t a , and a r e s e p a r a t e d from t h e s e a by narrow s t r i p s of l a n d and a l l have o u t l e t s t o the sea. The c o a s t a l l i n e of t h e N i l e D e l t a h a s , f o r some t i m e , b e e n u n d e r g o i n g a r a t h e r a c t i v e p r o c e s s of r e t r e a t . The High D a m a t Aswan h a s b r o u g h t t h e slow b u t c o n t i n u o u s p r o c e s s of b u i l d i n g t h e N i l e D e l t a t o an e n d . T h i s means t h a t t h e c o a s t a l l i n e is l e f t t o t h e e r o s i v e a c t i o n of t h e s h o r e c u r r e n t s . A d e t a i l e d d i s c u s s i o n of t h i s m a t t e r i s p r e s e n t e d i n a l a t e r c h a p t e r .
I
I
26OE
28 O
I - -
- 7
30
M E D I T E R R A N E A N
*
-+
- 28" c
Fig. 2 . 2 7 . Map showing t h e N i l e and i t s b r a n c h e s from A s s i u t Mediterranean Sea co as t
to the
57
REFERENCES Abu e l A t a a , A . , 1 9 7 8 . E g y p t and t h e N i l e a f t e r t h e High D a m ( t e x t i s i n A r a b i c ) , M i n i s t r y o f I r r i g a t i o n , C a i r o , E g y p t , 1 4 5 p p ( w i t h 18 p l a t e s ) . Beauchamp, R . S . , 1 9 6 4 . The R i f t V a l l e y L a k e s o f A f r i c a . V e r h . I n t . V e r . T h e o r . Angew. L i m n o l . 15: 9 1 - 9 9 . B e r r y , L . , and Whiteman, A . J . , 1 9 6 8 . T h e N i l e i n t h e S u d a n . G e o g r . J o u r n . 1 3 4 I : 1-37. H u r s t , H . E . , 1 9 2 7 . T h e L a k e P l a t e a u B a s i n o f t h e N i l e , 2nd p a r t , P h y s . D e p t . P a p e r 23, Government P r e s s , C a i r o , E g y p t , 6 6 p p . H u r s t , H . E . , a n d P h i l i p s , P . , 1931. The N i l e B a s i n , V o l . I : G e n e r a l d e s c r i p t i o n o f t h e B a s i n , m e t e o r o l o g y and t o p o g r a p h y o f t h e W h i t e N i l e B a s i n , P h y s . D e p t . P a p e r 2 6 , Government P r e s s , C a i r o , E g y p t , 1 2 8 p p . H u r s t , H . E . , 1 9 5 0 . The N i l e B a s i n , V o l . V I I I : T h e h y d r o l o g y o f t h e S o b a t and White N i l e and t h e t o p o g r a p h y o f t h e B l u e N i l e a n d A t b a r a , P h y s . D e p t . P a p e r 55, Government P r e s s , C a i r o , E g y p t , 1 2 5 p p . H u r s t , H . E . , B l a c k , R . P . , a n d S i m a i k a , Y . M . , 1 9 5 9 . The N i l e B a s i n , V o l . IX: The h y d r o l o g y o f t h e B l u e N i l e a n d A t b a r a a n d t h e Main N i l e T o Aswan, w i t h some r e f e r e n c e t o p r o j e c t s , N i l e C o n t r o l D e p t . P a p e r 1 2 , Government P r i n t i n g O f f i c e , C a i r o , Egypt, 207 pp. H u r s t , H . E . , B l a c k , R . P . , and S i m a i k a , Y.M., 1 9 6 6 . The Major N i l e P r o j e c t s , N i l e C o n t r o l D e p t . P a p e r 2 3 , Government P r i n t i n g O f f i c e , C a i r o , E g y p t , 1 5 9 p p . J o n g l e i I n v e s t i g a t i o n Team, 1 9 5 4 . The E q u a t o r i a l N i l e p r o j e c t and i t s e f f e c t i n t h e A n g l o - E g y p t i a n S u d a n . S u d a n G o v e r n m e n t , London. K a l i n i n , G . P . , 1 9 7 1 . G l o b a l h y d r o l o g y ( t r a n s l a t e d f r o m R u s s i a n , I s r a e l Program f o r S c i e n t i f i c T r a n s l a t i o n s L t d . ) . U . S . D e p t . o f Comm., N a t i o n a l T e c h n i c a l I n f o r m a t i o n S e r v i c e , S p r i n g f i e l d V a 22151, U.S.A. M o r a n d i n i , G . , 1 9 4 0 . R i c e r c h e L i m n o l o g i c h e . G e o g r a f i a - F i s i c a , V o l . 111, 1. M i s s i o n e d i s t u d i o a1 Lago T a n a , 319 p p . Rz6ska, J . ( e d i t o r ) , 1 9 7 6 . T h e N i l e , b i o l o g y o f a n a n c i e n t r i v e r . D r W . Junk B . V . P u b l i s h e r s , The H a g u e , T h e N e t h e r l a n d s , 4 1 7 p p . Stamp, D . L . , and Morgan, W.T., 1 9 7 2 . A f r i c a : a s t u d y i n t r o p i c a l d e v e l o p m e n t . J o h n Wiley and S o n s I n c . , N e w Y o r k , 5 2 0 p p . S u t c l i f f e , J . V . , 1 9 7 4 . A h y d r o l o g i c a l s t u d y o f t h e s o u t h e r n Sudd r e g i o n o f t h e Upper N i l e , B u l l . H y d r o . S c i . 1 9 : 237-255. T a l l i n g , J . F . , 1 9 6 6 . The a n n u a l c y c l e o f s t r a t i f i c a t i o n and p h y t o - p l a n k t o n growth i n L a k e V i c t o r i a ( E a s t A f r i c a ) , I n t . Rev. H y d r o b . 5 1 : 5 4 5 - 6 2 1 . T a l l i n g , J . F . and R z b s k a , J . , 1 9 6 7 . The d e v e l o p m e n t o f p l a n k t o n i n r e l a t i o n t o h y d r o l o g i c a l regime o f t h e Blue N i l e , J o u r n . Ecol. 55: 637-662. Thompson, K . , 1 9 7 5 . P r o d u c t i v i t y o f C y p e r u s p a p y r u s L . , I n ; P h o t o - s y n t h e s i s and p r o d u c t i v i t y i n d i f f e r e n t environments. (Ed.) J . P . Cooper; I . B . P . S y n t h e s i s S e r i e s 3 ; Cambridge U n i v e r s i t y P r e s s . Wayland, E . J . , 1 9 3 4 . R i f t s , r i v e r s , r a i n s and e a r l y man i n Uganda. J o u r n . Roy. A n t h r o p o l . I n s t . 6 4 : 333-352. 1 1 . Z a g h l o u l , S . S . , 1 9 8 2 . Water b a l a n c e o f L a k e V i c t o r i a and t h e e f f e c t o f g r a v i t y . M.Sc. T h e s i s , F a c . E n g r g . , C a i r o U n i v e r s i t y , G i z a , E g y p t , 239 p p . ( e x c l u d i n g annexes).
59
Chapter 3
CLIMATE OF THE NILE BASIN 3.1
HISTORICAL INTRODUCTION
T h e r e i s e v i d e n c e o f some c l i m a t i c c h a n g e s i n t h e N i l e B a s i n , e s p e c i a l l y i n Egypt and t h e S u d a n , i n t h e l a s t 25 000 y e a r s . I n Egypt t h e w e t p h a s e t h a t t e r m i n a t e d some 2 5 000 y e a r s BP ( b e f o r e p r e s e n t ) was f o l l o w e d by a d r y p h a s e t h a t l a s t e d a h o u t 7 000 y e a r s . The s u b s e q u e n t p e r i o d from 18 000 t o , s a y , 5 000 y e a r s BP w a s c h a r a c t e r i z e d by i t s h e a v y w i n t e r r a i n and by i n c r e a s e d f l o w i n t h e N i l e coming f r o m t h e E t h i o p i a n P l a t e a u . The g r a d u a l a r i d i t y which s w e p t o v e r E g y p t s i n c e t h e n w a s i n t e r r u p t e d by some w e t , t h o u g h s h o r t , i n t e r v a l s . T h e s e w e r e f r o m a b o u t 10 000 t o 8 000 y e a r s BP and f r o m a b o u t 6 000 t o 4 500 BP. The m o i s t i n t e r v a l s h a v e b e e n t e r m i n a t e d s i n c e a b o u t 2 500
years B.C.
(Butzer, K.W.,
1966 and 1 9 7 1 ) .
I n t h e S u d a n t h e p e r i o d f r o m 20 000 t o 1 5 000 y e a r s BP w a s v e r y a r i d . T h i s
was f o l l o w e d by a w e t p e r i o d t h a t l a s t e d f r o m 1 2 000 t o 7 000 y e a r s BP and by somewhat f l u c t u a t i n g i n t e r v a l s f r o m 7 000 t o 6 000 y e a r s BP. The c l i m a t e i n t h e i n t e r v a l from 6 000 t o 3 000 y e a r s BP c a n b e d e s c r i b e d as f a i r l y w e t . From 3 000 y e a r s a g o up t o t h e p r e s e n t ,
t h e climate i n t h e Sudan, l i k e t h a t i n Egypt, has
g r a d u a l l y b e e n becoming a r i d (Wickens, G . ,
1975).
A d e t a i l e d d e s c r i p t i o n o f t h e s e changes are beyond t h e s c o p e o f t h i s book.
Our i n t e r e s t h e r e i s f o c u s e d on t h e c l i m a t e as i t h a s b e e n i n t h e p a s t 100 y e a r s
or s o .
3.2
CLIMATIC REGIONS
A s h o r t d e s c r i p t i o n o f t h e c l i m a t e i n t h e a r e a o c c u p i e d by t h e N i l e B a s i n i s given i n V o l .
I of t h e N i l e Basin ( H u r s t , H . E .
and P h i l i p s , P . , 1 9 3 1 ) . I n t h i s
d e s c r i p t i o n t h e c l i m a t e h a s b e e n d i v i d e d i n t o t h r e e main t y p e s . T h e s e a r e : Type 1
- M e d i t e r r a n e a n c l i m a t e c o v e r i n g t h e a r e a from t h e sea coast t o a
little
s o u t h o f C a i r o . The a n n u a l r a i n f a l l d e c r e a s e s f r o m 150 t o 200 mm/yr o n t h e c o a s t t o a b o u t 2 5 t o 30 mm/yr a t C a i r o . Type 2
-
Desert or S a h a r a n c l i m a t e c o v e r i n g t h e a r e a f r o m a l i t t l e s o u t h of C a i r o t o A t h a r a . T h e r e i s p r a c t i c a l l y no r a i n f a l l i n t h i s a r e a .
Type 3
-
T r o p i c a l climate c o v e r i n g t h e a r e a s o u t h of Atbara. This type has f u r t h e r been sub-divided i n t o :
3a
- The S u d a n P l a i n s - T h e r e
is a steady increase i n r a i n f a l l south
o f t h e r a i n l e s s r e g i o n ( t y p e 2 ) . An a n n u a l d e p t h o f , s a y , 1 0 0 0 mm is reached i n t h e s o u t h .
60
3b
- The H i g h l a n d s of A b y s s i n i a - T h i s c o u l d be a r e g i o n o f r e l a t i v e l y heavy r a i n f a l l , s i n c e an a n n u a l d e p t h of 1 600 mm i s r e a c h e d i n some p l a c e s .
3c
- The H i g h l a n d s of t h e Lake P l a t e a t i - The a v e r a g e a n n u a l r a i n f a l l c o u l d b e i n t h e o r d e r o f 1 250 m m .
The c l i m a t e of t h e Sudan was d e s c r i b e d by I r e l a n d i n " A g r i c u l t u r e i n t h e Sudan" ( e d i t e d by T o t h i l l , J . D . ,
1 9 4 8 ) . The Sudan, h e m e n t i o n e d , l a y w h o l l y
w i t h i n t h e t r o p i c s between l a t i t u d e s 22'
and 3 O N .
I t i s almost e n t i r e l y lana-
l o c k e d and h a s a p r e d o m i n a n t l y c o n t i n e n t a l c l i m a t e . The e f f e c t o f t h e Red S e a i s q u i t e l i m i t e d and t h e r e a r e no l a k e s or i n l a n d w a t e r s u r f a c e s l a r g e enough t o p r o d u c e e v e n l o c a l c l i m a t i c e f f e c t s . Broadly s p e a k i n g ,
t h e r e f o r e , t h e Sudan i s
one v a s t p l a i n , i n t e r r u p t e d o n l y by t h e Marra Mountains o f D a r f u r and t h e Nuba Mountains o f s o u t h e r n K o r d o f a n . The cli.mate of t h e Sudan may be d i v i d e d i n t o 3 r e g i o n s . Kegion 1 i s s i t u a t e d n o r t h o f a b o u t l a t i t u d e 19'N.
In t h i s d e s e r t i c region the
d r y n o r t h e r l i e s p r e v a i l t h r o u g h o u t t h e y e a r and r a i n i s i n f r e q u e n t . I t e x p e r i e n c e s l a r g e d i u r n a l and a n n u a l v a r i a t i o n s i n t e m p e r a t u r e ,
c h a r a c t e r i s t i c s of a
desert climate. Region 2 i s s i t u a t e d s o u t h o f l a t i t u d e 19'N is t y p i c a l of a t r o p i c a l c o n t i n e n t ,
t o l a t i t u d e 3 O N . Here t h e c l i m a t e
though t h e n o r t h e r n p a r t is s e m i - a r i d .
Region 3 c o m p r i s e s t h e a r e a s a l o n g t h e Red S e a c o a s t and t h e e a s t e r n s l o p e s of t h e Red S e a h i l l s . L i k e r e g i o n 1, t h e n o r t h e r l i e s p r e v a i l t h r o u g h o u t t h e y e a r , e x c e p t t h a t t h e c l i m a t e i s p r o f o u n d l y a f f e c t e d by t h e m a r i t i m e i n f l u e n c e o f t h e Red S e a . The r a i n i s p a r t l y o r o g r a p h i c and p a r t l y c o n v e c t i o n a l . The c l i m a t e of A f r i c a w a s c l a s s i f i e d ( T r e w a r t h a , G . T . ,
1 9 6 2 ) , b a s e d on a
s i m p l i f i e d form o f t h e c l a s s i f i c a t i o n t h a t had o r i g i n a l l y been d e v e l o p e d by W.
Koppen. The r e s u l t s o f t h a t work have been used t o d e r i v e t h e map of t h e
c l i m a t e of A f r i c a (Stamp, D. and Morgan, W . ,
1 9 7 2 ) . The p a r t of t h e map c o v e r -
i n g t h e N i l e B a s i n i s shown i n F i g . 3 . 1 . The c a t e g o r i e s i n c l u d e d i n i t a r e : 1) The e q u a t o r i a l or t r o p i c a l r a i n f o r e s t c l i m a t e
- This
i s c h a r a c t e r i z e d by
a l m o s t c o n s t a n t h e a t , c o n s t a n t h u m i d i t y and c o n s t a n t r a i n f a l l . P l a n t growth t a k e s p l a c e throughout t h e y e a r , s o t h a t l u x u r i a n t v e g e t a t i o n i s t h e r u l e . The s u n h e a t c a u s e s e v a p o r a t i o n from l a k e s and m o i s t l a n d s u r f a c e s . The h e a t e d , s a t u r a t e d , a i r r i s e s and i s c o o l e d by c o n v e c t i o n s o t h a t r a i n f a l l s i n a l m o s t t h e same a r e a from which t h e m o i s t u r e o r i g i n a t e s ( c o n v e c t i v e rains). 2) The t r o p i c a l savannah
-
T h i s r e g i o n e x t e n d s from t h e t r o p i c a l r a i n f o r e s t on
one s i d e t o t h e d e s e r t m a r g i n s on t h e o t h e r . A c c o r d i n g l y , t h e savannah c l i m a t e v a r i e s g r e a t l y between t h e s e two l i m i t s . The v a r i a t i o n i s p r i m a r i l y i n t h e a n n u a l d e p t h of p r e c i p i t a t i o n , commonly 1 500 mm or more i n t h e
61
e q u a t o r i a l m a r g i n s t o 400 mm i n t h e s e m i - a r i d p a r t . The s e a s o n from November t o F e b r u a r y i s r e l a t i v e l y cool a n d d r y . T h i s i s f o l l o w e d by a h o t , d r y , s e a s o n w h e r e t h e h o t t e s t month i s A p r i l o r May. The coming of r a i n s from J u n e t o O c t o b e r , c a u s e s t h e l o w e r i n g o f t h e t e m p e r a t u r e . The amount o f t h e l o w e r i n g d e p e n d s on t h e amount o f r a i n f a l l . T h e s e a s o n a l d e p t h o f r a i n h a s a w i d e r e l a t i v e r a n g e b u t t h a t o f t h e m o n t h l y d e p t h i s much w i d e r . 3 , 4 ) The s e m i - a r i d
and a r i d c l i m a t e o r t h e s t e p p e and d e s e r t
l i m i t may b e t a k e n as t h e 400 mm/yr
isohyet.
-
The s a v a n n a h
I f less t h a n t h i s d e p t h , t h e
c l i m a t e may b e d e s c r i b e d as s t e p p e . I t i s r a t h e r d i f f i c u l t t o s a y where t h e s t e p p e c h a n g e s i n t o d e s e r t . On t h e p o l e w a r d m a r g i n s ,
3
low r a i n f a l l l i m i t
may b e t a k e n t o i n d i c a t e w h e r e t h e s t e p p e l a n d s f a d e i n t o t h e M e d i t e r r a n e a n Along t h e s o u t h e r n m a r g i n s , t h e r a i n y s e a s o n i s t h e h o t s e a s o n , w h e r e a s along t h e Mediterranean margins i t is t h e w i n t e r .
5 ) The H i g h l a n d s
-
T h e r e a r e two a r e a s whose c l i m a t e b e l o n g s t o t h i s c a t e g o r y ,
a r o u n d t h e E q u a t o r i a l L a k e s a n d a good p a r t o f t h e E t h i o p i a n P l a t e a u ( s e e map, F i g . 3.1.). T h e c l i m a t e h e r e i s v e r y much m o d i f i e d by t h e e l e v a t i o n . Some p a r t s r i s e t o t h e snow l i n e . The a n n u a l p r e c i p l t a t l o n e a s i l y r e a c h e s
1 500 m m . The m o i s t u r e i n d e x method h a s b e e n u s e d i n t h e h y d r o m e t e o r o l o g i c a l s u r v e y o f t h e catchments of Lakes V i c t o r i a ,
Kyoga a n d A l b e r t f o r t h e p u r p o s e o f c l a s s i f y -
i n g t h e c l i m a t e o f t h e L a k e P l a t e a u a r e a . The i n d e x h a s b e e n computed f o r s e v e r a l p o i n t s w i t h i n t h e a r e a from t h e e x p r e s s i o n I
m
100 ( S =____.__-.
-
0.6D)
(3.1)
PET
where Im
= moisture index,
S
= cumulated monthly s u r p l u s ,
(R
D
= cumulated monthly d e f i c i t ,
(PET
R
=
m
= s u b s c r i p t r e f e r r i n g t o month,
-
PET)
-
m'
R)m,
rainfall, and
PET = a n n u a l p o t e n t i a l e v a p o t r a n s p i r a t i o n The d i f f e r e n t t y p e s o f c l i m a t e c o r r e s p o n d t o t h e f o l l o w i n g I Type
Im value
__
100 and a b o v e
Perhumid Humid
Moist sub-humid
20
to
100 20
0
to
Dry sub-humid
-
20
to
0
Semi - a r i d
-
40
to
-20
Arid
-100
to
-40
values
62
Fig. 3.1.
C l i m a t o l o g i c a l r e g i o n s o f the N i l e B a s i n
63
The c o n t o u r l i n e s of e q u a l m o i s t u r e i n d e x v a l u e s i n t h e E q u a t o r i a l Lake P l a t e a u a r e as shown i n F i g . 3.2. From t h i s map one c a n e a s i l y see t h a t t h e g r e a t e r p a r t o f t h e p l a t e a u a r e a f a l l s i n t h e c l a s s of t h e d r y sub-humid c l i m a t e and o n l y a few s m a l l p a r t s f a l l i n t h e m o i s t sub-humid c l a s s . The i n f o r m a t i o n p r e s e n t e d on t h e map i n F i g . 3.2. d o e s n o t f u l l y a g r e e w i t h t h e c o r r e s p o n d i n g p a r t of t h e map i n F i g . 3 . 1 . They d i f f e r mainly i n t h a t p a r t s i t u a t e d w e s t of Lake V i c t o r i a ,
29'
E
30'
i n t h e b a s i n o f t h e Kagera and n o r t h o f i t .
31 '
32
33'
3 4'
35'
36'
F i g . 3.2. Contour l i n e s of e q u a l m o i s t u r e i n d e x v a l u e s f o r t h e E q u a t o r i a l Lake P l a t e a u a r e a (WMO, 1974)
64
3.3
NETWORK OF METEOROLOGICAL STATIONS
A d e s c r i p t i o n o f some o f t h e c l i m a t i c f e a t u r e s of t h e N i l e B a s i n a r e a i s
included i n t h e f o l l o w i n g s e c t i o n s of t h i s c h a p t e r . These f e a t u r e s i n c l u d e t h e t e m p e r a t u r e , h u m i d i t y , r a d i a t i o n , s u n s h i n e , c l o u d i n e s s , wind and g e n e r a l c i r c u l a t i o n of t h e a i r m a s s e s . T h i s d e s c r i p t i o n i s b a s e d on t h e d a t a o b s e r v e d a t some, or a l l , of t h e m e t e o r o l o g i c a l s t a t i o n s l i s t e d i n T a b l e 3.1.
TABLE 3.1
D a t a o f some o f t h e m e t e o r o l o g i c a l s t a t i o n s i n t h e N i l e B a s i n ( I r e l a n d , 1948; M i n i s t r y of War and M a r i n e , E g y p t , 1950; WMO, 1974)
Station Sidi Barrani S a 1 um ( Obs e r va t o r y )
Damietta Rosetta Mersa Matruh Edfina Port Said (Airport) Sirw Alexandria Atf Arish Sakha Mansura Damanhur Qurashiya Gemmeiza Tanta Zagazig Benha Delta Barrage C a i r o (Ezbekiya) Giza S u e z ( P o r t Tewfik) Helwan Fayum Siwa Beni Suef Minya ( A i r p o r t ) Hurghada Assiut Qena Qusseir Nag-Hammadi Luxor Dakhla Kharga Esna Deadalus KOQ Ombo Aswan
Country Egypt
Latitude N
31’ 31 31 31 31 31 31 31 31 31 31 31 31 31 30 30 30 30 30 30 30 30 29 29 29 29 29 28 27 27 26 26 26 25 25 25 25 24 24 24
38’ 33 25 24 22 18 17 14 12 11 07 07 03 02 51 48 47 35 28 11 03 02 56 52 18 12 04 05 14
11 10 08 03 39 29 26 18 55 29 02
Longitude E
25O 25 31 30 27 30 32 31 29 30 33 30 31 30 31 31 31 31 31 31 31 31 32 31 30 25 31 30 33 31 32 34 32 32 29 30 32 35 32 32
58’ 11 49 25 14 31 15 39 53 31 46 57 23 28 07 07 00 30 11 08 15 13 33 20 51 29 06 44 51 13 43 18 15 39 00 34 34 52 56 53
Altitude
metres 22 4 3 2 7 3 1 2 32 10 10 6 7 6 8 9 14 13 14 20 20 21 10 116 30 - 15 28 39 3 55 75 7 70 78 122 72 82 4 10 2 111
65
TABLE 3 . 1
(continued)
Station Wadi H a l f a P o r t Sudan Dongola Karima Tokar Atbara Khartoum Kassala J e b e l Aulia Wad-Medani El-Dueim E l Fasher Sennar Geneina E 1-0be i d S i nga Gallabat E l - Nahud El-Roseires Renk Ma1ak a 1 Addis Ababa Raga Gambe i 1a Akobo Wau Jimma Juba Tori t Loka Gulu Moroto Lira But i ab a Soroti Masindi Fort Portal Mubende Namulonge Eldoret K amp a 1a Entebbe Kitale Equator Kisumu Kericho Mbarara Kabale Bukoba Musoma Mwanza
Country
Latitude
__ Sudan
Ethiopia Sudan
Ethiopia Sudan
Uganda
Kenya Uganda Kenya Kenya Uganda Tanzania
21° 19 19 18 18 17 15 15 15 14 14 13 13 13 13 13 12 12 11 11 09 09 08 08 07 07 07 04 04 04 02 02 02 01 01 01 00 00 00 00 00 00 00 00 00 00 00 01 01 01 02
55 37 10 33 26 42 37 28 14 24 00 38 33 29 11 09 58 42 51 45 33 02 28 15 47 42 39 51 25 22 45 33 18 50 43 41 40 35 32 31 20 03 01 S 01 06 21 37 15 20 30 28
Longitude E
Altitude metres
31° 37 30 31 37 33 32 36 32 33 32 25 33 22 30 33 36 28 34 32 31 38 25 34 33 28 36 31 32 30 32 34 32 31 33 31 30 31 32 35 32 32 35
20 13 29 51 44 58 32 24 30 29 30 21 37 27 14 57 10 26 23 47 39 45 41 35 01 01 51 37 33 57 20 36 56 20 37 43 17 22 37 17 36 27 00
125 5 225 250 20 345 380 500 380 405 380 740 4 20 805 565 4 30 760 565 465 380 390 2450 460 450 400 435 1750 460 6 25 965 9 26 1524 1095 621 1127 1146 1539 1542 1148 2084 1230 1146 1896
35 34 35 30 29 31 33 32
33 35 20 39 59 49 48 55
2762 1146 2070 1443 1868 1137 1147 1140
66
The d a t a u s e d , p a r t o f which is i n c l u d e d i n t h e c l i m a t i c t a b l e s a v a i l a b l e i n t h i s book, are e x t r a c t e d from a number o f r e f e r e n c e s . Examples of t h e s e r e f e r e n c e s a r e : t h e p u b l i c a t i o n s o f t h e E a s t A f r i c a n M e t e o r o l o g i c a l Department (E.A.M.D.),
t h e C l i m a t o l o g i c a l Normals f o r E g y p t , t h e C l i m a t e s of A f r i c a and
some volumes o f t h e N i l e B a s i n . Each o f t h e c o u n t r i e s s h a r i n g t h e b a s i n o f t h e N i l e h a s i t s own n a t i o n a l network o f m e t e o r o l o g i c a l s t a t i o n s . However, t h e h y d r o m e t e o r o l o g i c a l p r o j e c t of t h e E q u a t o r i a l Lake a r e a had among i t s o b j e c t i v e s t h e s t r e n g t h e n i n g o f t h e p r e v i o u s l y e x i s t i n g networks i n Uganda, Kenya and T a n z a n i a , and l a t e r i n Rwanda and B u r u n d i . To f u l f i l l t h i s o b j e c t i v e , t w e n t y - f i v e m e t e o r o l o g i c a l s t a t i o n s have been e s t a b l i s h e d and t h i r t y e x i s t i n g s t a t i o n s have a l r e a d y been up-graded by t h e p r o v i s i o n of a d d i t i o n a l i n s t r u m e n t s . Moreover, 200 o r d i n a r y r a i n gauges have been i n s t a l l e d and 2 3 t o t a l i z e r s p l a c e d i n remote p l a c e s and i s l a n d s . F o r t h e e s t i m a t i o n of e v a p o r a t i o n from t h e E q u a t o r i a l L a k e s , a network of s i x s o l a r i m e t r e s f o r r a d i a t i o n measurement, s e v e n s t a t i o n s w i t h wind m a s t s f o r measurement of wind s p e e d , and e i g h t s t a t i o n s f o r t h e measurement of l a k e s u r f a c e water t e m p e r a t u r e , have been i n s t a l l e d . F o r t h e e s t i m a t i o n of e v a p o t r a n s p i r a t i o n ,
a network o f T h o r n t h w a i t e t a n k s
and some s p e c i a l l y s i m e t r e s have b e e n i n s t a l l e d a t a number o f l o c a t i o n s . An a u t o m a t i c w e a t h e r s t a t i o n h a d b e e n e s t a b l i s h e d on t h e Nabiyongo I s l a n d i n Lake V i c t o r i a , w i t h an a u x i l i a r y s t a t i o n a t E n t e b b e . The p r i n c i p a l s t a t i o n a t t h e l a t t e r h a s among i t s equipment t h e R u s s i a n 20 m 2 and GGI 3000 e v a p o r a t i o n pans (WMO, 1 9 7 4 ) . The m a j o r i t y o f t h e m e t e o r o l o g i c a l s t a t i o n s i n Egypt and t h e Sudan a r e s t a t i o n s of t h e s e c o n d o r d e r , where o b s e r v a t i o n s a r e t a k e n e v e r y day a t 0 8 . 0 0 , 1 4 . 0 0 and 20.00 h o u r s s t a n d a r d l o c a l t i m e . A t f i r s t - o r d e r s t a t i o n s , t h e o b s e r v a t i o n s a r e u s u a l l y t a k e n e i g h t t i m e s a day i n t h e s y n o p t i c h o u r s of o b s e r v a t i o n s , and a t t h i r d - o r d e r
s t a t i o n s , o b s e r v a t i o n s a r e taken a t 08.00 hours l o c a l t i m e
only. S c r e e n o b s e r v a t i o n s u s u a l l y c o m p r i s e t h e a i r t e m p e r a t u r e , maximum and minimum t e m p e r a t u r e s , a l l i n d e g r e e s c e n t i g r a d e , t h e b a r o m e t r i c p r e s s u r e i n m i l l i b a r s , t h e h u m i d i t y a s o b t a i n e d from t h e w e t and d r y b u l b t h e r m o m e t e r s , and t h e e v a p o r a t i v e c a p a c i t y of t h e a i r a s measured w i t h a P i c h e e v a p o r i m e t e r .
The
d u r a t i o n o f t h e b r i g h t s u n s h i n e i s measured m o s t l y by a Campbell-Stokes r e c o r d e r . The wind i s e x p r e s s e d by a number on t h e B e a u f o r t S c a l e when t h e wind f o r c e i s e s t i m a t e d , or i n k i l o m e t r e s p e r h o u r when i t s s p e e d i s measured by means o f an anemometer. The s o i l t e m p e r a t u r e i s measured a t a few l o c a t i o n s o n l y , and a t a d e p t h v a r y i n g from 0.60 m t o 2 . 1 0 m , whereas t h e g r a s s minimum t e m p e r a t u r e i s measured a t a h e i g h t o f 0 . 1 0 m above t h e ground l e v e l . The r i v e r and s e a t e m p e r a t u r e s a r e measured a t some s e l e c t e d s i t e s i n Egypt and t h e Sudan
67
The r a i n f a l l i s measured n o t o n l y a t t h e m e t e o r o l o g i c a l s t a t i o n s , b u t a l s o a t many o t h e r l o c a t i o n s . Most of t h e r a i n gauges i n s t a l l e d are c y l i n d e r i c a l i n form w i t h a c a t c h o f 200 c m 2 i n s u r f a c e a r e a and a r i m o f a b o u t 1 . 0 m h e i g h t from t h e ground s u r f a c e . The a n a l y s i s of t h e r a i n f a l l d a t a i s p r e s e n t e d i n Chapter 4 .
3.4
TEMPERATURE The mean d a i l y t e m p e r a t u r e f o r t h e months of t h e y e a r a t a number of s t a t i o n s
i n t h e N i l e B a s i n i s l i s t e d i n T a b l e 3 . 2 . These d a t a a r e b a s e d on t h e d a i l y mean t e m p e r a t u r e which is c a l c u l a t e d on t h e b a s i s of t h e number of o b s e r v a t i o n s taken e v e r y d a y . S o , f o r a l l s t a t i o n s i n Egypt and Sudan o b s e r v i n g t h r i c e d a i l y , t h e mean t e m p e r a t u r e i s o n e - f o u r t h
t h e sum o f t h e t e m p e r a t u r e s a t 0 8 . 0 0 , 1 4 . 0 0 ,
and 20.00 h o u r s p l u s t h e minimum t e m p e r a t u r e . F o r s t a t i o n s o b s e r v i n g t w i c e d a i l y , t h e t e m p e r a t u r e is t h e mean of t h e 0 8 . 0 0 h o u r and 20.00 h o u r o b s e r v a t i o n s , and for s t a t i o n s o b s e r v i n g once d a i l y t h e t e m p e r a t u r e is s i m p l y t h e mean of t h e maximum and minimum t e m p e r a t u r e s . The m e t e o r o l o g i c a l s t a t i o n s i n s t a l l e d by t h e h y d r o m e t e o r o l o g i c a l p r o j e c t i n t h e E q u a t o r i a l Lake P l a t e a u as w e l l a s t h e up-graded are spot-read
s t a t i o n s i n East Africa
v i s u a l l y d u r i n g t h e s y n o p t i c h o u r s : 06.00 Z*,
09.00 Z a n d 1 2 . 0 0 Z .
The mean d a i l y t e m p e r a t u r e shows a d i s t i n c t p a t t e r n c h a r a c t e r i s t i c of each p a r t i n t h e N i l e B a s i n . G e n e r a l l y s p e a k i n g , t h e c o o l e s t month i n Egypt i s January and t h e warmest i s J u l y , e x c e p t a l o n g t h e c o a s t s of t h e M e d i t e r r a n e a n and t h e Red S e a s , where August i s t h e warmest month. F o r t h e g r e a t e r p a r t of t h e Sudan, J a n u a r y i s t h e c o o l e s t month. The month w i t h t h e h i g h e s t mean d a i l y t e m p e r a t u r e changes r a p i d l y w i t h l a t i t u d e from J u l y i n Wadi H a l f a , s i m i l a r t o Egypt, t o May i n Wad Medani, A p r i l i n Malakal and F e b r u a r y i n J u b a down s o u t h . This main c y c l e i s f o l l o w e d , i n many p l a c e s , by a less pronounced c y c l e where t h e second minimum f a l l s i n August and t h e second maximum i n September or O c t o b e r . I n t h e E q u a t o r i a l Lake P l a t e a u t h e wave o f t h e mean d a i l y t e m p e r a t u r e
is q u i t e s i m i l a r t o t h a t i n t h e s o u t h e r n p a r t o f t h e Sudan. The primary maximum o c c u r s i n F e b r u a r y and t h e l o w e s t t e m p e r a t u r e i n J u l y . The s e c o n d a r y maximum t a k e s p l a c e i n O c t o b e r and i s f o l l o w e d by a s e c o n d a r y minimum i n November. The r a t i o of t h e mean d a i l y t e m p e r a t u r e i n t h e warmest month t o t h e mean d a i l y t e m p e r a t u r e i n t h e c o o l e s t month,
(Tmx /? mn ) ,
h a s been computed f o r a l l s t a t i o n s
given i n T a b l e 3 . 1 and p l o t t e d v e r s u s t h e l a t i t u d e ,
4.
Three curves a r e
o b t a i n e d a s shown i n F i g . 3 . 3 . I n t h e v e r y n o r t h e r n l a t i t u d e , a b o u t N 3 Z 0 ,
the
grouping of t h e s t a t i o n s i s n o t c l e a r . South of t h i s l a t i t u d e one c a n e a s i l y = Greenwich M e r i d i a n Time = mean d a i l y t e m p e r a t u r e i n t h e warmest month, and TmX= mean d a i l y t e m p e r a t u r e i n t h e c o o l e s t month mn
*Z
T
= G.M.T.
68
d i s t i n g u i s h o n e c u r v e f o r t h e s t a t i o n s l o c a t e d w e s t of t h e N i l e , a n o t h e r c u r v e
for t h e s t a t i o n s on t h e N i l e and i t s t r i b u t a r i e s and between t h e b r a n c h e s and a t h i r d c u r v e f o r t h o s e s t a t i o n s on t h e Red S e a c o a s t and e a s t o f t h e N i l e .
In
each c a s e t h e c u r v e c o n s i s t s o f a v e r y s h o r t r i s i n g limb f o l l o w e d by a l o n g f a l l i n g t a i l . The peak o c c u r s a t a p p r o x i m a t e l a t i t u d e s o f 28O, 27O and 30° n o r t h f o r t h e t h r e e c u r v e s i n t h e o r d e r d e s c r i b e d a b o v e . The c o r r e s p o n d i n g
(Tmx /? mn )
r a t i o is about 2.65,
2 . 5 and 2 . 0 5 r e s p e c t i v e l y .
The a n n u a l mean d a i l y t e m p e r a t u r e a t t h o s e s t a t i o n s l i s t e d i n T a b l e 3 . 2 have
been used f o r p l o t t i n g t h e mean a n n u a l i s o t h e r m o v e r t h e N i l e B a s i n . The map i n F i g . 3 . 4 . shows t h a t t h e mean of t h e a n n u a l mean d a i l y t e m p e r a t u r e i n c r e a s e s a l m o s t s t e a d i l y from a b o u t 19OC on t h e M e d i t e r r a n e a n S e a c o a s t i n t h e n o r t h t o a l m o s t 29OC i n A t b a r a down s o u t h . A mean t e m p e r a t u r e of 29 t o 29.5OC c o v e r s t h e b e l t from A t b a r a t o Khartoum. S o u t h o f Khartoum t h e t e m p e r a t u r e f a l l s , b u t s l o w l y , t o r e a c h 26OC a l o n g t h e s o u t h e r n f r o n t i e r of t h e Sudan. North-west
of
Lake V i c t o r i a t h e t e m p e r a t u r e d r o p s r a t h e r r a p i d l y t o r e a c h a b o u t 2l0C i n Entebbe and 20°C
i n F o r t P o r t a l . The topography of t h e h i g h l a n d s i n t h e e a s t e r n
p a r t o f t h e N i l e B a s i n c a u s e s t h e c o o l i n g of t h e mean t e m p e r a t u r e
t o about
17OC a s shown i n F i g . 3.4. The mean a n n u a l t e m p e r a t u r e h a s a s m a l l r a n g e of v a r i a t i o n . T h i s r a n g e v a r i e s from a b o u t 3OC i n t h e major p a r t o f t h e E q u a t o r i a l Lake P l a t e a u t o a maximum o f less t h a n 6 O C i n t h e c e n t r a l p l a i n s of t h e Sudan. T h i s narrow r a n g e i s produced by t h e r e l a t i v e l y s m a l l a n n u a l v a r i a t i o n o f radiation.
I n c o n t r a s t , t h e d i u r n a l r a n g e of t e m p e r a t u r e is q u i t e l a r g e . The
f i g u r e s p r e s e n t e d i n T a b l e 3 . 3 show t h e r a n g e t o b e l a r g e s t i n t h e n o r t h e r n p a r t o f t h e Sudan and t h e s o u t h e r n p a r t o f Egypt,
and s m a l l e s t i n t h e
M e d i t e r r a n e a n and Red S e a a r e a s and t h e Lake P l a t e a u a r e a .
TABLE 3 . 2
Mean d a i l y t e m p e r a t u r e a t c e r t a i n s t a t i o n s i n t h e N i l e B a s i n ( d a t a a r e from G r i f f i t h s , 1972; I r e l a n d , 1948; M i n i s t r y o f War and Marine, Egypt, 1950; WMO, 1974)
Mean d a i l y temperature i n
Station Jan.
Feb.
Mar.
Apr.
May
Jun.
Jul.
Aug.
OC
for Sep.
Oct.
Nov .
Dee.
Year
~
S i d i Barrani Salum ( O b s e r v a t o r y ) D ami e t t a Rosetta Mersa Matruh Port Said Alexandria Mansura Damanhur Tanta Zagazig D e l t a Barrage C a i r o (Ezbek i y a ) Giza S u e z ( P o r t Tewfik) Helwan Fay urn Siwa Beni Suef Minya Hurghada Assiut Qena Quss e i r Nag Hammadi Luxor Dakhla Kharga
12.3 11 3 13.2 15.2 12 4 13.7 13.7 13.4 13 6 11.6 11.5 13.0 12.3 11.2 13.8 12.3 11.6 10.7 12.4 12.2 15.8 11.7 13.2 17.8 1 2 .o 13 .O 12.3 13.1
13.2 12.3 14 . O 15.2 12.9 14 . 3 14.1 14 . O 14.2 12.3 12.6 14.0 13.5 12.5 14.6 13.5 13.2 12.6 14.1 14.1 16.2 13.3 15 .O 18.4 12.7 15.4 14.1 14.9
14.2 14.1 15.3 16 , 7 14.5 16.2 15.8 16.2 16.2 14.9 15.2 16 . O 16.3 15.4 17.1 16.4 16 1 15.8 16.9 17.1 18 . 7 17.1 19.4 20.7 16.6 19.4 18.1 19.1
16.6 16.8 18.3 19 . o 17.2 18.7 18.1 19.8 19.4 18.7 18.9 19.8 20 . 2 19.2 20.5 20.4 20.4 20.3 20.2 21.4 22.2 22.2 24.6 23.4 21.5 25 . o 23.4 23.9
19.3 19.4 22.2 22.0 19.9 21.8 2 1 .o 23.8 22.9 22.9 23.0 23.7 24.2 23.3 24.4 24.3 25.1 25.3 23.7 26.1 26.2 26.6 29.8 26.8 27.3 30.2 28.2 29 . o
22.0 22.3 24.2 24 . 5 22 9 24.6 23.6 2fi 3 25 2 25.4 25.7 26.2 26.8 26 . O 26.9 26.6 27.2 27.9 26.6 28 . O 28.5 28.8 31.7 28.9 28.5 31.4 30.4
23.8 24.3 26 . O 26.3 24 7 26.4 25.4 27 . 8 26.4 26.5 26.8 27.7 27.7 26.9 28.4 27.5 28.1 28.9 27.4 29 . O 29.5 29.4 32 .O 29.8 29.1 32.3 30.8
24.5 24.3 26.2 27.2 25.5 26.9 26.2 27 8 26.6 26.4 26.6 27.4 27.6 26.7 28.5 27.4 28.0 28.5 27.6 28.7 30 .O 29.1 32.1 30.3 29 4 32.1 30.5
23.5 23.0 24.6 26.3 24.4 25 . 8 25.3 26.1 25.2 24.4 24.5 25.2 25.3 24.3 26.3 25.4 25.6 26 . O 24.6 26.1 28.0 26.5 29.1 28.7 26.9 29.7 28.1
20.9 21.2 23.2 24.5 22.2 23.9 23.3 24 .o 23.5 22.1 22.4 23.2 22.7 22.0 24 . O 23.3 23.2 22.4 22.6 23.8 25.2 23.8 26 . O 26.7 24.6 26.8 24.9
17.4 17.8 19.8 21.2 18.7 20.4 19.9 2F. 3 19.8 18.3 18.4 19.2 18.7 18 . O 20.0 19 18.7 17.4 18.5 19.2 21.2 18.6 20.3 23.4 19.6 20.5 19.2
13.6 13.4 15.4 17.2 14.4 15.6 15,7 15.4 15.5 13.6 13.4 14.8 14 . O 13.2 15.4 14.1 13.5 12.3 13.5 14 . O 17.7 13.6 15 . O 19.6 14.6 15.1 13.8
18.4 18.4 20.2 21.3 19.1 20.7 2c.2 21.2 20.7 19 . 8 19.9 20.8 20.8 19.9 21.7 20.8 20.9 20.7 20.7 21.6 23.3 21.7 24.0 24.5 21.9 24 , 2 22.8
31.0
31.4
31.1
28.6
26 - 0
20.2
15 . O
23.6
.o
TABLE 3.2
4 0
(continued)
Mean d a i l y temperature i n OC f o r Station
Esna D e a d a l us Kom Ombo Aswan Wadi H a l f a P o r t Sudan Tokar Atbara Khar tourn Kassala W ad-Medani E l -Dueim Fasher Sennar Geneina El-Obeid Singa Gal l a b a t El-Nahud E l Roseires Renk Malakal Addis Ababa Raga Gambei l a Akobo Wau Jimma Juba Tori t
Jan.
Feb.
Mar.
Apr
May
Jun.
Jul.
Aug.
Sep.
Oct.
Nov.
Dec.
Year
13.1 21.8 14.9 15.5 15.9 23.5 24.4 22.2 23.6 25 .O 24.2 23.7 20.6 25 .O 22.4 21.0 25.7 26.0 23.4 26.2 24.7 27 .O 15.6 24.2 27.5 28.4 26.7 19.5 28.8 28.3
14.4 21.5 16.3 17.2 17.5 23.2 24.3 23.4 25 .O 26.1 25.2 25 . O 22.2 26.1 24.5 22.6 26.8 27.5 24.2 27.5 26.5 28.4 16.9 25.8 28.6 29.8 28 . O 20 .o 29.6 28.9
18.5 22.7 20.4 21.3 21.9 24.2 26 . O 26.6 28.2 28.8 28.2 27.8 25.3 29.1 25.8 25.8 29.6 29.4 25.6 29.8 28.1 30.4 18.2 27.4 29.9 30.9 29.5 20.0 29.5 28.8
23.3 24.6 25 . O 26.2 26.7 26.6 28 . O 30.4 31.4 31.6 31.0 30.9 28.4 32.0 29.1 29.3 32.0 30.6 30.4 31.6 31.9 31.0 18.0 28.8 29.4 30.2 29.8 20 .o 28.8 28.1
27.6 27.2 29.6 30.5 30.5 29.4 30.9 33.4 33.6 33.2 32.5 32.1 30.1 32.4 29.6 30.7 32.1 29.7 30.4 31.0 31.2 29.4 18.7 28.0 27.5 28 .O 28.4 19.5 27.4 26.8
29.7 28.3 31.2 32.9 32.2 32.3 33.4 34.8 33.6 32.1 31.9 31.8 30.5 31.2 29.4 30.2 30.3 26.6 30 . O 28.6 29.3 27.4 17.5 26.6 26.2 27.1 27.2 19 .o 26.5 25.9
30.1 29.7 31.4 33.2 32.2 34.5 35 . O 33.6 31.7 29.1 29.1 29.6 28.7 28.4 27.1 28.0 27.8 24.0 27.4 26.8 27.1 26.3 16.5 25.5 25.6 26.1 26.2 18 . O 25.5 24.8
30.1 30.3 31.4 33 . O 32.2 34.8 35.0 32.7 30.6 28.0 27.8 28.4 27 .O 27.4 25.4 27.0 26.8 23.8 26.6 26.4 26.3 26.2 16.1 25 , 2 25.6 26.1 26 . O 18.0 25.6 24.8
27.9 29.1 29.4 30.9 30.5 32.2 33.6 33.6 32.2 29.6 28.9 29.5 28.1 28.2 26.1 27.9 27.6 24.6 27.6 26.9 26.9 27 . O 16.3 25.8 25.9 26.9 26.6 18.5 26.4 25.8
24.2 28 .O 27.2 28.3 28.2 29.4 30.6 31.6 32.1 31.2 30.2 30.8 27.8 29.9 26.1 28.7 29.4 26 . O 28.4 28.0 28.5 27.8 16.5 26.6 26.5 27.4 27.4 18.0 27 .2 26.4
19.1 26.1 21.6 22.6 22.6 27.4 28.0 27.4 28.4 29.4 27.7 28.2 23.8 28.4 24.2 25.3 28.5 26 . O 24.8 27.8 27.3 27.6 15.1 25.3 26.8 28.2 27.5 18 . O 27.7 27 . O
14 .G 23.3 16.8 17.4 17.6 25 .O 25.6 28.2 25 .O 26.2 25.0 24.8 21.1 26 .O 23.2 22.0 26.4 25.6 23.7 26.5 26.5 27 . O 14.8 24.2 27.0 27.8 26.8 18.0 28.1 26.9
22.7 26 .O 24.6 25.8 25.7 28.6 29.6 29.5 29.6 29.2 28.5 28.6 26.1 28.7 26.1 26.6 28.6 26.6 26.9 28.1 27.9 28 . O 16.7 26.1 27.2 28.1 27.5 19 .o 27.6 26.9
TABLE 3.2
(continued)
S t a t ion
Gulu Lira Butiaba Soroti Masindi Fort Portal Entebbe Kisumu Bukoba Mwanza
Mean d a i l y t e m p e r a t u r e i n
OC
for
Jan.
Feb.
Mar.
Apr.
May
Jun.
Jul.
Aug.
Sep.
Oct.
Nov.
Dec.
Year
24.2 24.5 26.1 25.5 23.8 19.9 21.7 25.1 20.2 22.6
24.8 25.1 26.5 25.9 24.1 20.2 21.7 24.3 20.3 22.5
24.5 24.9 26.5 25.7 24.0 20.1 21.8 24.0 20.4 22.2
23.6 23.8 25.9 24.4 23.3 20.0 21.3 23.6 20.3 22.0
22.5 22.9 25.7 23.5 22.9 19.6 21.0 23.2 20.2 22.2
22.2 22.9 25.3 23.1 22.3 19.2 20.8 22.7 20.3 22.0
21.6 21.7 24.8 22.5 21.6 19.0 20.4 22.2 20.2 21.8
21.8 21.9 24.5 22.6 21.5 19.2 20.4 22.6 20.4 22.2
22.3 22.5 25.1 23.3 21.9 19.4 20.8 23.3 20.5 23.0
22.8 22.9 25.5 24.1 22.5 19.0 21.2 23.8 20.6 23.4
23.6 23.3 25.6 24.5 22.9 19.2 21.2 24.6 20.4 23.1
23.4 23.5 25.7 24.7 22.9 19.6 21.3 24.9 20.2 22.8
23.2 23.3 25.6 24.1 22.8 19.5 21.1 23.5 20.4 22.5
72 0
c
a,
.r(
1
o m
a
a,
k
5
Y
fi
;d
C
x
.d
rl
a,
a
o 0
9
9 P Y
a,
9
Y
73
Fig. 3 . 4 .
Map showing t h e mean annual isotherms i n t h e N i l e B a s i n area
4
TABLE 3.3
Mean d a i l y r a n g e of t e m p e r a t u r e a t a number of s t a t i o n s i n t h e N i l e
B a s i n ( I r e l a n d , 1948; M i n i s t r y of War
and M a r i n e , E g y p t , 1950; WMO 1974)
Station
Mean d a i l y r a n g e , i n OC, for Feb
Jan.
.
~
S i d i Barrani S a 1um ( Ob s e r va t o ry ) Damiet t a Rosetta Mersa Matruh Port Said A l e x a n d r ia Mansura Damanhur Tanta Zagazig D e l t a Barrage C a i r o (Ezbekiya) Giza S u e z ( P o r t Tewfik) Helwan Fayum Siwa Beni Suef Minya Hurghada Assiut Qena Qusseir Nag Hammadi Luxor Dakhla Kharga
10.6 9.8 9.8 5.8 9.2 8.2 7.9 12.6 12.1 13.6 14 .O 13.3 12.1 13.8 10.5 10.5 14.6 16.1 14.3 13.8 11.5 14 . O 15.8 8.8 15.0 1S.O 16.7 16.1
10.5 9.9 9.6 6.5 9.1 8.6 8.1 13.1 12.5 14.3 14.3 14.3 13.0 14.9 11 .o
11.5 14.9 16.4 15.4 14.5 12.4 15.3 17.5 8.7 16.8 19.4 17.7 17.1
Mar.
Apr.
__ 10.7 10.2 8.8 6.2 9.0 8.0 8.4 13.7 13.2 15.5 15.2 15.3 13.9 16.1 11.9 12.8 15.6 17.1 16.8 15.8 12.4 16.5 19.1 8.3 17.4 19.7 18.3 17.3
11.0 10.9 8.8 6.5 9.4 7.9 8.5 15.3 14.7 17.2 16.6 17.3 15.2 17.8 13.5 14.4 17.0 17.9 18.1 17.1 12.5 17.0 19.2 7.9 18.5 20.0 19.1 17.9
Jun .
May ~
11.6 9.8 8.8 5.7 8.8 7.9 8.2 15.7 14.8 17.4 16.8 17.6 15.7 17.9 13.9 15 .O 17 .O 18.0 18.2 17.0 12.1 16.6 18.0 7.6 17.5 19.5 18.3 16.8
Jul
.
Aug.
Sep.
Oct.
Nov.
Dec.
Year
8.5 9.7 8.7 4.5 G .6 8.0 6.8 14.0 12.4 15.2 14.8 14.6 13.3 15 .O 13.1 13.5 14.8 17.7 15.1 14.4 10.0 13.8 16.5 7.1 17.7 18.4 15.9 16.6
9.9 9.5 8.6 5.0 7.1 7.8 7.3 13.6 12.6 15.1 14.5 13.5 12.7 14.2 12.2 12.4 14.1 17.3 15.1 13.0 10 .o 12.8 16.1 6.9 14.6 17.8 15.5 15.4
10.4 10.6 8.3 5.8 8.4 7.6 8.2 12.9 13.0 14.7 14.4 13.7 13.0 14.4 11.8 11.7 14.3 17.6 14.7 12.7 11.0 12.1 16.3 7.3 15.3 18.2 16 .O 15.6
10.7 9.4 7.9 5.7 8.4 7.5 8.1 11.8 11.9 13.5 13.4 13.1 12.4 13.7 11.0 10.7 13.8 16.7 14.6 12.8 11.1 13.7 16.5 7.9 16.6 18.3 16.1 15.8
10.5 9.8 9.3 6.2 9.1 8.1 8.0 12.2 11.8 13.1 13.4 13.1 11.8 13.3 10.9 10.2 13.9 16.1 13.8 12.9 11.4 13.7 15.5 8.4 15.8 18.0 16.2 15.8
10.3 10.0 8.8 5.5 8.2 7.9 7.7 13.7 13.0 15.1 14.9 14.9 13.5 15.4 12.3 12.7 15.1 17.2 15.8 14.6 11.3 14.7 17.1 7.7 16.5 18.8 16.9 16.4
~
10.0 10.4 8.3 4.6 7.6 7.8 7.4 15.2 13.9 16.8 16.4 16.9 15.4 17.3 14 .O 14.9 16.3 18.5 17.7 16.3 11 .o 16.7 17.8 7.1 17.1 19.7 16.9 15.9
8.4 10.1 9.0 4.2 6.4 8.1 6.6 14.7 12.7 15.5 15.3 16.2 14.2 16.1 13.6 14.4 15.5 17.6 16.3 15.2 10.4 14.8 16.9 7.2 16.8 18.9 16.1 16.1
P
TABLE 3.3
(continued)
Mean d a i l y r a n g e , i n OC, f o r Station Jan.
Feb.
Mar.
Apr.
May
Jun. ~
Esna Deadalus I s l a n d K o m Ombo Aswan Wadi H a l f a P o r t Sudan Tokar Atbara Kh a r t o um Kassala Wad Medani El Dueim El Fasher Sennar Geneina El -0be i d Singa Gallabat E l -Nahud E l Roseires Renk Malakal Addis Ababa Raga Gambeila Akobo W au Juba Gulu Moro to
16.4 3.8 16.2 13.4 16.2 7.2 8.7 16.3 16.9 18.1 19.8 18.6 21.5 19.8 22.9 18.8 18.8 20 . o 16.9 20.6 19.1 17.5 17.0 22.8 18.6 16.4 18.2 17.2 15.5 15.0
17.1 3.8 17.4 14.3 17.6 8.0 9.2 17.2 17.6 19 .o 20.4 19.6 21.8 20.6 22.6 19.5 19.4 19.1 18.7 20.6 19.0 17.5 17.0 22.4 17.9 15.7 17.2 16.2 15.1 14.8
18.1 4.1 18.9 15.9 18.9 9 .o 10.2 18.1 19.0 19.1 20.8 20.3 22.2 21.2 21.2 19.9 20 .o 18.7 18.3 20 .o 19.5 17.4 16 .O 21.2 17.2 15.2 16.8 15.1 13.6 13.4
18.4 4.4 19.7 16.3 19.5 10.1 11.5 18.7 18.7 18.1 20.3 20.0 21.3 19.9 20.4 18.8 18.8 17.7 17.7 18.4 16.7 15.3 15 . O 16.9 14.8 13.7 14.7 13.6 11.6 12.2
19 .o 4.2 18.7 15.9 18.8 11.3 15.0 16.9 16.7 16 . O 17.4 17.6 18.8 17.3 19.3 16.8 16 .O 15.8 16.2 15.4 14.6 13.1 16 .O 14.4 12.2 11.9 13.2 12.1 10.5 12.1
18.0 4.5 18.7 15.9 18.2 13.2 17.4 16.1 15.0 14.6 15.2 15.4 17.2 15.6 17.3 14.6 14.6 13.7 13.5 13.1 12.3 11.3 13.0 12.9 11.1 11.4 12.1 12.0 10.5 12.3
Jul
.
Aug.
Sep.
Oct.
Nov.
Dec.
Year
16.5 5.1 17.8 14.7 16.8 11.9 13.1 14 .O 12.2 11.3
14.3 4.2 17.5 15.4 16.2 11.4 14.7 14.9 14.2 13.5 13.8 13.6 16 . O 13.7 14.8 13.4 12.9 12.5 12.5 12.6 11.5 10.7 11 .o 12.3 11.6 9.8 12.2 12.8 11.3 14.3
14.5 4.2 17.4 15.2 17.0 9.1 11.3 15.6 16.2 15.4 16.7 16.2 19.1 16.8 20.0 15.8 16.5 16.1 15.1 15.9 15.4 12.3 15 .O 14.0 13.8 11.3 13.5 14.0 12.0 13.6
16.4 4.2 16.3 14.1 16.5 7.4 10.1 15.5 16.3 16.3 18.6 17.3 21.3 18.7 20.5 17.9 18.6 19.5 16.3 17.4 18.3 16.3 18.0 19.6 15.7 13.8 16 .O 15.6 13.2 13.8
15.7 3.7 15.7 13.5 16.3 7 .O 8.9 15.9 16.7 17.3 19.3 17.9 21.8 19.3 22.1 18.8 18.5 10.2 18.2 20.4 18.8 17.7 17 .O 22.3 17.5 15.7 17.9 16.6 14.1 14 .O
16.8 4.3 17.8 15 .O 17.5 9.9 12.1 16.1 16.1 15.9 17.2 16.7 19.0 17.3 18.8 16.5 16.4 16.3 15.4 16.6 15.3 14.1 14.4 16.8 14.2 13.0 14.6 14.0 12.3 13.3
~
17.6 5.1 18.4 14.9 17.9 12.8 15.1 14.1 13.6 12.2 13.0 12.9 14.2 13.1 13 . O 12.1 11.9 11.1 10.8 11.2 10.1 10.8 9 .o 11.2 10.3 10.2 11.0 11.2 9.8 11.6
11.5 11.7 13.1 11.9 11.6 11.3 10.8 11 .o 10.4 11.1 8.9
9.7
9 .o 11.1 10.5 9.8 10 . 9 11.4 10.1 12.1
TABLE 3.3
(continued)
S t a ti o n
Lira Butiaba Mas i n d i F o r t Portal Mubende E l d o r et Entebhe Kitale Kericho Mharara I.:ab a 1e Bukoba Musoma Mwanza
Mean d a i l y r a n g e , i n 'C,
.~
tor
Jan.
Feb.
Mar.
Apr.
lay
Jun.
Jul.
Aug.
Sep.
Oct.
Nov.
Dec.
Year
17.2 7 9 14.2 14.6 10.4 16 0 9 8 10 0 15.2 12.6 14.1 10 7 8.8 8 8
16.5 7.5
14 9 7.2 12 8
12 8 7.3 11.5 11.9 9 .0 13 3 7.8 12.7 1: 8 11 . o 11.5 9 4 9.0 9.2
11.7 7.2 10.7 11.2 8.2 12.9 7.7 12 4 11 8 10.8 11.0 9.1 9.4 9.8
11.5 7.3 11.2 12.1 8.3 13.3 8.3
11.2 7.0 10.6 12.2 8.7 11.7 8.8
12.7
11 4
11.5
10.6 12.1 13.2 10.1 9.9 11.8
11.1 13.1 14.5 10.3 10.2 12.4
11.7 6.5 10.7 11.7 8.9 12.1 8.8 11.2 12.0 12.2 13.6 15.3
13.8 7 3 11.7 12.1 8 8 13 8 9 2 11 1 13 5 11 2 12.9 10.1
14.7 7.4 12.2 11.8 9.1 13.5 8.9 10 7 13 7 11.5 12.5 9.9 8.8 9.2
15.5 7.8 12.9 12.8 9.1 14.0 9 0 10.6 12.5 11.5 13.0 10.3 8.8 8.8
13.7 7.3 12.0 12.6 9.2 13.9 8.9 11 0 12.9 11.8 13.1 10.1 9.3 10 . o
14.1
14.6 10.6
16 8 9 7 10.4 15 4 12 5
13.8 10.5 9.0
9.3
1.3 5 10 0 15 9
8 8
11 5
14 6 1 : 8 13 1 10.3 9.2 9.4
9.5
11.2
7.4 11.3 12.2
a .O
14 .Q 9 4 10 . 7 i2 9 11.6 13 6 10.4 9.4 10.5
__
9.4 9.7
__
3.5
ATMOSPHERIC HUMIDITY The a t m o s p h e r i c h u m i d i t y i n t h e N i l e B a s i n area i s e x p r e s s e d m o s t l y i n terms
of
t h e r e l a t i v e h u m i d i t y . I n E g y p t and t h e S u d a n t h i s m e a s u r e m e n t i s made o n c e
or twice a day, except, of course, a t t h e f i r s t - o r d e r
s t a t i o n s . For s t a t i o n s
o b s e r v i n g t w i c e or more a d a y , t h e mean r e l a t i v e h u m i d i t y is t h e mean o f t h e r e l a t i v e h u m i d i t i e s m e a s u r e d a t 08 00 h o u r s and 20 00 h o u r s , and f o r t h o s e o b s e r v i n g o n c e d a i l y , i t i s s i m p l y t h a t m e a s u r e m e n t made a t 08 00 h o u r s . F o r t h e up-graded s t a t i o n s i n t h e c a t c h m e n t s o f t h e E q u a t o r i a l L a k e s , t h e measurements o f t h e r e l a t i v e h u m i d i t y a r e made a t 0 3 0 0 , 0 6 00 and 1 2 00 Z h o u r s , and t h e o t h e r s a r e a t 0 6 00 a n d 1 2 00 Z h o u r s o n l y . The d e v i a t i o n s from t h e mean d a i l y r e l a t i v e h u m i d i t y a v e r a g e d f o r J a n u a r y , A p r i l , J u l y and O c t o b e r f o r A l . e x a n d r i a , E g y p t , Khartoum a n d t h e S u d a n a r e shown i n F i g s . 3 . 5 ( a ) and 3 . 5 ( b ) r e s p e c t i v e l y . The g r a p h s f o r A l e x a n d r i a , and s i m i l a r l y f o r many s t a t i o n s , show t h a t t h e d a i l y mean is r e a c h e d t w i c e e v e r y d a y ; o n c e b e t w e e n 0 7 30 h o u r s and 10 30 h o u r s ( f o r e noon) and a n o t h e r t i m e b e t w e e n 17 30 h o u r s a n d 22 00 h o u r s ( a f t e r n o o n ) , dependi n g o n t h e l o c a t i o n and month o f t h e y e a r . T h i s i s n e a r l y s o e v e r y w h e r e i n t h e N i l e B a s i n , e x c l u d i n g Khartoum ( F i g . 3 . 5 ( b ) ) . G e n e r a l l y s p e a k i n g , t h e r e f o r e ,
in
t h e a b s e n c e of c o n t i n u o u s , o r f r e q u e n t , m e a s u r e m e n t s o f t h e r e l a t i v e h u m i d i t y , t h e a v e r a g e o f t h e o b s e r v a t i o n s made a t 08 00 h o u r s and 20 00 h o u r s , o r s i m p l y t h e o b s e r v a t i o n a t 08 00 h o u r s w i l l n o t b e t o o f a r f r o m t h e t r u e mean ( O l i v i e r , H.,
1 9 6 1 ) . The mean d a i l y r e l a t i v e h u m i d i t y f o r some s t a t i o n s i n E g y p t ,
the
h u m i d i t y a t 08 00 h o u r s i n t h e S u d a n a n d a t 0 6 00 h o u r s e l s e w h e r e , a r e i n c l u d e d i n Table 3 . 4 , The r e l a t i v e h u m id ity a t noon, H n ,
s e e m s t o bear a c e r t a i n relationship t o
t h e d a i l y mean h u m i d i t y , H m , o r t o t h e h u m i d i t y a t 08 00 h o u r s o r any o t h e r r e f e r e n c e hour'. thru'
T h i s r e l a t i o n s h i p i s shown g r a p h i c a l l y i n F i g s . 3 . 6 ( a ) and
( d ) f o r a number o f c o m b i n a t i o n o f s t a t i o n s f r o m some p a r t s o f t h e N i l e
Basin. Of i n t e r e s t i s F i g . 3 . 6 ( c ) , w h i c h shows two d i s t i n c t r e l a t i . o n s h i p s ; one for s t a t i o n s l o c a t e d a l o n g t h e M e d i t e r r a n e a n S e a c o a s t and t h e S u e z C a n a l , and t h e o t h e r f o r t h o s e s t a t i o n s l o c a t e d a l o n g t h e Red S e a c o a s t . F o r v a l u e s of H t h a n 80%, t h e r e l a t i v e h u m i d i t y a t n o o n , H n ,
less
i s much more f o r t h e s t a t i o n s on
t h e Red S e a t h a n f o r t h e s t a t i o n s o n t h e c o a s t o f t h e M e d i t e r r a n e a n S e a . The p l o t o f t h e HO6 h u m i d i t y v e r s u s t h e H12
r e l a t i v e humidity f o r t h e Equatorial
Lake P l a t e a u a r e a d o e s n o t e x h i b i t a s i n g l e r e l a t i o n s h i p f o r a l l t h e s t a t i o n s i n t h e a r e a . Among t h e f a c t o r s i n f l u e n c i n g t h e r e l a t i o n s h i p
are t h e geographic
l o c a t i o n o f t h e s t a t i o n , t h e a l t i t u d e , a n d t h e d i s t a n c e from t h e s t a t i o n t o t h e n e a r e s t l a k e . The g r a p h i c p l o t f o r t h e s t a t i o n s e a s t and s o u t h - e a s t o f Lake + R e l a t i v e h u m i d i t y a t 06 00 Z h o u r s
78
V i c t o r i a p r e s e n t s a much w i d e r s c a t t e r t h a n d o e s t h e p l o t f o r t h e s t a t i o n s i n and w e s t of t h e same l a k e .
t h e n o r t h , north-west
The t i m e s a r e n o t u n i f o r m a c r o s s t h e N i l e B a s i n a r e a and t h e e a r l y o b s e r v a t i o n o f h u m i d i t y i s t a k e n a t a t i m e when t h e r e l a t i v e h u m i d i t y i s c h a n g i n g r a p i d l y . I n some c a s e s t h i s i s done w i t h o u t h a v i n g a r e a d i l y a v a i l a b l e s l m u l taneous temperature t o s p e c i f y t h e c l i m a t e completely, a s i s t h e c a s e with t h e noon h u m i d i t y . I n t h i s c o n n e c t i o n , G r i f f i t h s ,
F.J.
(1972) recommends t h e u s e
Of
o t h e r p a r a m e t e r s t o d e s c r i b e t h e a t m o s p h e r i c h u m i d i t y a s t h e dew p o i n t or t h e a b s o l u t e h u m i d i t y . T h e s e two p a r a m e t e r s do n o t show t h e l a r g e d i u r n a l f l u c t u a t i o n t y p i c a l of t h e r e l a t i v e humidity c u r v e .
p
10
A
0
0
5
+
0
c
o
5
0
€ - 5
-
E
2
-10
ul
-
C .-0
-15
.5_
>
0
2
4
8
6
Local
b
n
10
14
12
standard
16
18
22
20
24
t i m e (hours )
Fig. 3.5(a) D e v i a t i o n s from mean d a i l y r e l a t i v e h u m i d i t i e s f o r A l e x a n d r i a , Egypt ( O l i v i e r , H . , 1961)
$ 20 2' 15
-
- lo aJ n
- 15 - 20
0
2
4
6
8
10
Local s t a n d a r d
1 2 14 16 18 time (hours)
20
22
24
Fig, 3.5(b) D e v i a t i o n s from mean d a i l y r e l a t i v e h u m i d i t i e s f o r Khartoum, Sudan ( O l i v i e r , H . , 1961)
1
1
1
~
TABLE 3.4
The mean d a i l y r e l a t i v e h u m i d i t y a t a number o f s t a t i o n s i n t h e N i l e B a s i n ( G r i f f i t h s , 1972; I r e l a n d , 1948; M i n i s t r y of War and M a r i n e , E g y p t , 1950; W M O ,
-
Station Jan. Salum ( O b s e r v a t o r y ) Mersa Matruh Edfina Port Said Alexandria Tanta Zagazig C a i r o (Ezbekiya) Giza S u e z ( P o r t Tewfik) Helwan Fayum Siwa M i ny a Assiut Qena Qusseir Nag Hammadi Luxor Dakhla Kharga Esna Deadalus I s l a n d Aswan Wadi H a l f a Merowe Atbara Khartoum
71 76 79 76 69 81 83 74 79 68 61 68 70 64 69 63 56 69 68 51 58 61 68 45 48 31 38 29
Feb
72 73 78 75 68 78 79 68 72 66 56 63 64 58 62 56 54 59 58 47 54 54 70 40 40 26 34 24
1974)
Mean d a i l y r e l a t i v e h u m i d i t y , i n p e r c e n t , f o r
Mar.
Apr.
69 74 74 73 68 76 75 65 67 63 52 58 61 52 54 44 52 48 46 41 46 41 74 32 31 20 27 16
66 72 70 73 69 68 67 58 60 60 45 50 56 43 41 31 52 40 34 35 40 32 70 27 24 14 18 15
May
70 74 68 73 72 61 60 52 53 59 41 42 52 39 36 27 52 38 30 32 38 27 77 27 22 15 19 22
Jun.
Jul.
Aug.
Sep.
Oct.
Nov.
Dec.
Year
67 77 70 75 74 63 62 55 56 61 44 46 53 42 37 28 50 40 32 30 38 25 78 25 21 18 24 34
70 81 73 77 77 70 68 61 62 62 51 51 55 46 42 31 52 44 34 30 38 27 77 26 23 23 32 47
76 80 73 76 75 74 73 65 68 65 54 57 58 51 46 32 51 45 37 33 38 31 76 29 26 22 33 52
74 77 71 73 69 77 78 69 72 67 58 62 60 58 55 44 53 54 46 37 44 42 78 33 30 23 30 46
74 75 74 72 69 79 80 72 73 68 59 64 63 61 62 53 56 58 53 43 48 49 79 35 34 22 28 31
73 75 78 73 70 81 82 74 78 69 62 69 66 65 67 59 58 64 60 47 55 52 73 40 39 24 31 30
70 74 79 76 70 80 83 76 81 68 62 72 70 68 69 63 57 71 66 53 60 59 68 45 42 26 34 29
71 76 74 74 71 74 74 66 68 64 54 58 61 54 53 44 54 52 47 40 46 42 74 34 32 22 29 31
m
0
TABLE 3 . 4
(continued) Mean d a i l y r e l a t i v e h u m i d i t y , i n p e r c e n t , for Station
Gallabatx KassalaX P o r t SudanX E l RoseiresX Wad MedaniX E l ObeidX E l FasherX Malakal Addis AbabaX AkoboX W au Jub a ToritX Mongall a Gulu+ Moroto+ Lira+ Butiaba+ Masindi+ Fort Portal+ Mubende+ Eldoret+ Entebbe K i tale+ Kericho+ Mbarara+ Kab a l e + Bukoba+ Musoma+ Mwanza+ x =
Jan.
Feb.
Mar.
Apr.
45 62 66 41 36 37 35 30 61 43 35 43 39 50 63 57 70 68 73 87 77 62 84 70 66 85 94 85 74 77
43 56 65 34 26 28 28 24 64 43 29 41 45 52 66 55 73 67 73 87 77 58 88 76 70 84 95 86 74 78
36 48 64 27 21 23 24 36 58 45 36 50 53 55 73 62 78 70 75 89 81 62 90 73 78 85 96 86 76 79
38 40 56 31 20 26 21 46 65 63 45 63 67 68 83 72 84 73 80 90 86 73 85 80 85 87 97 88 80
r e ? a t i v e h u m i d i t y a t 0 8 h r 30
-
81
May 53 40 45 48 31 41 31 58 63 75 64 85 73 75 85 75 89 74
81
95
88
76 78 83 83 86 95 85 79 77
Jun.
Jul.
Aug.
Sep.
Oct.
Nov.
Dec.
Year
68 49 37 66 48 56 47 75 76 79 73 77 75
78 66 39 79 67 73 65 84 86 84 78 82 79 84 87 79 90 79 86 90 82 82 91 85 85 79 95 76 70 66
80 72 41 83 77 79 74 87 86 85 80 83 79 83 88 76 89 80 86 90 84 82 92 84 81 80 92 82 69 63
77 65 47 80 70 69 61 85 76 84 76 77 74 81 85 69 86 78 83 90 83 72 88 79 78 82 91 82 70 61
67 48 66 70 50 48 37 80 56 78 71 71 72 78 85 66 82 77 81 89 94 66 85 74 74 84 93 81 69 66
54 52 68 49 36 33 31 63 59 71 55 63 62 73 74 63 79 74 78
49 60 69 42 39 36 34 34 62 58 46 52 53 57 70 62 74 71 77 88 84 67 85 72 75 85 95 83 64 76
57 55 55 54 43 46 41 58 68 67 57 66 64 70 79 68 78 74 79 89 83 71 87 78 78 84 96 83 72 72
81
84 75 87 75 83 88 80 79 87 84 84 82 95 79 73 68
+ = r e l a t i v e h u m i d i t y a t 06 00 2 h o u r s
88 81
67 88 73 72 85 94 82 67 68
81
70
LEGEND
o Sakha 60
50
Qurashiya x Gemrneiza Tanta z Zagazig 3 Cairo I Giza Fayurn +
M Minya r Assiut 1 Qena Y N a g - Harnrnadi A Luxor lsna -0. A s w a n
$ 40 C
I
30
20
10
0
Fig. 3.6(a) The mean d a i l y r e l a t i v e h u m i d i t y , H , v e r s u s t h e r e l a t i v e humid i t y a t n o o n , H n , for t h e i n l a n d s t a t i o n s i n Egyp?.
70
LEGEhD
x Karirna 60
+
0 3
v 50
c
1
A
$40
Y
o
Wadi-Halfa r Atbara A Khatourn 2 Kassala I Wad Medani 3 E l Fasher 0 Geneina M El-Obeid z
Gallabat
E l - Nahud
Renk Malakal Akobo Wau Loca Juba Raga
C
I
30
20
10
0 Fig. 3.6(b) The r e l a t i v e h u m i d i t y a t 08 00 h o u r s , Hg8, v e r s u s t h e r e l a t i v e h u m i d i t y a t noon, H for t h e i n l a n d s t a t i o n s i n t h e u d a n . n'
82
0
co
0
h
0 W
0 ul
0 -3
'10 '
H
0
z
0
-
0
m
o
M
5 m
._1
fi
rl 0
X
.
L >
--..-
s
n L L
n
0 n
n >L
>> ><
r
5
2
2o
A S O N D
50
"
"
"
"
"
"
J F M A M J J A S O N D
40
30
20
10
o
I a--
I~
"
3 ~ "
"
" -- ~
'
J F M A M J J A S O N D
J F M A M J
J A S O N D
Month Fig. 4.11.
Main patterns o f monthly rainfall in percentage of annual rainfall
152 Group I X c o m p r i s e s t h e r e m a i n i n g s t a t i o n s e x c e p t N o . 250, s i t u a t e d a t Addis Ababa. The r a i n f a l l i n t h e s e c o n d h a l f o f t h e y e a r f o r t h i s group d i f f e r s from t h a t f o r Groups V I I and VIII i n t h a t i t i s d i s t r i b u t e d n e a r l y l i k e a t r i a n g l e . The monthly r a i n f a l l p a t t e r n a t s t a t i o n No. 250 i s q u i t e s i m i l a r t o t h a t o f Group 1 1 1 , e x c e p t t h a t t h e t a i l s h e r e e x t e n d t o c o v e r t h e whole y e a r . The month t o month v a r i a t i o n , sometimes r e f e r r e d t o a s s e a s o n a l v a r i a t i o n , can be formulated,
among o t h e r s , by a harmonic f u n c t i o n .
The e x p r e s s i o n u s e d i n t h e h y d r o m e t e o r o l o g i c a l s u r v e y o f t h e c a t c h m e n t s o f Lakes V i c t o r i a , Kyoga and A l b e r t , r e a d s
(4.10) where.
R t = r a i n f a l l a t any month t ( t = 1, 2 , A.
....,
= c o n s t a n t e q u a l t o t h e a n n u a l r a i n f a l l d i v i d e d by 1 2 ,
A r = a m p l i t u d e o f t h e r t h harmonic ( r = 1, 2 ,
$
12)
....,
= p h a s e a n g l e of t h e rth harmonic ( r = 1, 2 , F o r t h e above-mentioned
6 ) , and
. . . .,
6)
hydrometeorological survey, t h e parameters A
and
0
w e r e c a l c u l a t e d u s i n g t h e r a i n f a l l i n e a c h y e a r from 1931 up t o and i n c l u d i n g
1970 f o r e a c h of t h e 22 g a u g i n g s t a t i o n s i n c l u d e d i n t h a t s u r v e y . S i n c e t h e r e a r e 12 monthly v a l u e s , R t , p a r a m e t e r s c a n n o t exceed 1 2 ; 6 f o r A A
....,
and $ r ( r = 1, 2 ,
f o r each y e a r , t h e maximum number o f
and 6 f o r
0 (0,
= z e r o ) . The v a l u e s o f
6 ) a v e r a g e d o v e r t h e p e r i o d 1931-1970 a r e g i v e n i n
T a b l e 4.7. More d e t a i l e d i n f o r m a t i o n a b o u t t h e s e harmonic p a r a m e t e r s , as w e l l a s t h e i r maximum and minimum v a l u e s , d a t e s o f o c c u r r e n c e and d i s t r i b u t i o n i n s p a c e c a n b e found i n P a r t 1, V o l .
I o f t h e s u r v e y r e p o r t (WMO, 1974).
From T a b l e 4.7 i t i s a p p a r e n t t h a t t h e f i r s t two p a i r s o f t h e harmonic p a r a -
meters, i . e . A1,
+1 and A 2 , $ 2 , a r e by f a r t h e most i m p o r t a n t o f a l l s i x p a i r s
i n t h e s e r i e s . The r e l a t i v e i m p o r t a n c e o f a c e r t a i n o s c i l l a t i o n c a n b e measured by t h e s q u a r e o f t h e a m p l i t u d e of t h e c o r r e s p o n d i n g s i n e t e r m . The e x p l a i n a b l e p o r t i o n of t h e v a r i a n c e i n a series i s t h e s u m o f
4
(A ) * , whereas t h e t o t a l
variance can be expressed a s
6
Var ( R ~= ) where
E
f c
r=l
+
E
(4.11)
i s t h e r e s i d u a l l e f t a f t e r f i t t i n g t h e s i x h a r m o n i c s , which i s sometimes
r e f e r r e d t o as t h e u n e x p l a i n a b l e p o r t i o n o f t h e t o t a l v a r i a n c e .
TABLE 4 . 7
The v a l u e s of t h e harmonic p a r a m e t e r s i n t h e model d e s c r i b i n g t h e s e a s o n a l v a r i a t i o n of r a i n f a l l a t t h e l i s t e d s t a t i o n s (WMO,
1974)
Gauging s t a t i o n
No.
-
145 146 152 163 172 191 196 198 202 214 222 229 231 232 233 234 238 242 243 249
f
ff
Name El-Doret’ Bugondo** Gulu Moroto Masindi Mbale Fort P o r t a l Entebbe Masaka Kalangala Mbarara Mumias Kisumu Kericho Kisii Bukoba Tarime Musoma Igabiro Mwanza Biharamulo Shanwa
Values of harmonic parameters A1
43.9 46.9 84.5 54.4 35.3 50.9 18.5 60.4 25.9 76.7 18.2 62.3 41.0 48.6 36.9 85.8 39 . O 49.5 56.3 68.5 62.0 72.6
$1
A2
275.4 250.5 235.1 270.2 241.4 260.5 177.8 331.5 344.0 329.9 71.1 282.4 336.6 301.6 290.4 0.8 8.1 2.7 30.5 37.3 32.3 35.9
15.9 50.4 34.1 9.6 37.8 23.4 72.4 69.8 55.7 74.4 40.0 50.9 33.8 41.3 57.5 86.1 52.7 36.0 47.2 30.4 39.9 13.2
92 182.4 212.3 233.4 188.0 217.7 205.8 214.8 181.8 194.8 170.3 213.1 196.1 197.2 196.0 206.3 190.7 196.5 201.5 206.0 190.6 209.8 193.5
A3 27.3 20.0 16.5 19.8 10.8 13.9 5.8 28.4 21.9 30.2 10.2 18.2 23.1 25.1 24.4 47.2 23.1 22.1 21.0 27.9 23.3 25.3
$3 116.1 57.2 78.9 126.1 112.6 80.3 90 .o 81.9 61.9 89.7 96.7 53.5 101.2 81.6 87.6 66.5 103.3 103.1 100.1 105.6 115.8 109.6
A4
94
A5
21.2 11.1 13.9 14.0 6.3 7.2 3.2 17.0 6.8 12.4 6.1 10.6 6.6 5.7 5.9 17.5 4.3 8.0 4.4 6.7 5.4 5.4
226.9 267.8 244.8 266.5 260.8 256.1 291.0 229.3 240.7 178.5 205.9 225.9 181.4 234.1 161.5 261.5 289.8 291.1 328.1 344.9 292.0 9.7
8.0 7.7 9.5 7.4 2.9 5.0 1.2 4.7 11.3 7.4 4.1 5.5 3.1 1.7 2.9 12.8 3.3 5.9 5.2 4.3 9.7 8.0
El-Doret s t a t i o n i s s i t u a t e d a t OO36.N and 35O26’E. I t s a l t i t u d e i s 2287 m. Bugondo s t a t i o n is s i t u a t e d a t 1°37’N and 33017’E. I t s a l t i t u d e is 1067 m .
$5 164.0 117.1 358.0 88.7 20.5 74.2 241.9 79.4 88.0 83.7 221.5 106.9 177.3 23.3 230.4 115.3 153.4 203.1 171.1 241.9 208.6 189.8
3.8 5.3 15.3 5.3 5.8 0.7 6.2 8.5 4.0 4.0 3.8 2.2 3.5 3.8 5.3 5.5 2.5 5.2 0.7 0.3 1.2 7.7
154
The harmonic t e r m r = 1 means t h a t t h e p e r i o d o f t h e o s c i l l a t i o n i s 1 2 months and r = 2 means t h a t t h e c o r r e s p o n d i n g o s c i l l a t i o n h a s a p e r i o d o f 6 months. The l a t t e r i s t h e b i - a n n u a l o s c i l l a t i o n r e s p o n s i b l e f o r t h e t w o p e a k s i n A p r i l and November. T h i s i s v i s i b l e i n t h e p a t t e r n Groups V I t o I X , e s p e c i a l l y i n Groups V I I and I X ( s e e F i g . 4 . 1 1 . ) .
The h y d r o m e t e o r o l o g i c a l s u r v e y of t h e c a t c h m e n t s o f Lakes V i c t o r i a , Kyoga and A l b e r t i n c l u d e d t h e p r o b a b i l i s t i c m o d e l l i n g o f t h e monthly r a i n f a l l a t t h e chosen s t a t i o n s . I t h a s been r e p o r t e d t h a t t h e d i s t r i b u t i o n f u n c t i o n t h a t s e r v e s
as a good f i t t o t h e o b s e r v e d d a t a i s t h e two-parameter Gamma f u n c t i o n (WMO, 1 9 7 4 ) . T h i s f u n c t i o n c a n b e w r i t t e n as
/x P(X) =
xY-l
O
by
,-X/B
dx (4.12)
rw
where P(X)
r(y) y and
= cumulative p r o b a b i l i t y = gamma f u n c t i o n , and
B = parameters of t h e d i s t r i b u t i o n fu n ct i on
The monthly v a l u e s o f t h e y and $ p a r a m e t e r s f o r e a c h o f t h e 22 s t a t i o n s i n t h e c a t c h m e n t s o f t h e above-mentioned
l a k e s a r e i n c l u d e d i n T a b l e 4 . 8 . An
a c c u r a t e e s t i m a t e o f t h e s e p a r a m e t e r s is n e c e s s a r y f o r d e t e r m i n i n g t h e monthly r a i n d e p t h c o r r e s p o n d i n g t o a c e r t a i n r e t u r n p e r i o d . F o r t h i s p u r p o s e o n e needs
a t a b l e g i v i n g t h e c u m u l a t i v e gamma d i s t r i b u t i o n l i k e t h e o n e c o m p i l e d from
E.S. P e a r s o n and H.O. H a r t l e y ( ( 1 9 5 4 ) and p r e s e n t e d by C.T. Haan i n h i s books a s T a b l e E.8 (Haan, C . T . ,
1 9 7 7 ) . The p r o c e d u r e f o r d e t e r m i n i n g t h e magnitude of an
e v e n t f o r any g i v e n r e t u r n p e r i o d u s i n g t h i s t a b l e is e x p l a i n e d i n t h e same r e f e r e n c e . W e h a v e used t h i s p r o c e d u r e f o r e s t i m a t i n g t h e monthly r a i n f a l l w i t h r e t u r n p e r i o d s of 1 . 0 1 , 1.11, 2 , 5 , 10, 2 0 , 50 and 100 y e a r s a t Bukoba and t h e r e s u l t s o b t a i n e d are i n c l u d e d i n T a b l e 4 . 9 . N e e d l e s s t o s a y , t h i s same p r o c e d u r e c a n b e a p p l i e d t o any o t h e r s t a t i o n whose d a t a c a n b e w e l l f i t t e d by t h e gamma d i s t r i b u t i o n function.
TABLE 4 . 8
Values of t h e parameters of t h e Gamma d i s t r i b u t i o n f u n c t i o n f i t t e d t o t h e monthly r a i n f a l l a t 22 s t a t i o n s i n t h e catchments of Lakes V i c t o r i a , Kyoga and A l b e r t (WMO, El-Dore t
Month
Y Jan. Feb Mar. Apr May June July Aug Sep Oct.
. .
. . Nov . Dec . Month
1.094 0.844 1.684 2.485 3.012 2.175 8.586 6.102 1.623 2.146 1.443 1.079
Jan. Feb . Mar. Apr May June July Aug Sep. Oct.
. .
Nov.
Dec
.
1.314 1.819 5.897 10.518 8.360 3.208 2.306 6.778 7.686 12.869 7.314 3.493
Gulu (145)
B
Y
B
Y
B
28.60 70.99 45.16 67.35 43.56 38.16 15.55 19.82 36.60 26.72 57.67 48,23
1.152 1.428 1.438 4.310 7.492 3.241 4.866 5.458 3.815 3.473 1.109 1.196
20.30 39.13 64.66 44.67 24,24 29.37 21.65 25.93 40.20 41.23 81.96 49.41
1.017 0.913 3.386 8,440 9.228 5.751 6.256 10 .060 4.358 7.021 1.476 1.280
21.69 52.48 28.29 20.38 19.71 26.33 27.08 24.52 41.39 24.22 64.50 36.49
F . P o r t a l (172)
Y
Bugondo
Entebbe (191)
Masaka (196)
B
Y
B
Y
B
33.26 43.58 24.40 18.62 17.35 25.16 27.37 18.04 23.70 16.74 22.83 26.37
1.817 2.910 7.287 9.829 6.394 3.240 2.499 2.928 2.021 3.229 3.904 2.469
41.65 31.07 24.30 27.73 42.66 33.88 28.49 27.81 38.08 31.59 39.29 46.55
1.284 2.756 4.581 5.902 3.041 1.392 1.669 1.774 3.144 5.398 4.538 2.211
42.69 23.68 26.56 31.85 60.19 37.49 24.03 31.76 29.27 20.31 23.17 44.49
1974)
Moroto (146)
Y 0.809 0.994 1.235 1.671 2.264 1.478 2.795 1.928 1.144 1.346 0.649 0.987
B 19.43 40.35 64.58 75.25 61.62 59.23 49.74 57.84 47.67 36.46 100.78 32.49
Kalangala (198)
Y
8
Masindi (152)
Y 1.202 1.303 3.428 7.252 0.963 3.632 4.092 7.109 6.925 6.353 1.945 1.382
0 33.17 42.83 34.52 21.11 29.41 26.52 28.18 19.29 19.99 21.63 58.30 42.90
Mbarara (202)
Y 1.595 2.313 7.224 5.758 2.877 0.862 0.931 2.447 3.407 4.249 3.689 2.359
B 33.40 29.34 13.93 21.11 29.71 34.28 22.58 23.38 27.27 23.52 32.17 31.49
Mbale (163)
Y
B
0.879 1.418 2.262 5.340 7.532 5.982 8.058 4.898 4.580 4.499 1.182 1.516
40.43 37.78 44.78 29.75 22.29 18.96 15.18 25.97 22.22 19.27 55.45 33.08
Mumias (214)
Y
B
1.068 1.809 3.672 6.222 15.310 6.920 4.441 7.274 6.160 5.176 2.109 2.034
55.73 52.32 42.31 40.56 17.45 24.49 30.56 21.22 24.91 27.16 56.50 43.81
TABLE 4.8
(continued)
Month
Jan. Feb. Mar, Apr. M aY June July Aug . Sep Oct. Nov Dec
.
. .
Kisumu (222)
Kericho (229)
K i s i i (231)
Bukoba (232)
Tarime (233) ~
Y
B
1.011 2.079 4.075 5.568 4.873 2.307 2.722 2.303 3.226 1.964 1.219 2.189
52.42 40.67 34.00 33.37 29.87 32.85 19.85 31.76 18.63 27.19 75.05 43.72
1.571 50.47 1.995 47.64 2.635 62.95 5.965 41.50 11.248 19.89 9.600 14.44 4.295 26.43 8.684 16.01 4.870 24.33 4.525 24.89 2.464 47.65 2.395 45.84
1.442 41.67 1.750 53.51 2.689 68.04 6.343 40.36 12.457 17.15 6.205 22.22 3.795 26.92 6.942 21.71 3.541 45.43 4.047 35.88 2.214 65.72 1.966 59.73
4.318 3.245 9.632 12.596 7.986 2.523 1.418 2.776 3.754 5.408 6.196 4.544
Musoma (234)
Igabiro (238)
Mwanza (242)
Biharamulo (243)
Y
B
Y
B
Y
B 34.52 48.95 24.27 29.64 40.14 34.18 38.41 25.98 29.40 26.44 29.42 42.92
~~
Y
3.331 26.62 2.529 41.45 5.311 32.52 10.664 21.81 6.005 27.77 1.846 40.50 1.501 39.04 3.297 22.07 4.266 21.22 3.565 34.46 3.177 46.89 3.714 36.19 Shanwa (249)
Month
Jan. Feb. Mar. Apr May June July Aug Sep. Oct. Nov Dec
. .
. .
Y
B
1.378 1.409 2.033 6.207 3.947 1.372 0.680 1.222 1.142 1.520 2.185 1.791
46.55 51.14 62.58 29.13 27.55 18.02 40.54 17.66 25.09 26.43 38.21 38.89
Y 3.173 2.708 5.296 7.866 2.740 0.886 0.896 1.785 2.499 5.661 3.555 4.199
B 28.57 38.05 29.55 24.75 40.24 25.63 16.80 13.13 24.99 15.82 34.22 28.14
Y 4.168 1.987 2.514 7.592 2.516 0.953 0.746 0.183 1.461 1.193 2.275 2.595
8 24.27 51.64 61.17 23.97 35.03 25.39 27.08 24.86 25.90 57.09 56.89 56.56
Y 3.758 5.022 5.163 5.667 1.899 0.775 0.897 1.054 1.141 2.897 4.620 5.339
0 24.63 22.26 29.07 32.76 42.58 17.20 7.24 20.34 36.64 23 * 23 26.73 19.94
B
Y
3.438 3.039 4.593 4.027 1.319 1.298 18.848 0.848 1.545 1.037 1.759 4.046
B 32.00 36.63 27.75 37.08 35.24 4.65 0.28 12.31 8.11 28.79 50.76 33.01
157
S t a t i s t i c a l d e s c r i p t o r s o f monthly r a i n f a l l a t Bukoba i n t h e p e r i o d
TABLE 4.9
1931-1970, and t h e e s t i m a t e d monthly r a i n d e p t h f o r v a r i o u s r e t u r n periods Statistical descriptor
-
X Jan. Feb . Mar. Apr May June July Aug . Sep . Oct. Nov. Dec .
.
4.2.3
149 159 234 373 321 86 54 72 110 143 182 195
S
cV
72.11 81.82 73.37 102.74 109.11 56.24 42.97 37.97 56.81 62.18 81.89 94.96
0.4838 0.5151 0.3139 0.2752 0.3404 0.6523 0.7887 0.5264 0.5148 0.4349 0.4492 0.4869
Cs
0.83 0.15 0.07 0.21 0.17 1.88 1.08 0.26 0.71 0.58 1.49 1.40
R a i n f a l l , mm/month f o r r e t u r n p e r i o d , y r 1.01
1.11
31 25 94 172 117 09 02 09 21 38 59 40
66 59 144 245 187 28 07 24 46 69 97 79
2
5
135 142 227 359 307 74 40 63 100 134 174 161
200 225 296 451 409 125 85 103 153 190 238 237
-
10
20
50
243 279 331 507 470 158 115 129 187 225 279 284
276 328 366 557 528 190 148 155 218 258 321 327
328 387 413 619 596 232 186 186 257 294 365 381
100
366 436 4 39 656 642 258 215 208 285 320 385 4 20
Daily v a r i a t i o n
The day-to-day
e v o l u t i o n of r a i n i n E a s t A f r i c a was i n v e s t i g a t e d by D . H .
Johnson, u s i n g t h e s o - c a l l e d r e g i o n a l i n d e x o f r a i n i n e s s (1962). D a i l y r a i n f a l l s
were p l o t t e d on maps f o r e a c h day from November 1958 t o May 1960. The w e t a r e a s
were l o c a t e d and e a c h was g i v e n a n i n d e x number. T h i s number, sometimes r e f e r red t o as c o n c e n t r a t i o n , i s t h e p e r c e n t a g e , i n t e n s o f p e r c e n t , o f s t a t i o n s r e p o r t i n g r a i n t o t h e t o t a l number of s t a t i o n s c o n f i n e d t o t h e l o c a t e d a r e a . A l l d a i l y maps w e r e s o a n a l y z e d . A s a m p l e of t h e r e s u l t s o b t a i n e d is shown i n F i g . 4.12. T h i s k i n d o f a n a l y s i s i s q u i t e s u b j e c t i v e and c e r t a i n l y o f l i m i t e d v a l u e f o r d e s i g n p u r p o s e s . Moreover, t h e e x t e n t of t h e s h o r t - p e r i o d
variations i n rain-
f a l l d i s t r i b u t i o n i s s o l a r g e t h a t c o r r e s p o n d i n g months and s e a s o n s i n s u c c e s s i v e y e a r s would sometimes e x h i b i t v e r y d i f f e r e n t c h a r a c t e r s . The o r i g i n a l p a p e r by Johnson s u p p o r t s t h i s c o n c l u s i o n v e r y s t r o n g l y (1962). The h o u r - t o - h o u r ,
o r t h e d i u r n a l v a r i a t i o n o f r a i n f a l l on t h e Lake P l a t e a u ,
t h e Sudan, and Egypt h a s a l r e a d y been d i s c u s s e d i n C h a p t e r 3.
158
Fig. 4.12. Sequences of d a i l y r a i n f a l l i n d e x e s f o r t h e combined r e g i o n s A and B (Lake V i c t o r i a and Uganda), C and D (most o f K e n y a ) , and E t o I ( m a i n l y Tanganyika) . J a n u a r y t o e a r l y A p r i l 1959 ( J o h n s o n , D . H . , 1962) 4.3
EXTREME RAINFALL INTENSITY FOR DURATIONS OF ONE DAY AND SHORTER
The maximum r a i n f a l l i n a day d u r i n g t h e y e a r s o f o b s e r v a t i o n o f each r a i n f a l l s t a t i o n i n Egypt and t h e Sudan up t o 1967 i s l i s t e d i n T a b l e 4 . 1 0 . S i n c e t h e p e r i o d of o b s e r v a t i o n a t t h e s e s t a t i o n s v a r i e s i n a w i d e r a n g e , from a mini-
mum of a b o u t 20 y e a r s t o a maximum of 70 y e a r s or more, i t i s n o t p o s s i b l e t o reduce t h e observed extreme i n t e n s i t i e s t o a s i n g l e r e t u r n p e r i o d . The p r o c e d u r e used i n t h e h y d r o m e t e o r o l o g i c a l s u r v e y of t h e c a t c h m e n t s of Lakes V i c t o r i a , Kyoga and A l b e r t i s b a s e d on f i t t i n g t h e Gumbel Type I e x t r e m a l f u n c t i o n t o t h e d i s t r i b u t i o n o f t h e e s t i m a t e d c o n t i n u o u s 24-hour maximum, T h i s c a n b e o b t a i n e d from t h e r e l a t i o n (WMO, 1974)
4
R'
Rx.
(4.13)
Rx = (Rx)obs
+
where (Rx)obs
i s t h e o b s e r v e d f i x e d day maximum and R ' i s t h e h i g h e s t d a i l y
r a i n f a l l on t h e p r e c e d i n g o r f o l l o w i n g d a y . T h i s p r o c e d u r e made i t p o s s i b l e t o d e v e l o p , among o t h e r s , g e n e r a l i z e d c h a r t s of 1-day e x t r e m e r a i n f a l l of 2 and 100 y e a r s r e t u r n p e r i o d s and a nomogram f o r e s t i m a t i o n o f 1-day e x t r e m e r a i n f a l l c o r r e s p o n d i n g t o any g i v e n r e t u r n p e r i o d . A comparison between t h e t w o maps i n F i g s . 4 . 1 3 a . and 4 . 1 3 b . shows t h a t , a l l o v e r t h e a r e a c o v e r e d by t h e s u r v e y , t h e 100-year 1-day e x t r e m e r a i n f a l l i s n e a r l y t w i c e as much as t h e 2 - y e a r 1-day e x t r e m e r a i n f a l l . I f t h i s r a t i o h o l d s e v e r y w h e r e i n t h e N i l e B a s i n , o n e may u s e t h e nomogram shown i n F i g . 4 . 1 3 c . , t o g e t h e r w i t h Table 4.10, a f t e r c o r r e c t i n g t h e t abul at ed r a i n f a l l s , f o r t h e
159
e s t i m a t i o n o f t h e r a i n d e p t h c o r r e s p o n d i n g t o any r e t u r n p e r i o d . A f i x e d 24hour r a i n f a l l c a n b e c o r r e c t e d t o a c o n t i n u o u s 24-hour
r a i n f a l l s i m p l y by
i n c r e a s i n g t h e f o r m e r by a b o u t 10-13% o f i t s v a l u e . A s a n example, c o n s i d e r t h e 1-day e x t r e m e r a i n f a l l t h a t o c c u r r e d i n t h e 50-
y e a r p e r i o d 1918-67 a t s t a t i o n s 27 i n Egypt and 88 i n t h e Sudan. The o b s e r v e d d e p t h s , (Rx)obs,
R
X
w e r e 7 2 and 118 mm r e s p e c t i v e l y . T h e s e c a n b e c o r r e c t e d t o g i v e
o f 80 mm f o r s t a t i o n 27 and 130 mm f o r s t a t i o n 88. By p l o t t i n g t h e s e t w o
v a l u e s on t h e 50-year
r e t u r n p e r i o d l i n e of t h e nomogram, F i g . 4 . 1 3 c . ,
and draw-
i n g a s t r a i g h t l i n e p a s s i n g t h r o u g h each p o i n t s u c h t h a t t h e 100-year r e t u r n p e r i o d l i n e r e a d s a 1-day r a i n f a l l t w i c e as much a s t h e 1-day r a i n f a l l t o b e r e a d on t h e 2-year
Station No.
r e t u r n p e r i o d l i n e . One a r r i v e s a t t h e f o l l o w i n g r e s u l t s : R a i n f a l l , mm/day, f o r r e t u r n p e r i o d s o f 2-yr
5-yr
10-yr
20-yr
25-yr
50-yr
100-yr
27
44
55
64
71
73
80
88
88
71
88
103
114
118
130
142
I t g o e s w i t h o u t s a y i n g t h a t t h e above r e s u l t s s h o u l d b e r e g a r d e d as a p p r o x i mate o n l y . The East A f r i c a n M e t e o r o l o g i c a l D e p a r t m e n t , EAMD, h a s t h e p r a c t i c e of e x p r e s s i n g t h e extreme r a i n f a l l i n t e n s i t y f o r d u r a t i o n s s h o r t e r t h a n 1 day, ( I x ) t , by t h e r e l a t i o n (4.14) where I o , a , and n a r e p a r a m e t e r s v a r y i n g w i t h t i m e and s p a c e , and t i s t h e d u r a t i o n o f t h e r a i n s t o r m f o r which t h e maximum i n t e n s i t y ,
( I ) i s computed.
The p a r a m e t e r n h a s b e e n found t o v a r y between 0 . 5 and 1 . 0 . The s t r a n g e t h i n g i n e q . 4 . 1 4 i s t h a t i t d o e s n o t i n c l u d e any t e r m c o n n e c t e d t o t h e r e t u r n p e r i o d . On t h e o t h e r hand, t h e o b s e r v e d e x t r e m e 1-hour monthly and a n n u a l r a i n f a l l a t a few s t a t i o n s , a f t e r b e i n g c o r r e c t e d , h a v e b e e n f i t t e d by Gumbel Type-I
e x t r e m a l f u n c t i o n and e x t r e m e h o u r l y r a i n f a l l c o r r e s p o n d i n g t o any
g i v e n r e t u r n p e r i o d d e t e r m i n e d . The c o r r e c t i o n o f a f i x e d 1-hour i n t e n s i t y t o a c o n t i n u o u s 1-hour i n t e n s i t y i s d o n e , s i m i l a r t o t h e 1-day r a i n f a l l , by m u l t i p l y i n g t h e f i x e d i n t e n s i t y by a b o u t 1.13. F urther i n v e s t i g a t i o n of t h e intensity-duration-frequency-relationships
of
t h e r a i n f a l l o n t h e c a t c h m e n t s o f t h e E q u a t o r i a l Lakes is e s s e n t i a l b e f o r e s u c h i n c o n s i s t e n c y i s r e s o l v e d . Moreover, i t is c e r t a i n l y u s e f u l t o e x t e n d s u c h an i n v e s t i g a t i o n t o c o v e r o t h e r p a r t s o f t h e N i l e B a s i n area.
TABLE 4.10
Maximum r a i n f a l l i n a day a s observeh i n Egypt and t h e Sudan ( N i l e C o n t r o l S t a f f , 1972)
Station
No. 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
mm
No.
P e r i o d of observation
Maximum rainfall i n a day, mm
1948-67 1912-67 1946-67 1912-67 1931-67 1912-67 1931-67 1947-67 1928-67 1928-67
54 75 121 67 55 85 56 76 60 85
194 1-67 1931-67 1928-66 1942-67 1903-67 1928-67 1910-67 1936-67 1931-67 1948-67 193 1-67 1933-67 1931-67 1918-66 1921-67 1918-67 1910-67
48 41 49 65 56 58 68 59 40 49 48 51 37 59 102 72 56
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
1941-67 1931-67 1946-67 1931-67 1921-67 1941-67 1943-67 1946-67 1955-67 1935-67 1931-67 1942-67 1948-67 1931-67 1931-67 1931-67 1954-67 1935-67 1941-64 1906-67 1908-67 1908-67 1919-67 1917-67 1913-67 1909-67 1908-67 1907-67
20 23 17 16 37 10 25 03 11 53 34 39 06 08 08 39 04 06 19 112 53 71 101 63 82 130 77 98
P e r i o d of observation
-
Maximum rainfall i n a day,
-
S t a ti o n
Station
Maximum rainfall i n a day,
No.
P e r i o d of observation
95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111
1942-67 1910-67 1914-67 1928-67 1942-67 1943-67 1905-67 1929-67 1911-67 1908-67 1911-67 1908-67 1918-67 1915-67 1915-67 1900-67 1906-67
128 115 105 87 97 193 130 105 99 110 170 108 171 97 107 116 145
113 114 115 116 117 118 119 120 121 122
1910-67 1915-67 1913-67 1906-67 1900-67 1915-67 1906-67 1919-64 1922-67 1932-67
127 178 114 140 144 126 130 160 140 164
112
-
mm
-
TABLE 4.10
(continued) Station
No.
Period of observation
Maximrainfall in a day, mm
29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47
1931-67 1914-67 1938-64 1926-67 1946-67 1938-67 1921-67 1946-67 1941-67 1936-67 1931-67 1932-67 1931-67 1907-67 1943-67 1931-67 1921-67 1928-67 193 1-67
39 75 72 24 28 32 28 70 27 28 53 32 50 32 49 33 142 16 44
Station
No. 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94
Period of observation
Maximum rainfall in a day, mm
Station
No.
Period of observation
Naximum rainfall in a day, mm
1914-67 1905-67 1908-67 1946-67 1894-1967 1920-67 1930-67 1905-67 1903-67 1905-67 1905-67 1929-67 1918-67 1905-67 1903-67 1902-67 1920-67 1914-67 1908-67
76 120 100 84 107 102 147 87 139 126 112 89 118 143 127 130 127 125 150
123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141
1922-67 1909-67 1906-63 1905-67 1913-67 1904-67 1944-67 1903-65 1907-67 1913-67 1905-67 1924-64 1925-67 1908-67 1924-67 1921-67 1929-64 1914-67 1916-65
150 160 148 129 123 125 108 150 140 131 162 128 156 121 116 136 107 125 137
0
0'1
09
A
a
-
0 y
0
08 5' 4
3
0'11
=
oz1
0 =
001
_.
3
z 9T
163
REFERENCES Brook, C . E . P . , 1924. The d i s t r i b u t i o n o f r a i n f a l l o v e r Uganda, w i t h a n o t e on Kenya Colony. Q u a r t . J o u r n . Roy. Meteo. S O C . , 5 0 : 325-338. Chow, V . T . , 1964. Handbook o f a p p l i e d h y d r o l o g y . McGraw H i l l Book Company, N e w York, 1 4 5 3 p p . C l a r k e , R . T . , 1973. M a t h e m a t i c a l models i n h y d r o l o g y . FA0 I r r i g a t i o n and d r a i n age p a p e r 1 9 , FAO, Rome. Haan, C . T . , 1977. S t a t i s t i c a l methods i n h y d r o l o g y . The Iowa S t a t e U n i v e r s i t y P r e s s , A m e s , USA,,378 p p . H u r s t , H . E . and B l a c k , R . P . , 1950. The N i l e B a s i n , f i r s t supplement t o Vol. V I , P h y s i c a l Department P a p e r 4 9 , S . O . P . P r e s s , C a i r o , 228 p p . H u r s t , H . E . , S i m a i k a , Y . M . , and B l a c k , R . P . , 1955. The N i l e B a s i n , second supplement t o V o l . V I , N i l e C o n t r o l Department P a p e r 4 , Government P r e s s , C a i r o , 206 pp. H u r s t , H . E . , S i m a i k a , Y . M . , and B l a c k , R . P . , 1957. The N i l e B a s i n , t h i r d s u p p l e ment t o V o l . VI, N i l e C o n t r o l Department P a p e r 9 , Government P r e s s , C a i r o , 198 p p . Huynh Ngoc P h i e n e t a l , 1980. R a i n f a l l d i s t r i b u t i o n i n n o r t h - e a s t e r n T h a i l a n d . Hydro. S c i . B u l . ( e d i t e d by R . T . C l a r k e ) , 2 5 . 2 : 167-182. 1948. A g r i c u l t u r e i n t h e Sudan ( e d i t e d by J . D . T o t h i l l ) . Ireland, A.W., C h a p t e r V: The C l i m a t e o f t h e Sudan, Oxford U n i v e r s i t y P r e s s , London. 1 9 6 2 . R a i n i n E a s t A f r i c a . Q u a r t . J o u r n . Roy. Meteo. S O C . , Johnson, D . H . , 88.375 : 1-19. K i t e , G . W . , 1 9 7 7 . Frequency and r i s k a n a l y s e s i n Hydrology. Water Resources P u b l i c a t i o n s , F o r t C o l l i n s , C o l o r a d o , 224 p p . K o t t e g o d a , N . T . , 1 9 8 0 . S t o c h a s t i c w a t e r r e s o u r c e s t e c h n o l o g y . The McMillan P r e s s L t d . , London, 384 pp Markovig, R . , 1 9 6 5 . P r o b a b i l i t y f u n c t i o n s o f b e s t f i t t o d i s t r i b u t i o n s of a n n u a l p r e c i p i t a t i o n and r u n o f f . Hydrology P a p e r 8 , C o l o r a d o S t a t e U n i v e r s i t y , F o r t C o l l i n s , C o l o r a d o , 34 p p . N i l e C o n t r o l S t a f f , 1963. The N i l e B a s i n , f o u r t h supplement t o V o l . V I , N i l e C o n t r o l Department P a p e r 18, G e n e r a l O r g a n i z a t i o n f o r Government P r i n t i n g O f f i c e s , C a i r o , 192 p p . N i l e C o n t r o l S t a f f , 1 9 6 9 . The N i l e B a s i n , f i f t h supplement t o Vol. V I , N i l e C o n t r o l Department P a p e r 2 5 , G e n e r a l O r g a n i z a t i o n f o r Government P r i n t i n g O f f i c e s , C a i r o , 169 p p . Nile C o n t r o l S t a f f , 1972. The N i l e B a s i n , s i x t h supplement t o V o l . V I , N i l e C o n t r o l Department P a p e r 29, G e n e r a l O r g a n i z a t i o n f o r Government P r i n t i n g O f f i c e s , C a i r o , 160 pp. P e a r s o n , E . S . , and H a r t l e y , H . O . ( e d i t o r s ) , 1954. B i o m e t r i k a T a b l e s f o r S t a t i s t i c i a n s . V o l . 1, Cambridge U n i v e r s i t y P r e s s . Rz6ska, J . ( e d i t o r ) , 1976. The N i l e , b i o l o g y of a n a n c i e n t r i v e r . D r W . Junk B . V . P u b l i s h e r s , The Hague, 417 p p . S h a h i n , M . M . , 1983. S t a t i s t i c a l m o d e l l i n g o f r a i n f a l l d a t a on t h e N i l e B a s i n . P a p e r p r e s e n t e d t o t h e c o n f e r e n c e on water r e s o u r c e s development i n Egypt (under p u b l i c a t i o n ) , C ai ro . UNESCO, 1 9 7 8 . World w a t e r b a l a n c e and w a t e r r e s o u r c e s o f t h e e a r t h . UNESCO, Paris. United S t a t e s Water R e s o u r c e s C o u n c i l , 1 9 7 7 . G u i d e l i n e s f o r d e t e r m i n i n g f l o o d flow f r e q u e n c y . R e v i s e d e d i t i o n , Washington, D . C . Y e v j e v i c h , V . , 1972. S t o c h a s t i c p r o c e s s e s i n h y d r o l o g y . Water R e s o u r c e s P u b l i c a t i o n s , F o r t C o l l i n s , C o l o r a d o , 276 p p . W M O , 1 9 7 4 . H y d r o m e t e o r o l o g i c a l s u r v e y o f t h e c a t c h m e n t s o f Lakes V i c t o r i a , Kyoga and A l b e r t , Vol. I : Meteorology and h y d r o l o g y of t h e b a s i n , P a r t 1, Geneva.
165
Chapter 5 FREE WATER SURFACE EVAPORATION E v a p o r a t i o n i s d e f i n e d as t h e t r a n s f e r o f m o i s t u r e i n t o t h e atmosphere from an open o r f r e e w a t e r s u r f a c e , a b a r e s o i l o r i n t e r c e p t i o n on a v e g e t a l c o v e r . The w a t e r - r e s o u r c e s
e n g i n e e r u s u a l l y r e g a r d s e v a p o r a t i o n of w a t e r as a l o s s ,
whether i t o c c u r s from r e s e r v o i r s , from n a t u r a l l a k e s , from b a r e s o i l , o r from land-carrying
crops.
S i n c e t h e r a i n f a l l on many p a r t s o f t h e N i l e B a s i n i s q u i t e s c a n t y , i t i s , t h e r e f o r e , n e c e s s a r y t o h a v e r e l i a b l e i n f o r m a t i o n a b o u t e v a p o r a t i o n losses. One s h o u l d n o t f o r g e t t h a t t h e mere e x i s t e n c e of some of t h e c o u n t r i e s s h a r i n g t h e N i l e w a t e r is a l m o s t e n t i r e l y d e p e n d e n t on t h e v a r i o u s i r r i g a t i o n schemes and on
t h e s t o r a g e works on t h e r i v e r . I n t h i s c h a p t e r t h e e v a p o r a t i o n from t h e f r e e w a t e r s u r f a c e s s u c h as l a k e s and r e s e r v o i r s i n t h e b a s i n i s t r e a t e d . E v a p o t r a n s p i r a t i o n from v e g e t a t e d and cropped s u r f a c e s i s t h e s u b j e c t matter of t h e n e x t c h a p t e r . I t is known t h a t e v a p o r a t i o n c a n b e measured by atmometers and c o n t a i n e r s of v a r i o u s s h a p e s and d i m e n s i o n s . I t c a n a l s o b e e s t i m a t e d from t h e w a t e r - b a l a n c e of t h e body of water o r t h e c a t c h m e n t area i n q u e s t i o n , p r o v i d e d t h a t s u f f i c i e n t l y a c c u r a t e d a t a c o v e r i n g a l l t h e terms i n t h e b a l a n c e e q u a t i o n o t h e r t h a n e v a p o r a t i o n a r e a v a i l a b l e . I n t h e a b s e n c e of a c t u a l measurements, o r where a d e q u a t e d a t a o f t h e b a l a n c e i t e m s a r e m i s s i n g , one u s u a l l y r e s o r t s t o evaporat i o n e s t i m a t e s , u s i n g one f o r m u l a o r a n o t h e r . Our i n t e r e s t h e r e does n o t e x t e n d t o t h e h o u r l y e v a p o r a t i o n and i t s c o r r e l a t i o n w i t h t h e c l i m a t e . We are n o t even c o n c e r n e d w i t h t h e d a i l y v a l u e s , b u t w e a r e c o n c e r n e d w i t h t h e summation o f t h e d a i l y v a l u e s on a monthly a v e r a g e b a s i s , and w i t h a n n u a l v a l u e s . Measurement of e v a p o r a t i o n a t a number o f s t a t i o n s i n t h e N i l e B a s i n d a t e s back t o t h e b e g i n n i n g of t h i s c e n t u r y . I n Egypt and t h e Sudan t h e P i c h e and t h e f l o a t i n g t a n k a r e s t i l l i n u s e , whereas t h e u s e o f t h e Wild e v a p o r i m e t e r h a s s t o p p e d s i n c e 1920. The f l o a t i n g t a n k h a s a s h a p e o f a cube o f 1 m s i d e . I t i s c o n s t r u c t e d o f i r o n and f l o a t e d on a r i v e r o r a l a k e by means of a wooden r a f t . The r i m of t h e t a n k r e m a i n s a few c e n t i m e t r e s above t h e l e v e l o f t h e o u t s i d e w a t e r . The t a n k i s f i l l e d up t o t h e same l e v e l o f t h e o u t s i d e w a t e r , and t h e e v a p o r a t i o n l o s s i s measured e v e r y day by a gauge which i n d i c a t e s t h e w a t e r l e v e l i n t h e t a n k . The d i f f i c u l t y w i t h t y p e of d e v i c e i s c a u s e d m a i n l y by waves s p l a s h i n g i n t o i t , e s p e c i a l l y on windy d a y s .
166
The e a r l y e x p e r i m e n t a t i o n o f e v a p o r a t i o n measurement u s i n g d i f f e r e n t d e v i c e s
w a s described i n V o l . I of t h e N i l e Basin (Hurst, H . E . ,
and P h i l i p s , P . ,
1931).
The c o n c l u s i o n s t h a t c a n b e drawn f o r t h e i r d e s c r i p t i o n a r e : t h e e v a p o r a t i o n from a f l o a t i n g t a n k 2 m s q u a r e c a n b e c o n s i d e r e d
i)
p r a c t i c a l l y a s t h e b e s t a v a i l a b l e a p p r o x i m a t i o n t o e v a p o r a t i o n from a n extended s u r f a c e of water, t h e r a t i o between e v a p o r a t i o n from a 2 m s q u a r e f l o a t i n g t a n k and e v a p o r a -
ii)
t i o n from a 1 m s q u a r e f l o a t i n g t a n k i s 0.88, and i i i ) t h e r a t i o between e v a p o r a t i o n from a 2 m s q u a r e f l o a t i n g t a n k and evaporat i o n from a P i c h e t u b e i s c l o s e t o 0 . 5 0 . The above c o n c l u s i o n s combined w i t h t h o s e r e p o r t e d by K e e l i n g a t a n e a r l i e r
t i m e were p r e s e n t e d i n Vol. I o f t h e N i l e B a s i n w i t h t h e a i m o f t r a n s o r m i n g t h e r e a d i n g s o f a l l t h e m e a s u r i n g d e v i c e s t o f r e e water s u r f a c e e v a p o r a t i o n ( H u r s t , H.E.,
and P h i l i p s , P . ,
1 9 3 1 ) . T h i s is l i s t e d i n T a b l e 5.1. The c l i m a t o l o g i c a l
normals f o r Egypt ( M i n i s t r y o f War and M a r i n e , Egypt,
1950) show some d i f f e r e n c e
between t h e normal e v a p o r a t i o n o v e r open w a t e r and t h e o r i g i n a l v a l u e s g i v e n i n T a b l e 5 . 1 f o r a l a r g e number of m e t e o r o l o g i c a l s c r e e n s . A few y e a r s l a t e r , a d i f f e r e n t set o f v a l u e s a p p e a r e d i n "The N i l e "
(Hurst, H . E . ,
1 9 5 2 ) . A summary
of t h e d a t a a v a i l a b l e i n t h e s e t w o r e f e r e n c e s a r e a l s o i n c l u d e d i n T a b l e 5 . 1 . The d a t a i n t h i s t a b l e h a v e been u s e d i n p r e p a r i n g t h e map shown i n F i g . 5 . 1 . , so a s t o g e t an o v e r a l l p i c t u r e o f t h e a n n u a l e v a p o r a t i o n a s deduced from t h e
Piche readings. As a l r e a d y m e n t i o n e d , t h e w a t e r - b a l a n c e method c a n b e used f o r e s t i m a t i n g t h e
e v a p o r a t i o n loss from a n open w a t e r where d i r e c t measurements are n o t a v a i l a b l e . The b a l a n c e e q u a t i o n f o r a body o f w a t e r , a l a k e f o r example, c a n b e s e t up f o r a c e r t a i n p e r i o d o f t i m e as
P1 + R + I - E 1 - O = A S where P1 = d i r e c t p r e c i p i t a t i o n on t h e l a k e ,
R
= run-off
from t h e l a k e c a t c h m e n t a r e a t o t h e l a k e i t s e l f ,
I
= i n f l o w t o t h e l a k e from r i v e r t r i b u t a r i e s ,
E l = e v a p o r a t i o n from t h e l a k e ,
0
= o u t f l o w from t h e l a k e , and
AS = change i n t h e volume of water s t o r e d i n t h e l a k e .
(5.1)
TABLE 5.1
Normal e v a p o r a t i o n o v e r open w a t e r i n t h e N i l e B a s i n , i n mm/day N i l e B a s i n Vol. I (1931)
Location
Mediterranean C o a s t Mersa Matruh Alexandria Port Said N i l e Delta Qurashiya Sakha C a i r o and neighbourhood C a i r o (Ezbekiya) Giza Helwan Tor Fayum Qasr el-Gebali Minya ( C e n t r a l Egypt) Upper Egypt A s s i u t (Upper Egypt) Aswan Oases Northern Sudan (from H a l f a t o Atbara) Wadi-Half a Merowe Atbara P o r t Sudan Khartoum and neighbourhood Khartoum Kassala Gallabat Wad Medani
Period of observation
Jan.
1920-29 1920-29 1920-29
3.2 1.8 1.5
1907-29 1907-29
Climatological Normals (1950)
Year
Jul.
Oct.
Year
3.8 2.1 2.5
3.5 3.0 2.3
4.2 2.2 2.6
3.6 2.0 2.2
1.0 1.0
2.5 2.3
2.4 2.8
1.6 1.7
2.2 1.9
1909-29 1920-27 1920-29 1905-29
1.2 1.5 2.5 3.2
2.7 3.8 6.5 4.4
3.7 4.1 7.6 5.3
1.9 2.3 5.0 3.7
2.3 2.8 5.4 4.2
1920-29 1920-29
1.9 1.4
4.8 4.2
6.4 5.0
3.4 2.8
4.1 3.3
3.8
1920-29 1920-29
1.8 3.8
5.5 8.7
7.1 10.0
3.4 7.8
4.5 7.5
4.6 7.1
Apr.
The N i l e (1952) Year
3.0 2.5 2.3 2.3 2.8 2.3
3.8
4.0 4.5 6.5 7.6
1905-29 1905-29 1905-29 1905-29
4.5 5.8 6.8 3.7
9.3 9.8 10.3 4.6
9.9 9.3 9.3 7.2
8.1 8.7 8.3 3.5
7.9 8.4 8.6 4.9
1905-29 1905-29 1905-29 1905-29
6.5 4.7 6.5 6.3
10.0 7.7
6.7 4.3
7.3 5.5
7.5 5.4
8.5 9.1
2.3 4.9
3.0 5.7
5.1 6.5
7.8
TABLE 5 . 1
(continued)
N i l e B a s i n Vol. I (1931) Location
C e n t r a l Sudan (Dueim t o R o s e i r e s ) Roseires Dueim Lake Tana El-Obeid El-Fasher S o u t h e r n Sudan (Malakal and s o u t h o u t s i d e t h e swamps) Malakal W au Mong a11a Lake A l b e r t Lake Edward Lake V i c t o r i a
P e r i o d of observation
Jan.
Apr.
Jul.
-
Oct.
Year
Climatological Normals (1950) Year
The N i l e (1952)
Year
6.3 1905-29 1905-29 1921-24 1907- 29 1918-29
6.8 7.6 4.1 7.2 5.2
8.1 9.5 5.1 9.2 7.9
2.5 4.9 1.2 4.7 4.3
3.4 5.8 2.2 6.3 6.0
5.3 6.9 3.0 6.7 5.8 3.4
1915-29 1906-29 1906-29
8.5 5.9 5.5
3.5
5.4 4.5 3.0
2.9
1.4 1.8 1.4
4.4
1.8 2.3 2.2 3.7
4.5 3.7 3.0
3.6
3.9 3.9 3.8
169
N
30'
25'
20°
15O
loo
10'
5O
5'
O0
21 O Fig. 5 . 1 .
Mean annual e v a p o r a t i o n from an open water s u r f a c e i n t h e N i l e Basin
170
I t is customary among t h e h y d r o l o g i s t s t o n e g l e c t t h e change i n s t o r a g e when
t h e p e r i o d o v e r which t h e g a i n s a r e b a l a n c e d w i t h t h e l o s s e s i s s u f f i c i e n t l y l o n g , s a y 1 y e a r o r more. When t h i s i s t h e case, AS i s p u t e q u a l t o z e r o and eq. 5 . 1 can be rewritten as E 1 = P
1
+ R + I - 0
(5.2)
The e v a p o r a t i o n from t h e Great Lakes i n t h e N i l e B a s i n has been e s t i m a t e d by
e q . 5.2 u s i n g t h e y e a r as a b a l a n c e p e r i o d . A b a l a n c e e q u a t i o n q u i t e s i m i l a r t o t h a t g i v e n by e q . 5 . 2 w a s used i n p r e -
p a r i n g t h e world water b a l a n c e (Baumgartner, A . , work t h e s o - c a l l e d
and R e i c h e l , E., 1 9 7 5 ) . I n t h i s
a c t u a l evaporation over t h e s u c c e s s i v e 5 degree l a t i t u d e
zones i s i n c l u d e d . I t is n o t common t o u s e t h e t e r m a c t u a l i n c o n n e c t i o n w i t h e v a p o r a t i o n . I n s t e a d , t h i s t e r m i s used j o i n t l y w i t h e v a p o t r a n s p i r a t i o n from a v e g e t a t e d o r a cropped s u r f a c e t o d i s t i n g u i s h i t from p o t e n t i a l e v a p o t r a n s p i r a t i o n . T h e r e f o r e , t h e d i s c u s s i o n o f t h e r e s u l t s o b t a i n e d by R e i c h e l and Baumgartner c o n c e r n i n g what t h e y have c a l l e d a c t u a l e v a p o r a t i o n w i l l b e presented i n the next chapter. I n a d d i t i o n t o a l l t h a t i s mentioned above, t h e r e h a s been a number o f d e t a i l e d e x p e r i m e n t s on e v a p o r a t i o n and e v a p o t r a n s p i r a t i o n from t h e N i l e B a s i n . S i n c e t h e b u l k of t h e a v a i l a b l e i n f o r m a t i o n is b a s e d , however, on t h e r e a d i n g s o f t h e P i c h e atmometer, i t i s l o g i c a l t o b e g i n by m e n t i o n i n g some remarks about t h e k i n d of d a t a one s h o u l d e x p e c t t o o b t a i n from such a n atmometer. I t i s i m p o r t a n t t o know t h i s b e f o r e any d e t a i l e d i n v e s t i g a t i o n c a n b e a t t e m p t e d . i)
T h e P i c h e , l i k e any o t h e r atmometer,
rather than h y d ro lo g i cal purposes,
i s mainly used f o r c l i m a t o l o g i c a l
t o c h a r a c t e r i z e t h e d r y i n g a b i l i t y of
t h e a i r under a g i v e n s e t o f c o n d i t i o n s . ii)
The p r o c e s s and c i r c u m s t a n c e s o f e v a p o r a t i o n from a P i c h e t u b e ( s u r f a c e a r e a from which e v a p o r a t i o n t a k e s p l a c e , two s i d e s = 13 cm2) a r e n o t
s t r i c t l y t h e same as t h o s e from a t a n k o r from a f r e e water s u r f a c e . The e x t e n t o f t h e a r e a a f f e c t e d by h u m i d i t y c a u s e d by e v a p o r a t i o n from a nearby w a t e r s u r f a c e i s a s t r o n g f a c t o r i n f l u e n c i n g t h e r e l a t i o n s h i p b e t ween t h e e v a p o r a t i o n from a P i c h e i n s t r u m e n t and t h e e v a p o r a t i o n from an open w a t e r s u r f a c e . Another s t r o n g f a c t o r i n f l u e n c i n g t h i s r e l a t i o n s h i p i s t h e d e g r e e of e x p o s u r e of t h e atmometer t o t h e s u r r o u n d i n g c l i m a t e . I n h i s book, "The N i l e " ( 1 9 5 2 ) , H u r s t d i s c u s s e d t h e u s e o f an e v a p o r i m e t e r i n the lakes area saying
"
...
The G r e a t Lakes, however, produce a l o c a l
c l i m a t e , so t h a t a P i c h e e v a p o r i m e t e r i n a town on t h e s h o r e s of Lake V i c t o r i a w i l l n o t i n d i c a t e t w i c e t h e e v a p o r a t i o n from t h e l a k e i t s e l f , but something a p p r e c i a b l y l e s s . T h i s i s b e c a u s e t h e damp atmosphere o v e r t h e
171
l a k e e x t e n d s some d i s t a n c e i n l a n d " .
T h i s i s o b v i o u s l y n o t t h e c a s e when
t h e evaporimeter i s p l a c e d a t a s t a t i o n n e x t t o a r i v e r of comparatively much s m a l l e r w i d t h . i i i ) A s mentioned i n V o l . I o f t h e N i l e B a s i n ( H u r s t , H . E . ,
and P h i l i p s , P . ,
1 9 3 1 ) , i t i s r e a s o n a b l e t o s u p p o s e t h a t t h e e v a p o r a t i o n from a l a r g e open
w a t e r s u r f a c e i s a l i t t l e less t h a n t h a t from t h e l a r g e s t t a n k s used i n t h e e x p e r i m e n t s ( h e r e 2 m x 2 m). The r a t i o between e v a p o r a t i o n from an e v a p o r i m e t e r and e v a p o r a t i o n from a f r e e w a t e r s u r f a c e w a s reviewed by S l e i g h t ( 1 9 2 7 ) , Hickox (1946) and l a t e r by O l i v i e r ( 1 9 6 1 ) . The r e s u l t s o b t a i n e d a r e summarized i n T a b l e 5 . 2 . R e l a t i v e rates of e v a p o r a t i o n showing e f f e c t o f pan s i z e on
TABLE 5 . 2
evaporation
Diameter of p a n , i n f e e t
Description
12
Compiled by R.B. S l e i g h t
1.00
9
6
4
3.39
2.26
2
1
1 . 0 0 9 1 . 0 8 9 1 . 1 7 5 1 . 2 0 2 1 . 2 6 0 1 . 2 8 4 1.589
Compared w i t h 1 8 0 0 - a c r e 1 . 0 9 9 1 . 1 0 8 1 . 1 9 6 1 . 2 9 0 1 . 3 2 0 1.383 1 . 4 1 0 1 . 7 4 5 r e s e r v o i r as u n i t y A f t e r t h i s h i s t o r i c a l r e v i e w w e s h a l l d e a l i n t h e r e m a i n i n g s e c t i o n s of t h i s c h a p t e r w i t h e v a p o r a t i o n from open w a t e r a t a number of s t a t i o n s i n t h e d i f f e r e n t p a r t s of t h e N i l e Basin. 5.1 5.1.1
LAKE PLATEAU AREA Lake V i c t o r i a
The mean a n n u a l e v a p o r a t i o n from Lake V i c t o r i a o v e r t h e p e r i o d from 1902 up t o and i n c l u d i n g 1 9 2 3 c a n b e found from T a b l e 5 . 1 as 3 . 6 mm/day. T h i s f i g u r e was o b t a i n e d from t h e h y d r o l o g i c b a l a n c e o f t h e l a k e f o r t h e same p e r i o d . The d i s t r i b u t i o n t h r o u g h o u t t h e y e a r w a s d e v e l o p e d by comparison w i t h t h e r e a d i n g s of t h e w e t b u l b t h e r m o m e t e r . When t h e b a l a n c e p e r i o d e x t e n d s up t o 1936, t h e a v e r a g e v a l u e s o f P1,
R + I and 0 become 1 1 5 1 , 276 and 311 mm/yr r e s p e c t i v e l y ,
t h u s g i v i n g a n a n n u a l mean e v a p o r a t i o n of 3 . 0 6 mm/day o n l y ( H u r s t , H . E . , Philips, P.,
and
1 9 3 8 ) . A t h i r d e v a p o r a t i o n r a t e which w a s g i v e n l a t e r by H u r s t
(1952) i s 3.8 mm/day.
T h i s i s b a s e d o n P1,
R + I and 0 of 1463, 239 and 314
mm/yr r e s p e c t i v e l y . A f o u r t h v a l u e c a n s t i l l b e found i n V o l . X of t h e N i l e Basin ( H u r s t , H . E . ,
Black, R.P.,
and S i m a i k a , Y.M., 1 9 6 6 ) . T h i s v a l u e o f
1150 mm/yr o r 3 . 1 5 mm/day i s b a s e d o n P1 = 1260, R
+ I = 190 and 0
= 300 mm/yr
r e s p e c t i v e l y . From t h e s e f i g u r e s one c a n e a s i l y see t h a t t h e e v a p o r a t i o n r a t e
172
from Lake V i c t o r i a v a r i e s from o n e b a l a n c e p e r i o d t o a n o t h e r . Each term i n t h e b a l a n c e e q u a t i o n u n d e r g o e s v a r i a t i o n w i t h t i m e . Next t o t h i s , t h e r e a r e d e f i n i t e l y some s a m p l i n g e r r o r s . An e s t i m a t e of t h e a n n u a l r a i n f a l l on s u c h a n enormous s u r f a c e o f w a t e r a s Lake V i c t o r i a c a n h a r d l y b e a c c u r a t e . An example o f t h i s , though from o u t s i d e t h e N i l e B a s i n and t o a s m a l l e r e x t e n t a s compared t o any of t h e E q u a t o r i a l L a k e s , i s t h e Lake H e f n e r i n t h e U n i t e d S t a t e s . During t h e 1950-51 e v a p o r a t i o n s t u d y from t h i s l a k e t h e r e were 2 1 e i g h t - i n c h d i a m e t e r gauges around t h e p e r i p h e r y of t h e 2200-acre
l a k e and o n e gauge on a r a f t i n
t h e c e n t r e . D u r i n g o n e s t o r m , r a i n f a l l r a n g e d from 0 . 1 i n c h on o n e s i d e o f t h e l a k e t o n e a r l y two i n c h e s on t h e o t h e r . During summer c o n v e c t i v e s h o w e r s , t h e a r e a l v a r i a b i l i t y o f r a i n f a l l may b e s o g r e a t t h a t t h e a v e r a g e r a i n f a l l v a l u e on t h e l a k e s u r f a c e i s s u f f i c i e n t l y i n a c c u r a t e t o i n v a l i d a t e t h e computed f i g u r e o f e v a p o r a t i o n (US G e o l o g i c a l S u r v e y , 1 9 5 4 ) . The w a t e r - b a l a n c e
of Lake V i c t o r i a w a s r e c e n t l y r e v i e w e d i n c o n n e c t i o n w i t h
t h e h y d r o m e t e o r o l o g i c a l network p r o j e c t i n t h e E q u a t o r i a l Lakes a r e a (Krishnamurthy, K . V . ,
and I b r a h i m , A . M . ,
1 9 7 3 ) . Based on t h e i n f o r m a t i o n
g a t h e r e d from t h e meagre network of r a i n gauges b e f o r e t h e commencement o f t h e p r o j e c t i n 1 9 6 7 , t h e b e s t a v a i l a b l e e s t i m a t e of P1 c a n b e t a k e n as 1420 mm/yr
+ -
10%. The i n f l o w p l u s r u n - o f f
t o t h e l a k e v a r i e d , a s r e p o r t e d , from 15 t o 18
m i l l i a r d m 3 / y r . T h i s f i g u r e i s somewhat d i f f e r e n t from t h e 1 2 . 6 m i l l i a r d m3/yr a d o p t e d by H u r s t and h i s co-workers (Hurst, H . E . ,
i n p r e p a r i n g Vol. X of t h e N i l e B a s i n
B l a c k , R . P . , and S i m a i k a , Y . M . ,
1 9 6 6 ) . I n t h e b a l a n c e p r e p a r e d by
Krishnamurthy and I b r a h i m t h e y e a r l y volumes o f 18 x
m 3 2 5% were u s e d t o r e p r e s e n t t h e r u n - o f f
lo9
m 3 2 5% and 23.6 x
lo9
t o and t h e o u t f l o w from Lake V i c t o r i a
r e s p e c t i v e l y . The e v a p o r a t i o n e s t i m a t e d from t h i s b a l a n c e l i e s i n t h e r a n g e of 3 . 6 5 and 4 . 5 0 mm/day. The l o w e r l i m i t o f t h i s r a n g e d o e s n o t d e p a r t s e n s i b l y from t h e a v e r a g e of t h e e v a p o r a t i o n r a t e s o b t a i n e d f o r d i f f e r e n t b a l a n c e r a t e s by H u r s t and h i s c o - w o r k e r s .
I t i s a l s o i n r e a s o n a b l e agreement w i t h t h e a v e r a g e
of t h e r e a d i n g s of t h e P i c h e e v a p o r i m e t e r s i n s t a l l e d by t h e East A f r i c a n M e t e o r o l o g i c a l S e r v i c e a t a number of p l a c e s n e a r t h e l a k e s h o r e s . The o l d work o f K e e l i n g b a s e d on t h e wet-bulb
d e p r e s s i o n a s an estimate o f t h e d a i l y evapora-
t i o n o v e r a c e r t a i n y e a r from a s t a n d a r d Wild i n s t r u m e n t was e x t e n d e d by O l i v i e r (1961) t o g i v e t h e mean d a i l y e v a p o r a t i o n o v e r a c e r t a i n month. K e e l i n g , a s a r e s u l t o f t h e e x p e r i m e n t s made i n Egypt and t h e Sudan, recommended a f a c t o r of 1 . 4 2 t o c o n v e r t t h e r e a d i n g o f a Wild i n s t r u m e n t t o P i c h e e v a p o r a t i o n . The e v a p o r a t i o n f o r m u l a d e v e l o p e d by O l i v i e r r e a d s
173
where
M
c
P
= f r e e water s u r f a c e e v a p o r a t i o n , i n mm/day,
from a s t a n d a r d t a n k a t a
l a t i t u d e $,
= a v e r a g e d e p r e s s i o n o f w e t b u l b i n OC f o r a p a r t i c u l a r month, and
L$N = r a t i o
L
- f o r l a t i t u d e ON a s t a k e n from t a b l e s ( f o r l a t i t u d e L$S r e a d ' s i x LO
months o n ' ) . The y e a r l y v a l u e s of c a t J i n j a and Entebbe a r e 36.1°C
and 30.1°C
respect-
i v e l y . T h e s e two p a l c e s a r e l o c a t e d a l m o s t on t h e e q u a t o r . Assuming t h a t c i s u n i f o r m l y d i s t r i b u t e d o v e r t h e 1 2 months o f t h e y e a r , and u s i n g t h e monthly v a l u e s L($ =
0)
g i v e n by O l i v i e r , t h e f r e e water e v a p o r a t i o n from a s t a n d a r d
t a n k c a n b e c a l c u l a t e d from e q . 5 . 3 . The r e s u l t s o b t a i n e d from c a l c u l a t i o n t o g e t h e r w i t h t h e a v e r a g e P i c h e e v a p o r a t i o n a t t h e same p l a c e s are g i v e n i n T a b l e 5 . 3 . From t h i s t a b l e i t a p p e a r s t h a t t h e computed t a n k e v a p o r a t i o n a t b o t h J i n j a and E n t e b b e , which are s i t u a t e d c l o s e t o t h e s h o r e s o f Lake V i c t o r i a ,
i s 20 t o 30% less t h a n e v a p o r a t i o n from t h e l a k e a s o b t a i n e d from t h e waterb a l a n c e method. A d d i t i o n a l l y , t h e r a t i o P i c h e e v a p o r a t i o n t o t a n k e v a p o r a t i o n needs t o b e c h a n g e d , a t l e a s t f o r t h e p l a c e s c o n s i d e r e d , from 1 . 4 2 t o 1 . 2 1 .
E s t i m a t e o f t a n k e v a p o r a t i o n and P i c h e e v a p o r a t i o n , f o r J i n j a and
TABLE 5 . 3
Entebbe
Place Jinja
Entebbe
Method
Evaporation,
i n mm/day,
for
f
J a n . F e b . Mar. Apr. May J u n e J u l y Aug. S e p . O c t . Nov. Dec. Year
a b
2.9 4.5
3.0 4.1
3.2 3.8
3.1 3.2
2.9 2.9 3.0 3.3
2.9 3.3
3.0 3.2
3.1 3.7
3.1 4.1
3.0 4.0
2.9 3.9
3.0 3.7
a
2.5 3.2
2.5 3.5
2.6 3.1
2.6 2.3
2.5 2.4 2.3 2.6
2.4 2.6
2.5 2.6
2.6 2.9
2.6 3.0
2.5 2.8
2.4 2.7
2.5 3.0
c
*a = Eq. 5 . 3 ; b = P i c h e , 1924-27 and 1932-34; c = P i c h e , 1924-30 and 1933-34 A s e r i e s o f e v a p o r a t i o n e x p e r i m e n t s u s i n g d i f f e r e n t t y p e s o f atmometers were c a r r i e d o u t f o r a number o f y e a r s i n E a s t A f r i c a n t e r r i t o r i e s , some o f which a r e l o c a t e d w i t h i n t h e b o u n d a r i e s o f t h e catchment a r e a o f Lake V i c t o r i a . A t e l e v e n s t a t i o n s , e x p r e s s i n g t h e a n n u a l e v a p o r a t i o n i n i n c h e s , i t was found t h a t
C l a s s A pan = 0 . 8 7 P i c h e
+
24
(5.4)
w i t h a c o r r e l a t i o n c o e f f i c i e n t = 0 . 9 5 . I t was c o n c l u d e d t h a t t h e P i c h e r e a d i n g s i n a Stevenson s c r e e n g i v e a s a t i s f a c t o r y e s t i m a t e of annual e v a p o r a t i o n a s measured by a c l a s s A p a n .
174
Comparison between t h e s o - c a l l e d Kenya pan and a c l a s s A pan were c a r r i e d o u t a t D a g o r e t t i h e a d q u a r t e r s , a few k i l o m e t r e s w e s t o f N a i r o b i . Using f o u r y e a r s o f r e c o r d (1384 d a y s ) , a c o r r e l a t i o n between d a i l y v a l u e s i n i n c h e s yielded the r e s u l t C l a s s A pan ( u n s c r e e n e d ) = 1 . 3 Kenya pan ( s c r e e n e d ) + 0 . 0 2
(5.5)
with a c o r r e l a t i o n c o e f f i c i e n t = 0 . 9 5 . Comparisons of monthly means o v e r two y e a r s , assuming l i n e a r r e l a t i o n s h i p , provided t h e f o l l o w i n g r e s u l t s : C l a s s A pan ( u n s c r e e n e d )
= 100
Class A pan ( s c r e e n e d )
=
86 ( r a n g e 84-88)
C l a s s A pan ( p a i n t e d )
=
84 ( r a n g e 82-86)
Kenya pan ( u n s c r e e n e d )
=
95 ( o n l y few months)
Kenya pan ( s c r e e n e d )
=
7 1 ( r a n g e 69-74)
Kenya pan ( p a i n t e d and s c r e e n e d ) =
69 ( r a n g e 65-72)
"Screen" i s a 1 - i n c h c h i c k e n w i r e mesh. " P a i n t e d " means t h a t t h e i n s i d e is p a i n t e d d u l l b l a c k w i t h b i t u m i n o u s p a i n t and t h e o u t s i d e w i t h aluminium p a i n t . F u r t h e r d e s c r i p t i o n o f t h o s e e x p e r i m e n t s is g i v e n i n T e c h n i c a l Note No. 8 3 o f t h e World M e t e o r o l o g i c a l O r g a n i z a t i o n (WMO, 1 9 6 6 ) . The two s t a t i o n s which a r e o f d i r e c t i n t e r e s t t o us a r e Kisumu and E n t e b b e . The P i c h e ( s c r e e n e d ) and Kenya p a n ( s c r e e n e d ) e v a p o r a t i o n a t Kisumu were 96 and 106 i n / y r and a t Entebbe 36 and 89 i n / y r ,
r e s p e c t i v e l y . The y e a r l y r a t i o s o f t h e c l a s s A pan ( u n s c r e e n e a
t o Kenya pan ( s c r e e n e d ) a t Kisumu and E n t e b b e w e r e 1 . 2 8 and 1 . 1 2 r e s p e c t i v e l y . Assuming t h e r a t i o l a k e e v a p o r a t i o n t o c l a s s A pan e v a p o r a t i o n a t 0 . 7 , t h e f r e e
water s u r f a c e e v a p o r a t i o n a t Kisumu and E n t e b b e becomes 6 . 6 mm/day and 4 . 8 7 mm/day r e s p e c t i v e l y . The mean a n n u a l P i c h e e v a p o r a t i o n i s 6 . 6 7 mm/day f o r Kisumu and 2 . 5 1 mm/day f o r E n t e b b e . The l a t t e r f i g u r e e m p h a s i z e s t h e abnormal i t y i n t h e r e a d i n g s o f t h e P i c h e atmometer a t E n t e b b e .
A n o t h e r a p p r o a c h t o e s t i m a t i n g e v a p o r a t i o n from a f r e e water s u r f a c e i s t h a t o f a p p l y i n g o n e o r a n o t h e r o f t h e e v a p o r a t i o n f o r m u l a s b a s e d on t h o s e meteorol o g i c a l f a c t o r s which a r e s t r o n g l y c o r r e l a t e d w i t h e v a p o r a t i o n . R i j k s (1969) h a s e s t i a m t e d t h e e v a p o r a t i o n from t h e m e t e o r o l o g i c a l d a t a measured o v e r a swamp a t t h e n o r t h e r n boundary of t h e C o t t o n R e s e a r c h S t a t i o n a t Namulonge (0°32'N,
32O37'E and 1100 m a l t i t u d e ) , Uganda. The r e s u l t s h e o b t a i n e d from
Penman's f o r m u l a a v e r a g e d o v e r t h e 5-day p e r i o d s ; 8-12 March, 22-26 March and 7-11 A p r i l , 1965, a r e 6 . 2 , 3 . 8 and 5 . 5 mm/day r e s p e c t i v e l y . Dagg, M., on Water
R e q u i r e m e n t s o f Crops i n E a s t A f r i c a (1972) h a s mentioned t h a t d i r e c t and r e l i a b l e measurements o f open w a t e r e v a p o r a t i o n are d e c e p t i v e l y d i f f i c u l t t o o b t a i n . Using t h e estimates o f R i j k s and Owen, 1965, and o f Woodhead, 1967,
175
from t h e Penman f o r m u l a , Dagg w a s a b l e t o c o n s t r u c t a map o f t h e e v a p o r a t i v e demand f o r E a s t A f r i c a w i t h m o d e r a t e c o n f i d e n c e ( 1 9 7 2 ) . P a r t o f t h i s map h a s been i n c o r p o r a t e d i n t h e map
o f t h e e v a p o r a t i v e demand f o r t h e N i l e B a s i n p r e -
p a r e d by us u s i n g t h e Penman method. T h i s method y i e l d s a n a v e r a g e r a t e o f 4 . 6 5 mm/day f o r t h e n o r t h e r n and n o r t h - w e s t e r n p a r t s o f t h e c a t c h m e n t a r e a of Lake V i c t o r i a . The mean a n n u a l P i c h e e v a p o r a t i o n a t J i n j a , Kampala and E n t e b b e , which r e p r e s e n t t h o s e p a r t s of t h e c a t c h m e n t , is 3 . 7 , 4 . 5 and 3 . 0 mm/day respectively. The above d i s c u s s i o n of t h e e v a p o r a t i o n from Lake Victoria and i t s catchment can b e concluded w i t h t h e f o l l o w i n g remarks. i)
The w a t e r - b a l a n c e
o f t h e l a k e a v e r a g e d o v e r d i f f e r e n t p e r i o d s g i v e s a mean
a n n u a l e v a p o r a t i o n i n t h e r a n g e o f from s l i g h t l y less t h a n 3 . 1 mm/day t o
3.8 mm/day w i t h a n o v e r a l l a v e r a g e o f 3 . 4 mm/day. S u b s t i t u t i n g h i g h e r annual r a i n f a l l depths i n t h e balance eq uat i on has
led t o evaporation
r a n g i n g from 3 . 6 5 t o 4 . 5 0 mm/day. ii)
The e v a p o r a t i v e demand f o r t h e c a t c h m e n t a r e a o f t h e Lake V i c t o r i a v a r i e s from o n e p a r t o f t h e c a t c h m e n t t o t h e o t h e r . The r a n g e o f v a l u e s o b t a i n e d from t h e Penman f o r m u l a b a s e d on t h e a v a i l a b l e m e t e o r o l o g i c a l o b s e r v a t i o n s
i s from a b o u t 4 . 4 mm/day t o s l i g h t l y more t h a n 6 . 0 mm/day. i i i ) The P i c h e r e a d i n g s a t t h e s t a t i o n s n e a r t h e s h o r e s of t h e l a k e need t o b e m u l t i p l i e d by a f a c t o r r a n g i n g from less t h a n 1 . 0 t o more t h a n 1 . 5 t o b r i n g them t o t h e i r e q u i v a l e n t s o f f r e e water s u r f a c e e v a p o r a t i o n . iv)
The c o n v e r s i o n f a c t o r o f t h e Wild i n s t r u m e n t
t o t h e Piche tube reading
seems t o b e less t h a n t h e 1 . 4 2 o r i g i n a l l y proposed by K e e l i n g . L a s t b u t n o t l e a s t , i t is worthwhile t o mention t h a t t h e hydro-meteorologic a l s u r v e y p r o j e c t o f Lakes V i c t o r i a , Kyoga and A l b e r t h a s set up f i v e c l a s s A e v a p o r a t i o n pans o n f i v e i s l a n d s i n Lake V i c t o r i a a s w e l l as n i n e c l a s s A evap o r a t i o n pans o n t h e s h o r e a t some o f t h e f i r s t o r d e r m e t e o r o l o g i c a l s c r e e n s ( s e e map, F i g . 5 . 2 . ) . B e s i d e s t h e s t a n d a r d c l a s s A p a n s , a R u s s i a n e v a p o r a t i o n pan 20 m2 i n s i z e h a s a l s o b e e n s e t up t o h e l p i n t h e d e t e r m i n a t i o n of evaporat i o n by t h e pan method. F u r t h e r m o r e , i t h a s been p r o p o s e d t h a t m e t e o r o l o g i c a l d a t a b e c o l l e c t e d f o r e s t i m a t i o n o f f r e e w a t e r s u r f a c e e v a p o r a t i o n by t h e masst r a n s f e r and e n e r g y b u d g e t t e c h n i q u e s . I n t h e a u t h o r ' s o p i n i o n t h e r e s u l t s t h a t c a n b e e x p e c t e d from t h e s e methods
s t i l l need t o b e s u p p o r t e d , and p r o b a b l y a d j u s t e d , by new f i g u r e s t o b e o b t a i n e d from t h e w a t e r - b a l a n c e
method. The m e t e o r o l o g i c a l network w i l l h e l p i n
o b t a i n i n g more a c c u r a t e f i g u r e s a b o u t t h e r a i n f a l l n o t o n l y on t h e catchment b u t on t h e l a k e i t s e l f . P r i o r t o t h e p r o j e c t , 58% o f t h e t o t a l i n f l o w t o t h e l a k e was measured and t h e f l o w from t h e ungauged a r e a s was e s t i m a t e d on t h e b a s i s of r a i n f a l l , c h a r a c t e r i s t i c s o f t h e c a t c h m e n t , and s i m i l a r i t y w i t h o t h e r
176
r i v e r s . The improvement o f t h e network by t h e p r o j e c t h a s b r o u g h t t h e measured i n f l o w t o a b o u t 90% o f t h e t o t a l i n f l o w (WMO, 1 9 7 4 ) .
Map showing t h e l o c a t i o n s o f e v a p o r a t i o n p a n s s e t up by t h e hydroFig. 5.2. m e t e o r o l o g i c a l network p r o j e c t on t h e i s l a n d s and a l o n g t h e s h o r e s o f Lake Victoria. 5.1.2
V i c t o r i a N i l e Basin
The w a t e r - b a l a n c e
o f Lake Kyoga w a s drawn by H u r s t (1952) c o n s i d e r i n g t h e
i n f l o w from t h e Upper V i c t o r i a N i l e as 2 0 . 9 m l r d m3/yr, t h e o t h e r t r i b u t a r i e s a s 3 . 5 m l r d m3/yr, l a k e and t h e r u n - o f f
t h e i n f l o w b r o u g h t by
and t h e sum of p r e c i p i t a t i o n and t h e
from t h e c a t c h m e n t t o i t a s 8 m l r d m 3 / y r .
The a n n u a l o u t -
f l o w from Lake Kyoga t o t h e L o w e r V i c t o r i a N i l e w a s c o n s i d e r e d a s 1 9 . 7 mlrd m3/
y r . These f i g u r e s l e a v e 1 2 . 4 m l r d m 3 / y r
f o r e v a p o r a t i o n from t h e l a k e (1760kmg
and e v a p o t r a n s p i r a t i o n from t h e swamps (4500 km2). The a v e r a g e r a t e s of evap o r a t i o n and e v a p o t r a n s p i r a t i o n h a v e been e s t i m a t e d a t 3 . 9 mm/day and 6.1 mm/ day, r e s p e c t i v e l y .
177
The mean monthly and a n n u a l P i c h e e v a p o r a t i o n a s o b s e r v e d i n t h e V i c t o r i a N i l e B a s i n are as f o l l o w s : Evaporation,
i n mm/day,
for
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sep.
Oct.
Nov.
D e c . . Year
7.4
7.7
6.0
4.3
3.5
4.1
3.9
3.7
4.2
4.5
5.0
4.9
4.9
The r e d u c t i o n f a c t o r t o b e a p p l i e d t o t h e above v a l u e s s o as t o c o n v e r t them i n t o open water e v a p o r a t i o n w a s d e s c r i b e d by H u r s t as " u n f o r t u n a t e l y v e r y uncert a i n " . A s a rough a p p r o x i m a t i o n h e a d o p t e d a f a c t o r of 0 . 6 7 t o t h e P i c h e evap o r a t i o n . T h i s f i g u r e i s t h e mean o f 0 . 8 3 , t h e f a c t o r proposed f o r Lake V i c t o r i a and 0 . 5 0 ,
t h e f a c t o r found f o r Egypt and t h e Sudan. I f t h i s f a c t o r i s a p p l i e d t o
t h e mean a n n u a l e v a p o r a t i o n , 3 . 3 mm/day e v a p o r a t i o n i s o b t a i n e d from t h e f r e e w a t e r s u r f a c e of Lake Kyoga. The method o f O l i v i e r (1961) g i v e s a mean a n n u a l e v a p o r a t i o n and a mean a n n u a l e v a p o t r a n s p i r a t i o n from t h e swamp, b o t h around S o r o t i , c l o s e t o t h e n o r t h e r n s h o r e o f Lake Kyoga, o f 7 . 0 and 4 . 7 mm/day r e s p e c t i v e l y . Assuming t h e r a t i o open water t o s t a n d a r d t a n k e v a p o r a t i o n t o b e 0 . 7 5 , t h e e v a p o r a t i o n from a f r e e water s u r f a c e i n t h e V i c t o r i a N i l e B a s i n becomes e q u a l
t o 5 . 2 5 mm/day. T h i s f i g u r e i s s l i g h t l y less t h a n t h e a v e r a g e v a l u e of 5 . 7 5 mm/ day e s t i m a t e d f o r t h e s u r r o u n d i n g s o f S o r o t i by t h e method o f Penman. Once more c o n s i d e r t h e w a t e r - b a l a n c e
drawn by H u r s t f o r Lake Kyoga and s u r -
r o u n d i n g s . I t i s n e i t h e r u n d e r s t a n d a b l e n o r j u s t i f i a b l y why t h e r a t e o f evapot r a n s p i r a t i o n from t h e swamps i s t a k e n as 1 . 6 1 t i m e s ( 6 . 1 : 3 . 9 )
t h e evaporation
r a t e from t h e l a k e i t s e l f . I f i t i s a r b i t r a r i l y assumed t h a t b o t h r a t e s are e q u a l , a g e n e r a l f i g u r e of 5 . 2 5 mm i s o b t a i n e d . From t h e f o r e g o i n g a n a l y s i s , o n e may draw t h e f o l l o w i n g c o n c l u s i o n s : i)
A mean a n n u a l e v a p o r a t i o n o f 3 . 9 mm/day seems t o b e f a i r l y s m a l l . T h i s f i g u r e n e e d s t o b e i n c r e a s e d t o a b o u t 5 . 2 5 mm/day.
ii)
The f a c t o r t o c o n v e r t P i c h e e v a p o r a t i o n t o f r e e w a t e r s u r f a c e e v a p o r a t i o n
i s n o t 0 . 6 7 a s o r i g i n a l l y s u g g e s t e d by H u r s t . T h i s f a c t o r n e e d s t o b e changed t o 1 . 0 5 or 1 . 1 0 . i i i ) The e v a p o r a t i v e demand o f t h e V i c t o r i a N i l e B a s i n r a n g e s from 5 . 5 mm/day t o 6 . 0 mm/day.
5.1.3
Lakes George and Edward, and Lake A l b e r t
I n t h e volumes o f t h e N i l e B a s i n up t o and i n c l u d i n g Vol I V t h e r e is no i n f o r m a t i o n a t a l l a b o u t e v a p o r a t i o n from Lakes George and Edward. I n V o l . V i t
i s mentioned t h a t no d i r e c t i n f o r m a t i o n a b o u t e v a p o r a t i o n from t h e s e two l a k e s
178
i s a v a i l a b l e . T h e i r s i z e and g e o g r a p h i c a l p o s i t i o n s u g g e s t , however, t h a t t h e e v a p o r a t i o n from Lakes George and Edward s h o u l d b e s i m i l a r t o t h a t from Lake Albert (Hurst, H . E . ,
and P h i l i p s , P . ,
1938).
E v a p o r a t i o n measurement a t B u t i a b a n e a r Lake A l b e r t s t a r t e d i n 1932 u s i n g t h e P i c h e atmometer. The mean monthly and y e a r l y P i c h e r e a d i n g s o v e r t h e p e r i o d 1932-1934 are as f o l l o w s : Evaporation,
i n mm/day,
for
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sep.
Oct.
Nov.
Dec.
Year
6.0
6.2
5.5
5.0
4.3
4.5
3.9
3.9
4.1
4.3
4.4
4.6
4.7
H u r s t assumed a r e d u c t i o n f a c t o r o f 0 . 7 so as t o deduce open water e v a p o r a t i o n from t h e P i c h e e v a p o r a t i o n . Based o n t h i s v a l u e f o r t h e r e d u c t i o n f a c t o r , t h e e v a p o r a t i o n from Lake A l b e r t would b e 3.3 mm/day. L a t e r , i n h i s book "The N i l e "
( 1 9 5 2 ) , H u r s t g a v e t h e b a l a n c e o f t h e Lakes
George and Edward. The t o t a l i n f l o w and r u n - o f f 2 . 2 m l r d m3/yr,
t o t h e l a k e s was assumed t o b e
t h e d i r e c t p r e c i p i t a t i o n 3.4 m l r d m 3 / y r and t h e o u t f l o w from
Edward t o t h e R i v e r S e m l e e k i 2 . 0 m l r d m3/yr. These f i g u r e s l e a v e a y e a r l y volume of 3 . 6 mlrd m 3 f o r e v a p o r a t i o n . The two l a k e s h a v e b e e n c o n s i d e r e d as one l a k e 2500 k m 2 i n s i z e , though Lake George i s a t a s l i g h t l y h i g h e r l e v e l t h a n Lake Edward. The mean a n n u a l d e p t h o f e v a p o r a t i o n i s 1440 mm, or a l m o s t 3 . 9 mm/day. I n t h e same r e f e r e n c e , i . e . "The N i l e "
( 1 9 5 2 ) , t h e b a l a n c e f o r Lake A l b e r t
w a s p r e p a r e d on t h e a s s u m p t i o n t h a t t h e a n n u a l volume o f d i r e c t p r e c i p i t a t i o n on the lake is 4.6 x
lo9
m 3 , t h e a n n u a l volume b r o u g h t by t h e Lower V i c t o r i a N i l e ,
R i v e r S e m l e e k i and t h e o t h e r t r i b u t a r i e s i s 2 5 . 0 x lo9 and t h e o u t f l o w from t h e l a k e i s 2 2 . 0 x lo9 m3/yr. T h e s e f i g u r e s are b a l a n c e d by a mean a n n u a l e v a p o r a t i o n of 3 . 9 mm/day. The same v a l u e f o r t h e e v a p o r a t i o n r a t e from Lake A l b e r t appears i n Vol. X of t h e N i l e Basin ( H u r s t , H . E . , Y.M.,
B l a c k , R . P . , and S i m a i k a ,
1 9 6 6 ) . I n t h e l a t t e r , t h e b a l a n c e was drawn between t h e i n f l o w t o and t h e
o u t f l o w from, t h e l a k e and t h e change i n s t o r a g e a v e r a g e d o v e r t h e p e r i o d 19401957. A c c o r d i n g t o t h e method o f O l i v i e r , e q . 5 . 3 , t h e w e t - b u l b d e p r e s s i o n measured
a t B u t i a b a g i v e s a mean a n n u a l e v a p o r a t i o n from a s t a n d a r d t a n k of a b o u t 4 . 2 mm/day. T h i s v a l u e c o r r e s p o n d s t o p r o b a b l y a f i g u r e o f a b o u t 3 . 5 mm/day f r e e w a t e r s u r f a c e e v a p o r a t i o n . The a n n u a l e v a p o r a t i o n measured by t h e P i c h e t u b e and t h e Kenya pan a t Gulu, a l m o s t 120 km n o r t h - e a s t
of B u t i a b a , h a v e b e e n r e p o r t e d
as 76 and 9 1 i n c h e s r e s p e c t i v e l y (WMO, 1 9 6 6 ) . T h e s e f i g u r e s c o r r e s p o n d t o n e a r l y 5 . 3 mm/day
P i c h e e v a p o r a t i o n and 5 . 6 mm/day open w a t e r e v a p o r a t i o n . The a u t h o r
h a s e s t i m a t e d t h e e v a p o r a t i v e demand a t some p l a c e s between t h e Lakes A l b e r t and Edward u s i n g Penman's m e t h o d . T h e e s t i m a t e s o b t a i n e d r a n g e f r o m 3 . 5 t o 3 . 9 mm/day.
179
The above r e v i e w may l e a d u s t o t h e f o l l o w i n g c o n c l u s i o n s : i)
The w a t e r b a l a n c e of Lake A l b e r t y i e l d s a mean a n n u a l e v a p o r a t i o n o f 3.9 mm/day from t h e l a k e . The s i m i l a r i t y and t h e n e a r n e s s of t h i s l a k e t o Lakes George and Edward s u g g e s t t h a t t h e e v a p o r a t i o n from t h e s e t w o l a k e s c a n a l s o b e c o n s i d e r e d a s 3.9 mm/day.
According t o t h i s f i g u r e , t h e P i c h e r e a d -
i n g a t B u t i a b a n e e d s t o b e m u l t i p l i e d by a r e d u c t i o n f a c t o r of 0 . 8 2 t o deduce t h e open w a t e r e v a p o r a t i o n . ii)
The e v a p o r a t i v e demand o f t h e c a t c h m e n t areas of Lakes George, Edward and A l b e r t v a r i e s c o n s i d e r a b l y from o n e l o c a t i o n t o t h e o t h e r . The r a n g e Of v a l u e s i s from a b o u t 3.5 mm/day, Albert,
t o more t h a n 5.5 mm/day,
somewhere between Lake Edward and Lake i n t h e neighbourhood o f t h e n o r t h e r n edge
of Lake A l b e r t ( s e e F i g . 5.3.).
F i g . 5.3.
Map of t h e e v a p o r a t i v e demand f o r t h e E q u a t o r i a l Lakes P l a t e a u
180
i i i ) The P i c h e e v a p o r a t i o n o f 4 . 7 mm/day a t B u t i a b a and o f 5 . 3 mm/day measured
a t Gulu needs t o b e m u l t i p l i e d by a f a c t o r o f from 1.1 t o 1 . 3 t o c o n v e r t i t t o t h e e v a p o r a t i v e demand o f t h e c a t c h m e n t a t t h e s e two l o c a t i o n s . BAHR EL JEBEL BASIN
5.2 5.2.1
Bahr e l J e b e l B a s i n ( o u t s i d e t h e swamps)
I n V o l . V o f t h e N i l e B a s i n i t i s mentioned t h a t a s m a l l t a n k 0 . 3 m s q u a r e
w a s f l o a t e d i n t h e r i v e r n e a r t h e d i s c h a r g e s i t e a t Nimule. E v a p o r a t i o n from t h a t t a n k was o b s e r v e d f o r some t i m e .
The d i f f i c u l t y i n m a i n t a i n i n g t h e t a n k t o
f u n c t i o n p r o p e r l y h a s l e d t o n e g l e c t i n g a l l t h e o b s e r v a t i o n s c o l l e c t e d from i t (Hurst, H.E.,
and P h i l i p s , P . ,
1938).
Both Volumes I and V o f t h e N i l e B a s i n g i v e t h e mean a n n u a l e v a p o r a t i o n from t h e P i c h e t u b e a t a number o f s t a t i o n s as f o l l o w s : S t a tion
Period of o b s erv ati o n
P i c h e e v a p o r a t i o n , mm/day
Malakal Masindi P o r t Lira Lerua Torit Jub a Mongalla Wau
1915-1934 1934 1933- 1934 1928- 1934 1922- 1934 1925-1928 and 1931-1934 1906-1930 1906-1929
9.0 4.2 5.0 4.9 7.1 7.0 6.1 7.4
H u r s t t o o k t h e mean o f t h e s e s t a t i o n s a f t e r e x c l u d i n g Malakal and Wau and m u l t i p l i e d t h e mean v a l u e by 0 . 5 7 t o deduce t h e open w a t e r e v a p o r a t i o n from t h e s o u t h e r n p a r t o f t h e Bahr e l J e b e l B a s i n . The r e s u l t s h e g o t were
E v a p o r a t i o n , i n mm/day,
for
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sep.
Oct.
Nov.
Dec.
Year
6.3
6.3
4.5
3.2
2.2
2.1
1.7
1.6
2.2
2.4
3.2
4.2
3.3
H e commented on t h e s e r e s u l t s s a y i n g " t h e whole q u e s t i o n of e v a p o r a t i o n from
open w a t e r in t h e s e s o u t h e r n d i s t r i c t s n e e d s i n v e s t i g a t i o n , b u t i s n o t e a s y on a c c o u n t o f t h e d i f f i c u l t y o f m a i n t a i n i n g f l o a t i n g t a n k s f r e e from wave a c t i o n . The d i s t r i b u t i o n t h r o u g h o u t t h e y e a r i s s i m i l a r a t a l l t h e s t a t i o n s and so t h e monthly mean v a l u e s t a k e n from a l l t h e s t a t i o n s hve b e e n r e d u c e d t o g i v e a t o t a l o f 1200 m m f o r t h e y e a r " . I n o r d e r t o e s t i m a t e t h e f r e e w a t e r s u r f a c e e v a p o r a t i o n from t h e Bahr e l J e b e l and i t s B a s i n o u t s i d e t h e swamps, t h e a u t h o r h a s worked o u t t h e f o r m u l a s of Penman and o f H a r g r e a v e s u s i n g t h e a v a i l a b l e m e t e o r o l o g i c a l d a t a a t M a l a k a l ,
181
Wau and Mongalla/Juba.
loo,
7O30' and 5'N,
These s t a t i o n s a r e s i t u a t e d c l o s e t o t h e l a t i t u d e s o f
respectively.
The f o r m u l a s u s e d r e a d as f o l l o w s : Penman's f o r m u l a H
E =
A 60
(1948)
+ y E (5.6)
A + v
where E
= f r e e water surface evaporation,
H
= n e t h e a t budget a t t h e s u r f a c e , i n cals/cm2.day,
i n mm/day,
A
= s l o p e o f t h e t a n g e n t o f t h e s a t u r a t e d vapour p r e s s u r e
y
= p s y c h r o m e t e r c o n s t a n t = 0 . 4 9 mm Hg/OC,
-
temperature curve
a t t h e mean a i r t e m p e r a t u r e , i n m m Hg/OC,
and
Ea = e v a p o r a t i o n c o r r e s p o n d i n g t o t h e h y p o t e t i c a l case of e q u a l a i r and water
temperatures. The q u a n t i t i e s H and E
c a n b e c a l c u l a t e d from t h e e q u a t i o n s
(5.7) and E
= 0.35 (e
s
-
e ) ( 1 + 0 . 0 1 u2) a
where
RA
= amount o f
e n e r g y r e a c h i n g t h e o u t e r l i m i t of t h e a t m o s p h e r e , i n c a l s / c m 2
(horizontal)
.
day,
= r e l a t i v e dura t io n of b r i g h t sunshine,
Ta = mean a i r t e m p e r a t u r e , i n d e g r e e s a b s o l u t e ,
e
= a c t u a l vapour p r e s s u r e i n t h e a i r ,
e
= s a t u r a t i o n vapour p r e s s u r e i n t h e a i r , i n mm Hg, and
i n mm Hg,
u2 = mean wind s p e e d i n m i l e s / d a y a t a h e i g h t o f 2 m above t h e w a t e r s u r f a c e . Hargreaves' E
formula (1956)
= 0 . 3 8 d (1 - hn) (T
-
32)
(5.9)
182
where Ev = monthly e v a p o r a t i o n from a class A p a n , i n i n c h e s ,
d
= monthly day-time c o e f f i c i e n t ,
hn = mean monthly r e l a t i v e h u m i d i t y a t noon, i n d e c i m a l s , and T
= mean monthly t e m p e r a t u r e , i n d e g r e e s F a h r e n h e i t
Eq. 5 . 9 i s b a s e d o n an a v e r a g e wind s p e e d o f 100 km/day.
Evaporation
i n c r e a s e s o r d e c r e a s e s a b o u t 9 % w i t h e a c h 50 km/day i n c r e a s e o r d e c r e a s e i n t h e wind s p e e d . F u r t h e r m o r e , where t h e s u n s h i n e i s m a t e r i a l l y less t h a n 90% one o u g h t t o correct t h e computed e v a p o r a t i o n as f o l l o w s : sunshine i n percent
30
correction i n percent
-34
40
50
60
70
80
90
-28
-24
- 20
-16
- 9
0
When o b s e r v a t i o n s on s u n s h i n e were n o t a v a i l a b l e , w e u s e d t h e e q u a t i o n devel o p e d by P a l a y a s o o t (1965) t o compute t h e s u n s h i n e , S , from t h e c l o u d ' c o v e r , C , scale 0
s
= 74.5
-
8 , as
+
9.5
c -
2.0
c2
(5.10)
Eq. 5 . 9 i s b a s e d on d a t a c o l l e c t e d f r o m l o c a t i o n s w i t h an a v e r a g e e l e v a t i o n o f 500 f t , s a y 150 m . S i n c e e v a p o r a t i o n i n c r e a s e s w i t h e l e v a t i o n , e q . 5 . 9 c a n b e c o r r e c t e d by i n c r e a s i n g t h e c a l c u l a t e d v a l u e s by 1%f o r e a c h 100 m i n c r e a s e i n elevation. The f i g u r e s o b t a i n e d from H a r g r e a v e s ' e q u a t i o n h a v e b e e n r e d u c e d by a f a c t o r of 0 . 7 so as t o c o n v e r t t h e c l a s s A pan e v a p o r a t i o n t o e v a p o r a t i o n from open w a t e r . The mean monthly e v a p o r a t i o n computed from e q . 5 . 6 and from e q . 5 . 9 and o b s e r v e d by t h e P i c h e i n s t r u m e n t i s l i s t e d i n T a b l e 5 . 4 . I n t h i s t a b l e , t h e s t a t i o n a t M a l a k a l i s c o n s i d e r e d a s r e p r e s e n t i n g t h e n o r t h e r n p a r t o f t h e Bahr
e l J e b e l and Mongalla/Juba as r e p r e s e n t i n g t h e s o u t h e r n p a r t , b o t h o u t s i d e t h e swamps. The s t a t i o n a t Wau c a n b e r e g a r d e d as a t r a n s i e n t p o i n t between t h e o u t s i d e and t h e i n s i d e of t h e swamps. The comparison between t h e e v a p o r a t i o n f i g u r e s p r e s e n t e d i n T a b l e 5 . 4 c a n be made somewhat e a s i e r when t h e y a r e p l o t t e d v e r s u s t h e months of t h e y e a r , as shown i n F i g . 5 . 4 . The r e m a r k a b l e f e a t u r e i n t h i s f i g u r e i s t h a t t h e t h r e e methods y i e l d , f o r a l l s t a t i o n s c o n s i d e r e d , e v a p o r a t i o n c u r v e s showing an i d e n t i c a l p a t t e r n o f v a r i a t i o n w i t h t i m e d u r i n g t h e y e a r . The e v a p o r a t i o n c u r v e , s t a r t i n g from J a n u a r y , e x h i b i t s a s l i g h t r i s e t i l l i t r e a c h e s t h e peak evaporat i o n i n February t h e n f a l l s s t e e p l y , e s p e c i a l l y i n t h e b e g i n n i n g , till t h e minimum e v a p o r a t i o n i s r e a c h e d between J u l y and August. From t h i s t i m e onwards,
183
t h e e v a p o r a t i o n i n c r e a s e s g r a d u a l l y t i l l J a n u a r y and F e b r u a r y i n t h e n e x t y e a r . TABLE 5 . 4
Mean monthly open water e v a p o r a t i o n and P i c h e e v a p o r a t i o n a t some s t a t i o n s i n t h e Bahr e l J e b e l B a s i n , o u t s i d e t h e swamps Evaporation,
Method
i n mm/day,
for
J a n . F e b . Mar. Apr. May J u n e J u l y Aug. S e p . O c t . Nov. Dec. Year 1. Malakal Penman 5.5 6.6 6 . 8 6.7 5.8 H a r g r e a v e s x 0 . 7 10.6 1 0 . 8 9 . 4 9 . 3 6 . 3 Piche 16.5 18.2 15.7 10.9 7.0
4.5 4.2 4.4
4.0 2.6 2.9
4.1 1.9 2.5
4.8 2.5 2.9
5.0 2.9 3.7
5.2 5.8 5.4 9.2 8.3 13.6
5.4 6.3 9.0
2 . Wau Penman 6 . 4 7.0 7.0 Hargreaves x 0 . 7 9 . 5 9 . 1 8 . 4 Piche 11.9 12.6 12.0
6.7 6.0 7.5 6.4 9.4 6 . 4
5.2 4.9 4.7
4.5 3.4 3.7
4.8 3.5 3.5
5.0 3.6 3.9
5.1
5.4 6 . 1 5 . 8 8.1 7.7 10.2
5.8 6.2 7.6
5.8 5.2 5.3 4.1 7.4 4.9
4.7 3.2 4.3
4.1 2.3 3.3
4.5 2.8 '3.4
4.9 3.2 4.4
5.3 4.6 7.1
5.5 5.0 7.0
3 . Mongalla/?u&
Penman 6.4 6.9 Hargreaves x 0 . 7 8 . 3 8 . 2 Piche 1 2 . 3 12.0
6.6 7.7 9.7
3.8
4.7
5.2
3.8
5.4
5.7 6.6 9.2
The f i g u r e s o b t a i n e d from t h e method o f Penman d i f f e r from t h o s e o b t a i n e d from t h e H a r g r e a v e s f o r m u l a and from t h e P i c h e e v a p o r i m e t e r i n two d i s t i n c t ways. The f i r s t i s t h a t t h e mean a n n u a l Penman e v a p o r a t i o n i n c r e a s e s from Mongalla/ J u b a t o Wau and d e c r e a s e s a t M a l a k a l , whereas t h e o t h e r two methods show i n c r e a s i n g e v a p o r a t i o n from t h e s o u t h t o t h e n o r t h . The s e c o n d is t h a t t h e v a r i a t i o n i n t h e mean monthly e v a p o r a t i o n e s t i m a t e d from t h e method of Penman i s s m a l l compared t o t h e v a r i a t i o n o f e v a p o r a t i o n e s t i m a t e d from t h e o t h e r t w o methods. The r a t i o o f t h e minimum t o t h e maximum e v a p o r a t i o n i n t h e Penman's method o f a b o u t 0 . 5 9 f o r b o t h Malakal and Mongalla/Juba and 0 . 6 4 f o r Wau. The
same r a t i o s b u t o b t a i n e d from t h e H a r g r e a v e s f o r m u l a and from t h e P i c h e t u b e a r e 0 . 1 8 and 0 . 1 4 f o r M a l a k a l , 0 . 3 7 and 0 . 2 8 f o r Wau, and 0 . 2 8 and 0 . 2 7 f o r Mongalla/Juba,
respectively.
The c l a s s A pan e v a p o r a t i o n e s t i m a t e d by t h e H a r g r e a v e s method h a s been m u l t i p l i e d by a f a c t o r of 0 . 7 to deduce t h e open w a t e r e v a p o r a t i o n . T h i s v a l u e s h o u l d b e m o d i f i e d t o 0 . 7 7 f o r M o n g a l l a / J u b a , 0.66 f o r Wau, and 0.60 f o r Malakal so a s t o y i e l d t h e same mean a n n u a l e v a p o r a t i o n as o b t a i n e d from Penman's
method. The m o d i f i e d v a l u e s l i e w i t h i n 0 . 6 0 - 0 . 8 1 , A pan f a c t o r ( L i n s l e y , R . K . ,
Kohler, M.A.,
t h e r a n g e found f o r t h e c l a s s
and P a u l h u s , J . L . ,
1958).
184
18
MALAKAL
16
14
t
/
!
;
/' /'
12
. %
0 D
10
g
8
C
.-0
6
[
L
e
Piche 9.0 - . _m.m_. l.d a y
/.-
I
x/'
>
w
2
01
12
2
10
g
8
.-E
6
E
4
. D
:
1
+,"
1
/.+-. '
1
+.
'.
I
I
I
I
1
I
I
I
\
I
>
12
MONGALLA I JUBA
>r
. E ,"
10
E
B
C
.O
6
a
4
z
c
0
+" /
/
,/'
>
w
2
Jan.
Fig. 5.4.
Feb.
Mar.
Apr.
May
Jun.
Jul.
Aug. Sep.
Oct.
Nov.
Mean monthly evaporation at Malakal, Wau and Mongalla/Juba
Dec.
185
The o l d d a t a g i v e n by K e e l i n g and t h e more r e c e n t o n e s g i v e n by O l i v i e r f o r t h e mean a n n u a l e v a p o r a t i o n from t h e s o - c a l l e d s t a n d a r d t a n k c a n b e summarized in the following: Station
K e e l i n g , mm/day
O l i v i e r , mm/day
Malakal Wau Juba
6.4 5.4 5.0
6.0 9.3 7.4
These v a l u e s , e x c e p t t h e o n e o b t a i n e d from O l i v i e r ' s f o r m u l a f o r Wau, seems t o b e i n r e a s o n a b l e agreement w i t h t h e f i g u r e s o b t a i n e d from t h e methods w e have already described. The r a t i o o f t h e P i c h e e v a p o r a t i o n t o t h e Penman e v a p o r a t i o n shows an i n c r e a s e from t h e s o u t h t o t h e n o r t h . T h i s r a t i o i s 1 . 2 7 f o r Mongalla/Juba,
1.31
f o r Wau and 1 . 6 7 f o r M a l a k a l .
5.2.2
Bahr e l J e b e l and Bahr e l Ghazal B a s i n s ( i n s i d e t h e swamps)
E v a p o r a t i o n o b s e r v a t i o n s from a 10 m s q u a r e open w a t e r t a n k f l o a t i n g i n Shambe Lagoon a r e mentioned i n V o l . X o f t h e N i l e B a s i n ( H u r s t , H . E . ,
R.P., and S i m a i k a , Y . M . ,
Black,
1 9 6 6 ) . The r e p o r t e d r e a d i n g s f o r t h e y e a r s 1948 and
1949 a r e 4 . 9 4 mm/day and 6 . 0 2 mm/day.
The d i f f i c u l t y i n m e a s u r i n g t h a t p a r t of
t h e r a i n f a l l t h a t r e a c h e d t h e t a n k i s p r o b a b l y one o f t h e f a c t o r s r e s p o n s i b l e for this difference. Unfortunately, t h e a v a i l a b l e metoeological d a t a f o r t h e a r e a i n s i d e t h e swamps ( c h a n n e l s , l a g o o n s , e t c . ) are n e i t h e r c o m p l e t e n o r q u i t e r e l i a b l e .
I t is
t h e r e f o r e n o t w o r t h w h i l e employing t h e s e d a t a f o r e s t i m a t i n g t h e e v a p o r a t i o n i n s i d e t h e swamps, a s t h i s was done f o r t h a t p a r t of t h e b a s i n o u t s i d e t h e swamps. One s h o u l d e x p e c t , however, less e v a p o r a t i o n from t h e f r e e w a t e r i n s i d e t h e swamps t h a n from o u t s i d e . T h i s argument i s s u p p o r t e d by t h e f i g u r e s g i v e n i n Vol. V o f t h e N i l e B a s i n ( H u r s t , H . E . ,
and P h i l i p s , P . , 1 9 3 8 ) . The a v e r a g e
t e m p e r a t u r e i n s i d e t h e swamps i s 2OC less t h a n o u t s i d e t h e swamps, t h e r e l a t i v e h u m i d i t y i s a b o u t 37% more, t h e c l o u d i n e s s i s 10% more and t h e wind f o r c e i s o n l y 20% of t h e wind f o r c e o u t s i d e t h e swamps. The q u e s t i o n t h a t t h e n a r i s e s i s by how much t h e e v a p o r a t i o n i n s i d e t h e swamps i s less t h a n t h e e v a p o r a t i o n o u t s i d e . I n a n a t t e m p t t o answer t h i s q u e s t i o n , O l i v i e r (1961) r e f e r r e d t o t h e o l d f i n d i n g s o f Ebermayer a b o u t t h e e v a p o r a t i o n i n s i d e and o u t s i d e t h e f o r e s t s i n Germany. Ebermayer s u g g e s t e d a r e d u c t i o n f a c t o r of 0 . 3 6 t o b e a p p l i e d t o t h e f r e e water e v a p o r a t i o n o u t s i d e t h e f o r e s t s t o deduce t h e e v a p o r a t i o n from i n s i d e the f o r e s t s .
I f t h i s r e d u c t i o n f a c t o r h o l d s i n t h e c a s e of t h e swamps, one
s h o u l d e x p e c t t h e f r e e water e v a p o r a t i o n from t h e swamps o f Bahr e l J e b e l and
186
Bahr e l Ghazal t o be about 2 . 1 mm/day. The Piche readings suggest t h e r a t i o o f 0 . 4 1 f o r t h e evaporation from t h e swamps i n p r o p o r t i o n t o t h e f r e e water surf a c e e v a p o r a t i o n . S u b s t i t u t i n g t h e d i f f e r e n c e i n t h e meteorological d a t a b e t ween t h e i n s i d e and t h e o u t s i d e of t h e swamps i n Hargreaves' formula, eq. 5 . 9 , one comes t o a r e d u c t i o n f a c t o r of about 0 . 5 . So one has t o expect a mean annual evaporation from i n s i d e t h e swamps i n t h e range of from 2 . 5 t o 3.0 mm/day. This range c o n s t i t u t e s about 50% only of t h e observed f l o a t i n g - t a n k e v a p o r a t i o n . The above d i s c u s s i o n can l e a d us t o t h e following general conclusions: i)
The open water evaporation o u t s i d e t h e swamps shows a gradual i n c r e a s e i n a n o r t h e r l y d i r e c t i o n from s l i g h t l y more than 5 mm/day a t Mongalla t o s l i g h t l y less than 6 mm/day a t Malakal.
ii)
The mean monthly evaporation a t a l l s t a t i o n s shows a well-defined p a t t e r n with time i n t h e y e a r .
iii) The range of v a r i a t i o n of t h e mean monthly evaporation estimated from t h e
Penman method is much s m a l l e r compared t o t h e same range i n t h e observed Piche readings o r t h e c l a s s A pan evaporation estimated from t h e Hargreaves formula. iv)
The r e d u c t i o n f a c t o r of t h e Piche readings t o deduce t h e f r e e water s u r f a c e evaporation i n c r e a s e s i n a n o r t h e r l y d i r e c t i o n from 1.27 a t Mongalla t o 1.67 a t Malakal.
v)
A reasonably a c c u r a t e measurement of t h e open water evaporation i n s i d e t h e
swamps is not a v a i l a b l e . I n d i r e c t e s t i m a t e s y i e l d a mean annual evaporat i o n i n t h e range of 2 . 5 t o 3.0 mm/day. 5.3
THE WHfTE NILE BASIN (from Malakal t o Khartoum)
Evaporation has been measured by means of f l o a t i n g tanks a t Malakal and Khartoum, a s w e l l a s by t h e Piche evaporimeter a t a number of s t a t i o n s . The f l o a t i n g tanks have been r e p o r t e d a s being not e n t i r e l y s a t i s f a c t o r y , though they g i v e more d i r e c t and u s e f u l information. The reason behind t h a t s t a t e of a f f a i r s was t h e d i f f i c u l t y i n p r o t e c t i n g t h e tanks completely from t h e e f f e c t s of s t r o n g winds. The evaporation from t h e White N i l e Basin has become important a f t e r t h e cons t r u c t i o n of t h e J e b e l Aulia dam, some f o r t y k i l o m e t r e s south of Khartoum. The s t o r a g e r e s e r v o i r , when f u l l , has a r e l a t i v e l y l a r g e s u r f a c e compared t o i t s volume. The mean monthly open water evaporation a t Khartoum, J e b e l A u l i a , Dueim, Rabak/Kosti, Renk and Malakal has been deduced from t h e Piche readings using a r e d u c t i o n f a c t o r of 0 . 5 . The mean annual f i g u r e s a r e presented both i n Table 5 . 1 and F i g . 5 . 1 .
H u r s t , i n Vol. VIII of t h e N i l e Basin (1950), commented on
t h e changing c l i m a t e a s one t r a v e l s south along t h e White Nile s t a r t i n g from Khartoum. The range of t h e Piche evaporation a t t h e northern end i s from 10 t o
187
20 mm/day. A t Dueim, 200 km s o u t h , t h e maximum is a b o u t t h e same b u t t h e minimum d e c r e a s e s t o 7 mm/day. A t M a l a k a l , 800 km s o u t h , t h e maximum remains a l m o s t t h e same and t h e minimum d r o p s f u r t h e r t o 3 mm/day and t h e e v a p o r a t i o n is less t h a n t h e minimum a t Khartoum f o r s e v e n months, from May up t o and i n c l u d i n g November. The mean monthly e v a p o r a t i v e demand f o r Khartoum e s t i m a t e d from t h e methods o f Penman and H a r g r e a v e s t o g e t h e r w i t h t h e P i c h e e v a p o r a t i o n i s i n c l u ded i n T a b l e 5 . 5 . TABLE 5 . 5
Mean monthly open water e v a p o r a t i o n a t Khartoum and P i c h e e v a p o r a t i o n from t h e White N i l e B a s i n Evaporation,
Method
J a n . F e b . Mar. Apr. May Penman 7.2 7.8 9.3 Hargreavesx0.7 10 . 1 11.6 13.6 Piche 12.0 1 4 . 0 17.0 Piche x 0.5+ 8 . 0 9 . 0 10.0
i n mm/day, f o r
J u n e J u l y Aug. S e p . O c t . Nov. D e c . Year
10.5 10.4 10.1 8 . 4 6 . 8 7 . 1 8 . 9 8 . 2 7 . 1 8 . 5 16.6 1 7 . 0 1 5 . 2 1 0 . 8 7 . 6 8 . 6 1 2 . 6 1 2 . 6 11.0 1 2 . 1 1 8 . 0 1 8 . 0 16.0 1 3 . 0 1 0 . 0 1 2 . 0 1 4 . 0 1 3 . 0 1 2 . 0 1 4 . 1 10.0 8 . 0 7 . 0 5 . 0 3 . 0 4 . 0 5 . 0 7 . 0 7 . 0 7 . 0
+Average o f P i c h e r e a d i n g s a t Khartoum, J e b e l A u l i a , Dueim, R a b a k / K o s t i , Renk and M a l a k a l . O t h e r d a t a b e l o n g t o Khartoum o n l y . The d a t a a b o u t Malakal i s included i n Table 5 . 4 . The mean monthly e v a p o r a t i o n g i v e n i n T a b l e 5 . 5 shows t h e same p a t t e r n f o r a l l methods u s e d . T h e r e a r e two s e a s o n s f o r t h e peak e v a p o r a t i o n , though t h e second peak h a s a n a v e r a g e v a l u e e q u a l t o a b o u t 75% o f t h e v a l u e of t h e f i r s t peak. The f i r s t peak o c c u r s i n April-May and t h e s e c o n d i n October-November. The p r o p o r t i o n o f t h e minimum monthly e v a p o r a t i o n t o t h e maximum monthly evap o r a t i o n v a r i e s from 0 . 6 5 i n t h e method o f Penman t o 0 . 4 5 i n t h e method of Hargreaves t o 0 . 5 5 f o r t h e P i c h e e v a p o r a t i o n . The r e s u l t s o b t a i n e d from t h e Hargreaves f o r m u l a when m u l t i p l i e d by 0 . 7 a r e a b o u t 434. h i g h e r t h a n t h o s e o b t a i n e d from t h e Penman method whereas t h e P i c h e r e a d i n g s need t o b e reduced by a b o u t 40% t o become e q u a l t o t h e Penman e v a p o r a t i o n . A d d i t i o n a l e s t i m a t e s o f e v a p o r a t i o n from a s t a n d a r d t a n k a r e 11.3 and 1 0 . 5 mm/day as o b t a i n e d from t h e O l i v i e r method and t h e d a t a of K e e l i n g , r e s p e c t i v e l y .
A t Khartoum o b s e r v a t o r y , c o m p a r i s o n s w e r e made among e v a p o r a t i o n d a t a from ( 1 ) a 1 2 - f t d i a m e t e r t a n k , 4 - f t d e e p ; ( 2 ) a c l a s s A p a n ; ( 3 ) a P i c h e atmometer; ( 4 ) l a k e e v a p o r a t i o n computed by Penman e q u a t i o n , and ( 5 ) l a k e e v a p o r a t i o n by K o h l e r ' s method, which i s b a s e d on c o r r e c t i n g class A pan e v a p o r a t i o n f o r h e a t t r a n s f e r t h r o u g h i t s s i d e s and b o t t o m . The r e s u l t s o b t a i n e d f o r t h e y e a r s 1961 and 1962 are g i v e n i n T a b l e 5 . 6 (WMO, 1 9 6 6 ) .
188
TABLE 5 . 6
Comparison between e v a p o r a t i o n o b t a i n e d from d i f f e r e n t methods and e v a p o r i m e t e r s a t Khartoum (WMO, 1966)
Year
1960
Month
1 2 - f t pan Piche
1 2 - f t pan Penman
1 2 - f t pan (Kohler)
Jan. Feb. Mar. Apr . May June July Aug . Sep. Oct. Nov Dec.
0.68 0.60 0.65 0.62 0.64 0.67 0.62 0.65 0.69 0.69 0.64 0.61
0.50 0.43 0.48 0.46 0.53 0.53 0.54 0.60 0.61 0.55 0.45 0.47
1.23 1.13 1.18 1.25 1.37 1.20 1.06 1.14 1.17 1.31 1.25 1.16
1.07 0.99 1.02 0.97 0.99 1.03 0.94 1.00 1.01 1.05 0.99 0.98
Annual
0.64
0.51
1.20
1.oo
Jan. Feb Mar Apr . May June July Aug . Sep. Oct. Nov . Dec
0.67 0.66 0.64 0.62 0.59 0.61 0.68 0.74 0.69 0.67 0.66 0.69
0.51 0.50 0.50 0.49 0.48 0.58 0.78 0.63 0.63 0.53 0.50 0.50
1.20 1.24 1.21 1.28 1.36 1.24 0.97 0.96 1.20 1.25 1.26 1.22
1.06 1.01 1.00 0.98 0.97 0.95 0.98 1.03 1.04 1.01 1.00 1.03
Annual
0.65
0.55
1.20
1.00
.
.
196 1
1 2 - f t pan C l a s s A pan
.
The f i g u r e s l i s t e d i n T a b l e 5 . 6 c a n b e c o n v e r t e d t o o t h e r f i g u r e s g i v i n g t h e r a t i o between e v a p o r a t i o n from a f r e e water s u r f a c e and e v a p o r a t i o n a s o b s e r v e d by an e v a p o r i m e t e r o r e s t i m a t e d from a f o r m u l a . T h i s h a s been done u s i n g t h e d a t a i n T a b l e 5 . 2 where t h e r a t i o o f 1 2 - f t pan d i a m e t e r t o an e x t e n d e d w a t e r s u r f a c e is 1 . 0 9 9 . The monthly a v e r a g e r a t i o s found f o r 1960 and 1961 o f t h e 1 2 - f t p a n : P i c h e and t h e 1 2 - f t p a n : Penman were e a c h d i v i d e d by 1 . 0 9 9 t o deduce t h e c o r r e s p o n d i n g f r e e w a t e r s u r f a c e : P i c h e and f r e e water s u r f a c e : Penman. By t h i s p r o c e d u r e two sets of mean monthly e v a p o r a t i o n v a l u e s from t h e f r e e w a t e r s u r f a c e a t Khartoum h a v e b e e n d e v e l o p e d . The new set o f d a t a o r i g i n a t i n g from P i c h e r e a d i n g s i s somewhat on t h e low s i d e , and t h a t o r i g i n a t i n g from Penman's method i s somewhat on t h e h i g h s i d e . A f a i r compromise i s t o t a k e t h e a r i t h m e t i c mean of t h e s e two s e t s . A c c o r d i n g l y , one c a n l i s t t h e f r e e w a t e r s u r f a c e e v a p o r a t i o n
a t Khartoum a s f o l l o w s :
189
Evaporation,
i n mm/day,
for
Jan.
Feb.
Mar.
Apr.
May
Jun.
Jul.
Aug.
Sep.
Oct.
Nov.
Dec.
Year
6.74
7.16
8.85
9.94
10.60
9.64
7.78
6.05
7.21
8.62
7.49
6.49
8.04
The f o r e g o i n g a n a l y s i s l e a d s u s t o t h e c o n c l u s i o n t h a t t h e f r e e w a t e r s u r f a c e e v a p o r a t i o n from t h e White N i l e B a s i n r a n g e s from s l i g h t l y less t h a n 6 mm/ day a t Malakal t o 8 mm/day a t Khartoum i n a n o r t h e r l y d i r e c t i o n o v e r a d i s t a n c e of a b o u t 800 km. The mean monthly e v a p o r a t i o n a t Khartoum h a s two p e a k s ; t h e f i r s t peak o f 10.6 mm/day o c c u r r i n g i n O c t o b e r . The minimum t a k e s p l a c e i n August and i n December where t h e mean monthly e v a p o r a t i o n f a l l s t o 6.0 and 6.5 r e s p e c t i v e l y . E v a p o r a t i o n from open w a t e r a t Malakal shows a s i n g l e peak
mm/day
o n l y . The e x p e r i m e n t s c o n d u c t e d a t Khartoum when combined w i t h o t h e r e x p e r i ments show t h a t t h e Penman e v a p o r a t i o n i s a b o u t 10% less t h a n t h e e v a p o r a t i o n from a f r e e w a t e r s u r f a c e . The r e d u c t i o n f a c t o r o f t h e P i c h e r e a d i n g s needs t o b e i n c r e a s e d t o 0.55-0.60 i n s t e a d of 0.5 t o deduce t h e open w a t e r e v a p o r a t i o n .
5.4
THE ETHIOPIAN PLATEAU
5.4.1
The S o b a t B a s i n
E s t i m a t i o n o f f r e e w a t e r e v a p o r a t i o n from t h e S o b a t B a s i n h a s been c o n f i n e d t o t h r e e s t a t i o n s , namely: Gambeila, Akobo and M a l a k a l . A s a m a t t e r of f a c t t h e s t a t i o n a t Malakal b e l o n g s t o t h e White N i l e B a s i n . However, s i n c e i t i s s i t u a t e d c l o s e t o t h e mouth of t h e S o b a t on t h e White N i l e , i t s d a t a c a n b e used t o c o m p l e t e t h e e s t i m a t e s of e v a p o r a t i o n from t h e c a t c h m e n t o f t h e S o b a t . The a v e r a g e P i c h e r e a d i n g s o v e r d i f f e r e n t p e r i o d s f o r Gambeila and Akobo a r e l i s t e d i n T a b l e 5.7 whereas t h e d a t a b e l o n g i n g t o Malakal have a l r e a d y been i n c l u d e d i n T a b l e 5.4. TABLE 5.7
P i c h e e v a p o r a t i o n a t Gambeila and Akobo i n t h e S o b a t B a s i n ( H u r s t ,
H.E., Station
1950) Evaporation,
Period
*
i n mm/day,
for
J a n . F e b . Mar. Apr. May J u n e J u l y Aug. S e p . O c t . Nov. Dec. Year
Gambeila
a b
9.0 10.0 1 1 . 0 8.0 9.8 9.6
8.0 5.0 7.5 4.0
4.0 2.5
3.0 2.2
3.0 2.1
3.0 2.6
4.0 3.3
5.0 4.3
7.0 5.9
5.9 5.1
Akobo
c
10.3 11.8 10.9
8.8 5.3
3.9
2.9
2.4
3.1
3.6
4.6
7.8
6.3
*a = 1909-34; b = 1950-57;c= 1950-57 ( i n 1950 from J a n u a r y t o March o n l y )
190
The mean a n n u a l e v a p o r a t i o n as g i v e n i n T a b l e 5 . 7 i s n o t r e a l l y c o n s i s t e n t w i t h t h e r e g r e s s i o n r e l a t i o n between t h e a n n u a l r a i n f a l l , R i n mm, and t h e P i c h e e v a p o r a t i o n , i n mm/day, which w a s d e v e l o p e d by H u r s t i n V o l . X o f t h e N i l e B a s i n ( 1 9 6 6 ) . The r e g r e s s i o n e q u a t i o n Epiche
= 10.3
-
0.0076 (R
-
714)
(5.11)
gives E f o r Gambeila and Akobo as 6 . 6 and 8 . 9 mm/day r e s p e c t i v e l y . The d i s piche c r e p a n c y between t h e s e f i g u r e s and t h e o b s e r v e d P i c h e e v a p o r a t i o n may throw some d o u b t on t h e v a l i d i t y o f e q . 5 . 1 1 , o r t h e d a t a i n T a b l e 5 . 7 , o r b o t h . The a v a i l a b l e m e t e o r o l o g i c a l d a t a of t h e S o b a t B a s i n , e x c e p t f o r M a l a k a l , are i n a d e q u a t e t o a l l o w f o r e s t i m a t i n g t h e f r e e water e v a p o r a t i o n from any f o r m u l a . W e t h e r e f o r e need t o a p p r o a c h t h e problem from a d i f f e r e n t a n g l e . The w e i g h t e d a v e r a g e P i c h e e v a p o r a t i o n ( t o t a l 32 y e a r s o f r e c o r d ) i s 5 . 7 mm/day f o r Gambeila. Assuming a l i n e a r p r o p o r t i o n between t h e e v a p o r a t i o n a t t h e two s t a t i o n s , a n a v e r a g e P i c h e e v a p o r a t i o n o f 7 . 0 4 mm/day, o v e r t h e same p e r i o d , c a n b e o b t a i n e d f o r Akobo. The t w o s t a t i o n s are s i t u a t e d o u t s i d e t h e Machar swamps and a p p a r e n t l y have s i m i l a r c o n d i t i o n s a f f e c t i n g t h e f r e e w a t e r e v a p o r a t i o n t o t h e s t a t i o n s o u t s i d e t h e swamps i n t h e Bahr e l J e b e l and Bahr e l Ghazal B a s i n s . From t h e d a t a p r e s e n t e d i n Table 5 . 4 i t is obvious t h a t t h e r e d u c t i o n f a c t o r t o t h e P i c h e r e a d i n g s a t Mongalla and Wau h a s a n a v e r a g e o f 0 . 7 5 . Applying t h i s f a c t o r t o t h e e s t i m a t e d P i c h e e v a p o r a t i o n f o r Gambeila and Akobo o n e o b t a i n s a mean a n n u a l open water e v a p o r a t i o n o f 4 . 3 and 5 . 3 mm/day r e s p e c t i v e l y . F u r t h e r m o r e , i f w e assume t h a t t h e s e f i g u r e s are d i s t r i b u t e d throughout t h e y e a r s i m i l a r t o t h e P i c h e r e a d i n g s i n Table 5 . 7 , one can o b t a i n t h e mean monthly e v a p o r a t i o n as f o l l o w s :
Station
E v a p o r a t i o n , i n mm/day,
for
J a n . Feb. Mar. Apr. May J u n e J u l y Aug. S e p . O c t . Nov. Dec. Year
Gambeila
6.6
7.7
8.0
6.0
3.5
2.5
2.1
2.1
2.2
2.8
3.6
5.0
4.3
Akobo
8.7
9.9
9.2
7.4
4.4
4.5
3.3
2.4
2.0
2.6
3.9
6.5
5.3
The S o b a t B a s i n i n c l u d e s t h e Machar swamps. The s u r f a c e a r e a o f t h e s e swamps
i s e x c e e d i n g l y v a r i a b l e . The J o n g l e i i n v e s t i g a t i o n team r e p o r t e d t h a t when t h e r a i n s were below t h e n o r m a l , t h e y became d r y , and when t h e y were above t h e n o r m a l , t h e swamps c o v e r e d a b o u t 20.000 km2. T h i s team used what t h e y r e p o r t e d
as t h e maximum s u r f a c e a r e a f o r e s t i m a t i n g t h e e v a p o r a t i o n from t h e Machar swamps. From t h e w a t e r - b a l a n c e o f t h e swamps t h e a n n u a l e v a p o r a t i o n w a s estimat e d a t 0 . 9 5 m o r a b o u t 2 . 6 mm/day. H u r s t condemned t h i s f i g u r e as b e i n g
19 1
a b s u r d l y l o w . I n s t e a d , i n V o l . X o f t h e N i l e B a s i n (1966), h e e s t i m a t e d t h e annual volume o f w a t e r d i s a p p e a r i n g by e v a p o r a t i o n from t h e Machar swamps a t 9 . 9 mlrd m 3 . H e f u r t h e r assumed t h a t t h i s volume corresponded t o 6.700 km2 s u r f a c e of t h e swamps. I f t h i s is r e a l l y so, t h e mean annual f r e e w a t e r s u r f a c e evaporat i o n becomes 4 . 1 mm/day and n o t 5.0 mm/day as computed by H u r s t . The f i g u r e of 4 . 1 mm/day seems t o b e f a i r l y c o n s i s t e n t w i t h t h o s e found by u s as 4 . 3 and 5 . 3 mm/day e v a p o r a t i o n a t Gambeila and Akobo r e s p e c t i v e l y . 5.4.2
The B l u e N i l e B a s i n
The open water e v a p o r a t i o n deduced from t h e P i c h e r e a d i n g s a t a number of places i n t h e Blue N i l e Basin is a l r e a d y included i n Table 5 . 1 .
The mean
annual e v a p o r a t i o n as g i v e n i n t h i s t a b l e v a r i e s from 3 . 0 mm/day f o r Lake Tana t o 6 . 5 mm/day a t Wad Medani and e v e n t u a l l y t o 7 . 8 mm/day a t Khartoum. Rz6ska, upon c o m p i l i n g some h y d r o l o g i c a l d a t a from H u r s t ‘ s book
,
“The N i l e “
(1976) concluded t h a t t h e mean annual l o s s e s o f open water measured i n t a n k s w e r e 1 . 0 9 (0.54-1.29)
m f o r Lake Tana and 2.30 m f o r t h e r e a c h from R o s e i r e s up
t o Wad Medani. Lake Tana h a s a s u r f a c e area o f about 3150 km2 and a catchment o f about 13400 km2. The i s o h y e t a l maps show a d i r e c t r a i n f a l l o f 1 . 2 m i n a normal y e a r o v e r t h e l a k e and about 1 . 3 m o v e r t h e catchment. The run-off
c o e f f i c i e n t f o r t h i s catchment a s o b t a i n e d from “The Water
Resources o f The E a r t h ” (1974) is 0 . 1 5 . S i n c e t h e o u t f l o w from Lake Tana i n a normal y e a r i s known t o b e a b o u t 3 . 5 mlrd m 3 , one c a n e a s i l y a r r i v e a t an annual d e p t h o f e v a p o r a t i o n o f 0 . 9 m, from t h e water b a l a n c e , e q . 5 . 2 . T h i s depth c o r r e s p o n d s t o a mean annual e v a p o r a t i o n o f 2 . 5 mm/day, which i s 20% less t h a n t h e f i g u r e p r e s e n t e d i n T a b l e 5 . 1 . The i n a c c u r a c i e s i n v o l v e d i n t h e r a i n d e p t h s , i n t h e estimate of t h e o u t f l o w from t h e l a k e , and p r o b a b l y t h e run-off c o e f f i c i e n t from t h e catchment o b v i o u s l y a f f e c t t h e a c c u r a c y of t h e estimate of evaporation. H u r s t , i n Vol. VIII of t h e N i l e B a s i n ( 1 9 5 0 ) , mentions t h a t t h e n e x t p o i n t ( n e x t t o Lake Tana) where t h e r e i s any h y d r o l o g i c a l i n f o r m a t i o n i s a t R o s e i r e s i n t h e Sudan. We h a v e t r i e d t o estimate t h e f r e e water s u r f a c e e v a p o r a t i o n from t h e a v a i l a b l e m e t e o r o l o g i c a l d a t a f o r t h r e e s t a t i o n s , namely: Addis Abbaba, R o s e i r e s and Wad Medani. The computed e v a p o r a t i o n , t o g e t h e r w i t h t h e a v a i l a b l e Piche readings a r e l i s t e d i n Table 5 . 8 . The e v a p o r a t i o n from t h e Blue N i l e B a s i n , whether computed from t h e formula o f Penman o r t h a t o f H a r g r e a v e s , or measured by t h e P i c h e i n s t r u m e n t , shows an i n c r e a s e i n a n o r t h e r l y d i r e c t i o n from Addis Abbaba t o Wad Medani and e v e n t u a l l y t o Khartoum.
192
TABLE 5.8
Computed f r e e s u r f a c e w a t e r e v a p o r a t i o n and measured P i c h e e v a p o r a t i o n a t some s t a t i o n s i n t h e B l u e N i l e B a s i n
Station and Metho'
Evaporation, J a n . Feb. Mar. Apr.
May
i n mm/day,
for
J u n e J u l y Aug. S e p . O c t . Nov. Dec. Year
Addis Abbaba
a
b
Roseires a b C
d Wad Medani a b C
d
5.2 7.3
5.3 7.5
5.4 7.7
5.7 8.5
5.6 8.3
4.9 7.0
4.2 5.6
4.1 5.3
4.7 6.5
5.0 6.9
5.1 7.1
6.3
6.5
7.8
8.3
7.6
7.4
7.9
8.1
6.6
6.4 7.2 8.4 4.2
5.0 3.9 5.0 2.5
5.1 4.0 5.1 2.6
5.3 4.6 5.8 2.9
5.5 4.8 6.8 3.4
5.7 5.8 6.3 6.3 8.4 7.9 8.8 11.4 1 0 . 6 4.4 5.7 5.3
5.8 6.3 7.8 9.2 8.9 8.3 6.4 7.8 8.7 10.2 12.9 12.5 11.3 8.1 14.3 16.5 20.4 21.5 20.4 18.7 11.3 7.1 8.2 10.2 10.8 10.2 9.4 5.6
5.3 5.7 6.6 3.3
6.3 6.5
1 0 . 0 10.7 11.9 12.3 9.2 13.6 14.7 15.8 16.2 13.2
6.8
5.2 7.3
5.0 7.1
7.5 6.6 5.6 7.0 9.5 9.2 8.1 9.2 7 . 1 11.4 14.6 13.4 14.7 3.6 5.7 7.3 6.7 7.3
*a = Penman; b = H a r g r e a v e s x 0.7; c = P i c h e ; d = P i c h e x 0.5 The f r e e w a t e r e v a p o r a t i o n computed by Penman's f o r m u l a h a s been p l o t t e d v e r s u s t h e t i m e i n months f o r t h e s t a t i o n s a t Addis Abbaba, Roseires, Wad Medani and Khartoum. The f o u r e v a p o r a t i o n c u r v e s i l l u s t r a t e d i n F i g . 5.5. p r e s e n t some marked f e a t u r e s . The r a t i o of t h e minimum e v a p o r a t i o n t o t h e maximum e v a p o r a t i o n f o r Addis a b b a b a , b e i n g 0.72, i s c e r t a i n l y h i g h compared w i t h t h e a v e r a g e o f t h e c o r r e s p o n d i n g r a t i o s f o r t h e o t h e r s t a t i o n s , which i s a b o u t 0 . 6 0
(0.57-0.64). The e v a p o r a t i o n c u r v e s f o r Addis Abbaba and R o s e i r e s are s i m i l a r t o t h o s e p l o t t e d f o r t h e s t a t i o n s i n t h e Bahr e l J e b e l B a s i n ( F i g . 5.4.) i n t h a t e a c h c u r v e h a s a s i n g l e p e a k . The o t h e r two c u r v e s f o r Wad Medani and Khartoum show two peak e v a p o r a t i o n r a t e s , t h e f i r s t o c c u r s i n April-May
and t h e s e c o n d ,
t h e s m a l l e r of t h e two, i n O c t o b e r . The f i g u r e s l i s t e d i n T a b l e s 5.6 and 5.8 i n d i c a t e t h a t t h e r a t i o o f t h e mean a n n u a l e v a p o r a t i o n computed by t h e Penman t o t h e mean a n n u a l e v a p o r a t i o n r e a d from t h e P i c h e t u b e i s n e a r l y t h e same f o r b o t h Khartoum and R o s e i r e s , 0.60 and
0.59 r e s p e c t i v e l y . The Same r a t i o c a n b e found from T a b l e 5.4 a s 0 . 6 0 f o r Malakal a l s o . I t m i g h t b e t h a t f o r t h o s e p a r t s o f t h e White N i l e and t h e B l u e N i l e B a s i n s o u t s i d e t h e swamps t h e f a c t o r 0 . 6 c a n r e a s o n a b l y b e u s e d t o c o n v e r t
t h e mean a n n u a l P i c h e e v a p o r a t i o n t o t h e c o r r e s p o n d i n g Penman e v a p o r a t i o n . Note t h a t t h i s i s s l i g h t l y d i f f e r e n t from t h e f a c t o r o f 0.5 used by H u r s t f o r c o n v e r t i n g t h e Piche readings t o f r e e water surface evaporation.
193
10
9
a x
0 D
- 7 E E C
.-
.-0
z6 n
> w
x
,..
*
.........J(‘. ......... .
5
x...........x.”
4
3
Jan.
Feb.
Mar.
Apr.
May.
Jun.
Jul.
Aug.
Sep.
Oct.
Nov.
Dec.
F i g . 5.5. Mean monthly e v a p o r a t i o n from open water a t some s t a t i o n s i n t h e Blue N i l e B a s i n e s t i m a t e d from t h e Penman method
The Penman: t h e P i c h e r a t i o f o r t h e mean monthly e v a p o r a t i o n h a s been found t o v a r y from as small a s 0 . 4 4 t o a s l a r g e a s 1 . 0 f o r R o s e i r e s and Khartoum s t a t i o n s . The r a n g e o f v a r i a t i o n f o r Malakal i s even much w i d e r s i n c e t h e mini-
m u m and t h e maximum v a l u e s f o r t h i s r a t i o a r e 0.33 and 1.66 r e s p e c t i v e l y . I n a d d i t i o n to t h e s t a t i o n s a t Addis Abbaba, R o s e i r e s , Wad Medani and Khartoum, t h e r e are t h r e e o t h e r s t a t i o n s i n t h e B l u e N i l e B a s i n f o r which t h e P i c h e r e a d i n g s o v e r t h e p e r i o d 1921-1950 a r e a v a i l a b l e . These a r e : S i n g a and S e n n a r , n e a r l y h a l f w a y between R o s e i r e s and Khartoum, and Kurmuk, t o which t h e nearest s t a t i o n is Roseires.
194
The a v e r a g e o f t h e monthly c o e f f i c i e n t s f o r Khartoum and R o s e i r e s h a v e been m u l t i p l i e d by t h e P i c h e e v a p o r a t i o n a t S i n g a and a t S e n n a r whereas t h e monthly c o e f f i c i e n t s f o r R o s e i r e s a l o n e have b e e n m u l t i p l i e d by t h e P i c h e e v a p o r a t i o n a t Kurmuk ( H u r s t , N . E . ,
B l a c k , R.P. and S i m a i k a , Y.M., 1959). The r e s u l t s
o b t a i n e d a r e l i s t e d i n T a b l e 5.9. TABLE 5.9
F r e e w a t e r s u r f a c e e v a p o r a t i o n f o r S i n g a , Sennar and Kurmuk i n t h e Blue N i l e B a s i n
Station Singa Sennar Kurmuk 5.4.3
E v a p o r a t i o n , i n mm/day, f o r J a n . Feb. Mar. Apr. May J u n e J u l y Aug. S e p . O c t . Nov. D e c . Year 7.2 7.8 7.2
8.3 9.5 7.9
8.8 9.7 8.3 11.4 12.2 9.2 8.2 6.7 4.6
8.2 9.5 3.7
5.8 6.6 3.1
3.7 4.2 2.6
3.8 5.0 2.8
5.8 7.2 2.9
8.6 9.6 4.8
7.5 7.9 6.9
7.0 7.4 5.3
The A t b a r a B a s i n
While d i s c u s s i n g t h e hydrology o f t h e A t b a r a B a s i n i n Vol.
IX o f t h e N i l e
B a s i n , H u r s t mentions t h a t t h e r e i s v e r y l i t t l e i n f o r m a t i o n a b o u t t h e evaporat i o n from t h i s b a s i n . Though Lake Tana i s j u s t o u t s i d e t h e A t b a r a B a s i n , t h e s u r r o u n d i n g c o u n t r y is s i m i l a r t o t h a t o f t h e A t b a r a c a t c h m e n t . H u r s t used t h i s c h a r a c t e r i s t i c t o g e t h e r w i t h t h e a v e r a g e o f t h e P i c h e r e a d i n g s a t Kassala and G a l l a b a t , reduced by a f a c t o r of 0.5, i n o r d e r t o s k e t c h a n approximate p i c t u r e o f t h e open w a t e r e v a p o r a t i o n i n o r n e a r t h e A t b a r a B a s i n ( H u r s t , H . E . , R.P.,
and Simaika, Y . M . ,
Black,
1959). The f i g u r e s o b t a i n e d by H u r s t and t h e f i g u r e s
w e have e s t i m a t e d by t h e Penman method d i r e c t l y f o r A t b a r a and i n d i r e c t l y as an
a v e r a g e f o r K a s s a l a and G a l l a b a t are g i v e n i n T a b l e 5.10. The f o r e g o i n g d i s c u s s i o n o f t h e f r e e water s u r f a c e e v a p o r a t i o n from t h e E t h i o p i a n P l a t e a u may l e a d t o t h e f o l l o w i n g c o n c l u s i o n s : i)
The e v a p o r a t i o n from Lake Tana i s n o t known t o a f a i r d e g r e e of a c c u r a c y . The i n e d a q u a t e i n f o r m a t i o n about t h e hydrology of t h i s l a k e p o i n t s t o a mean a n n u a l e v a p o r a t i o n o f 3.0-3.5 mm/day.
ii)
The mean a n n u a l e v a p o r a t i o n from t h e Machar swamps i n t h e S o b a t B a s i n may b e t a k e n a s a b o u t 4.1 mm/day.
i i i ) The open w a t e r e v a p o r a t i o n f o r t h e B l u e N i l e B a s i n i n c r e a s e s g r a d u a l l y i n
a n o r t h - w e s t e r l y d i r e c t i o n . The mean annual e v a p o r a t i o n a t Kurmuk i s 5.3 mm/day,
i n c r e a s i n g t o a b o u t 6.3 mm/day a t Roseires, 7.0-7.5 f o r S i n g a ,
S e n n a r and Wad Medani and becomes 8.0 mm/day a t Khartoum. The a v e r a g e f o r Kassala and G a l l a b a t i n t h e A t b a r a b a s i n i s n e a r l y 6.7 mm/day. T h i s f i g u r e i n c r e a s e s t o 8.0 mm/day a t A t b a r a .
195
iv)
The e v a p o r a t i v e demand f o r t h e White N i l e B a s i n and t h e E t h i o p i a n P l a t e a u
v)
The r e d u c t i o n c o e f f i c i e n t f o r t h e P i c h e r e a d i n g s , on an a n n u a l b a s i s ,
i s p r e s e n t e d i n F i g . 5.6. d e c r e a s e s from 0.8 f o r t h e S o b a t B a s i n t o a b o u t 0 . 6 f o r t h e b a s i n s of t h e B l u e N i l e and t h e A t b a r a . The monthly c o e f f i c i e n t v a r i e s i n a wide r a n g e w i t h an i n d i c a t i o n t o a heavy s e a s o n a l component.
TABLE 5.10
Open water e v a p o r a t i o n i n t h e A t b a r a B a s i n
Station + Method
*
P i c h e x 0.5 a b
Evaporation,
i n mm/day,
for
J a n , Feb. Mar. Apr. May J u n e J u l y Aug. S e p . O c t . Nov. Dec. Year
3.3
1.1 2.4
1.4 2.8
2.2 4.3
2.4 5.2
3.2 5.3
3.1 5.3
7.5 10.6 9.3
9.0 8.8
8.6
8.6
9.9
6.5
5.6
8.0
8.6
6.6
3.3
3.3
5.5
6.6
6.3 6.7
3.9 5.6
4.8 6.5
5.1 7.8
5.5
6.2
6.7
7.3
5.7 4.1 8.3 7.1
2.4 5.2
1.1
Penman (direct) C
(indirect) b
9.6 8.2
4.3
*a = Lake T a n a ; b = K a s s a l a and G a l l a b a t ; c = Atbara
196
1 a,
m
Y 4
n
&
a
.A .A
5
a,
w
9 c w 0
4
U
a,
4 .A
Y a,
z .r(
a,
c E
9
d
U
a,
4
n
c,
k
a,
rl
a,
c m
9
w
v1
0
3
.A
a,
n
k
9
+ 0I
m
P
I a,
U
>
.A
0
k
+ a > w
'9 M
M
.rl
r4
197
5.5 5.5.1
THE M A I N N I L E
The Main N i l e from Khartoum t o Aswan
The mean monthly a s w e l l a s t h e mean a n n u a l e v a p o r a t i o n a s o b s e r v e d by t h e P i c h e i n s t r u m e n t a t a number of l o c a t i o n s from Khartoum t o Aswan c a n b e found i n
Vol. I X of t h e N i l e B a s i n ( H u r s t , H . E . ,
Black, R.P.,
and S i m a i k a , Y . M . ,
1959). A
summary of t h e s e d a t a i s g i v e n i n T a b l e 5.11. TABLE 5.11
P i c h e e v a p o r a t i o n f o r a number of s t a t i o n s on t h e Main N i l e from Khartoum t o Aswan
Station
Khartoum* Zeidab Atbara Merowe Dongola Wadi H a l f a Aswan
Evaporation, J a n . F e b . Mar. Apr. May
13.0 16.0 1 0 . 0 12.0 13.8 16.2 11.6 13.7 8.4 10.9 8.8 1 0 . 9 7.6 9 . 1
19.0 15.0 18.9 16.8 13.5 14.4 12.9
20.0 16.0 20.8 20.0 15.6 18.1 16.8
19.0 16.0 20.8 21.2 17.9 19.4 18.5
i n mm/day,
for
J u n e J u l y Aug. S e p . O c t . Nov. Dec. Year
18.0 17.0 20.2 21.0 17.0 21.5 21.6
14.0 15.0 18.5 18.6 17.8 19.4 19.5
11.0 13.0 16.1 16.7 16.6 17.5 19.2
12.0 13.0 16.6 19.0 15.8 18.2 18.1
15.0 13.0 16.4 17.6 14.1 15.9 15.5
14.0 12.0 14.8 14.0 10.9 11.7 10.8
13.0 10.0 13.5 11.5 9.1 8.6 7.3
15.0 14.0 17.2 16.8 14.0 15.4 14.7
*Mean o f t h r e e s t a t i o n s a t Khartoum
H u r s t m e n t i o n s t h a t t h e e v a p o r a t i o n v a l u e s f o r Dongola a r e d e f i n e t l y lower t h a n woulti h a v e been e x p e c t e d from t h e o b s e r v a t i o n s a t t h e o t h e r s t a t i o n s . T h i s s t a t i o n was c l o s e d and t h e o b s e r v a t i o n s c o v e r e d 7 y e a r s o n l y . F u r t h e r m o r e , i t i s n o t p o s s i b l e now t o s a y w h e t h e r t h e d i f f e r e n c e i s r e a l o r due o n l y t o some c i r cumstance c o n n e c t e d w i t h t h e i n s t r u m e n t and i t s e x p o s u r e . The f r e e w a t e r s u r f a c e e v a p o r a t i o n a t Khartoum and A t b a r a h a s a l r e a d y been g i v e n i n t h e l a s t two s e c t i o n s . The long-term m e t e o r o l o g i c a l d a t a a v a i l a b l e f o r Merowe, Wadi H a l f a and Aswan have been worked o u t f o r e s t i m a t i n g t h e open w a t e r e v a p o r a t i o n f o r t h e s e s t a t i o n s u s i n g t h e Penman and H a r g r e a v e s methods. The r e s u l t s o b t a i n e d a r e l i s t e d i n T a b l e 5.12. I n a d d i t i o n t o t h e c o m p u t a t i o n r e s u l t s p r e s e n t e d i n T a b l e 5.12, i t may b e w o r t h w h i l e g i v i n g h e r e t h e mean a n n u a l e v a p o r a t i o n from a s t a n d a r d t a n k a s o b t a i n e d Prom t h e method of O l i v i e r . T h i s i s 1 0 . 5 f o r Khartoum, 1 1 . 0 f o r Wadi H a l f a and 10.8 mm/day f o r Aswan. The mean monthly v a l u e s f o r Aswan, a s an example, are as f o l l o w s :
Month
J a n . F e b . Mar. Apr. May
E v a p o r a t i o n , mm/day 4.9
6.4
9.5
J u n e J u l y Aug. S e p . O c t . Nov. D e c .
12.6 14.1 16.0 15.1 14.7 13.2 10.9
7.3
5.1
198 TABLE 5.12
Estimated f r e e water s u r f a c e evaporation f o r Merowe, Wadi Halfa and Aswan on t h e Main N i l e ~
Station Method
+
Evaporation, mm/day,
~
f
J a n . Feb. Mar. Apr. May
~~
for
June J u l y Aug. Sep. Oct. Nov. Dec. Year
Merowe a b
5.7 7.6
6 . 5 7 . 6 8 . 9 9.6 9 . 8 9 . 4 9 . 3 9 . 1 8 . 2 6 . 7 8 . 8 11.7 1 5 . 5 20.3 19.2 1 7 . 1 17.5 15.7 1 4 . 1 11.2
5.6 8.0 8 . 6 13.9
Wadi Halfa a b
4.5 4.3
5.8 5.8
7 . 1 8 . 7 9 . 3 9 . 8 9 . 6 9 . 3 8 . 6 7.7 8 . 3 12.0 14.6 15.8 14.7 1 4 . 3 1 2 . 8 10.4
6.1 7.7
4.0 7.5 4 . 8 10.5
Aswan a b
3.5 5.3
4.6 6.1
6 . 2 7.7 8 . 5 9 . 4 9 . 3 8 . 8 8 . 2 6 . 5 9 . 4 12.5 15.4 16.6 1 6 . 6 15.6 13.7 1 1 . 5
4.5 8.2
3.7 6 . 8 5 . 7 11.4
~
*a = Penman; b = Hargreaves x 0 . 7 The River Atbara i s t h e l a s t t r i b u t a r y of t h e Main N i l e . The annual flow downstream of t h e confluence i s t h e sum of t h e flow above i t , a t Hassanab, and t h e supply of t h e Atbara. The annual flow a t Mongalla, some 810 km downstream of t h e confluence, a t Hassanab and a t Atbara, has been averaged over a period of 30 y e a r s , 1943-72. The mean annual l o s s i n t h e reach from j u s t downstream of the confluence t o Dongola f o r t h e period considered i s about 1060 m i l l i o n m 3 . This f i g u r e i s most d e f i n i t e l y only an approximate one, a s i t v a r i e s i n a very wide range from -6236 m i l l i o n m 3 / y r
i n 1958 t o +5623 m i l l i o n m3/yr i n 1964. Our
f i g u r e is a l s o high compared t o t h a t obtained by Hurst
-
800 m i l l i o n m 3 / y r - a s
an average f o r t h e p e r i o d 1912-1952, f o r t h e reach from j u s t downstream of t h e confluence down t o Wadi H a l f a , 1210 km downstream of Atbara. Anyhow, using our f i g u r e and assuming an average width of 500 m f o r t h e r i v e r between t h e r i s i n g and t h e f a l l i n g s t a g e s , one g e t s an evaporation depth of 2.62 m / y r .
This f i g u r e
corresponds t o a mean annual evaporation of 7 . 2 mm/day, which is about 10% less than what we have obtained from Penman's method f o r Atbara and Merowe. The reach of t h e Main N i l e from Atbara t o Aswan i s c h a r a c t e r i z e d by t h e almost c o n s t a n t r a t i o between t h e Penman and t h e Piche e v a p o r a t i o n . This i s 0.465 f o r Atbara, 0.476 f o r Merowe, 0.487 f o r Wadi Halfa and 0.463 f o r Aswan, with a mean of 0.475 f o r t h e e n t i r e r e a c h . The pan evaporation f i g u r e s obtained from t h e Hargreaves method need t o be m u l t i p l i e d by a c o e f f i c i e n t of 0.44 i n s t e a d of 0 . 7 t o reduce them t o Penman's e v a p o r a t i o n . On t h e o t h e r hand, t h e evaporation from a s t a n d a r d tank computed by t h e O l i v i e r formula needs t o be c o r r e c t e d by a f a c t o r of 0.68 f o r Wadi Halfa and 0 . 6 3 f o r Aswan. One should not f o r g e t t h a t a l l t h e s e r a t i o s a r e based on mean annual e v a p o r a t i o n . For t h e comp a r i s o n between t h e mean monthly evaporation a s obtained from t h e d i f f e r e n t methods, r e f e r e n c e i s made t o F i g . 5 . 7 .
199
22.
.,
23 -
+ ,.
'.
/+ '.
18 -
c
,
''+-
'+
.\
ASWAN
+.
.'
x- --x-
m a n 6 . 8 mm.lday
I
2
I
I
I
22 30
+
/
.
I
I
I
,.-. + .'
'-
+' \.
I
'..+-.
t
I
-+.,
WADI
- HALFA
\.
21 x
b
19 17
$15 i13
.-
2 11
L
0 P
Z 9
w
7 c;
Jan.
Fig. 5 . 7 .
Feb.
Mar.
Apr.
May.
Jul.
Jun.
Aug.
Sep.
Oct.
Nov.
Dec.
Mean monthly e v a p o r a t i o n a t Atbara, Wadi H a l f a , and Aswan
200
5.5.2
The N i l e from Aswan t o G i z a , t h e O a s e s , and t h e Red S e a C o a s t
I n t h i s s t r e t c h o f t h e r i v e r , i n t h e o a s e s o f t h e Libyan d e s e r t , and a l o n g t h e c o a s t o f t h e Red S e a , t h e r e i s a r e a s o n a b l e number of m e t e o r o l o g i c a l s c r e e n s . T i l l t h e t i m e of w r i t i n g t h i s book, t h e p u b l i s h e d d a t a o f t h e s e s c r e e n s a r e up t o 1945 o n l y ( M i n i s t r y of War and Marine, screens,
1 9 5 0 ) . F o r most o f t h e
t h e d a t a up t o t h e n have t h e r e f o r e been u s e d . T h i s c o v e r s a p e r i o d o f ,
s a y , 20 t o 4 0
y e a r s f o r each s t a t i o n .
O t h e r t h a n t h e s t a t i o n a t Aswan, w e h a v e s e l e c t e d f o u r s t a t i o n s o n , o r v e r y near to,
t h e r i v e r : Q e n a , A s s i u t , Minya and G i z a ; two a l o n g t h e Red S e a C o a s t :
Q u s s e i r and Hurghada; a s e m i - d e s e r t i c
o a s i s : E l Fayum, and El-Kharga O a s i s .
S i n c e t h e c l i m a t o l o g i c a l normals f o r E l Dakhla O a s i s a r e n e a r l y t h e same as t h o s e f o r E l Kharga, t h e e s t i m a t e d e v a p o r a t i o n f o r t h e l a t t e r i s assumed t o r e p r e s e n t t h e f r e e w a t e r s u r f a c e e v a p o r a t i o n f o r t h e two o a s e s . Each o f t h e s t a t i o n s c o n s i d e r e d h e r e i s e q u i p p e d w i t h a P i c h e i n s t r u m e n t . A d d i t i o n a l l y , two c l a s s A pans h a v e b e e n i n s t a l l e d a t G i z a and E l Kharga s t a t i o n s s i n c e 1957 and 1964 r e s p e c t i v e l y . The e v a p o r a t i o n d a t a o b t a i n e d from t h e s e two pans h a v e , among o t h e r s , b e e n r e p o r t e d by t h e a u t h o r i n an e a r l i e r work ( S h a h i n , M . M . ,
1 9 7 0 ) . Some of t h e s e d a t a , t o g e t h e r w i t h t h e o b s e r v e d P i c h e
e v a p o r a t i o n and t h e f r e e water s u r f a c e e v a p o r a t i o n e s t i m a t e d by t h e methods of Penman and H a r g r e a v e s , are g i v e n i n T a b l e 5 . 1 3 . F u r t h e r m o r e ,
t h e l i n e s of equal
e v a p o r a t i v e demand f o r t h e a r e a c o n s i d e r e d are p l o t t e d on t h e map, F i g . 5 . 8 . From t h i s map, t h e g r a d u a l d e c l i n e i n t h e e v a p o r a t i v e demand from Wadi H a l f a i n
a northerly d i r e c t i o n t o Giza can e a s i l y be d et ect ed. This is a l s o t r u e f o r the c o a s t of t h e Red S e a , e x c e p t a t Hurghada, n e a r l y h a l f w a y between Q u s s e i r and S u e z , where a p o s i t i v e jump i n t h e e v a p o r a t i v e demand c a n b e s e e n . With t h e e x c e p t i o n o f t h e s t a t i o n s a t Q u s s e i r and Hurghada, which a r e s i t u a t e d a t t h e c o a s t , t h e r a t i o of t h e Hargreaves e v a p o r a t i o n t o t h e e v a p o r a t i o n e s t i m a t e d by t h e Penman method i s v e r y n e a r l y c o n s t a n t f o r a l l t h e s t a t i o n s .
In
f a c t , i t v a r i e s from 2 . 0 1 t o 2 . 2 1 w i t h an o v e r a l l a v e r a g e of 2 . 0 8 . The same r a t i o b u t f o r Q u s s e i r and Hurghada i s 1 . 5 4 and 1 . 6 6 r e s p e c t i v e l y . T h i s marked r e d u c t i o n i s o b v i o u s a s t h e h u m i d i t y a t noon, which i s a p a r a m e t e r i n t h e H a r g r e a v e s e q u a t i o n , d o e s n o t d i f f e r c o n s i d e r a b l y from t h e mean of t h e day a s i n t h e c a s e of t h e s t a t i o n s i n t h e i n t e r i o r . The pan e v a p o r a t i o n computed by H a r g r e a v e s ' e q u a t i o n i s s e e n t o b e less t h a n t h e o b s e r v e d c l a s s A pan e v a p o r a t i o n f o r El-Kharga by a b o u t 1696, and more t h a n t h e o b s e r v e d pan e v a p o r a t i o n a t G i z a by 27%. T h i s emphasizes t h e v e r y s t r o n g e f f e c t o f t h e h u m i d i t y a t noon i n H a r g r e a v e s ' method compared t o t h e mean humid i t y which i n f l u e n c e s t h e pan e v a p o r a t i o n .
201
TABLE 5.13
Observed and e s t i m a t e d e v a p o r a t i o n f o r t h e N i l e from Aswan t o G i z a , t h e Oases, and t h e Red Sea Coast
Station and Method* Qena __
a b C
Kharga (Oasis) a b C
d Assiut a b C
Qusseir (sea coast) a b C
Minya a b C
Hurghada (sea coast) a b C
Fay urn (semi-oasis) a b C
Giza __
a b C
d
Evaporation, i n mm/day, f o r Jan. Feb. Mar. Apr.
2.1 3.2 3.3
2.9 4.3 4.2
May
June July Aug. S e p . O c t . Nov. Dec. Year
5.0 6.6 8.4 9.2 8.9 8.0 7.2 5.2 7.0 10.4 13.3 14.0 13.4 13.0 10.2 7.4 6.5 9.2 10.9 12.4 11.8 11.4 8.6 6.1
3.7 5.2 4.4
2.2 5.8 3.5 8.7 3.4 7.7
2.4 3.5 5.2 6.9 8.6 9.4 9.2 8.5 7.7 5.7 3.6 2.4 6.1 3.5 4.5 6.8 9.6 11.5 13.1 13.2 12.7 10.2 8.3 5.8 4.0 8.6 9.5 11.6 16.3 20.3 24.2 25.1 24.5 23.9 22.6 19.8 13.7 9.9 18.4 6.4 8.5 12.0 16.1 20.0 23.3 21.7 19.5 18.2 14.4 9.9 6.8 14.7 1.8 2.9 3.9
3.1 3.8 5.1
4.3 6.6 8.3 8.7 8.5 8.2 6.4 6 . 1 9.2 11.8 12.9 12.6 11.5 9.3 7.6 10.8 13.7 15.6 13.9 12.8 10.6
4.9 6.6 7.1
3.1 4.2 8.0
3.9 4.6 5.6
5.1 6.2 7.3 8.2 8.5 8.0 7.3 5.5 6.2 7.6 8.9 8.4 8.3 7.3 9.6 10.0 11.3 12.9 11.5 11.9 11.2
5.4 4.4 6.0 5.0 9.4 8.8
3.2 4.5 7.9
5.9 6.4 9.8
1.9 2.5 4.2
2.6 3.2 5.7
3.8 4.7 7.1
4.1 6.4 7.3
2.0 2.9 3.7
4.8 7.0 8.8
5.3 7.0 7.7 7.7 6.9 7.4 11.0 11.7 11.5 10.5 9.8 14.2 15.2 13.7 11.8
5.6 7.9 8.7
3.4 4.7 4.9
3.3 4.6 5.5
2.0 5.5 3.1 7.9 3.7 9.1
3.7 4.4 5.6 7.0 8.3 9.5 9.6 9.0 7.7 6.2 5.2 3.8 6.7 4.8 5.2 6 . 4 8.0 10.3 10.8 11.0 10.5 8.7 7.0 5.8 5.1 7.8 10.7 11.5 13.6 15.5 17.6 21.5 19.5 19.8 17.6 13.7 11.5 10.4 15.2
1.5 2.7 3.4
2.6 3.5 4.4
3.9 5.3 6.0
1.5 2.7 3.3 2.8
2.6 3.2 4.2 3.9
3.3 5.2 6.7 7.9 7.4 4.6 7.0 9.0 10.2 9.7 5.3 7.2 9.1 9.2 8.4 5.8 8.1 10.3 11.4 10.4
5.4 7.6 8.2 8 . 2 7.3 8.2 11.8 13.5 12.3 10.8 8.6 11.8 13.3 11.8 10.1 6.2 8.5 7.0 9.5
5.5 9.1 8.0
4.0 6.6 6.5
3.0 4.6 4.5
1.6 2.8 3.2
4.9 7.6 7.6
5.0 6.9 5.5 8.1
3.5 5.7 4.8 6.1
2.3 4.1 3.5 4.1
1.5 3.0 2.9 3.0
4.4 6.2 5.9 7.0
*a = Penman; b = Hargreaves x 0.7; c = P i c h e ; d = c l a s s A pan
20 2
N
El Bahariya,
-- 6.0 m m . l d a y 25'
E
G
Y
P
T
Scale
0 50 100
Fig. 5 . 8 . Evaporative demand for t h e Main N i l e from Atbara t o G i z a , t h e c o a s t a l l i n e o f t h e Red S e a , and t h e Libyan d e s e r t o a s e s
203
Another example o f t h e r o l e p l a y e d by h u m i d i t y i n e v a p o r a t i o n e s t i m a t e s i s t h e r e s u l t s o b t a i n e d from O l i v i e r ' s method
*
Evaporation, J a n . Feb.
Mar.
Apr.
May
Jun.
i n mm/day, Jul.
Aug.
for Sep.
O c t . Nov. Dec. Year
a b
5 . 0 2 6 . 7 1 1 0 . 7 5 1 4 . 0 3 1 4 . 6 6 1 7 . 4 3 1 6 . 7 1 1 5 . 6 2 1 2 . 9 7 1 0 . 4 2 6 . 3 5 5 . 1 1 11.31 1 . 7 6 2 . 4 8 4 . 0 2 5 . 5 9 7 . 6 8 8 . 8 3 6 . 7 1 5 . 9 2 4 . 7 8 3 . 5 6 2 . 4 8 1.81 4 . 6 4
*a
= E l Kharga O a s i s s t a t i o n ; b = G i z a s t a t i o n
where t h e r a t i o o f t h e mean a n n u a l e v a p o r a t i o n f o r E l Kharga t o G i z a i s 2 . 4 4 . The r a t i o o f t h e Penman e v a p o r a t i o n a t t h e s e two s t a t i o n s i s 1 . 3 9 o n l y . F u r t h e r a n a l y s i s and d i s c u s s i o n o f t h e r e l a t i o n s h i p s between t h e e v a p o r a t i o n f i g u r e s o b t a i n e d f o r t h e d i f f e r e n t p a r t s o f t h e N i l e B a s i n c a n b e found i n section 5.6. 5.5.3
The N i l e D e l t a Area from t h e Apex t o t h e M e d i t e r r a n e a n S e a C o a s t
F o r t h e e v a p o r a t i o n s t u d y i n t h e area e x t e n d i n g from t h e apex o f t h e d e l t a t o t h e M e d i t e r r a n e a n S e a C o a s t , e l e v e n s t a t i o n s h a v e b e e n s e l e c t e d . These a r e : C a i r o , n e a r t h e a p e x o f t h e d e l t a ; S u e z , a t t h e f a r t h e s t end o f t h e Red S e a ; Z a g a z i g , El S i r w , T a n t a , E d f i n a and E l T a h r i r , i n s i d e and o u t s i d e t h e D a m i e t t a and R o s e t t a b r a n c h e s o f t h e N i l e and P o r t S a i d , A l e x a n d r i a , E l Kasr and S a l l u m , a l o n g t h e M e d i t e r r a n e a n C o a s t . The e v a p o r a t i o n f i g u r e s o b t a i n e d from Penman's method, from H a r g r e a v e s ' method, c o r r e c t e d by a f a c t o r o f 0 . 7 , and from t h e P i c h e i n s t r u m e n t , are l i s t e d i n T a b l e 5 . 1 4 . The s t a t i o n s a t E l T a h r i r and E l Kasr have each b e e n p r o v i d e d w i t h a c l a s s A pan s i n c e 1964. The a v e r a g e pan evap o r a t i o n from t h e p e r i o d 1964-1968 i s a l s o i n c l u d e d i n T a b l e 5 . 1 4 .
In this table
t h e e v a p o r a t i o n d a t a r e p r e s e n t i n g C a i r o are f o r Almaza A i r p o r t . A c t u a l l y , t h e r e a r e f o u r s t a t i o n s n e a r , or i n , C a i r o i t s e l f ; Almaza, H e l i o p o l i s , Eebekiya and Abbassiya.
No P i c h e e v a p o r a t i o n d a t a a r e a v a i l a b l e f o r t h e l a s t s t a t i o n . The
a v e r a g e o f t h e f i r s t t h r e e s t a t i o n s i s 70% of Almaza a l o n e . A c c o r d i n g l y , t h e Penman f i g u r e o f 4 . 9 mm/day o b t a i n e d f o r Almaza A i r p o r t h a s been r e d u c e d t o 4 mm/day i n o r d e r t o r e p r e s e n t t h e C a i r o a r e a . T h i s l a s t f i g u r e h a s been u s e d , among o t h e r s , f o r p r e p a r i n g t h e map of t h e e v a p o r a t i v e demand f o r t h e N i l e d e l t a a r e a shown i n F i g . 5 . 9 . F i g . 5 . 9 shows t h a t t h e e v a p o r a t i v e demand d e c r e a s e s from t h e e a s t t o t h e c e n t r e o f t h e d e l t a a r e a and t h e n i n c r e a s e s i n a w e s t e r l y d i r e c t i o n . The evap o r a t i o n i n c r e a s e s g r a d u a l l y from 4 . 4 mm/day for A l e x a n d r i a t o 4 . 9 mm/day f o r Sallum on t h e M e d i t e r r a n e a n C o a s t . The e v a p o r a t i o n d e c r e a s e s from t h e apex o f the d e l t a t o t h e c e n t r e then i n creas es i n a n or t her l y d i r e c t i o n t o t h e c o a s t .
204
TABLE 5.14
Evaporation d a t a f o r t h e N i l e d e l t a area from t h e apex t o t h e Mediterranean S e a Coast
Station and Method* Almaza (airport) a b C
Suez (Canal) a b d Zagazig a b C
Tanta _ _
a b C
Evaporation, i n mm/day, f o r J a n . Feb. Mar. Apr.
May
June J u l y Aug. S e p . O c t . Nov. Dec. Year
2.3 2.8 7.1
3.3 3.7 7.7
4.1 6 . 2 7.1 7.7 7.0 6.7 5.3 7.9 10.7 12.0 10.8 9.8 8.8 11.2 14.2 14.1 12.5 10.9
3.3 4.1 7.2
1.9 2.7 5.2
4.9 7.0 9.8
2.3 2.8 5.1
3.1 3.5 5.8
4.6 5.7 7.0 7.9 7.9 7.4 6.3 5.3 6.4 8.5 11.0 10.8 10.2 8.4 7.6 10.1 12.5 13.7 13.2 12.4 10.8
5.0 3.6 6.3 4.1 8.8 6.5
2.2 3.2 5.2
5.2 6.7 9.3
1.6 2.4 1.9
2.6 3.2 2.5
3.5 4.6 3.2
4.9 6.9 4.5
6.3 7.0 6.9 9.2 10.2 9.6 5.6 5.9 5.2
6.4 8.5 4.4
3.0 4.1 2.3
1.7 2.6 1.8
4.4 6.2 3.7
1.5 1.8 2.1
2.1 2.2 2.8
2.8 3.4 3.8
4.2 5.5 5.6
5.3 6.4 7.9 10.3 7.2 7.8
5.8 4.6 7.5 5.7 5.7 4.5
3.6 5.3 3.6
2.6 3.2 2.7
1.5 2.1 2.0
3.8 5.3 4.5
1.5 2.1 3.8
2.8 2.5 4.8
3.6 3.5 6.8
5.3 6.0 7.3 6.9 5.8 5.6 7.2 9.2 8.6 7.7 9.1 10.8 12.5 11.4 10.3
4.9 6.4 8.7
3.2 4.5 6.3
2.5 3.1 4.2
1.6 2.3 3.3
4.3 5.2 7.7
1.9 1.9 2.9
2.6 2.3 3.5
3.7 2.9 4.4
4.8 6.3 7.4 4.0 5.7 7.0 5.3 6.0 6.3
6.6 5.6 6.2 5.0 5.2 4.7
4.5 3.8 4.2
3.1 2.5 3.3
1.7 4.5 2.0 4.2 2.7 4.6
1.6 1.8 3.1
2.1 2.2 3.5
3.1 3.0 4.6
4.3 4.3 5.9
5.2 6.7 6.9 5.5 7.1 7.0 6.9 7.2 7.0
6.2 5.9 6.5
4.6 4.7 5.6
3.3 3.9 4.6
2.4 2.7 3.8
1.5 1.8 3.0
4.0 4.2 5.1
2.1 2.1 5.9
2.8 3.7 2.7 3.1 6 . 2 7.7
5.2 3.7 7.4
6.2 4.5 8.5
7.0 4.9 8.8
7.1 6.6 5.5 5.7 5.8 5.4 9 . 2 9.1 10.0
4.1 5.0 9.3
3.2 2.9 7.8
2.1 2.1 5.5
4.6 4.0 8.0
2.3 2.4 4.8
3.0 2.8 5.2
3.6 3.4 5.0
4.8 4.2 5.3
5.3 4.6 5.0
6.8 6.6 5.6 5.3 5.0 5.4 4.7 4.7 5.0
5.5 5.1 5.5
4.0 4.3 5.2
3.2 3.1 4.9
2.1 2.5 4.9
4.4 4.0 5.0
1.9 1.7 4.8
2.6 2.1 5.8
3.6 3.3 7.0
4.8 3.7 8.4
5.9 6.9 8.3 6.6 5.4 4.4 5.5 7.4 5.5 4.6 8.2 10.4 11.0 10.6 9.6
3.6 3.3 7.8
2.6 2.2 5.4
1.7 1.7 5.1
4.7 3.8 7.8
6.1 9.0 6.7
4.8 4.3 7.0 6.5 9.2 8.9
5.2 4.0 7.2 5.8 3.5 3.1
El Tahrir a b e E l Sirw
a b C
Edf i n a a b C
Port S a i d (airport) a b C
Alexandria a b C
E l Kasr a b e
7.4 7.4 6.1
*a = Penman; b = Hargreaves x 0.7; c = P i c h e ; d = P i c h e (Port T e w f i k ) ; e = C l a s s A pan
205
(continued)
TABLE 5.14
S t a tion and Method'
E v a p o r a t i o n , i n mm/day, J a n . Feb. Mar. Apr.
Sallum (Observatory) a b C
2.4 3.0 2.3 2.8 6.6 7.3
4.1 3.6 7.9
May
5.4 5.6 4.8 4.5 8.9 7.9
for
J u n e J u l y Aug. S e p . O c t . Nov. D e c . Year
7.6 7.5 9.4
8.5 8.6 9.7
7.6 5.7 6.6 5.3 8.2 7.9
3.9 4.2 7.2
3.1
3.1 6.9
2.3 2.6 7.1
4.9 4.7 7.9
*a = Penman; b = H a r g r e a v e s x 0.7; c = P i c h e I n o t h e r words, t h e e v a p o r a t i o n from t h e d e l t a h a s a bowel s h a p e w i t h d e c r e a s i n g e v a p o r a t i o n towards t h e c e n t r e .
Map of t h e e v a p o r a t i v e demand f o r t h e N i l e d e l t a a r e a and t h e c o a s t F i g . 5.9. of t h e M e d i t e r r a n e a n S e a
5.6
ANALYSIS OF THE OBSERVED AND THE COMPUTED EVAPORATION FOR THE NILE BASIN General
I n many i n s t a n c e s i n t h e p r e v i o u s s e c t i o n s , t h e o b s e r v e d P i c h e e v a p o r a t i o n a t a number of s t a t i o n s i n t h e N i l e B a s i n i s g i v e n . T h i s e v a p o r a t i o n h a s b e e n m u l t i p l i e d by t h e r e d u c t i o n f a c t o r proposed by H u r s t t o deduce t h e open w a t e r e v a p o r a t i o n for t h e whole b a s i n ( s e e F i g . 5 . 1 ) .
206
The e v a p o r a t i o n measured from t h e USWB c l a s s A pan a t f o u r l o c a t i o n s i n Egypt i s g i v e n i n s e c t i o n s 5.5.2 and 5.5.3. The supposed c l a s s A pan evaporat i o n h a s been computed f o r n e a r l y a l l t h e s t a t i o n s i n t h e b a s i n , u s i n g t h e Hargreaves formula. The r e s u l t s have been reduced by a f a c t o r 0.7 i n o r d e r t o c o n v e r t t h e pan e v a p o r a t i o n i n t o f r e e w a t e r s u r f a c e e v a p o r a t i o n . The e v a p o r a t i o n from t h e s o - c a l l e d
' s t a n d a r d t a n k ' has been computed, b u t
f o r a fewer number o f s t a t i o n s u s i n g O l i v i e r ' s method. The f r e e water s u r f a c e e v a p o r a t i o n h a s been computed d i r e c t l y u s i n g t h e Penman formula f o r most o f t h e s t a t i o n s where t h e r e l e v a n t c l i m a t o l o g i c a l s t a n d a r d s a r e a v a i l a b l e . The r e s u l t s a r e p r e s e n t e d n o t o n l y i n t a b u l a r form, b u t a l s o i n maps f o r q u i c k e r and e a s i e r u s e . The maps shown i n F i g s . 5.3.. 5.6.. 5.8., and 5.9. have been combined t o g e t h e r t o compose t h e g e n e r a l map, F i g . 5.10. 5.6.1
The f r e e w a t e r s u r f a c e e v a p o r a t i o n computed by Penman's formula
For t h e p u r p o s e o f e s t i m a t i n g t h e open w a t e r e v a p o r a t i o n f o r t h e Nile B a s i n , t h e d i r e c t method t h a t h a s been used is t h a t o f Penman. The n e c e s s a r y e q u a t i o n s have been g i v e n e a r l i e r . The new or m o d i f i e d v e r s i o n o f t h e Penman e q u a t i o n employs a wind f u n c t i o n o t h e r t h a n t h a t g i v e n i n e q . 5.8 which w e used i n o u r calculation. When combined w i t h t h e f i n d i n g s of Hickox, t h e e x p e r i m e n t s conducted a t t h e Khartoum O b s e r v a t o r y i n 1961 and 1962 showed t h a t t h e mean annual open water e v a p o r a t i o n w a s a b o u t 10% l a r g e r t h a n t h a t o b t a i n e d from Panman's method. I f w e t r u s t t h e r e s u l t s o f t h o s e e x p e r i m e n t s , t h e computed e v a p o r a t i o n from Penman's formula needs t h e n t o b e a d j u s t e d . The monthly and y e a r l y a d j u s t m e n t f a c t o r s a r e as follows: Adjustment F a c t o r January February March April May June July August September October November December Year
1.106 1.078 1.087 1.051 1.242 1.110 0.924 0.955 1.078 1.165 1.142 1.083 1.092
207
25'
20'
15'
Fig. 5.10. Mean annual free water evaporation in the Nile Basin estimated from Penman's method
20 8
T o t h e b e s t o f o u r knowledge t h e r e s u l t s o f t h e Khartoum e x p e r i m e n t s a r e p r o b a b l y t h e o n l y o n e s which may h e l p t o j u d g e t h e a c c u r a c y of Penmah's f o r m u l a f o r e s t i m a t i n g t h e f r e e water s u r f a c e e v a p o r a t i o n from t h e N i l e B a s i n .
Accor-
d i n g l y , t h e r e s u l t s presented i n t h e previous s e c t i o n e i t h e r i n t a b u l a r o r g r a p h i c a l form may b e d e s c r i b e d as f a i r l y a c c u r a t e . More a c c u r a t e e v a p o r a t i o n f i g u r e s may b e o b t a i n e d u s i n g o u r r e s u l t s t o g e t h e r w i t h t h e a p p r o p r i a t e a d j u s t ment.
5.6.2
R e l a t i o n s h i p between e s t i m a t e d e v a p o r a t i o n from a c l a s s A pan and e s t i mated f r e e water s u r f a c e e v a p o r a t i o n
I n o r d e r t o estimate e v a p o r a t i o n from a c l a s s A p a n , t h e method o f Hargreaves h a s b e e n u s e d . The e s t i m a t e d pan e v a p o r a t i o n h a s b e e n r e d u c e d by a c o n s t a n t f a c t o r , 0 . 7 , s o as t o deduce t h e open w a t e r e v a p o r a t i o n . Most o f t h e e v a p o r a t i o n f i g u r e s t h u s o b t a i n e d h a v e proved t o b e l a r g e r t h a n t h e f r e e water s u r f a c e evap o r a t i o n e s t i m a t e d d i r e c t l y from Penman's method. T h i s d i s c r e p a n c y l e a d s us t o the conclusion t h a t e i t h e r t h e reduction f a c t o r , 0.7,
i s , f o r most o f t h e s t a -
t i o n s c o n s i d e r e d , l a r g e r t h a n i t s h o u l d b e , o r t h e H a r g r e a v e s e v a p o r a t i o n i s not e q u a l t o t h e pan e v a p o r a t i o n . The s i m p l e g r a p h i c a l p l o t o f t h e H a r g r e a v e s e v a p o r a t i o n ( w i t h o u t any reduct i o n ) , Y , v e r s u s t h e Penman e v a p o r a t i o n , X , f o r 30 d i f f e r e n t s t a t i o n s , s u g g e s t s t h e p o s s i b i l i t y of h a v i n g t h e p a i r s X and Y d i v i d e d i n t o f o u r g r o u p s . The f i r s t g r o u p c o m p r i s e s 18 i n t e r i o r s t a t i o n s . The s e c o n d g r o u p i s f o r t h e t h r e e s t a t i o n s a l o n g t h e Red S e a C o a s t and t h e t h i r d i s f o r t h e s i x s t a t i o n s a t , o r n e a r t o , t h e c o a s t o f t h e M e d i t e r r a n e a n S e a . The f o u r t h g r o u p c o m p r i s e s t h e t h r e e s t a t i o n s l o c a t e d o u t s i d e t h e swamps o f t h e Bahr e l J e b e l and Bahr e l Ghazal B a s i n s . T h i s grouping h a s t h e advantage t h a t i t reduces t h e scatter of t h e p o i n t s a b o u t t h e r e s p e c t i v e r e g r e s s i o n l i n e , though does n o t e l i m i n a t e i t . F u r t h e r g r o u p i n g may s t i l l r e d u c e f u r t h e r t h e s c a t t e r . I n view o f t h e r a t h e r a p p r o x i m a t e n a t u r e of t h e c l i m a t o l o g i c a l d a t a used for e s t i m a t i n g t h e e v a p o r a t i o n , a t l e a s t f o r some of t h e s t a t i o n s , i t h a s been found u n n e c e s s a r y t o d e s c r i b e t h e r e g r e s s i o n o f Y on X by a p o l y n o m i a l o f a d e g r e e h i g h e r t h a n 2 . Under t h i s c o n d i t i o n t h e r e g r e s s i o n e q u a t i o n s of t h e f o u r groups are :
1st g r o u p
Y = 1 . 8 3 9 + 0.924 X + 0.125 X2
2nd g r o u p
Y = 1 . 2 3 9 x 1.331 X
3rd group
Y = 1.222 + 0.822 X
4 t h group
Y =-6.271
+
1.489 X
whereX andY are i n mm/day and r
+ + + XY
( I x y = 0.951)
(5.12a)
0.020 X2
( r X y = 0.950)
(5.12b)
0 . 0 4 1 X2
(rXy= 0 . 9 5 3 )
(5.12~)
0.187 X2
( r X y= 0 . 9 4 7 )
( 5 .12d)
i s t h e c o r r e l a t i o n c o e f f i c i e n t betweenX and Y .
209
The f i t of t h e r e g r e s s i o n l i n e s g i v e n by e q . 5 . 1 2 t o t h e r e s p e c t i v e group of s t a t i o n s is shown i n F i g . 5 . 1 1 a . t h r u '
c.
E s t i m a t i o n o f t h e f r e e water s u r f a c e e v a p o r a t i o n by t h e method o f Penman h a s a l r e a d y b e e n d e s c r i b e d by t h e e q u a t i o n s 5 . 6 , 5 . 7 , and 5 . 8 . I t r e q u i r e s a knowledge of t h e temperature,
t , t h e mean r e l a t i v e h u m i d i t y , h , t h e short-wave
r a d i a t i o n , R A , t h e wind s p e e d , u, and t h e r e l a t i v e d u r a t i o n o f t h e b r i g h t sun-
! shine, !
.
E s t i m a t i o n o f e v a p o r a t i o n from a c l a s s A pan u s i n g t h e H a r g r e a v e s
method, e q . 5 . 9 r e q u i r e s a knowledge o f t h e t e m p e r a t u r e , t , t h e r e l a t i v e h u m i d i t y a t noon, h n , and a c o e f f i c i e n t , d , which depends o n t h e d u r a t i o n of t h e day-time.
C o r r e c t i o n o f t h e f i g u r e s o b t a i n e d from e q . 5 . 9 r e q u i r e s , however, a
knowledge o f t h e s u n s h i n e ,
,
t h e wind s p e e d , u, and t h e e l e v a t i o n of t h e s t a -
t i o n above t h e mean l e v e l o f t h e s e a . S i n c e R A , i n t h e case o f Penman's method, and d , i n t h e c a s e o f H a r g r e a v e s ' method, a r e a v a i l a b l e f o r a g i v e n month and t h e g e o g r a p h i c a l l a t i t u d e o f any p o i n t , t h e b a s i c d i f f e r e n c e i n t h e c l i m a t o l o g i c a l p a r a m e t e r s needed f o r t h e s e two methods is c o n f i n e d t o h and h
.
Both humi-
d i t i e s a r e r e c o r d e d a t e v e r y m e t e o r o l o g i c s c r e e n , though hn a t a s l i g h t l y fewer number o f s c r e e n s .
I t i s q u i t e a p p a r e n t t h a t t h e v a l u e of h a v i n g t h e pan e v a p o r a t i o n f i r s t comp u t e d by t h e H a r g r e a v e s method t h e n c o n v e r t e d i n t o a f r e e water s u r f a c e evaporat i o n , using e i t h e r F ig . 5 . 1 1 . , o r Table 5.13, is questionable! 5.6.3
R e l a t i o n s h i p between o b s e r v e d e v a p o r a t i o n from a c l a s s A pan and estimated f r e e water surface evaporation
From t h e s t a t i o n s f o r which t h e f r e e w a t e r s u r f a c e e v a p o r a t i o n h a s been e s t i m a t e d by t h e Penman method, t h e r e a r e f i v e s t a t i o n s o n l y f o r which c l a s s A pan e v a p o r a t i o n i s a v a i l a b l e . These a r e : E l Kharga o a s i s , E l - T a h r i r ,
E l Kasr
and G i z a i n Egypt and Khartoum O b s e r v a t o r y i n t h e Sudan. The monthly and y e a r l y r a t i o s of t h e Penman e v a p o r a t i o n t o t h e pan evaporat i o n f o r t h e s e r a t i o s are l i s t e d i n T a b l e 5 . 1 5 , from which i s e v i d e n t t h a t t h e r a t i o , E penman : E pan A ( p a n c o e f f i c i e n t ) v a r i e s from o n e month t o t h e o t h e r . The l a r g e s t v a l u e o f t h e pan c o e f f i c i e n t o c c u r r s i n J u l y f o r t h e t h r e e n o r t h e r n s t a t i o n s and i n August f o r t h e two s o u t h e r n s t a t i o n s . The r a t i o of t h e maximum pan c o e f f i c i e n t t o t h e minimum pan c o e f f i c i e n t v a r i e s c o n s i d e r a b l y from one s t a t i o n t o t h e o t h e r . For t h e f i v e s t a t i o n s i n t h e i r t a b u l a t e d o r d e r , t h i s r a t i o
i s 2 . 2 7 , 1 . 5 3 , 1 . 4 2 , 1 . 1 6 and 1 . 3 7 r e s p e c t i v e l y . The mean c o e f f i c i e n t of t h e pan a l s o v a r i e s from o n e l o c a t i o n t o t h e o t h e r . The mean a n n u a l r e l a t i v e h u m i d i t y f o r t h e s t a t i o n s i n t h e i r t a b u l a t e d o r d e r i s 7 2 . 3 , 6 6 . 8 , 7 1 . 7 , 3 1 . 4 and 3 2 . 1 , respectively.
The mean a n n u a l v a l u e of t h e pan c o e f f i c i e n t and t h e a m p l i t u d e of
t h e mean monthly c o e f f i c i e n t b o t h seem t o some e x t e n t , t o b e d e p e n d e n t o n t h e mean a n n u a l h u m i d i t y .
2 10
25 24
~~~
LEGEND S X V
2:
A
0 22
a Y
21
A
20
0
3
19
C
a
L 2
18 17
J
I 3
. E A
Wad - Medani Khartoum Atbara Merowe Wadi - H a l f a Aswan Qena Asyirt E l -Kharga Oasis Minya ( A i r Port ) Fay u m Giza Almaze ( A i r Port ) A d d i s Abbaba Tanta Zagazi g El - Tahrir Rose ires
16
E 15 C
.g 1 4 2
$13 > a12 > CI
E
11
:
= 10 9
a7 -7-
6 5 4 -
1
2
3
4 5 6 7 Penman evaporation,
8
9
1 0 1 1
1 2 1 3 1 4
mm./day
Fig. 5.11a. Relationship between evaporation computed by the Hargreaves formula and evaporation computed by the Penman formula for the interior stations in the Nile Basin
16 LEGEND 15
15
s Kusseir
14 -
14 0
13
=-.12
0
E
11
-
Port S a i d
13
a t or n e a r t he coast
v Edfina
1 El Ka s r A
A
D
Sallum
?
'2
E 11
-
-
E . 10 -
E
. 1c
3
C
.0_
.-
12
2
a 9 8
a
- 9
9 -
0
9
8-
E
7 -
a 8
a >
1
d
D
2ul
:
E!6 =
D
=
5
3t
4
21 0
6-
L
54 -
1-
1
I
I
I
I
I
I
I
4 5 6 7 8 P e nma n e v a por at i on, m m . l d a y
2
3
I
9
I
10
F i g . 5.11b R e l a t i o n s h i p between e v a p o r a t i o n computed by t h e Hargreaves f o r m u l a and t h a t b y t h e Penman f o r m u l a f o r t h e s t a t i o n s a t o r n e a r t h e Red S e a C o a s t and t h e M e d i t e r r a n e a n Sea Coast
0
I
I
I
I
I
I
2 3 4 5 6 7 8 9 10 Penman e v a p o r a t i o n , m m . / d a y r i g . s . 1 1 ~ . H e l a t i o n s n i p between e v a p o r a z i o n comput e d by H a r g r e a v e s ' f o r m u l a and t h a t by Penman's f o r mula f o r s t a t i o n s j u s t o u t s i d e t h e swamps o f t h e Bahr e l J e b e l Basin 1
:: CL
212
TABLE 5.15
Monthly and y e a r l y r a t i o s of Penman e v a p o r a t i o n t o C l a s s A pan evaporation
E penman : E pan A , f o r S t i t i n n
1_.11--__
El-Kasr El-Tahrir Giza Kharga O a s i s Khartoum
J a n . F e b . Mar. Apr.
May
J u n e J u l y Aug. S e p . O c t . Nov. D e c . Year
.396 .395 .536 .375 .556
,720 .555 .650 .430 .450
.663 .584 ,693 .404 .525
.448 .583 .667 .412 .532
,414 .529 .569 .433 .540
.571 .582 .642 .429 .490
.755 .605 .712 .424 .643
.623 .563 .653 .436 .670
.563 ,563 .617 .423 .583
.462 .508 .574 .396 .532
.481 .595 .561 .364 .518
.333 .485 .500 .353 .546
.603 .558 .629 .415 .538
The o b s e r v e d pan e v a p o r a t i o n , E h a s b e e n p l o t t e d v e r s u s t h e f r e e w a t e r surP’ f a c e e v a p o r a t i o n e s t i m a t e d from t h e method o f Penman, X . The p o l y n o m i a l r e g r e s sion equations f i t t e d t o the p l o t t e d points are: El-Kasr
E
El-Tahrir
E
Giza
E
Kharga O a s i s Khartoum
where E
P
E E
P P
P P
P
= 2.082
+
1.652 X - 0.068 X 2
= 0.856
+
1.592 X
= 0.520
+
1.687 X - 0.040 X2
= 2.407
+
1.696 X
-
+
0.005 X 2
0.046 X 2
=15.613 - 2.667 X + 0.316 X 2
(r
EP I
(rEp, (rEp,
= .963) = ,993) X = .993)
(5.13)
( r E p , X = .995)
( rEp, X = .945)
and X a r e i n mm/day.
T h e s e r e g r e s s i o n r e l a t i o n s when f i t t e d t o t h e r e s p e c t i v e p a i r s o f E
and X P g i v e t h e l i n e s shown i n F i g . 5.12. Though e q s . 5.13 a r e somewhat d i f f e r e n t from t h o s e d e v e l o p e d e a r l i e r by t h e a u t h o r w h i l e a n a l y z i n g t h e e v a p o r a t i o n pan d a t a i n Egypt ( S h a h i n , M.M., 1970), b o t h s e t s of e q u a t i o n s , f o r t h e u s u a l r a n g e of X , s t i l l y i e l d a l m o s t t h e same v a l u e s o f E
5.6.4
P
.
R e l a t i o n s h i p between c l a s s A pan e v a p o r a t i o n and P i c h e e v a p o r a t i o n
The r a t i o c l a s s A pan e v a p o r a t i o n t o P i c h e e v a p o r a t i o n f o r G i z a , Kharga and Khartoum s t a t i o n s i s g i v e n i n T a b l e 5.16. The same r a t i o , a v e r a g e d o v e r t h e p e r i o d from J u n e 1954 up t o and i n c l u d i n g May 1960 for t h e Lod A i r p o r t ,
Israel
(WMO, 1966) i s a l s o i n c l u d e d i n T a b l e 5.16 f o r c o m p a r i s o n . The c o n c l u s i o n one c a n draw from t h e f i g u r e s l i s t e d i n t h i s t a b l e i s t h a t t h e mean a n n u a l r a t i o of c l a s s A pan t o P i c h e e v a p o r i m e t e r i n s c r e e n v a r i e s from a b o u t 0.8 f o r Kharga and Aswan (mean r e l a t i v e h u m i d i t y a b o u t 32%) t o 1.03 f o r G i z a (mean r e l a t i v e humid i t y a b o u t 72%). Even f o r t h e Lod, which i s s i t u a t e d o u t s i d e t h e N i l e B a s i n ,
213
23
LEGEND
22 o
21
A
20
+
z
x
Kharga Oasis Khartoum E l - Kasr E l - Tahrir Giza
19 18 17 16 1s >
. 0
u 14
E E 13
El
.- 1 2 r
2 CL 0
0 > a l
3 In 0
5
11
10
9
8 7 6 S
4
3
2
0
1
2
3 4 Penman
S 6 7 8 9 evaporation, rnm I day
1 0 1 1
12
Fig. 5.12. R e l a t i o n s h i p between observed c l a s s A pan e v a p o r a t i o n and f r e e water s u r f a c e e v a p o r a t i o n e s t i m a t e d from t h e Penman method
214
t h e r a t i o i s 1 . 0 6 . T h i s r a t i o as o b t a i n e d , b u t i n d i r e c t l y ,
from t h e 2-year e v a -
p o r a t i o n s t u d i e s i n Kenya and Uganda i s 3 . 3 f o r E n t e b b e , 1 . 6 f o r b o t h E q u a t o r and Gulu, and 1 . 4 7 f o r Kisumu. The P i c h e e v a p o r a t i o n a t E n t e b b e h a s b e e n r e p o r t e d a s b e i n g abnormal. I f w e n e g l e c t t h i s s t a t i o n , w e w i l l s t i l l b e l e f t with t h e r a t h e r high r a t i o f o r t h e o t h e r s t a t i o n s . This supports our opinion t h a t t h e P i c h e r e a d i n g s i n t h e E q u a t o r i a l Lakes a r e a are g e n e r a l l y s m a l l and much more v a r i a b l e t h a n are t a n k s .
TABLE 5 . 1 6
Monthly and a n n u a l r a t i o s o f class A pan e v a p o r a t i o n t o P i c h e evaporation
Station Giza a* b* Kharga Khartoum Lod ( A i r p o r t )
E pan A : E p i c h e , f o r J a n . F e b . Mar. Apr. .54 .92 .67 .75 .73
May
J u n e J u l y Aug. S e p . O c t . Nov. Dec. Year
.57 .60 .59 . 6 1 .67 . 7 3 .74 .69 . 9 5 1 . 0 1 . 9 8 .99 1 . 0 7 1 . 2 0 1 . 2 4 1 . 1 2 .74 .74 .79 .83 . 9 3 .89 . 8 2 .81 - 7 4 .76 .77 .82 .87 1 . 0 1 1.09 .90 .88 1 . 0 0 1 . 0 5 1.13 1 . 2 9 1 . 3 5 1 . 3 4 1 . 1 5
.62 .98 .73 .79 .94
.58 .92 .72 .73 .74
. 5 3 .64 .86 1 . 0 3 .69 .SO .75 .82 .68 1.06
*a = P i c h e u n s c r e e n e d ; b = P i c h e s c r e e n e d The r e s u l t s o b t a i n e d from t h e Namulonge C o t t e n R e s e a r c h S t a t i o n , Uganda, a r e q u i t e i m p r e s s i v e and i l l u m i n a t i n g . A g r a p h i c a l p l o t of t h e s e r e s u l t s h a s been p r e s e n t e d by O l i v i e r ( ( 1 9 6 1 ) and r e p r o d u c e d h e r e i n F i g . 5 . 1 3 .
c u
Tank
e v a p o r i m e t e r , i n c h e s per 24 h o u r s . Fiducial limits 9 : 1 .
Fig. 5.13. Comparison of r e a d i n g s o f P i c h e and t a n k e v a p o r i m e t e r s a t Namulonge C o t t o n R e s e a r c h S t a t i o n , Uganda
215
5.6.5
R e l a t i o n s h i p between P i c h e e v a p o r a t i o n and e s t i m a t e d f r e e water s u r f a c e evaporation
We have a l r e a d y mentioned a t t h e b e g i n n i n g o f t h i s c h a p t e r t h a t t h e P i c h e e v a p o r i m e t e r d o e s n o t , s t r i c t l y , p r e s e n t " f r e e water s u r f a c e " c o n d i t i o n s . The r e a s o n s b e h i n d t h i s a r e , a s g i v e n by O l i v i e r ( 1 9 6 1 ) , t h a t t h e P i c h e e v a p o r a t i o n t a k e s p l a c e from a p o r o u s b l o t t i n g p a p e r and t h e r e f o r e t h e s u r f a c e from which
water e v a p o r a t e s i s n o t as a u n i f o r m " f r e e " w a t e r s u r f a c e . F u r t h e r m o r e , r a d i a t i o n a b s o r p t i o n i s p r o b a b l y d i f f e r e n t f o r t h i s t y p e o f e v a p o r i m e t e r compared w i t h a p l a i n w a t e r s u r f a c e . A d d i t i o n a l l y , t h e area o f t h e e v a p o r a t i o n s u r f a c e
i s i n f i n i t e l y s m a l l compared w i t h any s u r f a c e a r e a o f a r e s e r v o i r or a l a k e . However, i n areas where o n l y P i c h e r e a d i n g s are a v a i l a b l e , t h e s e have t o b e used, w i t h s u i t a b l e c o r r e c t i o n s . I n o t h e r words,
the Piche evaporation has t o
be m u l t i p l i e d by a c o e f f i c i e n t i n o r d e r t o b r i n g i t t o i t s e q u i v a l e n t o f a f r e e water s u r f a c e . For t h i s purpose, Table 5 . 1 7 h a s been prepared t o g i v e t h e monthly and t h e a n n u a l P i c h e c o e f f i c i e n t s f o r 30 s t a t i o n s i n t h e N i l e B a s i n . For o t h e r s t a t i o n s , e s p e c i a l l y i n Uganda and Kenya, where t h e a v a i l a b l e d a t a allow f o r a n i n d i r e c t c o m p u t a t i o n o f t h e open w a t e r e v a p o r a t i o n , o n l y t h e annual c o e f f i c i e n t o f e a c h s t a t i o n h a s been e s t i m a t e d a s f o l l o w s :
Station
Piche Coefficient
Station
Piche C o e f f i c i e n t
Gambei 1a Akobo Gulu Eldoret Soroti Butiaba
0.78 0.84 1.07 1.10 1.12 1.17
Masindi P o r t Jinja Entebbe K amp a 1 a Kisumu Narok
1.20 1.30 1.85 1.35 1.03 1.00
The a r e a l d i s t r i b u t i o n o f t h e mean a n n u a l P i c h e c o e f f i c i e n t o v e r t h e b a s i n of t h e N i l e c a n b e s e e n from t h e map i n F i g . 5 . 1 4 . Examination o f t h i s map shows t h a t t h e mean a n n u a l v a l u e o f t h e P i c h e c o e f f i c i e n t changes i n two p r i n c i p a l d i r e c t i o n s . The f i r s t d i r e c t i o n i s s o u t h - n o r t h where t h e c o e f f i c i e n t d e c r e a s e s from a p r o b a b l e maximum o f , s a y , 1 . 3 around t h e n o r t h e r n s h o r e of Lake V i c t o r i a ( n e g l e c t i n g t h e r e a d i n g s a t E n t e b b e ) t o a minimum of about 0 . 3 5 f o r E l Kharga Oasis i n E g y p t . T h i s i s f o l l o w e d by an i n c r e a s e i n t h e P i c h e c o e f f i c i e n t up t o a b o u t 0 . 9 which i s r e a c h e d a t , or n e a r , t h e c o a s t a l l i n e of t h e M e d i t e r r a n e a n S e a between D a m i e t t a and A l e x a n d r i a . The second d i r e c t i o n i s e a s t - w e s t where t h e P i c h e c o e f f i c i e n t d e c r e a s e s from about 0 . 6 5 t o a b o u t 0 . 5 0 f o r Khartoum, Merowe, H a l f a and Aswan on t h e Main N i l e and t o 0.35 f u r t h e r t o t h e w e s t f o r E l Kharga O a s i s . T h i s p a t t e r n i s i n t e r r u p t e d by t h e r a t h e r h i g h c o e f f i c i e n t i n t h e s t r i p between Q u s s e i r on t h e Red Sea C o a s t and Qena on t h e N i l e R i v e r . T h i s i n t e r r u p t i o n i s p r o b a b l y due t o t h e s e v e r a l t o r r e n t s and Wadi's which i n t e r s e c t t h i s s t r i p . The upheaval i n t h e
TABLE 5.17
Monthly and a n n u a l P i c h e c o e f f i c i e n t s f o r t h e N i l e B a s i n E
Station
Sallum Alexandria Po rt Said Edfina Sirw Tanta Zagazig Giza Almaza ( A i r p o r t ) Suez Fayum lllinya ( A i r p o r t ) Hurghada AsyCit E l Kharga ( O a s i s ) Qena Qusseir Aswan Wadi H a l f a Merowe Atbara Khartoum Kassala/Gallabat Wad Medani Sennar Singa Roseires Kurmuk Malakal Wau Mongalla/Juba
Jan.
Feb.
Mar.
Apr.
.364 .479 .356 . 5 16 .655 .7 14 ,842 .455 ,324 .451 ,441 .452 ,346 ,462 .25 3 . 6 36 .388 .461 .511 .491 .399 . 5 18 .598 ,406 ,530 .535 .463 .459 .333 .5 38 .520
.411 .577 .452 ,600 .743 .750 1.040 .6 19 .429 ,534 .59 1 .456 .383 .608 .302 ,690 .696 .505 .532 .474 .383 .446 ,562 .382 ,573 ,494 ,442 . 4 34 ,363 .555 .575
.5 19 .720 .481 .674 .84 1 ,737 1.094 .623 .466 .605 ,650 .5 35 .412 ,566 ,319 .769 .531 ,481 .493 .452 ,397 .466 .55 1 .382 .531 .5 14 .494 .490 .4 33 .583 ,680
,607 .906 .703 .729 ,906 .750 1.089 ,722 .554 .564 .628 .54 1 ,452 ,611 .340 .7 17 .6 20 ,458 .48 1 ,445 . 5 10 .497 .578 ,428 ,595 .545 .5 13 .508 ,606 .713 ,784
May .709 1.060 .729 .754 1.050 .7 36 1.125 .7 36 .500 .560 .644 .493 .472 .606 ,355 ,771 ,646 .459 .479 ,453 .447 .558 .577 .4 36 .579 .583 .576 .573 .829 .9 37 1.061
Penman
’
Epiche’ for
June
July
Aug.
Sep.
Oct.
Nov.
Dec.
Year
.809 1.447 .795 .931 1.175 ,821 1.186 .859 .546 .577 .617 .507 .442 ,558 ,375 .742 .6 36 .435 .455 .467 .497 ,536 .6 35 .444 ,699 ,695 ,762 .77 1 1.023 1.106 1.093
.876 1.404 .772 .986 1.213 .9 10 1.327 .881 .560 .598 .695 .562 .492 . 6 12 .376 .754 .739 .477 ,495 .505 .497 .556 .651 .566 .8 32 .844 1.000 1.010 1.380 1.216 1.243
.927 1.120 .725 .954 1.270 1.018 1.455 .857 .615 .597 .723 .585 ,455 .641 .356 .702 .6 72 ,458 .531 .557 . 5 34 .550 .688 .803 . 8 19 .831 1.000 .995 1.640 1.371 1.324
.722 1.000 ,550 .821 1.191 1.022 1.486 .go9 ,522 .583 .688 .644 .4 38 .604 .347 .837 .652 .453 .473 ,479 ,5 18 .601 .590 .887 .758 .760 .9 14 .903 1.656 1.282 1.113
.542 ,769 .441 .717 1.071 1.000 1.290 .729 .483 .568 .615 .562 .453 .690 .288 .852 .574 . 4 19 .484 .466 .604 .575 .640 .658 .729 .731 .809 . 8 10 1.351 1.095 .963
.449 .653 .4 10 .6 32 .940 ,963 1.304 .657 .458 ,554 ,667 .600 .452 .694 .263 .841 .500 .417 .521 .479 ,4 39 .535 .635 .452 .640 .601 .648 .649 .627 .701 .746
.324 .429 .382 .500 .6 30 .750 ,944 ,5 17 .365 .423 .500 .541 .365 .541 .242 .647 .405 .507 .465 ,487 . 4 15 .499 .594 . 4 18 .551 .546 .509 .503 .426 .598 .6 20
.620 .880 .575 .784 .978 .844 1.189 ,746 .500 .559 .645 .600 .44 1 ,604 .332 .753 .602 .463 .487 .476 .465 .536 .632 .476 .573 .591 .594 .58 1 ,600 .763 .786
217
B
N
30'
25'
20°
15O
1 oo
5'
0'
21
'
Fig. 5.14. D i s t r i b u t i o n o f t h e mean a n n u a l c o e f f i c i e n t f o r c o n v e r t i n g t h e P i c h e e v a p o r a t i o n t o f r e e w a t e r e v a p o r a t i o n u s i n g Penman's method
218
v a l u e o f t h e P i c h e c o e f f i c i e n t a r o u n d Qena t o t h e w e s t i s c o u n t e r - b a l a n c e d by t h e d r o p around Hurghada o n t h e c o a s t of t h e Red S e a between Q u s s e i r and S u e z . T h e r e i s a r a p i d i n c r e a s e i n t h e v a l u e of t h e c o e f f i c i e n t a l o n g t h e M e d i t e r r a nean C o a s t from P o r t S a i d , i n t h e e a s t , t o A l e x a n d r i a ,
i n the w e s t . This
i n c r e a s e may b e due t o t h e h i g h r e l a t i v e h u m i d i t y b r o u g h t up l o c a l l y by t h e c o n t i n u o u s i r r i g a t i o n and t h e v e r y many c a n a l s
i n t h e N i l e D e l t a . West of
A l e x a n d r i a i n t h e d i r e c t i o n o f S a l l u m , t h e P i c h e c o e f f i c i e n t d e c r e a s e s from a b o u t 0 . 8 8 f o r t h e f o r m e r t o a b o u t 0 . 6 2 f o r t h e l a t t e r . I n g e n e r a l , t h e mean monthly c o e f f i c i e n t o f t h e P i c h e e v a p o r i m e t e r u n d e r g o e s a s e a s o n a l c y c l e . The a m p l i t u d e o f t h i s c y c l e v a r i e s from one s t a t i o n t o t h e o t h e r . Based on t h e f i g u r e s l i s t e d i n T a b l e 5 . 1 7 t h e maximum v a l u e of t h e c o e f f i c i e n t o c c u r s m o s t l y i n J u l y and A u g u s t . I n 84% of t h e s t a t i o n s i n v e s t i g a t e d t h e peak h a s been found t o t a k e p l a c e i n t h e three-month
p e r i o d July-September.
The minimum v a l u e o f t h e
monthly c o e f f i c i e n t o c c u r s f o r 90% o f t h e s t a t i o n s i n t h e three-month December-February.
period
The f r e q u e n c y h i s t o g r a m s o f t h e o c c u r r e n c e of t h e maximum
and t h e minimum v a l u e s of t h e P i c h e c o e f f i c i e n t f o r a l l s t a t i o n s a r e shown i n F i g s . 5 . 1 5 a and 5 . 1 5 b , r e s p e c t i v e l y . The r a t i o of t h e maximum t o t h e minimum, e x c l u d i n g t h e very h i g h v a l u e a t U a l a k a l , v a r i e s from 1 t o 3 . 5 . The f r e q u e n c y d i s t r i b u t i o n of t h i s r a t i o f o r t h e s t a t i o n s i n v e s t i g a t e d i s shown i n F i g . 5 . 1 5 ~ .Most o f t h e s t a t i o n s t h a t e n j o y a f a i r l y s t a b l e v a l u e f o r t h e P i c h e c o e f f i c i e n t , i . e . t h e r a t i o of t h e maximum monthly t o t h e minimum monthly c o e f f i c i e n t i s i n t h e r a n g e o f from 1 . 0 t o 1 . 5 , a r e t h o s e o n t h e N i l e R i v e r i n t h e a r i d z o n e . The l e s s s t a b l e r a t i o , from 1 . 5 t o 2 . 0 , t a k e s p l a c e i n t h o s e s t a t i o n s i n t h e N i l e D e l t a and V a l l e y a r e a s where i n t e n s i v e i r r i g a t e d a g r i c u l t u r e i s p r a c t i s e d . The l e a s t s t a b l e r a t i o , i . e . l a r g e r t h a n 2 . 0 i s a c h a r a c t e r i s t i c of t h e s t a t i o n s a t , o r c l o s e t o , t h e M e d i t e r r a n e a n C o a s t and j u s t o u t s i d e t h e swamps o f t h e Bahr e l J e b e l , Bahr e l Ghazal and t h e S o b a t B a s i n s . I n o r d e r t o i l l u s t r a t e t h e p a t t e r n of v a r i a t i o n of t h e P i c h e monthly c o e f f i c i e n t , t h e f i g u r e s l i s t e d i n T a b l e 5 . 1 7 have f i r s t been c o n v e r t e d t o modular v a l u e s . T h i s i s done by d i v i d i n g t h e c o e f f i c i e n t f o r each month by t h e mean annual c o e f f i c i e n t f o r th e s t a t i o n considered. F i g . 5 . 1 6 shows smooth c u r v e s f i t t e d t o t h e monthly c o e f f i c i e n t s v e r s u s t h e months o f t h e y e a r f o r s i x s t a t i o n s o n l y a s an example. These c u r v e s p r e s e n t o t h e r c h a r a c t e r i s t i c s i n a d d i t i o n t o t h e s e a s o n a l c y c l e , t h e months i n which t h e maximum and t h e minimum v a l u e s o c c u r , and t h e r a t i o of t h e maximum t o t h e minimum.
219
16
-
7
1412 210-
10r
8
QI
-
3
2 6 aJ 3 5 4
L:
8 -
-
LI
4 -
2
0
21
1
1
1
1
1
l
l
F i g . 5.15a. Frequency h i s t o gram of t h e month o f o c c u r r e n c e of t h e maximum v a l u e o f t h e Piche c o e f f i c i e n t
0
1.0
-
6 -
l
l
1
0
-_I
-
"
'
"
1
~
'
"
~
F i g . 5.15b. Frequency h i s t o g r a m o f t h e month o f o c c u r r e n c e o f t h e minimum v a l u e of t h e Piche c o e f f i c i e n t
1.5 2.0 2.5 3.0 3 . 5 4.0 4 . 5 5.0 M a x . coef'icient I Min. c o e f f i c i e n t
F i g . 5 . 1 5 ~ . Frequency h i s t o g r a m o f t h e r a t i o of t h e maximum monthly P i c h e c o e f f i c i e n t t o t h e minimum monthly P i c h e c o e f f i c i e n t
220
Curves s i m i l a r t o t h o s e f o r Khartoum and Minya A i r p o r t i n d i c a t e t h e p r e s e n c e o f two c y c l e s i n s t e a d o f o n e . The r i s i n g l i m b s o f t h e c u r v e s a p p r o a c h , o r e q u a l , t h e modular v a l u e 1 . 0 between t h e end of March and m i d - A p r i l .
The same modular
v a l u e i s e q u a l l e d or a p p r o a c h e d by t h e f a l l i n g l i m b s o f t h e c u r v e s i n t h e p e r i o d between mid-September and mid-November.
I’
\
0.4
0.3l
1
1
2
I
3
1
4
I
5
I
6
I
7
I
8
I
9
I
10
I
11
I
12
Month Fig. 5.16. V a r i a t i o n o f t h e modular v a l u e o f t h e P i c h e c o e f f i c i e n t w i t h t h e month o f t h e y e a r f o r some e v a p o r a t i o n s t a t i o n s i n t h e N i l e B a s i n
221
REFERENCES Baumgartner, A . , and R e i c h e l , E . , 1975. The w o r l d w a t e r b a l a n c e , R . Oldenburg V e r l a g , Munich. Dagg, M . , 1972. E a s t A f r i c a : i t s p e o p l e s and r e s o u r c e s ( e d i t e d by W.T.W. Morgan). C h a p t e r 10: Water r e q u i r e m e n t s o f c r o p s , 119-125. Oxford U n i v e r s i t y P r e s s , London, N e w York. H a r g r e a v e s , G . H . , 1956. I r r i g a t i o n r e q u i r e m e n t s b a s e d on c l i m a t i c d a t a . Paper No. 1105 J o u r n . I r . & D r . D i v . , ASCE, V o l . 8 2 , No. IR-3: 1-10. Hickox, G . H . , 1946. E v a p o r a t i o n from a f r e e w a t e r s u r f a c e . T r a n s . A m e r . S O C . C i v . E n g r s . , 111, P a p e r No. 2266, 1-33. H u r s t , H . E . , 1950. The N i l e B a s i n , V o l . V I I I : The h y d r o l o g y of t h e S o b a t and White N i l e and t h e topography o f t h e B l u e N i l e and A t b a r a . P h y s i c a l Department p a p e r No. 5 5 , The Government P r e s s , C a i r o , E g y p t , 125 p p . H u r s t , H . E . , 1952. The N i l e , a g e n e r a l a c c o u n t of t h e r i v e r and t h e u t i l i z a t i o n o f i t s w a t e r s . C o n s t a b l e , London, 326 p p . H u r s t , H . E . , B l a c k , R . P . , and S i m a i k a , Y . M . , 1959. The N i l e B a s i n , Vol. I X : The h y d r o l o g y o f t h e Blue N i l e and A t b a r a and of t h e main N i l e a t Aswan, w i t h some r e f e r e n c e t o p r o j e c t s . N i l e C o n t r o l Department P a p e r No. 1 2 , The General o r g a n i z a t i o n f o r Government P r i n t i n g O f f i c e s , C a i r o , E g y p t , 206 p p . H u r s t , H . E . , B l a c k , R . P . , and S i m a i k a , Y . M . , 1966. The N i l e B a s i n , Vol. X , The m a j o r N i l e p r o j e c t s . N i l e C o n t r o l Department P a p e r No, 2 3 , G e n e r a l Organizat i o n f o r Government P r i n t i n g O f f i c e s , C a i r o , 217 p p . H u r s t , H . E . , and P h i l i p s , P . , 1931. The N i l e B a s i n , Vol. I , G e n e r a l d e s c r i p t i o n o f t h e b a s i n , m e t e o r o l o g y and topography of t h e White N i l e B a s i n . P h y s i c a l Department P a p e r No. 2 6 , Government P r e s s . C a i r o , 128 p p . H u r s t , H . E . , and P h i l i p s , P . , 1938. The N i l e B a s i n , Vol. V, The hydrology o f t h e Lake P l a t e a u and Bahr e l J e b e l . P h y s i c a l Department P a p e r No. 3 5 , S c h n i d l e r s ' P r e s s . C a i r o , 235 p p . K r i s h n a m u r t h y , K . V . , and I b r a h i m , A . H . , 1 9 7 3 . H y d r o m e t e o r o l o g i c a l s t u d i e s o f Lakes V i c t o r i a , Kyoga and A l b e r t , G e o p h y s i c a l nomograph 1 7 : Man-Made Lakes (Ackermann, W . C . , e t a l : e d i t o r s ) , A . G . U . , Washington D . C . , 272-277. L i n s l e y , R . K . , K o h l e r , M . A . , and P a u l h u s , J . L . , 1 9 5 8 . Hydrology f o r e n g i n e e r s . M c G r a w - H i l l Book Company I n c . , N e w York, London, 340 p p . M i n i s t r y o f War and M a r i n e , E g y p t , 1950. C l i m a t o l o g i c a l normals for Egypt. M e t e o r o l o g i c a l Department of E g y p t . C , Tsoumas & Co. P r e s s , C a i r o . O l i v i e r , H . , 1961. I r r i g a t i o n and C l i m a t e , Edward Arnold ( p u b l i s h e r s ) L i m i t e d , London, 250 p p . P a l a y a s o o t , P . , 1 9 6 5 . E s t i m a t i o n o f pan e v a p o r a t i o n and p o t e n t i a l e v a p o t r a n s p i r a t i o n of r i c e i n t h e c e n t r a l p l a i n o f T h a i l a n d by u s i n g v a r i o u s f o r m u l a s b a s e d on c l i m a t o l o g i c a l d a t a . M.Sc. t h e s i s p r e s e n t e d t o t h e Utah S t a t e U n i v e r s i t y , Utah, U.S.A. Penman, H . L . , 1 9 4 8 . N a t u r a l e v a p o r a t i o n from open w a t e r , b a r e s o i l and g r a s s . P r o c e e d i n g Roy. S O C . A g r i c . , 1 9 3 : 120-145. R i j k s , D . A . , 1969. E v a p o r a t i o n from a p a p y r u s swamp. Q u a r t . J o u r n . Roy. Meteo. S O C . , 9 5 : 643-649. Rzbska, J . ( e d i t o r ) , 1 9 7 6 . The N i l e , Biology o f an a n c i e n t r i v e r . D r W . Junk B . V . P u b l i s h e r s , The Hague, The N e t h e r l a n d s , 417 p p . S h a h i n , M . M . , 1 9 7 0 . A n a l y s i s of e v a p o r a t i o n pan d a t a i n U . A . R . ( E g y p t ) . Annual B u l l e t i n o f ICID, N e w D e l h i , I n d i a , 53-69. S l e i g h t , R . B . , 1 9 2 7 . D i s c u s s i o n o f Houk's p a p e r : E v a p o r a t i o n on U n i t e d S t a t e s r e c l a m a t i o n p r o j e c t s . T r a n s . A m e r . S O C . C i v . E n g r s . , 9 0 , 303-316. US G e o l o g i c a l S u r v e y , 1 9 5 4 . Water l o s s i n v e s t i g a t i o n s : Vol. I - Lake Hefner s t u d i e s . G e o l o g i c a l Survey P r o f e s s i o n a l P a p e r No. 269. USSR N a t i o n a l Committee f o r IHD, 1 9 7 4 . World w a t e r b a l a n c e and w a t e r r e s o u r c e s o f t h e e a r t h ( t r a n s l a t e d from R u s s i a n ) , UNESCO, P a r i s .
222
WMO, 1966. Measurement and e s t i m a t i o n of e v a p o r a t i o n and e v a p o t r a n s p i r a t i o n . T e c h n i c a l n o t e N o . 83, Geneva, 1 2 1 p p . WMO, 1974. H y d r o m e t e o r o l o g i c a l s u r v e y o f t h e c a t c h m e n t s of Lakes V i c t o r i a , Kyoga and A l b e r t , V o l . I : M e t e o r o l o g y and h y d r o l o g y of t h e b a s i n , P a r t s 1 &2, Geneva.
223
Chapter 6
EVAPOTRANSPIRATION
E v a p o t r a n s p i r a t i o n i s t h e t o t a l q u a n t i t y o f w a t e r consumed by e v a p o r a t i o n and t r a n s p i r a t i o n . E v a p o r a t i o n h a s a l r e a d y b e e n d e f i n e d i n C h a p t e r 5 . T r a n s p i r a t i o n i s s i m p l y e v a p o r a t i o n f r o m t h e p l a n t . I t i s a p r o c e s s by w h i c h w a t e r v a p o u r is r e l e a s e d t o t h e atmosphere t h r o u g h s u r f a c e p o r e s i n p l a n t f o l i a g e , mainly
through stomata1 o p e n i n g s . A s m a l l p o r t i o n o f t h e e m i t t e d moisture,
generally
l e s s t h a n lo%, comes f r o m t h e y o u n g e r p l a n t s t e m s . R e l e a s e o f m o i s t u r e by t r a n s p i r a t i o n o c u r s p r i n c i p a l l y d u r i n g t h e day-time h o u r s o f t h e growing s e a s o n . P r o b a b l y n o t more t h a n 5 t o 10% o f t h e d a i l y t r a n s p i r a t i o n t a k e s p l a c e d u r i n g t h e n i g h t . The r a t e o f t r a n s p i r a t i o n u s u a l l y r e a c h e s a maximum v a l u e s h o r t l y a f t e r n o o n , a n d a minimum j u s t b e f o r e s u n r i s e . E v a p o t r a n s p i r a t i o n c a n b e d e f i n e d as t h e sum o f t h e volumes o f w a t e r u s e d p e r u n i t a r e a by t h e v e g e t a t i v e g r o w t h i n t r a n s p i r a t i o n and t h a t e v a p o r a t e d from t h e s o i l , snow or i n t e r c e p t e d p r e c i p i t a t i o n o n a g i v e n area i n any s p e c i f i e d t i m e . Evapotranspiration is usually expressed i n u n i t s of depth per u n i t o f t i m e . The t e r m c o n s u m p t i v e u s e , commonly u s e d i n i r r i g a t i o n h y d r o l o g y , i s e s s e n t i a l l y synonymous w i t h e v a p o t r a n s p i r a t i o n . The d e f i n i t i o n o f e v a p o t r a n s p i r a t i o n i m p l i e s t h a t t h e f a c t o r s a f f e c t i n g i t are e s s e n t i a l l y t h o s e a f f e c t i n g e v a p o r a t i o n and t r a n s p i r a t i o n . So t h e r a t e o f e v a p o t r a n s p i r a t i o n d e p e n d s o n t h e c l i m a t e , crop r a i s e d , s t a g e o f p l a n t development, d e n s i t y of v e g e t a t i v e cover, s o i l m o i s t u r e s u p p l y , s a l i n i t y a n d l e n g t h of g r o w i n g s e a s o n . F a c t o r s i n c l u d e d i n climate a r e : solar r a d i a t i o n , temperature,
day-time h o u r s , d u r a t i o n of sun-
s h i n e , h u m i d i t y and wind s p e e d . When e v a p o t r a n s p i r a t i o n t a k e s p l a c e f r o m a s o i l s u r f a c e c o m p l e t e l y c o v e r e d by a c t i v e l y g r o w i n g v e g e t a t i o n and t h e r e i s no l i m i t a t i o n i n s o i l m o i s t u r e , i t i s r e f e r r e d t o as p o t e n t i a l e v a p o t r a n s p i r a t i o n . I n o t h e r words, p o t e n t i a l evapot r a n s p i r a t i o n can b e c o n s i d e r e d as t h e upper l i m i t of t h e a c t u a l evapotranspirat i o n . A c c o r d i n g t o T h o r n t h w a i t e , p o t e n t i a l e v a p o t r a n s p i r a t i o n d e p e n d s o n l y on t h e amount o f s o l a r e n e r g y r e c e i v e d by t h e e a r t h ' s s u r f a c e , c o n s e q u e n t l y t h e resulting temperature,
and n o t o n t h e k i n d o f p l a n t ( T h o r n t h w a i t e , C . W . ,
1948).
The most common m e t h o d s u s e d f o r d e t e r m i n i n g t h e e v a p o t r a n s p i r a t i o n a r e t h e water-balance,
t h e t a n k and l y s i m e t e r e x p e r i m e n t s , s o i l m o i s t u r e d e p l e t i o n
s t u d i e s i n f i e l d p l o t s , c o r r e l a t i o n w i t h e v a p o r a t i o n from a pan o r from an open
water b o d y , a n d m e t h o d s b a s e d o n t h e p h y s i c s o f t h e v a p o u r t r a n s f e r a n d / o r t h e heat energy-balance.
224
A g e n e r a l i d e a a b o u t t h e a c t u a l and p o t e n t i a l e v a p o t r a n s p i r a t i o n from t h e
N i l e B a s i n c a n b e drawn from a t l e a s t t w o s o u r c e s . R e i c h e l and Baumgartner
(1975) gave t h e a c t u a l e v a p o t r a n s p i r a t i o n f o r t h e d i f f e r e n t 5 - d e g r e e l a t i t u d e zones of t h e g l o b e , which w a s b a s e d on t h e s i m p l e e q u a t i o n E T = P - R
(6.1)
where ET = a c t u a l e v a p o t r a n s p i r a t i o n , P
= p r e c i p i t a t i o n , and
R
= run-off The t h r e e v a r i a b l e s , E T , P, and R , a r e a l l e x p r e s s e d i n a d e p t h u n i t p e r
y e a r . For t h e 5 - d e g r e e l a t i t u d e zones c o v e r e d by t h e N i l e Basin t h e v a l u e s o f these variables are as follows:
Latitude, degree north
south
Precipitation, P, m/yr
Run-off,
R, m / y r
Actual evapotranspiration, ET, m / y r
-7 -5
35-30 30-25 25-20 20-15 15-10 10- 5 5- 0
151 29 31 1 51 741 1203 1329
5 55 233 34 2
158 34 30 146 686 970 987
0- 5
1488
34 3
1145
1
The s e c o n d s o u r c e o f i n f o r m a t i o n i s t h e two maps, F i g s , 6 . 1 . and 6 . 2 . ,
which
g i v e t h e a c t u a l e v a p o t r a n s p i r a t i o n and t h e p o t e n t i a l e v a p o t r a n s p i r a t i o n redrawn from t h e Water R e s o u r c e s o f t h e E a r t h (Korzun, V . ,
e t a l , 1978). Evidently
t h e s e two maps g i v e more d e t a i l e d i n f o r m a t i o n t h a n t h e v e r y g e n e r a l f i g u r e s g i v e n by R e i c h e l and Baumgartner. As an e x a m p l e , t h e 5 - d e g r e e l a t i t u d e zone e x t e n d i n g from t h e e q u a t o r up n o r t h shows an a c t u a l e v a p o t r a n s p i r a t i o n i n c r e a s i n g f r o m , s a y , 300 mm/yr
i n t h e e a s t t o a b o u t 1250 m m / y r
i n the w e s t a t the
s h o r e of Lake Edward. For t h i s z o n e , t h e above t a b u l a t e d f i g u r e s g i v e an a c t u a l e v a p o t r a n s p i r a t i o n o f 987 mm/yr. The c o u n t r i e s s h a r i n g t h e N i l e water h a v e b e e n u s i n g s e v e r a l b u t a l s o d i f f e r e n t p r a c t i c e s t o d e t e r m i n e or t o e s t i m a t e e v a p o t r a n s p i r a t i o n .
I t may, t h e r e f o r e ,
b e w i s e r t o g i v e a n a c c o u n t o f t h e p r a c t i c e s u s e d and t h e r e s u l t s o b t a i n e d t h e r e f r o m on a c o u n t r y - w i s e b a s i s i n s t e a d o f a r i v e r b a s i n - w i s e b a s i s .
225
Fig. 6 . 1 . Lines of equal a c t u a l e v a p o t r a n s p i r a t i o n , mm/yr, from t h e N i l e Basin and surroundings (redrawn from t h e World Water Balance and Water Resources o f the Earth (Korzun, V . , e t a l , 1978)
226
30'
20'
Fig. 6 . 2 . L i n e s o f equal p o t e n t i a l e v a p o t r a n s p i r a t i o n , mm/day, from t h e N i l e B a s i n and surroundings (redrawn from t h e World Water Balance and Water Resources o f t h e Earth (Korzun, V . , e t a l , 1978)
227
EVAPOTRANSPIRATION STUDIES I N THE ARAB REPUBLIC OF EGYPT
6.1 6.1.1
Tank and l y s i m e t e r e x p e r i m e n t s
The r e s u l t s o f t a n k e x p e r i m e n t s a t t h e a g r o m e t e o r o l o g i c a l s t a t i o n a t Giza have been r e p o r t e d by Omar, M . H .
(1960). D u r i n g t h e 3-year p e r i o d , 1957-1959,
p o t e n t i a l e v a p o t r a n s p i r a t i o n was measured from t h r e e e v a p o t r a n s p i r o m e t e r s of t h e t y p e known as " m o d i f i e d e v a p o t r a n s p i r o m e t e r "
d e v e l o p e d by Mather. T h i s was a
tank w i t h a n o p e n i n g a t t h e bottom where a t u b e was f i x e d i n o r d e r t o e n a b l e t h e measurement o f p e r c o l a t i o n water. Two t a n k s w e r e c y l i n d e r i c a l , a b o u t 60 cm i n d i a m e t e r e a c h , and t h e t h i r d t a n k h a d a r e c t a n g u l a r c r o s s - s e c t i o n
130 x 90 cm.
The t a n k s w e r e p l a n t e d w i t h l i b y a g r a s s and i n s t a l l e d i n t h e s t a t i o n ' s g r a s s f i e l d . Care was t a k e n t o m a i n t a i n t h e same v e g e t a t i o n l e v e l i n s i d e and o u t s i d e t h e t a n k s by c u t t i n g t h e g r a s s when n e c e s s a r y . The d i f f e r e n c e i n a r e a of t h e e v a p o r a t i n g s u r f a c e of t h e t a n k s used was r e p o r t e d t o b e o f no s i g n i f i c a n t influence. Results generally bore t h i s o u t . O t h e r measurements were made by Popoff e v a p o t r a n s p i r o m e t e r s which have a s m a l l e r a r e a (500 cm2) compared t o M a t h e r ' s e v a p o t r a n s p i r o m e t e r s
.
The former
were r e p o r t e d t o g i v e r e s u l t s a l m o s t 10% l a r g e r t h a n t h o s e o b t a i n e d from t h e b i g g e r t a n k s . A comparison between t h e e v a p o t r a n s p i r a t i o n from t h e s m a l l t a n k s and t h e b i g g e r t a n k , a l l p l a n t e d w i t h c o t t o n , c a n b e s e e n from F i g . 6.3.
-
"...',
Legend
---- Average
- Large -
,'
of s m a l l tonks. tank.
I
I
Feb.
I
I
Mar.
Apr.
May.
I
I
I
Jun.
Jul.
Aug.
Month F i g . 6.3. Comparison between p o t e n t i a l e v a p o t r a n s p i r a t i o n from s m a l l and l a r g e tanks r a i s i n g c o t t o n a t G i z a (Omar, M . H . , 1960)
228
Z e i n e l Abedin e t a1 (1967) compared t h e e v a p o t r a n s p i r a t i o n from f i e l d p l o t s t o t h a t from w e i g h a b l e l y s i m e t e r s ,
a l l r a i s i n g l a t e m a i z e , u n d e r two d i f f e r e n t
i r r i g a t i o n t r e a t m e n t s a t t h e e x p e r i m e n t a l farm o f C a i r o U n i v e r s i t y . The s o i l m o i s t u r e c h a n g e s were d e t e c t e d by a n e u t r o n m o i s t u r e metre. A summary o f some of t h e r e s u l t s o b t a i n e d from t h i s i n v e s t i g a t i o n i s p r e s e n t e d i n T a b l e 6 . 1 . From t h i s summary i t i s e v i d e n t t h a t t h e e v a p o t r a n s p i r a t i o n from t h e l y s i m e t e r s as
w e l l as t h e f i e l d p l o t s r e s p o n d s t o t h e f r e q u e n c y o f i r r i g a t i o n , c o n s e q u e n t l y t h e a v a i l a b l e s o i l m o i s t u r e . The e v a p o t r a n s p i r a t i o n from t h e l y s i m e t e r was 12.7% l a r g e r t h a n t h a t from t h e f i e l d p l o t i n t h e t r e a t m e n t t h a t r e c e i v e d more f r e q u e n t i r r i g a t i o n , and 2 2 . 9 % i n t h e o t h e r t r e a t m e n t . T h e s e d i f f e r e n c e s i n t h e amount o f e v a p o t r a n s p i r a t i o n r e s u l t e d i n an i n c r e a s e i n t h e c r o p y i e l d i n t h e
l y s i m e t e r t h a n i n t h e f i e l d p l o t by 1 0 . 5 and 5 . 2 % f o r t r e a t m e n t s 1 and 2 respectively. TABLE 6 . 1
E v a p o t r a n s p i r a t i o n from l y s i m e t e r s and f i e l d p l o t s r a i s i n g maize under two d i f f e r e n t i r r i g a t i o n t r e a t m e n t s , F a c u l t y of A g r i c u l t u r e E x p e r i m e n t a l Farm, C a i r o U n i v e r s i t y , Egypt ( 2 . e l Abedine, A . , Abdallah, M . ,
and Abdel-Samie,
1967)
Treatment 1
Year 1966
depth of r o o t zone
24.07-01.08 02.08-09.08 10.08-20.08 21.08-04.09 05.09-17.09 18.09-01.10 02.10-11.10 12.10-05.11
cm
40 60 80 100 120 140 140 140
Total Yield, ton/ha
6.1.2
Treatment 2
-
e vapo transpiration,
mm
Year 1966
Cm
lysimeter
plot
34.4 40.8 57.6 76.6 80.2 72.8 82.8 147.3
31.8 39.2 59.2 62.4 67.0 77.0 72.8 116.2
24.07-07.08 08.08-20.08 21.08-07.09 08.09-25.09 26.09-11.10 12.10-05.11
592.5
525.6
Total
4.30
depth of r o o t zone
transpiration,
m
lysimeter
plot
52.6 50.6 75.2 119.6 119.2 141.4
48.8 64.2 73.8 67.2 88.2 112.0
558.6
454.2
3.63
3.45
40 80 100 130 140\ 140
3139
S o i l moisture depletion s t u d i e s i n controlled f i e l d p l o t s
I s r a e l s e n , O.W.
( 1 9 5 6 ) mentioned t h a t t h e e a r l y measurements o f consumptive
u s e were made o n s e l e c t e d f i e l d p l o t s o f i r r i g a t e d c r o p s where t h e w a t e r t a b l e
was a t a c o n s i d e r a b l e d e p t h below t h e s u r f a c e . The p r o c e d u r e was t o measure t h e volume o f water a p p l i e d t o t h e p l o t a t each i r r i g a t i o n and t o measure any s u r f a c e run-off
t h a t might o c c u r . I n o r d e r t o a v o i d p e r c o l a t i o n of w a t e r below t h e
229
p l a n t r o o t z o n e , i t w a s n e c e s s a r y t o a p p l y t h e w a t e r i n small d e p t h s , n o t e x c e e d i n g f i v e i n c h e s i n a s i n g l e i r r i g a t i o n on o r d i n a r y s o i l s . P r e c i s e measurements o f t h e change i n s o i l m o i s t u r e w e r e n o t u n d e r t a k e n i n most o f t h e e a r l y studies. The f i e l d p l o t method was a d o p t e d f o r t h e f i r s t t i m e i n Egypt i n 1948 by t h e t h e n M i n i s t r y o f P u b l i c Works, nowadays t h e M i n i s t r y o f I r r i g a t i o n . A few y e a r s
l a t e r , El-Shal,
( 1 9 5 4 ) i n v e s t i g a t e d t h e w a t e r u s e by c o t t o n and maize a t
M.I.
S h e b i n e l Kom, El-Menufiya G o v e r n r a t e , by m e a s u r i n g t h e c h a n g e s i n s o i l m o i s t u r e u s i n g t h e s o i l t u b e . I n view o f t h e p r o x i m i t y o f t h e water t a b l e t o t h e ground s u r f a c e , t h e c r o p w a t e r u s e was p a r t l y s u p p l i e d t h r o u g h c a p i l l a r y movement o f t h e s u b s o i l t o t h e r o o t z o n e . S i n c e t h a t p a r t c o u l d n o t b e d e t e r m i n e d , i t was n o t p o s s i b l e a t t h a t t i m e t o d e t e r m i n e t h e a c t u a l w a t e r u s e by e i t h e r c r o p . Consumptive u s e o f w a t e r by v a r i o u s c r o p s h a s b e e n d e t e r m i n e d by i n t e n s i v e s o i l m o i s t u r e s t u d i e s . S o i l s a m p l e s h a v e b e e n t a k e n a t 10 c m i n t e r v a l s b e f o r e and a f t e r e a c h i r r i g a t i o n , w i t h some s a m p l e s i n between t h e s u c c e s s i v e i r r i g a t i o n c y c l e s . The d e p t h from t h e s a m p l e s t a k e n v a r i e d between 60 and 90 c m . S t a n d a r d l a b o r a t o r y p r a c t i c e s have b e e n a p p l i e d f o r d e t e r m i n i n g t h e m o i s t u r e c o n t e n t i n t h e s o i l s a m p l e s . The q u a n t i t y o f water removed from each l a y e r of s o i l was computed by t h e e q u a t i o n
d =
(Pfc
-
Pr)
’
Ps
.
D
100
where d
= d e p t h o f w a t e r removed, i n c m ,
pfc = s o i l moisture co n t en t a t f i e l d c a p a c i t y , i n per cent , pr ps
D
= remaining s o i l m o i s t u r e c o n t e n t ,
i n percent,
= a p p a r e n t s p e c i f i c g r a v i t y o f s o i l , and = d e p t h of l a y e r c o n s i d e r e d , i n c m . The t o t a l d e p t h o f w a t e r removed from t h e r o o t zone c o u l d t h u s b e o b t a i n e d
by summing up t h e m o i s t u r e e x t r a c t e d from t h e s u c c e s s i v e l a y e r s d u r i n g a cert a i n t i m e p e r i o d . T h a t d e p t h w a s c o r r e c t e d f o r t h e e x t r a c t i o n o f m o i s t u r e from t h e l a y e r s below t h e s a m p l i n g d e p t h and f o r t h e consumption o f water i n t h e i n t e r v a l o f t i m e from t h e moment o f i r r i g a t i o n a p p l i c a t i o n t i l l t h e f i e l d c a p a c i t y of t h e s o i l m o i s t u r e had b e e n r e a c h e d . The main f i n d i n g s from t h o s e e x t e n s i v e e x p e r i m e n t s which l a s t e d more t h a n t e n y e a r s c a n b e summarized as f o l l o w s : i)
The s o i l m o i s t u r e i n t h e s u r f a c e l a y e r r e a c h e s t h e s a t u r a t i o n c a p a c i t y upon t h e c o m p l e t i o n o f l a n d i r r i g a t i o n . T h i s m o i s t u r e l e v e l d r o p s r a p i d l y i n t h e f i r s t few days a f t e r i r r i g a t i o n f o l l o w e d by a less r a p i d d e c r e a s e
2 30
i n t h e s u b s e q u e n t d a y s . A s t u d y o f t h e c y c l i c change i n s o i l m o i s t u r e h a s shown t h a t t h e m o i s t u r e c o n t e n t i n t h e t o p 10 c m o f s o i l f e l l below t h e w i l t i n g p o i n t 18 d a y s a f t e r i r r i g a t i o n a p p l i c a t i o n , whereas t h e u n d e r l y i n g l a y e r s behaved d i f f e r e n t l y , e v e n w i t h e l o n g a t e d c y c l e s o f 30 days o r more. Moreover, t h e m o i s t u r e d e p l e t e d from t h e s u r f a c e 10 c m o f s o i l was negat i v e l y c o r r e l a t e d w i t h t h e a t m o s p h e r i c r e l a t i v e h u m i d i t y . The t o t a l m o i s t u r e removed from t h e u p p e r 30 c m o f s o i l was s t r o n g l y p o s i t i v e l y c o r r e l a t e d w i t h t h e a v e r a g e a i r t e m p e r a t u r e ( Z . e l Abedine, A . , M.,
ii)
and A b d a l l a h ,
1949).
F u r t h e r s t u d i e s h a v e shown t h a t t h e s i g n i f i c a n t c y c l i c c h a n g e s i n S o i l m o i s t u r e are c o n f i n e d t o t h e t o p 50 c m o f s o i l , and t h e r a t e of m o i s t u r e d e p l e t i o n d e c r e a s e s w i t h d e p t h below t h e s u r f a c e . T h i s c a n r e a d i l y b e s e e n from F i g . 6 . 4 . The amount o f m o i s t u r e d e p l e t e d from t h e r o o t zone v a r i e s from o n e i r r i g a t i o n c y c l e t o a n o t h e r , d e p e n d i n g o n t h e s t a g e of p l a n t growth a s w e l l a s t h e c l i m a t o l o g i c a l c o n d i t i o n s . F i g . 6 . 5 . i l l u s t r a t e s t h e c y c l i c v a r i a t i o n i n m o i s t u r e c o n t e n t i n t h e t o p 50 cm of s o i l , a s o b s e r v e d by El-Warith
M.,
(Khafagi, A . ,
Z . e l Abedine, A . ,
Shahin, M . ,
and E l - W a r i t h ,
1964), i n a f i e l d p l o t r a i s i n g c o t t o n a t S i d s .
50 aJ
E
al
-B 40
-5
B 4c
36
x
n
2
-. 3 2
2
aJ
c
2
c
30
c
28
C U 0
E3 2 4
?
c
.-ln
.- 20
E
I
.-
0
cn
44
3
10
0 4 8 (14-5-1958) Time f o l l o w i n g
I 12
16
20
24 28 (11-6-1958) irrigation application, days
Fig. 6.4. M o i s t u r e d e p l e t i o n from s o i l l a y e r s of a f i e l d p l o t r a i s i n g co t to n a t S i d s under an e l o n g a t e d i r r i g a t i o n t r e a t ment ( K h a f a g i , A , , Z . e l - A b e d i n e , A . , S h a h i n , M . , and E l - W a r i t h , M . , 1 9 6 4 )
E" 20
-
2 -.1 6
Mar. Apr. May. Jun. Jul. Aug. Month
Fig. 6.5. C y c l i c v a r i a t i o n of t h e moisture content i n t h e top 50 cm. o f s o i l i n a f i e l d p l o t r a i s i n g c o t t o n a t S i d s (Khafagi, A , , Z . el-Abedine, A , , Shahin, M., and E l - W a r i t h , M., 1964)
231
i i i ) The m o i s t u r e d e p l e t i o n c h a r a c t e r i s t i c c u r v e s o f t h e s o i l l a y e r s from which t h e p l a n t e x t r a c t s i t s m o i s t u r e u s e c a n b e o b t a i n e d by p l o t t i n g t h e d a i l y l o s s i n m o i s t u r e from e a c h l a y e r v e r s u s t h e t i m e s d u r i n g t h e growing s e a s o n of t h e p l a n t . Four sets o f c h a r a c t e r i s t i c c u r v e s are shown i n F i g . 6 . 6 . These s e t s r e p r e s e n t t h e d a i l y m o i s t u r e removal from t h e i n d i v i d u a l l a y e r s o f f i e l d p l o t s r a i s i n g c o t t o n , w h e a t , maize and berseem ( E g y p t i a n c l o v e r ) a t Giza and S i d s . Each c h a r a c t e r i s t i c c u r v e assumes t h e s h a p e o f a f o r c e d oscillation,
t h e a m p l i t u d e o f which damps w i t h d e p t h . The crown p o i n t o f
t h e c u r v e b e l o n g i n g t o any l a y e r l a g s b e h i n d t h e crown p o i n t o f t h e c u r v e belonging t o t h e o v erly in g l a y e r . iv)
The c o n t r i b u t i o n o f e a c h l a y e r t o t h e s e a s o n a l consumptive u s e i s s i m p l y t h e i n t e g r a t i o n o f t h e corresponding c h a r a c t e r i s t i c c u r v e , o r simply t h e t h e a r e a u n d e r i t . T h i s c o n t r i b u t i o n e v i d e n t l y d e c r e a s e s w i t h d e p t h below t h e s u r f a c e . N e v e r t h e l e s s , l a y e r s d e e p e r t h a n 5 0 cm s t i l l c o n t r i b u t e t o t h e t o t a l consumptive u s e , though by smaller amounts. The c o n t r i b u t i o n o f t h e s u c c e s s i v e l a y e r s , uz, i n p r o p o r t i o n t o t h e t o t a l e x t r a c t i o n , u, when p l o t t e d v e r s u s t h e d e p t h below s u r f a c e , z , y i e l d s t h e s e t o f c u r v e s shown i n F i g . 6 . 7 . (El-Shal,
M.I.,
1 9 6 6 ) . These c u r v e s c a n a p p r o x i m a t e l y be d e s -
c r i b e d by t h e g e n e r a l e q u a t i o n
where A and a are c o n s t a n t f o r e a c h c r o p , and a l m o s t t h e same f o r t h e f o u r c r o p s p r e s e n t e d i n F i g . 6 . 7 . F o r A = 0 . 3 5 , a = 3 . 5 , and 0 . 8 m a s d e p t h of moisture e x t r a c t i o n , t h e c o n t r i b u t i o n i n percent of t he t o t a l e x t r a c t i o n
i s 55 f o r t h e f i r s t q u a r t e r , 2 5 . 3 f o r t h e s e c o n d , 1 2 . 4 f o r t h e t h i r d and 7 . 3 f o r t h e f o u r t h q u a r t e r . These r e s u l t s a r e i n agreement w i t h t h e b a s i c f i n d i n g of Schockley, D . R . ,
t h a t most i r r i g a t e d c r o p s have s i m i l a r e x t r a c -
t i o n p a t t e r n s . H i s c o n c l u s i o n t h a t t h e f o u r s u c c e s s i v e q u a r t e r s of t h e r o o t z o n e , s t a r t i n g from t h e s u r f a c e , c o n t r i b u t e by 4 0 , 3 0 , 20 and 10% t o t h e t o t a l e x t r a c t i o n , i s n o t i n agreement w i t h ours. The f i g u r e s w e o b t a i n e d a r e more i n l i n e w i t h v a l u e s o b t a i n e d by S t a n b e r r y , C . O . Department o f A g r i c u l t u r e ,
(United S t a t e s
1955) from t h e a n a l y s i s o f t h e d a t a from 28
a l f a l f a t e s t s c o n d u c t e d i n 10 s t a t e s i n USA. Those were 4 7 , 2 6 , 17 and 10% f o r t h e f i r s t , s e c o n d , t h i r d and f o u r t h q u a r t e r o f t h e e x t r a c t i o n d e p t h , respectively.
232
2.5
I
Mar.
> n
.
1
May.
Apr.
I
I
I
1
Jun.
Jul.
Aug
Month
D
2.5
I
I
I
I
1
I
c.
0
al
.-> In In
U aJ
3
In
al
5
Nov.
Dec.
Jan. Feb. Month
Aug.
Sep.
Oct.
Mar.
Apr
Nov.
Month
2.5 2,0
I
-
I
I
I
I
I
I
I
Berseem
1.5 -
1.0
-
0.5
Oct.
Nov.
Dec.
Jan.
Feb.
Mar.
Apr.
May.
Month
Fig. 6 . 6 . M o i s t u r e d e p l e t i o n c h a r a c t e r i s t i c c u r v e s f o r t h e t o p 50 cm l a y e r s o f s o i l u n d e r f o u r d i f f e r e n t c r o p s , a v e r a g e f o r G i z a and S i d s s t a t i o n s ( E l - S h a l ,
M.I., 1966)
233
u l u , i n percent Fig. 6.7.
P a t t e r n o f s e a s o n a l m o i s t u r e e x t r a c t i o n by some c r o p s i n Egypt
A f u r t h e r s u p p o r t f o r our i d e a t h a t t h e m o i s t u r e e x t r a c t i o n p a t t e r n i s more o r l e s s e x p o n e n t i a l r a t h e r t h a n l i n e a r , a s was o r i g i n a l l y proposed by S c h o c k l e y , came from t h e a n a l y s i s o f t h e d a t a c o l l e c t e d from e x p e r i m e n t s a t S c o t t s B l u f f S t a t i o n , N e b r a s k a , USA ( U n i t e d S t a t e s Department o f Agriculture,
1 9 5 5 ) . A summary of some of t h e r e s u l t s i s a s f o l l o w s ' :
Depth,
Moisture e x t r a c t i o n i n percent of t o t a l e x t r a c t i o n f o r
I L
0-1 1-2 2-3 3-4
v)
Oats
Beets
Potatoes
Average
64 16 12
62 19 12 7
57 23
61.0 19.3 12.3 7.3
8
13 7
The c o r r e c t e d c y c l i c loss of m o i s t u r e when d i v i d e d by t h e d u r a t i o n of each
c y c l e g i v e s t h e d a i l y l o s s o r t h e c r o p w a t e r u s e p e r d a y . D a i l y , monthly and s e a s o n a l consumptive u s e o f w a t e r by a l a r g e number o f c r o p s i n Egypt
are g i v e n i n T a b l e s 6 . 2 up t o and i n c l u d i n g 6 . 8 . S p e c i f i c a t i o n s o f t h e s e tables are:
6.2 6.3 6.4 6.5 6.6 6.6 6.6 6.6 6.7 6.8
Number o f exTeriments
Number o f ex2erinent s t a t i o n s
13 7 6 1
4 4 3
1 1 1
1 2 4
1
1 1 1 1 2 2
Crop i n v e s t i g a t e d Cotton Wheat L a t e maize E a r l y maize Fenugreek Chick p e a E g y p t i a n Lupine Lentil B e r s e e m (Egyptian c l o v e r ) C i t r u s trees
Sr. No.
N
Consumptive u s e of w a t e r f o r c o t t o n i n Egypt
TABLE 6.2
Expt. S t a t . &
From
To
Investigator Sakha
22.03.1957 01.04.1957 11.04.1957 21.04.1957 01.05.1957 11.05.1957 21.05.1957 01.06.1957 11.06.1957
El-Shala
9 10 10 10 10 10 11 10 10
31.03.1957 10.04.1957 20.04.1957 30.04.1957 10.05.1957 20.05.1957 31.05.1957 10.06.1957 20.06.1957
Consumptive u s e , daily
1.19 1.19 1.63 2.29 2.29 3.68 4.27 5.38 6.12
rnm
From
monthly
10.71 51.10 106.67
21.06.1957 01.07.1957 11.07.1957 21.07.1957 01.08.1957 11.08.1957 20.08.1957 01.09.1957 11.09.1957
To
30.06.1957 10.07.1957 10.07.1957 30.07.1957 10.08.1957 20.08.1957 31.08.1957 10.09.1957 14.09.1957
Interval days
10 10 10 11 10 10 11 10 4
Sakha
18.03.1958 21.03.1958 01.04.1958 11.04.1958 21.04.1958 01.05.1958 11.05.1958 21.05.1958 01.06.1958
El-Shala
2 11 10 10 10 10 10 11 10
10.03.1958 31.03.1958 10.04.1958 20.04.1958 30.04.1958 10.05.1958 20.05.1958 31.05.1958 10.06.1958
1.34 1.34 1.34 1.72 2.60 2.60 3.69 4.42 6.36
17.42
56.60 111.52
11.06.1958 21.06.1958 01.07.1958 11.07.1958 21.07.1958 01.08.1958 11.08.1958 21.08.1958 01.09.1958
20.06.1958 30.06.1958 10.07.1958 20.07.1958 31.07.1958 10.08.1958 20.08.1958 31.08.1958 06.09.1958
10
10 10 10 11 10 10 11 6
Sakha
3 A . El-Warithb
17.03.1958 21.03.1958 01.04.1958 17.04.1958 01.05.1958 14.05.1958 01.06.1958
Season
3
20.03.1958 31.03.1958 16.04.1958 30.04.1958 13.05.1958 31.05.1958 11.06.1958
11 16
14 13 18 11
1966;
7.11 7.54 7.67 6.74 4.18 3.35 2.10 2.10 2.10
mm
monthly
186.10 226.24 98.40 29.40
6.73 7.09 7.20 7.30 4.90 4 .OO 3.00 3.00 3.00
201.80 198.90 103.00 18.00
b = Abdel-Warith,
1.60 1.60 1.60 2.10 2.10 3.50 4.30
22.40 55 .OO 90.30
12.06.1958 01.07.1958 05.07.1958 23.07.1958 01.08.1958 10.08.1958 01.09.1958
30.06.1958 04.07.1958 22.07.1958 31.07.1958 09.08.1958 31.08.1958 14.09.1958
19 4 18 9 9 22 13
5.20 5.25 7.55 5.80 5.80 3.40 3.40
146.10 209.10 127.00 44.20
694.10 mm
180 d a y s M.I.,
daily
707.24 mm
172 d a y s
Season
Consumptive u s e ,
708.62 mm
176 d a y s
Season
a = El-Shal,
Interval days
w
Y.,
1965
TABLE 6 . 2
Sr.
No.
(continued)
Expt. S t a t . &
I n v e s t i g a t or Sakha
A.
El-Warith
b
From
To
Interval days
12.03.1960 21.03.1960 01.04.1960 12.04.1960 01.05.1960 18.05.1960 01.06.1960
20.03.1960 31.03.1960 11.04.1960 30.04.1960 17.05.1960 31.05.1960 19.06.1960
9 11 11 19 17 14 19
Season
05.03.1958 11.03.1958 21.03.1958 01.04.1958 11.04.1958 21.04.1958 (‘1.05.1958 11.05.1958 2 1.05 .1958
El-Shala
Seasou
A . El-Warithb
05.03.1958 19.03.1958 01.04.1958 03.04.1958 16.04.1958 01.05.1958
Season
a = El-Shal,
1.10 1.10 1.10 2.20 2.80 4.90 5.40
monthly 22 .oo 53.90 116.20
From
To
Interval days
20.06.1960 01.07.1960 05.07.1960 20.07.1960 01.08.1960 08.08.1960 01.09.1960
30.06.1960 04.07.1960 19.07.1960 31.07.1960 07.08.1960 31.08.1960 03.09.1960
11 4 15 12 7 24 2
10.03.1958 20.03.1958 31.03.1958 10.04.1958 20.04.1958 30.04.1958 10.05.1958 20.05.1958 31.05.1958
5 10 11 10 10 10 10 10
11
1966;
daily 6.80 6.80 7.60 5.60 5.60 3.70 3.70
mm
monthly 177.40
208.40 128.00 7.40
0.85 0.92 1.59 2.14 2.42 2.48 2.97 3.35 4.65
30.94
70.40
114.35
01.06.1958 11.06.1958 21.06.1958 01.07.1958 11.07.1958 21.07.1958 01.08.1958 11.08.1958 21.08. 1958
10.06.1958 20.06.1958 30.06.1958 10.07.1958 20.07.1958 31.07.1958 10.08.1958 20.08.1958 31.08.1958
10
10 10 10 10 11 10 10 11
6.26 6.87 7.30 7.32 6.55 5.78 4.21 3.57 2.09
204.30
202.28
100.79
723.06 mm 18.03.1958 31.03.1958 02.04.1958 15.04.1958 30.04.1958 17.05.1958
14 13 2 13 15 17
b = Abdel-Warith, M . ,
1.80 2.00 2.00 2.70 3.00 3.90
51.20 84.10
18.05. 1958 01.06 1958 15.06 1958 01.07 1958 15.07 1958 0 1 . 0 8 . 1958
31.05.1958 14.06.1958 30.06.1958 14.07.1958 31.07.1958 30.08.1958
14 14 16 14 17 30
4.50 5.30 6.40 7.30 5.90 3.47
129.30 176 .80 202.50 104.10
748.00 mm
179 days
M.I.,
Consumptive u s e ,
713.30 mm
179 days
Giza
6
daily
mm
175 days
Giza
5
Consumptive u s e ,
1965
N cn W
TABLE 6 . 2
Sr. No.
(continued)
Expt. S t a t . &
Investigator Giza
7
N
w m
A.
E l Samie & Y . Barrada'
Season Sids
El-Shala
Season Sids 9
a = El-Shal, M . I . ,
From
To
Interval days
01.03.1958 23.03.1958 01.04.1958 16.04.1958 01.05.1958 03.05.1958 19.05.1958 01.06.1958 04.06.1958
23.03.1958 31.03.1958 15.04.1958 30.04.1958 02.05.1958 18.05.1958 31.05.1958 03.06.1958 22.06.1958
23 8 15 15 2 16 13 3 19
Consumptive u s e , daily 1.0 1.0 1.0 1.8 1.8 5.5 5.4 5.4 5.5
mm
monthly
31 .OO 42 .OO
161.80
From
To
23.06.1958 01.07.1958 07.07.1958 16.07.1958 28.07.1958 01.08.1958 06.08.1958 21.08.1958 30.08.1958 01.09.1958
30.06.1958 06.07.1958 15.07.1958 27.07.1958 31.07.1958 05.08.1958 20.08.1958 29.08.1958 31.08.1958 03.09.1958
Interval days 8 6 9 12 4 5 15 9 2 3
4 10 11 10
10.03.1957 20.03.1957 31.03.1957 10.04.1957 20.04.1957 30.04.1957 10.05.1957 20.05.1957 31.05.1957
10
10 10
10 11
0.74 0.74
1.11
1.75 1.99 2.35 2.78 3.42 5.33
22.57
60.90
120.63
01.06.1957 11.06.1957 21.06.1957 01.07.1957 11.07.1957 21.07.1957 01.08.1957 11.08.1957 21.08.1957
10.06.1957 20 .06.1957 30.06.1957 10.07.1957 20.07.1957 31.07.1957 10.08.1957 20.08.1957 31.08.1957
10 10 10 10 10 11 10 10 11
4.7 4.7 7.3 10.4 10.8 10.8 6.4 3.2 3.2 3.2
mm
monthly 158.30
261.90
185 .20 9.60
6.25 7.10 7.26 7.30 6.52 5.93 5.22 3.63 2.04
206.10
203.43
110.94
724.57 mm
178 days 03.03.1958 11.03.1958 21.03.1958 01.04.1958
daily
849.80 mm
187 days 06.03.1957 11.03.1957 21.03.1957 01.04.1957 11.04.1957 21.04.1957 01.05.1957 11.05.1957 21.05 .1957
Consumptive u s e ,
7 10 11
10.03.1958 20.03.1958 31.03.1958 10.04.1958
1966; c = Abd El-Samie,
10
A.G.,
0.79 0.79 1.37 1.63
28.50
and Barrada, Y., 1960
01.06.1958 11.06.1958 21.06.1958 01.07.1958
10.06.1958 20.06.1958 30.06.1958 10.07.1958
10 10 10 10
6.12 6.90 7.63 7.33
206.50
TABLE 6 . 2
Sr. NO
-
'
(continued)
Expt. S t a t . &
Investigator El-Shala
9
From
To
Interval days
11.04.1958 21.04.1958 0 1.05.1958 11.05.1958 21.05.1958
20.04.1958 30.04.1958 10 .05 .1958 20.05.1958 31.05.1958
10 10 10 10 11
Season
A.
03.03.1958 23.03.1958 01.04.1958 09.04.1958 27.04.1958 01.05.1958 17.05.1958
El-Warith
Season
11
El-Warith
59.40
122.84
To
11.07.1958 21.07.1958 01.08.1958 11.08.1958 21.08.1958
20.07.1958 31.07.1958 10.08.1958 20.08.1958 31.08.1958
Interval days 10 11 10 10 11
22.03.1958 31.03.1958 08.04.1958 26.04.1958 30.04.1958 16.05.1958 31.05.1958
b
22.02.1959 01.03.1959 17.03.1959 01.04.1959 14.04.1959 01.05.1959 17.05.1959 28.05.1959
Season
Consumptive u s e , mm daily monthly 1.76 5.62 4.88 4.14 2.46
202.70
117.26
737.20 mm 20 9 8 18 4 16 15
1.30 2 .oo 2 .oo 2.20 3.20 3.20 5 .OO
44.00
68.40 126.20
01.06.1958 16.06.1958 01.07.1958 17.07.1958 01.08.1958 03.08.1958 01.09.1958
15.06.1958 30.06.1958 16.07.1958 31.07.1958 02.08.1958 31.08.1958 02.09.1958
15 15 16 15 2 29
183 days
Sids
A.
1.98 2.33 3.16 3.69 4.94
From
181 days
Sids
10
Consumptive u s e , mm daily monthly
6.60 5.80 7.00 5.60 5.60 3.50 3.50
186.00 196 .OO 112.70 3.50
736.80 mm 28.02.1959 16.03.1959 31.03.1959 13.04.1959 30.04.1959 16.05.1959 27.05.1959 31.05.1959
7 16 15 13 17 16 11 4
183 days
0.80 0.80 1.40 1.40 2.60 3.40 5.40 6.50
5.60 33.80 62.40
139.80
01.06.1959 07.06.1959 19.06.1959 28.06.1959 01.07.1959 06.07.1959 15.07.1959 28.07.1959 01.08.1959
06.06.1959 18.06.1959 27.06.1959 30.06.1959 05.07.1959 14.07.1959 27.07.1959 31.07.1959 23.08.1959
6 12 9 3 5 9 13 4 23
6.50 7.40 8.40 8.20 8.20 8.10 6.30 4.20 4.20
228.00
212.60 92 .OO
774.20 mm
a = E l - S h a l , M . I . , 1966;
b = Abdel-Warith,
M.,
1965
N
w
-1
TABLE 6 . 2
Sr. No.
Expt. S t a t . & Investigator Sids
12
N
(continued)
El-Shala
W
From
To
09.03.1960 11.03.1960 21.03.1960 01.04.1960 11.04.1960 20.04.1960 01.05.1960 11.05.1960 21.05.1960
10.03.1960 20.03.1960 31.03.1960 10.04.1960 20.04.1960 30.04.1960 10.05.1960 20.05.1960 31.05.1960
l3
El-Shala
22.02.1960 01.03.1960 11.03.1960 21.03.1960 01.04.1960 11.04.1960 21.04.1960 01.05.1960 11.05.1960 21.05.1960
Season
a = El-Shal, M.I.,
1 10 11 10 10 10 10 10 11
Consumptive u s e , mm daily monthly
1.06 1.06 1.06 1.51 1.63 2.70 3.29 4.37 5.05
23.32
58.40
132.15
From
To
01.06.1960 11.06.1960 21.06.1960 01.07.1960 11.07.1960 21.07.1960 01.08.1960 11.08.1960 21.08.1960
10 .06.1960 20.06.1060 30.06 .1960 10 .07.1960 20.07.1960 31.07.1960 10.08.1960 20.08.1960 31.08.1960
Interval days 10 10 10 10 10 11 10 10 11
195 davs
1966
Consumptive u s e , mm daily monthly 5.61 7.36 7.92 7.62 6.10 5.75 4.02 3.29 3.29
208.90
200.45
109.29
732. 5 1 nun
175 days
Season Ma1 lawi
Interval days
29.02.1960 10.03.1960 20.03.1960 31.03.1960 10.04.1960 20.04.1960 30.04.1960 10.05.1960 20.05.1960 31.05.1960
7 10 10 11 10 10 10 10 10 11
1.20 1.20 1.38 1.66 1.82 1.97 2.96 3.51 4.57 5.03
8.40
44.06
67.50
136.13
01.06.1960 11.06.1960 21.06.1960 01.07.1960 11.07.1960 21.07.1960 01.08.1960 11.08.1960 21.08.1960 01.09.1960
10 .06.1960 20 .06 ,1960 30 .06 .1960 10.07.1960 20 .07.1960 31.07.1960 10.08.1960 20.08.1960 31.08.1960 04.09.1960
10 10
10
10 10 11 10 10 11 4
5.84 7.72 8.26 7.72 6.48 5.73 4.35 3.43 3.33 3.28
218.20
205.03
114.43 13.12
806. 87 nun
TABLE 6.3 Sr.
No
C o n s u m p t i v e u s e of w a t e r for w h e a t i n Egypt
Expt. S t a t &
Investigator Sakha
15.11.1957 21.11.1957 01.12.1957 11.12.1957 21.12.1957 01.01.1958 11.01.1958 21.01.1958 01.02.1958 11.02.1958
El-Shala
Season
To
Interval
20.11.1957 30.11.1957 10.12.1957 20.12.1957 31.12.1957 10.01.1958 20.01.1958 31.01.1958 10.02.1958 20.02.1958
days
5 10 10 10 11 10 10 11 10 10
Consumptive u s e , mm daily monthly
1.81 1.81 1.81 1.73 1.60 1.60 1.60 1.60 1.60 1.71
27.15 53.00 49.60
From
21.02.1958 01.03.1958 11.03.1958 21.03.1958 01.04.1958 11.04.1058 21.04.1958 01.05.1958 11.05.1958
To
28.02.1958 10.03.1958 20.03.1958 31.03.1958 10.04.1958 20.04.1958 30.04.1958 10.05.1958 16.05.1958
Interval days
8 10 10 11 10 10 10
10 6
22.11.1958 01.12.1958 11.12.1958 21.12.1958 01.01.1959 11.01.1959 21.01.1959 01.02.1959 11.02.1959
El-Shala
Season
30.11.1958 10.12.1958 20.12.1958 31.12.1958 10.01.1959 20.01.1959 31.01.1959 10.02.1959 20.02.1959
9 10 10 11 10 10 11 10 10
1.79 1.79 1.79 1.70 1.63 1.63 1.63 1.68 2.27
16.11 54.50 50.53
21.02.1959 01.03.1959 11.03.1959 21.03.1959 01.04.1959 11.04.1959 21.04.1959 01.05.1959 11.05.1959
28.02.1959 10.03.1959 20.03.1959 31.03.1959 10.04.1959 20.04.1959 30.04.1959 10.05.1959 12.05.1959
8 10 10 11 10 10 10 10 2
17.11.1957 01.12.1957 19.12.1957 01.01.1958 24.01.1958 01.02.1958
d
a = El-Shal,
30.11.1957 18.12.1957 31.12.1957 23.01.1958 31.01.1958 28.02.1958
169 d a y s
Season M.I.,
2.64 2.68 3.15 3.87 2.57 2.24 2.24 1.89 1.80
54.22 100.87 70.50 29.10
2.27 2.27 2.34 2.95 2.95 2.09 1.72 1.72 1.72
57.69 78.61 67.66 20.69
345.79 mm
172 d a y s
Giza
Consumptive u s e , mm daily monthly
385.04 mm
182 d a y s
Sakha
Shahin
From
1966; d = S h a h i n , M., 1959
14 18 13 23 8 28
1.56 1.56 1.92 1.92 2.10 2.10
21.84 53.04 60.76 58 .80
01.03.1958 05.03.1958 01.04.1958 04.04.1958 01.05.1958
04.03.1958 31.03.1958 03.04.1958 30.04.1958 04.05.1958
4
27 3 27 4
2.10 2 .80 2 .80 1.70 1.70
84 .OO 54.30 6 .80
339.54 mm
TABLE 6 . 3 Sr. No.
Expt. S t a t . &
Investigator Giza
4
El-Shala
Sids
El-Shala
6 El-Shala
a = El-Shal,
Interval
To
days
10.11.1958 20.11.1958 01.12.1958 11.12.1958 21.12.1958 01.01.1959 11.01.1959 21.01.1959 01.02.1959
20.11.1958 30.11.1958 10.12.1958 20.12.1958 31.12.1958 10.01.1959 20.01.1959 31.01.1959 10.02.1959
10 10
10
10 11 10
io
11 10
Consumptive use, daily 1.91 1.91 1.88 1.58 1.58 1.58 1.66 1.70 1.70
mm
month 1y 38 .20 51.98 51.10
Interval
From
To
days
11.02.1959 21.02.1959 01.03.1959 11.03.1959 21.03.1959 01.04.1959 11.04.1959 21.04.1959 01.05.1959
20.02.1959 28.02.1959 10.03.1959 20.03.1959 31.03.1959 10.04.1959 20.04.1959 30.04.1959 10.05.1959
10 8 10 10 11 10 10 10 10
09.11.1957 01.12.1957 11.12.1957 21.12.1957 01.01.1958 11.01.1958 21.01.1958 01.02.1958
10.11.1957 10.12.1957 20.12.1957 31.12.1957 10.01.1958 20 .O 1.1958 31.01.1958 10.02.1958
1 10 10 10 10 11
I0
10 11 10
1.79 1.79 1.79 1.79 1.79 1.60 1.60 1.60 1.60 1.98
37.59 53.40
11.02.1958 21.02.1958 01.03.1958 11.03.1958 21.03.1958 01.04.1958 11.04.1958 21.04.1958 01.05.1958
20.02.1958 28.02.1958 10.03.1958 20.03.1958 31.03.1958 10.04.1958 20.04.1958 30.04.1958 09.05.1958
10 8 10 10 11 10 10 10 9
M. I., 1966
daily 1.85 2.46 2.46 2.48 2.63 2.63 2.63 2.63 1.68
mm
monthly 55.18 78.33 70.90 16.80
2.36 2.36 2.49 3.01 3.01 2.83 2.56 2.30 1.90
62.28 88.11
76.90 17.10
384.98 mm
181 days
09.11.1958 11.11.1958 21.11.1958 01.12.1958 11.12.1958 21.12.1958
Consumptive u s e ,
370.49 mm
182 days
Season Sids
1
0
From
Season
5
h3
(continued)
10.11.1958 20.11.1958 30.11.1958 10.12.1958 20.12.1958 31.12.1958
1 10 10 10 10 11
2 .oo 2 .oo 2 .oo 1.68 1.61 1.62
42 .OO 50.70
11.02.1959 21.02.1959 01.03.1959 11.03.1959 21.03.1959 01.04.1959
20.02.1959 28.02.1959 10.03.1959 20.03.1959 31.03.1959 10.04.1959
10 4 10 10 11 10
2.24 2.23 2.24 2.37 3.16 3.16
57.44 80.90
TABLE 6 . 3
Sr.
No.
6
(continued)
Expt. S t a t . &
To
Interval days
01.01.1959 11.01.1959 21.01.1959 01.02.1959
10.01.1959 20.01.1959 31.01.1959 10.02.1959
10 10 11 10
Investigator El-Shala
Season Ma11awi
7
From
El-Shala
Season
a = E l - S h a l , M.I.,
Consumptive u s e , daily 1.63 1.63 1.63 1.72
mm
monthly
50.53
From
To
11.04.1959 20.04.1959 21.04.1959 30.04.1959 01.05.1959- 08.05.1959
Interval days 10 10 8
178 days
1966
daily 2.52 1.74 1.74
mm
monthly 74.20 13.92
369.69 mm
180 days 15.11.1957 21.11.1957 01.12.1957 11.12.1957 21.12.1957 01.01.1958 11.01.1958 21.01.1958 01.02.1958 11.02.1958
Consumptive u s e ,
20.11.1957 30.11.1957 10.12.1957 20.12.1957 31.12.1957 10.01.1958 20.01.1958 31.01.1958 10.02.1958 20.02.1958
5 10 10 10 11 10 10 11 10 10
1.61 1.61 1.61 1.56 1.56 1.614 1.614 1.614 1.93 2.06
24.15 48.86 50.03
20.02.1958 01.03.1958 11.03.1958 21.03.1958 01.04.1958 11.04.1958 21.04.1958 01.05.1958 11.05.1958
28.02.1958 10.03.1958 20.03.1958 31.03.1958 10.04.1958 20.04.1958 30.04.1958 10.05.1958 12.05.1958
8 10 10 11 10 10 10 10 4
2.66 3.25 3.30 3.40 2.78 2.52 2.52 1.50 1.50
61.18 102.90 78.20 18.OO
383.22 mm
Consumptive u s e of w a t e r for l a t e maize i n Egypt
TABLE 6.4 Sr.
No.
Expt. S t a t . &
Interval days
From
To
30.07.1959 01.08.1959 11.08.1959 21.08.1959 01.09.1959 11.09.1959 21.09.1959
31.07.1959 10.08.1959 20.08.1959 31.08.1959 10.09.1959 20.09.1959 30.09.1959
Investigator Sakha
El-Shala
Season Giza
El-Shala
Season Giza
E l -Gibali Season Giza
El-Gibalie Season a = hl-Shal, M . I . ,
1 10 10 11 10 10 10
N Ip N
Consumptive u s e , mm daily monthly
3.06 3.06 3.06 4.61 5.06 6.086 6.36
3.06
111.87
175 .86
Interval From
To
01.10.1959 11.10.1959 21.10.1959 01.11.1959 11.11.1959 21.11.1959 01.12.1959
10.10.1959 20.10.1959 31.10.1959 10.11.1959 20.11.1959 30.11.1959 08.12.1959
days
10 10 11 10 10 10 8
10.08.1959 20.08.1959 31.08.1959 10.09.1959 20.09.1959 30.09.1959
8 10 11 10 10 10
2.93 2.93 4.49 5.43 6.25 6.886
102.13
185.66
01.10.1959 10.10.1959 11.10.1959 20.10.1959 21.10.1959 31.10.1959 01.11.1959 10.11.1959 11.11.1959 20.11.1959 21.11.1959 30.11.1959
10 10 11 10 10 10
27.07.1962 31.07.1962 09.08.1962 23.08.1962 31.08.1962
18 4 9 14 8
3.155 5.59 5.59 5.905 6.64
79.15
01.09.1962 12.09.1962 01.10.1962 04.10.1962
11.09.1962 30.09.1962 03.10.1962 21.10.1962
11 19 3 18
122 d a v s
53.67 8.40
6.99 6.76 4.07 2.26 1.48 1.48
182.27
52.20
6.64 3.19 3.91 3.225
147.33 58.05
186.10 470.63 mm
104 d a y s 07.07.1963 01.08.1963 11.08.1963 25.08.1963 01.09.1963
167.25
522.26 mm
120 d a y s 09.07.1962 28.07.1962 01.08.1962 10.09.1962 24.08.1962
6.67 5.006 4.59 1.936 1.936 1.495 1.05
520.11 mm
131 d a y s 02.08.1959 11.08.1959 21.08.1959 01.09.1959 11.09.1959 21.09.1959
Consumptive u s e , mm daily monthly
24 10 14 7 13
31.07.1963 10.08.1963 24.08.1963 31.08.1963 13.09.1963 ~-
1966; e = E l - G i b a l i , A . ,
1966.
3.445 3.445 4.995 6.39 6.39
82.68
149.11
14.09.1963 30.09.1963 01.10.1963 04.10.1963 05.10.1963 17.10.1963 18.10.1963 31.10.1963 Oi. 11.1963 06.11.1963
17 4 13 14 6
5.20 5.20 5.04 3.315 3.315
152.30
132.73 19.89
536.72 mm
TABLE 6 . 4 Sr.
No.
(continued)
Expt. S t a t . & Investigator Sids
To
28.07.1959 01.08.1959 11.08.1959 21.08.1959 01.09.1959 11.09.1959 21.09.1959
5
E l -Shala
31-07.1959 10.08.1959 20.08.1959 31.08.1959 10.09.1959 20.09.1959 30.09.1959
Interval days 3 10 10 11 10 10 10
Consumptive u s e , daily
mm
2.95 2.95 3.12 4.68 4.68 6.25 6.45
monthly 8.85
112.18
173.80
Sids Shenouda El-Gibali Tawdros & Gamal Season
01.10.1959 11.10.1959 21.10.1959 01.11.1959 11.11.1959 21.11.1959 01.12.1959
10 10 11 10 10 10
8
Consumptive u s e ,
mm
daily 6.52 6.26 4.73 3.36 2.19 1.15 1.04
15.07.1964 27.07.1964 01.08.1964 08.08.1964 20.08.1964 01.09.1964
monthly
179.83 67 .OO 8.32
549.98 mm 26.07.1964 31.07.1964 07.08.1964 19.08.1964 31.08.1964 12.09.1964
12 5 7 12 12 12
3.62 2.66 2.66 5.74 6.21 3.75
56.74
162.02
13.09.1964 25.09.1964 01.10.1964 07.10.1964 19.10.1964
24.09.1964 30.09.1964 06.10.1964 18.10.1964 31.10.1964
12 6 6 12 19
5.56 5.31 5.31 5.52 3.31
143.58
163.99
5 2 6 . 3 3 mm
Consumptive u s e o f w a t e r for e a r l y maize i n Egypt ~
Shenouda El-Gibali Tawdros & Gamal
a = El-Shal,
10.10.1959 20.10.1959 31.10.1959 10.11.1959 20.11.1959 30.11.1959 08.12.1959
Interval days
109. days
Sids
Season
To
From
133 days
Season
TABLE 6 . 5
From
16 04.1964 01.05.1964 07.05.1964 19.05.1964 01.06.1964
30.04.1964 06.05.1964 18.05.1964 30.05.1964 31.05.1964 11.06.1964
15 6 12 12 1 11
1.85 1.85 3.06 6.25 5.93 5.93
~~
27.75
128.75
12.06.1964 24.06.1964 01.07.1964 06 .07.1964 18.07.1964 30.07.1964 01.08.1964
= Shenouda, E l - G i b a l i ,
~
12 7 5 12 12 2 8
6.91 7.47 7.47 5.44 5.60 3.20 3.20
200.44
176.23 25.60
558.77 mm
115 days
M.I., 1966; f
23.06.1964 30.06.1964 05.07.1964 17.07 .1964 29.07.1964 31.07.1964 1 8 . 0 8 .1964
Tawdros and Gamal, 1966
L N W
Month
&
No.
Investigator Assiut
Nov. Dec. Jan. Feb. Mar.
El-Gibalig
1
Fenugreek+
1965 1965 1966 1966 1966
TABLE 6.7
Nov. 10-Mar.
Chicken p e a
+
++
E g y p t i a n Lupin
Lentil
++
daily
monthly
daily
monthly
daily
monthly
daily
monthly
1.541 2.087 1.880 2.394 1.709
30.82 64.70 58.24 67.03 12.88
1.414 1.721 1.360 1.923 0.735
28.28 53.34 42.16 53.85 8.09
1.532 2.065 1.852 2.325 1.221
30.65 64.00 57.40 65.10 18.31
1.405 1.778 1.417 1.729 0.819
28.11 55.12 43.94 53.59 12.29
Season
+ = Growth s e a s o n :
L L
Consumptive u s e , i n mm, f o r t h e c r o p s
Expt. S t a t .
Sr.
Sr. No.
N
Consumptive u s e of w a t e r f o r some w i n t e r c r o p s i n Egypt
TABLE 6.6
233.67 11; ++ = Growth s e a s o n :
185 .72
235.46
193.05
Nov. 10-Mar. 15
Consumptive u s e of w a t e r f o r Berseem ( E g y p t i a n c l o v e r ) i n Egypt
Expt. S t a t . &
From
To
Interval days
10.11.1957 28.11.1957 01.12.1957 22.12.1957 01.01.1958 01.02.1958 07.02.1958 17.02.1958
27.11.1957 30.11.1957 2 1.12.1957 31.12.1957 31.0 1.1958 06.02.1958 14.02.1958 26.02.1958
17 3 21 10 31 6 8 10
Investigator Giza
Shahin
d
Season
Consumptive u s e , mm daily monthly
1.82 2.15 2.50 1.45 0.80 0.45 1.65 3.15
37.39 67 .OO 24.80
From
To
Interval days
27.02.1958 01.03.1958 28.03.1958 01.04.1958 07.04.1958 15.04.1958 01.05.1958
28.02.1958 27.03.1958 31.03.1958 06.04.1958 16.04.1958 30.04.1958 07.05.1958
2 27 4 6 10 16 7
214 days ( n o r e c o r d s w e r e a v a i l a b l e between O c t . 15 and Nov. 10)
g = E l - G i b a l i , M.H., 1969; d = S h a h i n , M . ,
1959
Consumptive u s e , daily
3.50 3.95 2.80 2.50 4.10 4.35 1.85
mm
monthly
57.70 117.85
117.40 12.95
435.09 mm
TABLE 6 . 7
Sr. No.
(continued)
Expt. S t a t . &
Investigator Sids
El-Shala
Season
TABLE 6 . 8 Sr.
No.
To
20.10.1957 01.11.1957 11.11.1957 21.11.1957 01.12.1957 11.12.1957 21.12.1957 01.01.1958 11.01.1958 21.01.1958 01.02.1958 11.02.1958
31.10.1957 10.11.1957 20.11.1957 30.11.1957 10.12.1957 20.12.1957 31.12.1957 10.01.1958 20.01.1958 31.01.1958 10.02.1958 20.02.1958
11 10 10 10 10 10 11 10 10 11 10
Consumptive u s e , daily
mm
1.564 1.564 1.530 1.526 2.047 2.068 2.261 1.659 0.757 0.757 1.229 1.700
monthly 17.204
46.200
66.021
32.487
From
To
21.02.1958 01.03.1958 11.03.1958 21.03.1958 01.04.1958 11.04.1958 21.04.1958 01.05.1958 11.05.1958 21.05.1958 01.06.1958
28.02.1958 10.03.1958 20.03.1958 31.03.1958 10.04.1958 20.04.1958 30.04.1958 10.05.1958 20.05.1958 31.05.1958 07.06.1958
Interval days 8 10 10
11
10 10 10 10 10 11 7
Consumptive u s e , daily
mm
monthly
1.821 2.671 2.671 4.953 4.433 3.859 4.337 4.165 4.091 2.940 1.558
43.858
107.903
126.290
114.900 10.906
565.769 mm
230 days
Consumptive use of w a t e r f o r c i t r u s t r e e s i n Egypt
Expt. S t a t .
From
&
Investigator Giza
1
Interval days
From
d Shahin
Interval
To
days
,
01.01.1957 01.02.1957 01.03.1957 17.03.1957 01.04.1957 29.04.1957 01.05.1957 29.05.1957 01.06.1957
31.01.1957 28.02.1957 16.03.1957 31.03.1957 28.04.1957 30.04.1957 28.05.1957 31.05.1957 12.06.1957
a = E l - S h a l , M . I . , 1966; d = S h a h i n , M . ,
1959
31 28 16 15 28 2 28 3 12
Consumptive u s e , mm daily monthly 0.72 1.47 2.25 2.35 2.81 2.87 2.87 3.43 3.53
22.32 41.16 71.25 84.42 90.65
From
To
03.07.1957 22.07.1957 01.08.1957 07.08.1957 01.09.1957 23.09.1957 01.10.1957 09.10.1957 01.11.1957
21.07.1957 31.07.1957 06.08.1957 31.08.1957 22.09.1957 30.09.1957 08.10.1957 31.10.1957 26.11.1957
Interval days 19 10 6
25 22 8 8 23 26
Consumptive u s e , daily 3.93 3.91 3.91 3.82 3.50 3.41 3.41 2.80 2.33
mm
monthly
121.25 118.96 104.28 91.68
TABLE 6 . 8
Sr. No.
N D b
(continued)
Expt. S t a t . &
Investigator d Shahin
m
From
To
13.06.1957
30.06.1957 0 2 .07.19 57
0 1.07.1957
Interval days 18 2
Consumptive u s e , daily 3.74 3.74
mm
monthly 109 .68
From
To
27.11.1957 01.12.1957
30.11.1957 31.12.1957
Interval days 4 31
01.01.1958 01.02.1958 01.03.1958 04.03.1958 01.04.1958 01.05.1958 09.05.1958 28.05.1958 01.06.1958 16.06.1958
Giza
d Shahin
31.01.1958 28.02.1958 03.03.1958 31.03.1958 30.04.1958 08.05.1958 27.05.1958 31.05.1958 15.06.1958 30.06.1958
31 28 3 28 30 8 19 4 15 15
0.91 1.38 1.38 2.17 2.40 2.72 3.48 3.50 3.58 3.62
28.21 38.64 64.90 72.00 101.88 108 .OO
01.07.1958 07.07.1958 0 1.08.1958 27.08.1958 01.09.1958 10.09.1958 0 1.10.1958 09.10.1958 01.11.1958 01.12.1958
06.07.1958 31.07.1958 26.08.1958 31.08.1958 09.09.1958 30.09.1958 08.10.1958 31.10.1958 30.11.1958 31.12.1958
6 25 26 5
9 21
8 23 30 31
2.06 1.72
mm
monthly 68.82 53.32
3.68 3.86 3.77 3.58 3.50 3.36 3.36 2.93 2.15 1.79
118.58 115.92 102.06 94.27 64.50 55.49
964.45 mm
Annual Delta Barrage
El-Nokrashy
daily
977.77 mm
Annual
2
Consumptive use,
h
01.01.1959 01.02.1959 15.02.1959 01.03.1959 07.03.1959 28.03.1959 01.04.1959 19.04.1959 01.05.1959 19.05.1959 30.05.1959 01.06.1959
31.01.1959 14.02.1959 28.02.1959 06.03.1959 27.03.1959 31.03.1959 18.04.1959 30.04.1959 18.05.1959 29.05.1959 31.05.1959 13.06.1959
31 14 14 6 21 4 18 12 18 11 2 13
1.59 1.59 2.12 2.12 2.53 2.62 2.62 5.30 3.74 6.07 4.48 4.48
49.29 51.64 76.33 110.76 143.08
11.07.1959 27.07.1959 01.08.1959 10.08.1959 23.08.1959 01.09.1959 06.09 .1959 21.09.1959 01.10.1959 06.10.1959 21.10.1959 01.11.1959
26.07.1959 31.07.1959 09.08.1959 22.08.1959 31.08.1959 05.09.1959 20.09.1959 30.09.1959 05.10.1959 20.10.1959 31.10.1959 09.11.1959
16 5 9 13 9 5 15 10 5 15 11 9
4.22 4.57 4.57 3.65 3.34 3.34 2.86 3.02 3.02 2.28. 1.53 1.53
142.35 118.59 89.80 74.83
(continued)
TABLE 6 . 8
~~
Sr. No.
Expt. S t a t .
Interval
From
&
To
Investigator E l -Nokrashy
h
14.06.1959 26.06.1959 27.06.1959 30.06.1959 01.07.1959 10.07.1959
days
13 4 10
Consumptive u s e , mm daily monthly
5.10 5.20 5.20
197.44
Interval
From
To
10.11.1959 30.11.1959 01.12.1959 24.12.1959 25.12.1959 31.12.1959
days
21 24 7
1.29 0.625 1.59
40.86 26.13
1121.10 mm
Annual Delta Barrage
E 1-Nokrashy
Consumptive u s e , mm daily monthly
h
01.01.1961 01.02.1961 07.02.1961 01.03.1961 07.03.1961 01.04.1961 22.04.1961 01.05.196 1 08.05.1961 19.05.1961 01.06.1961 13.06.1961 25.06.1961 01.07.1961 07.07.1961
Annual
h = El-Nokrashy, M . A . ,
31.01.1961 06.02.1962 28.02.1961 06.03.1961 31.03.1961 21.04.1961 30.04.1961 07 .05.1961 18.05.1961 31 .05.1961 12.06.1961 24.06.1961 30.06.1961 06.07.1961 18.07.1961
31 6 22 6 25 21 9 7 11 13 12 12 6 6 12
1.42 1.42 2.02 2.02 2.77 2.81 4.07 4.07 6.70 6.07 4.35 5.31 5.27 5.27 4.64
44.02 52.56 81.37 95.64
181.10
149.54
19.07.1961 01.08.1961 13.08.1961 25.08.196 1 01.09.1961 07.09.1961 19.09.1961 01.10.1961 21.10.1961 0 1.11.196 1 06.11.1961 26.11.1961 01.12.1961 26.12.1961
31.07.1961 12.08.1961 24.08.1961 31.08.1961 06.09.1961 18.09.1961 30.09.1961 20.10.1961 31.10.1961 05.11.1961 25.11.1961 30.11.1961 25.12.1961 31.12.1961
13 12 12 7 6 12 12 20 11 5 20 5
25 6
4.14 3.84 4.22 3.33 3.33 3.79 2.53 1.60. 1.88 1.88 1.50 0.80 0.80 1.42
141.10
120.03
95.82 52.68
43.40 28.52
1085 .80 m m
1963
P N
4
248
The c o n s u m p t i v e u s e d a t a l i s t e d i n T a b l e s 6 . 2 t h r u ' 6 . 8 , t o g e t h e r w i t h t h e Same t y p e o f d a t a f o r o t h e r c r o p s n o t i n c l u d e d i n t h i s s u r v e y , s h o u l d , no d o u b t , form t h e b a s i s o f t h e i r r i g a t i o n r e q u i r e m e n t s f o r a g r i c u l t u r e i n E g y p t . B e f o r e u n d e r t a k i n g t h i s s t e p , i t was f e l t n e c e s s a r y t o compare t h e s e d a t a w i t h t h e u s e r e q u i r e m e n t s f o r t h e same k i n d o f c r o p s r a i s e d o u t s i d e Egypt u n d e r more o r less
s i m i l a r c l i m a t i c and s o i l c o n d i t i o n s . Comparison may, however, b e e x t e n d e d t o c o n s u m p t i v e u s e r e q u i r e m e n t s f o r t h e same c r o p s i n Egypt as o b t a i n e d by o t h e r methods whenever n e e d e d . The d a t a i n T a b l e 6 . 2 h a v e b e e n u s e d f o r d e v e l o p i n g t h e consumptive u s e c u r v e s f o r c o t t o n i n Sakha, n o r t h e r n c e n t r a l p a r t o f t h e Delta, Giza, apex of t h e D e l t a , and S i d s and M a l l a w i , Middle Egypt ( S h a h i n , M . ,
and E l - S h a l ,
M.I.,
1 9 6 9 ) . T h e s e c u r v e s a r e shown i n F i g . 6 . 8 a . By n e g l e c t i n g t h e d i f f e r e n c e s i n w a t e r u s e produced m a i n l y by t h e d i f f e r e n c e s i n t e m p e r a t u r e , l e n g t h o f growing s e a s o n , d e p t h t o w a t e r t a b l e , f e r t i l i z e r s , and f r e q u e n c y and q u a n t i t y o f i r r i g a t i o n a p p l i c a t i o n s , one c a n i m m e d i a t e l y see t h a t F i g . 6 . 8 a . t o Fig. 6.8b.
i s q u i t e comparable
f o r Mesa and Tempe A r i z o n a , a s g i v e n by E r i e ( 1 9 6 3 ) , and t o
F i g . 6 . 8 ~ .f o r t h e s o u t h e r n p a r t o f B u l g a r i a ( S h a h i n , M . ,
e t a l , 1973).
O l i v i e r e s t i m a t e d t h e c o n s u m p t i v e u s e o f w a t e r f o r c o t t o n as f o l l o w s ( 1 9 6 1 ) : Growing s e a s o n Location
to
from
Station
Seasonal consumptive u s e ,
mm
Lower Egypt ( N i l e Delta and G i z a )
2 0 . 2 . -10.4
2 0 . 8 . - l o . 11
Middle Egypt ( f r o m G i z a t o Assiut)
20.2.-05.4
5.8.-20.10
Assiut
1085
Upper Egypt ( f r o m A s s i u t t o s o u t h e r n b o r d e r of Egypt)
2 0 . 2 , -05.4
5 . 8 . -15.10
Aswan
1465
Giza
7 10
T h e s e v a l u e s were o b t a i n e d from h i s f o r m u l a
cu $ = c x
LO
where C
= a v e r a g e d e p r e s s i o n o f wet-bulb
L0/L2
= cyclic (radiation/latitude) by O l i v i e r (1961)
).
i n OC f o r a p a r t i c u l a r month, and
f a c t o r f o r t h e p a r t i c u l a r month ( t a b u l a t e d
249
8 7 -
6
-
Sakha S t a t .
5 -
4 3 2 1 -
OL
Feb.
I
Apr.
Mar.
May
Jun.
Jul.
Aug.
Sep.
Jun.
Jul.
Aug.
Sep.
1
Month
, Feb.
I
I
Apr.
Mar.
May
A-
-*-
Month
9,
I
1
-X-
I
I
-v-0-
-3-
2 vy
5 4 -
3 -
"0
A
.&%@
2 1
1
I
Feb.
Apr.
Mar.
May
y +
I
I
Jun.
I
Aug.
Jul.
1957 1958 1958 1959 1959 1960
Sep
Month
0'
Feb.
I
,
I
I
Mar.
I
Apr.
I
I
I
May
I
Jun.
I
Jul.
I
I
Aug.
Sep.
Month Fig. 6.8a. Consumptive u s e o f water f o r c o t t o n a t some l o c a t i o n s i n Egypt (Shahin, M . , and E l - S h a l , M . I . , 1969)
250
h
s!
0.4
.
a!
'0
blossoms u s u a l l y matur
L
.- 0.3
.-C
$ 0.2
First
3 al
.> c
1
0.1
L1
5
+
b l ossorn
1
Seasonal
1
1
use 41.2"
g 0.0
0
Fig. 6.8b. Mean consumptive use f o r c o t t o n a t Mesa and Tempe, Arizona, 1954-1962 ( E r i e , L . J . , 1963)
80
LEGEND 1965 o 1964 3 1963 -.X 1962 _..+
70
----
.?A-
60 50 40
30 20
10
O
'
16 2 0
Apr.
& 16 20 31 lb ioo; May
Jun.
o;
Zb 31
Jul.
10 20 31 10
Aug.
o; o;
Sep.
16
2;
Oc!.
F i g . 6 . 8 ~ . Consumptive u s e for c o t t o n i n t h e southern p a r t of B u l g a r i a (Shahin, M., e t a l , 1973)
25 1
The f i g u r e o b t a i n e d by O l i v i e r f o r Lower Egypt and G i z a i s e x a c t l y l i k e t h a t g i v e n i n T a b l e 6 . 2 . The f i g u r e f o r M a l l a w i c a n b e compared t o t h e a v e r a g e of t h e consumptive use a t G i z a and A s s i u t . T h i s g i v e s a f i g u r e a l m o s t 10% l a r g e r t h a n t h e measured w a t e r u s e by c o t t o n a t M a l l a w i . The f i g u r e s o b t a i n e d from O l i v i e r ' s method f o r wheat a r e as f o l l o w s : Growing s e a s o n
Location Lower Egypt Middle Egypt Upper Egypt
From
To
25.10 10.10 5.10-30.11
15.5 20.4 10.4-10.5
Station
Seasonal consumptive u s e , mm
Giza Assiut Aswan
400 472 800
The f i g u r e s o b t a i n e d by O l i v i e r are a b o u t 8% and 10% l a r g e r t h a n t h e f i g u r e s i n T a b l e 6 . 3 f o r wheat a t G i z a and M a l l a w i r e s p e c t i v e l y . The c o n s u m p t i v e u s e f o r wheat a t D u j a i l a h e x p e r i m e n t a l s t a t i o n , I r a q , was d e t e r m i n e d by means o f t h e w a t e r - b a l a n c e method. The s e a s o n a l w a t e r u s e i s 485 mm w i t h a p r o b a b l e e r r o r o f from 10 t o 20% ( h u m a n s , J . H . ,
e t a l , 1 9 6 3 ) . The
d a i l y consumptive u s e from t h i s e x p e r i m e n t s t a t i o n i s p l o t t e d v e r s u s t h e t i m e d u r i n g g r o w t h , s o as
t o compare i t w i t h t h e c h a r a c t e r i s t i c u s e c u r v e s d e r i v e d
from t h e d a t a i n T a b l e 6 . 3 . F i g s 6 . 9 a and 6 . 9 b show t h e c h a r a c t e r i s t i c c u r v e s f o r Egypt and I r a q , r e s p e c t i v e l y . I n order t o i l l u s t r a t e the e f f e c t s of the moisture level i n the s o i l , the q u a n t i t y of f e r t i l i z e r s on t h e e v a p o t r a n s p i r a t i o n f o r wheat and t h e w a t e r u s e e f f i c i e n c y , Haise, R . ,
and V i e t s , F . , have u s e d u n p u b l i s h e d d a t a from Marvin
E . J e n s e n , A m a r i l l o e x p e r i m e n t s t a t i o n , USDA, B u s h l a n d , T e x a s , f o r t h e p e r i o d
1955-1956
( 1 9 5 7 ) . These d a t a a r e p r e s e n t e d i n T a b l e 6 . 9 . The i n t e r e s t i n g f e a t u r e
a b o u t t h e f i g u r e s i n t h i s t a b l e is t h a t e x p e r i m e n t M - 1 was r u n under i r r i g a t i o n c o n d i t i o n s v e r y s i m i l a r t o t h o s e i n Egypt b e f o r e t h e c o n s t r u c t i o n of t h e High Aswan D a m . The i r r i g a t i o n p r a c t i c e t h e n w a s t o s t o p i r r i g a t i n g t h e w i n t e r c r o p s f o r 6 weeks,
from 25 December t o 5 F e b r u a r y e a c h y e a r . T h i s p e r i o d was known as
the winter closure of canals.
I n t h e post-dam c o n d i t i o n , i . e . from 1965 and
onwards, t h e d u r a t i o n o f t h e c a n a l c l o s u r e w a s r e d u c e d t o a b o u t 3 weeks. From t h a t e x p e r i m e n t , w e see t h a t t h e a v e r a g e consumptive u s e f o r wheat under t h e t h r e e d i f f e r e n t n i t r o g e n t r e a t m e n t s i s a b o u t 500 mm f o r t h e s e a s o n . T h i s f i g u r e
i s v e r y c l o s e t o t h e one o b t a i n e d by O l i v i e r f o r A s s i u t i n t h e s o u t h e r n p a r t o f Middle E g y p t . I t a l s o a g r e e s f a i r l y w e l l w i t h t h e r e s u l t o b t a i n e d from t h e D u j a i l a h e x p e r i m e n t s t a t i o n i n I r a q . The e s t i m a t e o f t h e s e a s o n a l w a t e r u s e by t h e i n d i a n c o r n and Dura maize r a i s e d i n Lower Egypt ( f r o m 5 . 7 - 3 0 . 8
t o 15.10-
3 0 . 1 1 ) u s i n g t h e O l i v i e r method, i s 350 mm. The same method g i v e s a s e a s o n a l consumption o f 950 mm by f l o o d sorghum r a i s e d a t Aswan ( f r o m 2 0 . 8 t o 1 5 . 1 2 ) .
252
4
I
I
I
0'
I
I
I
1 1
x
. 0
E E
-0-
-x-
1957 -1958 1958 -1959
al !A
1
,
01 c
Q
5 C
0 J
I
I
I
I
1
I
I
I
_ _ _ _ _ _ ~ ~ ~
Nov
Dec
Jan
Feb
Mar
Apr
May
Month Fig. 6 . 9 a . Consumptive use o f water f o r wheat a t some l o c a t i o n s i n Egypt ( S h a h i n , M . , and E l - S h a l , M . I . , 1 9 6 9 )
.5 . 0 Q 77
I
I
I
E
4.0
measurements! o pped
al
111
3.0
al
.->
E
3
g
a
0
2.0 1.0
N o v . Dec. 1957
I
I
Jan
I
I
Feb
Mar
I
Apr.
May
1958
Fig. 6.9b. Consumptive u s e of w a t e r f o r wheat a t D u j a i l a h , I r a q (Boumans, J . H . , e t a l , 1963)
25 3
TABLE 6 . 9
Evapotranspiration (ET)
and w a t e r u s e e f f i c i e n c y (W.U.E.)
of winter
wheat f o r v a r i o u s n i t r o g e n and m o i s t u r e l e v e l s ( H a i s e , H . R . .
Viets, F.G.,
Nitrogen app 1i e d lbs/acre 0
80 120
and
1957) Moisture level*
M-1
M- 3
M-2
M-4
ET, in
W.U.E. bu/in
ET, in
W.U.E. bu/in
ET, in
W.U.E. bu/in
ET, in
W.U.E. bu/in
19.4 19.7 20.3
0.87 0.92 0.86
21.6 24.2 23.9
1.03 1.08 1.18
22.9 24.8 28.3
1.28 1.67 1.51
23.6 30.4 30.2
1.42 1.51 1.74
*M-I: NO s p r i n g a p p l i c a t i o n M-2: One 4 - i n c h a p p l i c a t i o n a t j o i n t i n g s t a g e , March 28 M-3: One 4 - i n c h a p p l i c a t i o n p r i o r t o b o o t s t a g e , A p r i l 16 and a 4 - i n c h a p p l i c a t i o n a t t h e f l o w e r i n g s t a g e , May 1 5 M-4: One 4 - i n c h a p p l i c a t i o n a t j o i n t i n g s t a g e , March, 2 8 ; one 4 - i n c h a p p l i c a t i o n a t e a r l y b o o t s t a g e , A p r i l 3 0 , and a 4 - i n c h a p p l i c a t i o n j u s t a f t e r f l o w e r i n g , May 15
The d a t a l i s t e d i n T a b l e s 6 . 4 and 6 . 5 f o r l a t e c o r n and e a r l y c o r n a r e p r e s e n t e d g r a p h i c a l l y i n F i g s . 6 . 1 0 a and 6 . 1 1 a , r e s p e c t i v e l y . F i g . 6 . 1 0 a can be compared t o F i g , 6 . 1 0 b f o r z e a maize grown i n C e n t r a l C a l i f o r n i a (FAO, 1 9 7 1 ) . The l a t t e r consumes s e a s o n a l l y 500 mm which i s a b o u t 5% l e s s t h a n t h e c o r r e s p o n d i n g f i g u r e f o r Sakha and G i z a s t a t i o n s , where t h e c l i m a t i c c o n d i t i o n s a r e a l m o s t i d e n t i c a l . Moreover, t h e c o n s u m p t i v e u s e f o r sorghum a t Mesa, A r i z o n a ( f r o m J u l y t o O c t o b e r ) a s found by H a r r i s and c o m p i l e d by Blaney (1957) i s 2 0 . 4 i n c h e s o r 520 mm f o r t h e s e a s o n . T h i s f i g u r e i s i n f u l l agreement w i t h o u r s f o r t h e s t r e t c h from Sakha t o S i d s . The r e s u l t s o b t a i n e d by H a r r o l d , L . L . ,
and
D r e i b e l b i s , F . R . , ( 1 9 5 9 ) a b o u t e v a p o t r a n s p i r a t i o n from a l y s i m e t e r 8 . 4 m 2 i n s u r f a c e a r e a n a a r C o s h o c t o n , O h i o , r a i s i n g c o r n i n t h e p e r i o d from May t o O c t o b e r a r e shown g r a p h i c a l l y i n F i g . 6 . 1 1 b . T h i s f i g u r e can b e compared t o F i g . 6 . 1 1 a showing t h e u s e f o r e a r l y c o r n a t S i d s , Middle EgyDt. The a v e r a g e e v a p o t r a n s p i r a t i o n from t h e l y s i m e t e r for t h e two y e a r s 1949 and 1953 i s 570 mm p e r s e a s o n . The c o r r e s p o n d i n g s e a s o n a l u s e a t S i d s i n 1964 was 558 m m . The cons u m p t i v e u s e f o r c o r n h a s been r e p o r t e d by B l a n e y , H . F . , and C r i d d l e , W . D . , ( 1 9 6 6 ) as 4 4 0 , 525 and 740 mm/season f o r D a v i s , C a l i f o r n i a , Manden, N . Dakota, and R e d f i e l d . S . D a k o t a .
254
$ 8
.
U
E 6
Explanation
$ 4
1
Y 2
0
1959
x
1962
+
1963
c
a E O
0'
Jul
Aug
Sep.
Oct.
Nov
Dec
I
Month
Fig. 6.10a. Consumptive u s e o f w a t e r f o r l a t e c o r n at some l o c a t i o n s i n Egypt ( S h a h i n , M . , and E l - S h a l , M.I., 1969)
0
F i g . 6.10b. (FAO, 1971)
Month
Consumptive u s e of w a t e r f o r z e a m a i z e a t C e n t r a l C a l i f o r n i a
255
Apr
V
May
Jun
Aug
Jul
Month
Fig. 6.11a. Consumptive u s e o f w a t e r f o r e a r l y c o r n a t S i d s , Egypt (Shenouda, E l - G i b a l i , Tawdros and Gamal, 1966).
e
May
Jun.
0 V
Jul. Month
Aug.
1
Sep.
Fig. 6.11b. Consumptive u s e o f w a t e r f o r c o r n from a l y s i m e t e r n e a r Coshoctcil, Ohio ( H a r r o l d , L . L . , and D r e i b e l b i s , F . R . , 1959)
The s t r a n g e s t f i g u r e s f o r water u s e by c o r n ( m a i z e ) f o r Egypt one c a n meet a r e t h o s e o b t a i n e d by Doorenbos and P r u i t t (1977). The e s t i m a t e d f i g u r e s a r e b a s e d on t h e c l i m a t o l o g i c a l d a t a o f C a i r o , which i s v e r y c l o s e t o G i z a s t a t i o n . Using t h e i r own method, t h e y came up w i t h t h e f o l l o w i n g monthly and s e a s o n a l (mid-May t o mid-September)
figures:
May
June
July
August
September
S e a s o n , mm
50 50
150 170
310 340
255 280
70 70
855 9 10
These e s t i m a t e s a r e c e r t a i n l y t o o h i g h and need t o b e r e d u c e d by a b o u t 35%. F u r t h e r m o r e , t h e y c o n t r a d i c t t h e r a n g e o f v a l u e s g i v e n i n t h e same r e f e r e n c e . The s e a s o n a l consumptive u s e r a n g e s from 400 t o 750 and f o r sorghum from 300 t o 650 mm.
The a u t h o r does n o t i n t e n d t o condemn t h e method Doorenbos and P r u i t t
have d e v e l o p e d f o r e s t i m a t i n g t h e e v a p o t r a n s p i r a t i o n , b u t t h e c r o p c o e f f i c i e n t s ,
256 a t l e a s t f o r some o f t h e c r o p s , undoubtedly need d r a s t i c c h a n g e s . The consumptive u s e c u r v e o f berseem as shown i n F i g . 6 . 1 2 a , h a s an u n d u l a t i n g s h a p e which c o n s i s t s of a number of c o n n e c t e d c u r v e s , e a c h h a v i n g a b a s e w i d t h e q u a l t o t h e t i m e i n t e r v a l between two c o n s e c u t i v e c u t t i n g s . The number o f c u t t i n g s d u r i n g t h e growing s e a s o n i s u s u a l l y t h r e e t o f o u r , a f t e r which t h e l a n d r a i s i n g berseem i s l e f t f o r s e e d d e v e l o p i n g . The marked d e c l i n e i n t h e
*
w a t e r use by t h i s c r o p d u r i n g J a n u a r y and F e b r u a r y i s c a u s e d , i n a d d i t i o n t o t h e low t e m p e r a t u r e and t h e f i r s t c u t t i n g of t h e c r o p , by t h e w i n t e r c l o s u r e a l r e a d y mentioned i n c o n n e c t i o n w i t h t h e consumptive u s e f o r w h e a t . As berseem c u l t i v a t i o n i s c o n f i n e d t o a r a t h e r l i m i t e d number o f c o u n t r i e s , one c a n h a r d l y f i n d any i n f o r m a t i o n a b o u t i t s u s e o f water i n t h e l i t e r a t u r e o f a g r i c u l t u r a l h y d r o l o g y . The r e s u l t s o b t a i n e d from e x p e r i m e n t s on berseem i r r i g a t i o n u s i n g s a l i n e w a t e r i n T u n i s i a (Combremont,
R.,
1972) a r e p r e s e n t e d i n F i g . 6 . 1 2 b . T h e r e , t h e
growing s e a s o n o f berseem is a b o u t 30 d a y s s h o r t e r t h a n t h a t i n E g y p t . The c r o p s e a s o n a l u s e o f water i s 530 nun i n T u n i s i a and 570 nun i n t h e c e n t r a l p a r t o f Middle Egypt.
7J -2.53
E E . l
5 0
I
Oct.
Nov
Dec
Jon
Feb
Mar
May
Apr
Jun
Month Fig. 6.12a. Consumptive u s e o f w a t e r f o r berseem a t two l o c a t i o n s i n Middle Egypt ( S h a h i n , M . , and E l - S h a l , M . I . , 1969)
.
E E 6
c
Oct
V
rig. 6 . 1 2 b .
1972)
Nov
Dec
Jon
Feb
Mar
I
Apr
,
May
I
Jun.
Month Consumptive u s e of water f o r berseem a t T u n i s i a (Cambremont.
R.,
25 7 The d a t a l i s t e d i n T a b l e 6 . 8 are p r e s e n t e d g r a p h i c a l l y , a s shown i n F i g . 6 . 1 3 a . Using t h e c l i m a t o l o g i c a l d a t a o f C a i r o and t a k i n g t h e c r o p c o e f f i c i e n t by Doorenbos and P r u i t t f o r c i t r u s t r e e s , which a r e c l e a n c u l t i v a t e d and p r o v i d e a l m o s t 50% ground c o v e r , one c a n o b t a i n c u r v e (1) shown i n F i g . 6 . 1 3 b . The a n n u a l consumption o f t h e s p e c i f i e d o r a n g e t r e e s u s i n g t h i s method amounts t o 1215 nun. T h i s f i g u r e i s n e a r l y 17% l a r g e r t h a n t h e a v e r a g e w a t e r u s e by t h e same c r o p r a i s e d i n t h e D e l t a b a r r a g e a r e a and G i z a s t a t i o n . The consumptive use o f w a t e r f o r o r a n g e s r a i s e d i n t h e S a l t R i v e r V a l l e y i n USA i s p r e s e n t e d by c u r v e ( 2 ) i n F i g . 6 . 1 3 b . T h i s c u r v e compares f a i r l y w e l l w i t h t h e two c u r v e s shown i n F i g . 6 . 1 3 a .
I
. GIZA
2
STAT
.a;b--o
*-0-
O0
+
I
+ '
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1
Uo-+.*
+-,p4t-
5 3 -
+ . o
C
3
O----3
m
0
9
a,
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I
0
V
9
I
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e .r( a,
m k
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z
r o rml m a ,
3s
M
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n
0
0
a,
I
k
a
.r(
c,
> r(
rl
rl m
0
w
.r(
w
0
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c
0
w
ri
w
c,
w
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Ld .r( r l k
c u
m u
hrl
l a ,
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0
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9
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II
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'
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rlrl
a,o m c w a, mm k
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3
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m . - 3 x
c mr o
P a,*
0
Z
f
m 3
rl
2
a,
w 3
c,
h a,
B
c,
a,
9 w a,
E
m
c
.r(
P
a
a a,
rl 0
c
.r(
c, m
c
a,
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U
a,
w w
9
U
c, m
e
m
c,
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9 w h
0
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rl
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26 3
TABLE 6 . 1 1 a
Consumptive u s e c o e f f i c i e n t s f o r some c r o p s i n Egypt t o b e u s e d w i t h t h e B l a n e y - C r i d d l e
Consumptive u s e c o e f f i c i e n t f o r
Crop Jan. Cotton Wheat E a r l y Corn L a t e Corn Berseem C i t r u s Orchards Fenugreek & L u p i n Chickpea & L e n t i l S u g a r Cane F i e l d Beans P o t a t o e s (summer) Potatoes ( f a l l ) Snap Beans Cow P e a s Squash Cucumber
formula
Feb.
Mar.
-
-
.49
.59
.26 .66
-
.31 .35 .56 .42 .29 .54
-
.86
-
-
-
-
-
.42 .51 .57 .46 .72 .73 .63
.72 .55 .26 .18 .87 .67 .77
-
-
Apr.
.39 .49 .36
-
.82 .57
-
.84 .48 1.01
-
.30
May
June
July
.66 .31 .71
1.07
1.03
-
.64 .60
-
-
.90
-
.69
-
.35
-
1.06
-
.25 .64
-
-
1.00
-
.80
-
Aug.
.56
Sep.
.51
-
Oct.
-
-
-
.87 .51
-
.60 .91
-
1.13
.70
.64
.59
.80 .31 .56
1.32
1.00
-
1.36
-
.99 .44 .30
-
1.37
-
-
-
.60 1.02 .63
-
-
-
-
-
-
-
.37
.19 .80
-
1.01 .87
.79
Nov.
Dec.
.46
.37 .42 .38 .34 .79
.47
-
.62 .32 .59 .50 .41
.49 1.14
.98 .78
-
o r year
0.71 0.51 0.79 0.88 0.56 0.54 0.46 0.37 0.91 0.58 0.77 0.59 0.77 0.64 0.87 0.69
TABLE 6.11b
Consumptive use coefficients for some crops in EgyDt to be used with Thornthwaits's formula
Consumptive use coefficient for Crop Jan. Cotton Wheat Early Corn Late Corn Berseem Citrus Orchards Fenugreek & Lupin Chickpea & Lentil Sugar Cane Field Beans Potatoes (summer) Potatoes (fall) Snap Beans Cow Peas Squash Cucumber
2.15
1.49 1.47 2.86 2.13 1.22 2.44
-
3.77
Feb.
2.78
1.65 2.46 2.32 1.88 3.44 3.44 2.97
-
Mar.
Apr.
May
June
July
Aug.
Sep.
0.75 1.89
0.72 .89 .37
0.98 .42 .96
1.33
1.23
0.66
0.73
-
2.18 1.51 0.40 0.26 2.39 1.84 2.06
-
1.59 1.14
-
1.68 .96 2.02
0.55
-
.98 .88
-
1.32
-
1.01
0.52
-
-
1.28
-
.50 .82
-
1.28
-
-
-
0.99
-
-
-
-
Oct.
-
1.01 .59
.56 1.00
-
1.54
.78
.76
1.25
1.40 .28 1.00
1.63
2.80
1.79
-
-
1.52
-
-
1.18 .52 .36
-
0.71 1.21 0.75
-
0.78
-
2.14 1.85
-
0.34 1.43
-
1.41
Nov.
Dec.
.97
1.67
.82 .86 .44 .48 1.62
1.00 2.33
-
-
-
2.43 1.14 1.43 1.21 1.46
-
3.49 2.78
-
Season o r year 0.97 1.54 0.93 1.08 1.29 0.94 1.96 1.56 1.85 2.08 1.51 2.01 1.82 0.83 1.36 1.16
266
-
b) R a d i a t i o n methods
The r a d i a t i o n methods d e v e l o p e d f o r e s t i m a t i n g p o t e n t i a l
e v a p o t r a n s p i r a t i o n a r e e i t h e r b a s e d on t h e h e a t e n e r g y b u d g e t , t o g e t h e r w i t h some e m p i r i c a l a p p r o x i m a t i o n s t o u t i l i z e t h e a v a i l a b l e c l i m a t i c d a t a , or t h e y a r e c o m p l e t e l y e m p i r i c a l . I n t h e f i r s t g r o u p i s t h e f o r m u l a of Penman, which w e a l r e a d y p r e s e n t e d i n C h a p t e r 5 as a means f o r e s t i m a t i n g e v a p o r a t i o n from a f r e e water s u r f a c e , E
.
The same f o r m u l a when u s e d f o r e s t i m a t i n g t h e p o t e n t i a l eva-
potranspiration reads:
ET
= a . E
P
(6.4)
0
where a i s a c o e f f i c i e n t v e r y i n g w i t h t h e month. A l a r g e number of e m p i r i c a l f o r m u l a s , a l l b a s e d on one form
G-
a n o t h e r of
r a d i a t i o n , have been d e v e l o p e d f o r e s t i m a t i n g p o t e n t i a l e v a p o t r a n s p i r a t i o n . The g e n e r a l form of t h e e q u a t i o n embracing t h e s e f o r m u l a s can b e w r i t t e n a s
ET
P
= K .
C .
(R+a)
I n 1961, T u r c , L .
ET
P
= 0 . 4 0 (-
t
(6.5) (1961), presented h i s formula:
15)(R + 50)
+
mm/month
(6.5.1)
where
t = mean monthly t e m p e r a t u r e i n d e g r e e s c e n t i g r a d e , and R = mean incoming r a d i a t i o n i n gm c a l s / c m 2 . d a y . R c a n b e computed from t h e t h e o r e t i c a l r a d i a t i o n , R A , r e a c h i n g t h e e a r t h ' s
+
atmosphere u s i n g t h e r e l a t i o n R = R ( 0 . 1 8 A t a g e of p o s s i b l e s u n s h i n e .
0.0062 S ) , where S i s t h e p e r c e n -
The f o r m u l a d e v e l o p e d by J e n s e n and H a i s e (1963) i s :
ET
P
= (0.014 T
-
0.37) R
so
(0.35
+
0 . 6 1 S)
inches/month
(6.5.2)
where T
= mean monthly
Rso
= s o l a r r a d i a t i o n on c l o u d l e s s d a y s ,
temperature i n degrees F a h r e n h e i t ,
S
= possible sunshine expressed decimally.
Christiansen, J . E . ,
and
(1969) c o n s i d e r e d a = 0 i n e q . 6 . 5 i n o r d e r t o e s t i m a t e
e v a p o r a t i o n or e v a p o t r a n s p i r a t i o n .
H e and h i s co-workers
h a v e d e v e l o p e d an
e x t e n s i v e number of e q u a t i o n s , i n which k i s k e p t a s a d i m e n s i o n l e s s c o n s t a n t and t h e p r o d u c t kc r e p r e s e n t s t h e r a t i o o f t h e e n e r g y u t i l i z e d i n t h e e v a p o r a -
267
t i o n p r o c e s s t o t h e energy a v a i l a b l e a t t h e o u t e r s u r f a c e of t h e atmosphere. The c o e f f i c i e n t C i s t h e p r o d u c t o f a l a r g e number o f s u b - c o e f f i c i e n t s ,
each
r e l a t e d t o a c l i m a t i c or o t h e r p a r a m e t e r t h a t i s l i k e l y t o a f f e c t e v a p o r a t i o n o r e v a p o t r a n s p i r a t i o n . The f o r m u l a d e v e l o p e d by C h r i s t i a n s e n w h i c h r e l a t e s ET
t o e x t r a t e r r e s t r i a l r a d i a t i o n a s a base is
ETP = 0 . 3 2 4 R A . CTT
.
CWT
.
CHT
.
CST
.
CEl
inches/month
P
(6.5.3)
where RA = e x t r a t e r r e s t r i a l r a d i a t i o n i n e q u i v a l e n t e v a p o r a t i o n i n i n c h e s / m o n t h , CTT = 0 . 1 7 4 + 0 . 4 2 8 ( T / T ) + 0 . 3 9 8 ( T / T o ) 2
T
= t h e mean a i r t e m p e r a t u r e i n d e g r e e s F a h r e n h e i t
CWT = 0 . 6 7 2
+ 0.406
= \ k =
s T \% .
- 0 . 0 7 8 (W/W
(W/Wo) ,\\
s \ \
U
0
I
aJ
>
0
L
z
b
0
c r
.c
I:
Y
i
Y
All d i s c h a r g e s g i v e n h e r e o r e m e a n s for t h e p e r i o d 1929 - 1933, they a r e in mlrd m3/yr.
i
H u r s t a l s o gave an e s t i m a t e o f t h e g r o s s volume of flow from P i b o r Port to t h e mouth on t h e S o b a t , a d i s t a n c e of 312 km, a t 3 . 6 mlrd m 3 / y r
and t h e n e t a t
3.1, a s a l r e a d y m e n t i o n e d . T h i s means t h a t t h e loss i s a b o u t 0 . 5 mlrd m 3 / y r ,
or
a b o u t 14% o f t h e g r o s s f l o w .
8.7.2.1.4
The S o b a t below t h e Pibor-Baro
Below t h e Pibor-Baro
Junction
j u n c t i o n , t h e main stream i s known a s t h e S o b a t . I t
f l o w s a d i s t a n c e o f a b o u t 350 km i n a n o r t h - w e s t e r l y
direction before it joins
373
t h e White N i l e . The S o b a t below t h e j u n c t i o n h a s a s u b - b a s i n of a b o u t 36 800 km2 i n a r e a , which b r i n g s t h e t o t a l d r a i n a g e b a s i n area t o 187 200 km2. The S o b a t h a s a n a v e r a g e s l o p e of 3 cm/km i n t h e low-flow i n t h e high-flow
s e a s o n and 4 cm/km
s e a s o n . The main t r i b u t a r i e s a r e t h e Khor F u l l u s , Nyading,
Twalor a n d Wakau. They j o i n t h e main r i v e r a t d i s t a n c e s o f 1 6 , 239, 290 and 307 km from t h e mouth, r e s p e c t i v e l y . I n a normal y e a r t h e S o b a t a t a head c a r r i e s 1 2 . 4 mlrd m3/yr.
Of t h i s amount, 3 . 1 m l r d m3 are s u p p l i e d by t h e P i b o r and t h e
rest by t h e Bar0 ( s e e s e c t i o n 8 . 7 . 2 . 1 . 3 ) .
The a v e r a g e r a i n f a l l on t h e S o b a t sub-
b a s i n c a n b e t a k e n a s 780 mm/yr. T h i s f i g u r e is t h e a v e r a g e of t h e a n n u a l r a i n d e p t h s a t Kodok, s t a t i o n 1 1 7 , M a l a k a l , s t a t i o n 118, Abwong, s t a t i o n 1 2 0 , and
Nasser, s t a t i o n 123. F o r t h e s e s t a t i o n s i n t h e i r o r d e r , t h e mean r a i n f a l l f o r t h e p e r i o d 1938-1967 w a s 738, 8 1 9 , 7 6 3 and 894 mm/yr. c o e f f i c i e n t a t 4%, t h e run-off
Assuming t h e a n n u a l r u n - o f f
s h o u l d t h e n b e 0 . 0 4 x 0 . 7 8 x 36.8 o r 1 . 1 5 mlrd m3
p e r y e a r . T h i s f i g u r e i s s l i g h t l y h i g h e r t h a n t h e 1.08 m l r d m3/yr r e p o r t e d i n Vol. V I I I of t h e N i l e B a s i n a s a mean f o r t h e p e r i o d 1934-1947 ( H u r s t , H . E . , 1 9 5 0 ) . The sum o f t h e f l o w a t t h e S o b a t h e a d and t h e r u n - o f f
i s 1 3 . 5 5 mlrd m3/yr.
Reducing t h i s amount by a b o u t 5% f o r t h e n e t conveyance l o s s , t h e amount t h a t f i n a l l y r e a c h e s t h e mouth on t h e White N i l e i s 1 2 . 9 mlrd m3/yr. The d i s c h a r g e s o f t h e S o b a t a t H i l l e t D o l e i b , 9 km above t h e mouth, have been measured s i n c e 1911. The d i s c h a r g e - g a u g e measurements f o r t h e r a t i n g c u r v e can b e s e e n from F i g . 1 9 , Appendix E. The a v e r a g e hydrograph a t H i l l e t D o l e i b c o r r e s ponding t o an a n n u a l volume o f 1 2 . 9 mlrd m3/yr i s shown i n F i g . 8 . 3 4 .
5
F
; m
20
0"
J
Fig. 8.34.
F
M
A
M
'
"
J J Month
A
I
S
"
O
'
N
D
'
The d i s c h a r g e hydrograph of t h e S o b a t a t H i l l e t D o l e i b , n e a r mouth
The r e l a t i o n between t h e d i s c h a r g e s measured a t t h e S o b a t mouth, i n f a c t a t
H i l l e t D o l e i b , Y, and t h e d i f f e r e n c e between t h e d i s c h a r g e a t Malakal less t h e d i s c h a r g e s of t h e Bahr e l G h a z a l , J e b e l and Zeraf as a t M a l a k a l , X, h a s been
374
examined f o r t h e p u r p o s e o f c h e c k i n g t h e c o n s i s t e n c y o f t h e r e s u l t s . The l i n e a r r e g r e s s i o n r e l a t i o n found i s
Y =
-
0.4255
-+ 0 . 9 8 2 7 X
(8.6)
w i t h a c o r r e l a t i o n c o e f f i c i e n t o f 0 . 9 6 1 9 . S i n c e t h e mean f l o w a t Malakal f o r t h e p e r i o d 1905-1966 was 2 8 . 8 2 mlrd m3/yr and t h e sum o f t h e J e b e l and Z e r a f w a s 14.74 mlrd m3/yr,
t h e mean of Y must b e 1408 m l r d m3/yr.
same p e r i o d , as o b t a i n e d from e q . 8 . 6 , h a s t o b e 14.76
The mean o f X o v e r t h e 1 . 8 4 o r i n t h e r a n g e of
between 1 6 . 6 0 a n d 1 2 . 9 2 mlrd m3/yr ( c o n f i d e n c e l e v e l = 9 5 % ) . T h i s r e s u l t shows t h a t t h e f i g u r e 2 9 . 2 m l r d m3/yr,
though on t h e low s i d e , y e t n e e d s n o t b e
rejected.
8.7.2.2
The d i s c h a r g e s a t Malakal
The monthly and a n n u a l d i s c h a r g e s o f t h e White N i l e a t Malakal f o r t h e p e r i o d 1912-73 are g i v e n i n T a b l e 7 , Appendix D ( C a i r o U n i v e r s i t y I n s t i t u t e of Technology,
-
Massachusetts
1 9 7 7 ) . The g a u g e - d i s c h a r g e measurements u s e d f o r p r e -
p a r i n g t h e r a t i n g c u r v e o f t h e White N i l e a t Malakal are shown i n F i g . 2 0 , Appendix E . The a n n u a l f l o w volume a t Malakal i n t h e p e r i o d i n v e s t i g a t e d showed two maxima; t h e e a r l i e r one took p l a c e i n 1918 and w a s s h a r p , and t h e s e c o n d i n 1964 and w a s b r o a d e r . The two f l o w volumes i n t h e i r o r d e r o f o c c u r r e n c e were 44.35 and 4 8 . 6 4 mlrd m 3 / y r
respectively.
Each o f them i s a l m o s t 80% of t h e r e c o r d e d
maximum a t Mongalla on t h e Bahr e l - d e b e l
(see section 8.5.1.2).
The l o w e s t mini-
mum o b s e r v e d a t Malakal w a s 23.32 m l r d m3/yr a n d i t t o o k p l a c e i n 1940. T h i s i s s l i g h t l y less t h a n t h e minima which w e r e o b s e r v e d i n 1 9 1 3 , 1922 and 1950 and t h e i r v a l u e s w e r e 2 3 . 8 3 , 23.59 and 2 3 . 7 5 mlrd m3/yr.
The a v e r a g e of t h e s e f o u r
minima i s s l i g h t l y more t h a n 1 . 5 t i m e s t h e minimum a t Mongalla on t h e Bahr e l J e b e l . The mean f l o w volume a t M a l a k a l , b e i n g 2 9 . 4 4 mlrd m3/yr,
is a b o u t 6% less
t h a n t h e mean f l o w volume a t N o n g a l l a f o r t h e same p e r i o d 1912-1973.
These f i g -
u r e s show t h e i n t e r a c t i o n between t h e c o n t r i b u t i o n s o f t h e B a h r e l - G h a z a l ,
Jebel
and Zeraf on one hand and t h e c o n t r i b u t i o n s of t h e B a r o , P i b o r and t h e S o b a t on r t h e o t h e r . Of s p e c i a l i n t e r e s t is t h a t t h e mean a n n u a l f l o w a t Malakal is s l i g h t l y less t h a n t h e mean a n n u a l f l o w a t Mongalla. T h i s r e s u l t means t h a t t h e g a i n from t h e 750 000 km2, which c o m p r i s e t h e s u b - b a s i n s o f t h e G h a z a l , J e b e l , Z e r a f , B a r o , P i b o r and t h e S o b a t , a l l downstream o f Mongalla, a r e a few p e r c e n t
less t h a n t h e l o s s which t a k e s p l a c e i n t h e swamps and t h e l o w - l y i n g a r e a s . An i m p o r t a n t f e a t u r e i n t h e a n n u a l f l o w series a t Malakal is t h a t t h e r a t i o o f t h e maximum t o t h e minimum is a b o u t 2 : l ( s e e F i g . 8 . 3 5 ) , whereas t h i s r a t i o , e x c e p t f o r Mongalla, i s a b o u t 4 : l ( s e e F i g . 8 . 2 1 ) .
375
Fig. 8.35. G r a p h i c a l p l o t o f t h e a n n u a l d i s c h a r g e s e r i e s of Malakal i n t h e p e r i o d 1912-1973
The s t a t i s t i c a l p r o p e r t i e s of t h e 12-monthly series and o f t h e a n n u a l s e r i e s have been examined and t h e r e s u l t s p r e s e n t e d i n T a b l e 8 . 1 2 . These r e s u l t s show t h a t t h e monthly d i s c h a r g e s e r i e s , e x c e p t t h a t o f March, c o n t a i n a r a t h e r s t r o n g s e r i a l c o r r e l a t i o n between t h e i r e l e m e n t s ; t h e s e r i a l c o r r e l a t i o n i n t h e a n n u a l
series i s even s t r o n g e r . The c o r r e l a t i o n c o e f f i c i e n t which i s s i g n i f i c a n t l y d i f f e r e n t from t h e z e r o , a t a c o n f i d e n c e l e v e l of 95%, i s l i m i t e d h e r e t o l a g 2 o r 3. I t h a s been f o u n d , however, t h a t one c a n remove a good d e a l of t h e dependence from any of t h e s e r i e s by f i t t i n g a l i n e a r a u t o r e g r e s s i v e model t o i t . The r e s i d u a l s l e f t a f t e r f i t t i n g t h e model have shown t h e y a r e u n c o r r e l a t e d a t t h e same l e v e l o f s i g n i f i c a n c e . T h i s c a n b e s e e n from T a b l e 8.13. The o r d e r o f t h e model w e l l f i t t i n g t h e monthly series i s t h e f i r s t + , whereas t h e s e c o n d o r d e r
++
is a b e t t e r f i t t o t h e a n n u a l s e r i e s . I n t h i s r e s p e c t t h e d i s c h a r g e series of Malakal r e s e m b l e t h o s e o f Mongalla. The p r o b a b i l i t y d i s t r i b u t i o n f u n c t i o n of good f i t t o t h e monthly and a n n u a l
series of Malakal i s t h e P e a r s o n Type I11 o n l y . F i g . 8 . 3 6 . shows t h e f i t of t h i s d i s t r i b u t i o n f u n c t i o n t o t h e a n n u a l d i s c h a r g e s e r i e s . F o r t h i s s e r i e s and f o r t h e 12-monthly
o n e s , t h e 100 a n d t h e 200- y e a r d i s c h a r g e s computed by t h i s func-
tion are: Discharge, lo,j ,3 Jan. Feb. Mar. Apr. May 100-yr 200-yr
J u n e J u l y Aug. Sep. O c t . Nov. D e c .
Year
5563 4465 4914 2940 2723 2949 3514 4125 4763 5 5 3 3 5574 5699 49060 6094 5014 5652 3221 2910 3094 3684 4268 5082 2928 6009 6125 52660
+ Model p a r a m e t e r i s t h e f i r s t s e r i a l c o r r e l a t i o n c o e f f i c i e n t r l ( s e e T a b l e 8.12)
++ Model p a r a m e t e r s awe a
1
= 1 . 0 5 1 3 and a2 = -0.3499
The b a s i c s t a t i s t i c a l d e s c r i p t o r s and s e r i a l c o r r e l a t i o n c o e f f i c i e n t s of t h e monthly and annual d i s c h a r g e s of
TABLE 8 . 1 2
t h e White N i l e a t Malakal f o r t h e p e r i o d 1912-1973
Month of t h e y e a r
I tern
B a s i c s t a t i s t i ca 1 descriptor
i,
106 m 3
s, 106 m 3 C
CV
Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sep.
Oct.
Nov.
Dec.
Year
2446 9 39 0.3839 1.4868 5.7$71
1735 1704 732 786 0.4219 0.4614 2.2417 3.0920 8.6520 13.7243
1484 404 0.2723 2.0042 7.9193
1650 350 0.2120 1.1324 4.3800
2015 309 0.1525 1.0071 3.7999
2505 312 0.1246 1.3408 4.4741
2871 370 0.1287 1.5958 5.9292
3080 470 0.1525 1.9511 8.9434
3402 609 0.1789 1.7958 8.3041
3310 628 0.1898 2.0020 9.5429
3158 789 0.2500 1.3147 6.9154
29438 5671 0.1926 1.7325 5.9970
0.4934 0.2832 0.2296 v.0390 0.0285 0.1015 0.0720 0.1442 0.1305 0.0046 0.1468 0.0174 0.0642 0.0230 0.2570
0.6058 0.3385 0,2464 0.0321 0.0348 0.1166 0.1201 0.0989 0 .0794 0.0342 0.0861 0.0861 0.0708 0.0310 0.0980
0.2808 0.0966 0.0540 0 .0064 0.0058 0.0214 0.0459 0.0202 q.0186 0.0593 0.1119 9.1119 0.0856 6.0500 6.0851
0.6661 0.3905 0.2622 0.1548 0.2050 0.2133 0.2212 0.2071 0.1226 0.0445 .0600 0.0600 0.0987 0.0259 0.0725
0.6498 0.4005 0.2977 0.1755 0.1894 0.2395 0.2442 0.2261 0.1808 0 . 1317 0.0234 0.1392 0.1347 0.1629 0.0257
0.6193 0.3590 0.2877 0.0453 0.0215 0.1474 0.1696 0.1309 0.1534 0.1054 c.0536 0.1214 6.1616 6.1879 6.0702
0.7472 0.4872 0.3361 0.1710 0.1435 0.2142 0.2016 0.1338 0.0930 0.0398 4.0782 0.1368 0.1681 6.1571 6.0788
0.7649 0.5115 0.3029 0.2172 0.1778 0.1595 0.1470 0.1023 0.1014 0.0402 .0829 0.1822 0.2101 .1128 0.0145
0.6785 0.4623 0.2718 0.1660 0.1009 0.0773 0.0718 0.0473 6.0157 0.0440 .lo56 0.1655 0.1689 6.0399 0.0490
0.6987 0.5010 0.3794 0.2955 0.1624 0.0563 0.0237 0.0644 .0803 0.0763 1283 0.1330 0.1076 0.0233 0.0604
0.6708 0.4449 0.2809 0.0901 0.0620 0.0399 0.0271 0.0320 q.0441 0.0660 4.1279 1156 0.1237 6.0009 0.0721
0.5145 0.2916 0.2321 0.0450 0.0592 0.0986 0.0422 0 .OQ74 0.0378 0.0871 1721 0.0200 0.0494 0.0424 0.1849
0.8010 0.5162 0.3218 0.1675 0.1035 0.1257 0.1347 0.0997 0 .0268 0.0397 G.0957 0.1482 0.1324 6.0146 0.0826
Serial correlation coefficient rl r2
r3 r4 r5 r6 r7 r8 r9 r10 rll r12 '13 r14
r.
-
p
p
p
p
p q.
0.
G.
W
9
TABLE 8 . 1 3
S e r i a l c o r r e l a t i o n c o e f f i c i e n t s of t h e r e s i d u a l s l e f t from f i t t i n g a u t o r e g r e s s i v e models t o t h e d i s c h a r g e
series a t Malakal
Month 8 Year January February March April May June July August qeptember October November December Year
Serial correlation coefficients
-
2
rl
-
.018 -.025 .090 -.020 .011 .bo6
r3
,053 ,065 .009 .096 -.013 ,008 -.030 0 16 .016
-. l o 6 -.ill -.124 -.134
.006
,189 .153 .026 .015 .062 .238 -.071 -.140 .053 .009 .076 .162
.019
.152
.152
-.
.061 -.039 0 18 .004
.
-
r4
rs
-.196 -.0 38 -.192 -.0 35 .010 .009 -.151 .lo3 -. l o 1 -.027 -.217 -.059 -.189 .110 :005 -.ooo .024 -.033 -. l o 9 .011 -.149 .0 19 .096 .027 .032
.043
r6 -128 .lo8 0 39 .093 .143 .225 -.192 -.029 -0 15 -.080 .013 .079
.
.
.086
r7
r8
.076 .018
.lo9 .035 0 10 .099 .073 .037
-
.028
.022 .053 .041
.088
.052 .037 .064 .051 .008 .022
rg .026
r 10
-.029 -.075
. -.051
-.050 -.018
-
.021 .116
,034 .002 .040 .091 -.071 .025 .034 .052 -.065 .054
.060
.046
.lo6 -.052 -.097 .054 .068 .079
.010 .024 .081
-.116 -.0 39 -.0 39 .002
-
-
-
11
-.210 .096 -.070
.090 .005 -0 35
-.
.om
-.066 -.023 -.066 -.067 .213 .064
'12 .055 .001 -.063 .011 .152 .069 .082 .042 -.077 .054
c
-
.ooo
-.112 .121
13
14
-.076
-.071
-.ill -.0 17 -.048 -.063 -.225 -.149 -.120 -.131 .lo2 -
-.026
'15 .182
-.053 -.070 -.134 .041 .008 .048
.210
-
.133 -.139 .lo7 .001 .035 .096 -.049 .010 .031
.150 .124 .037 .028
.lo8 .lo3 .088
,056 .154
.239
378
55
50
45
-E 40 01
2 -35
Q
m 0
L
U
.%'
D
-0 3
2
30
25
Q
20 15 1 0.01
I
1
I
1
1
0.1 0.2 0.5 1
I
2
5
1
I
1
1
1
1
1
I
I
1
10 20 30 4 0 50 60 70 80 90 95 Non exceedance probability, '10
1
8
98 99
99.9
Fig. 8.36. F i t o f t h e P e a r s o n Type I 1 1 f u n c t i o n t o t h e d i s t r i b u t i o n of t h e annual d i s c h a r g e s a t Malakal i n t h e p e r i o d 1912-1973 8.7.3
The White N i l e from Malakal t o j u s t above t h e j u n c t i o n w i t h t h e Blue Nile -
B e l o w t h e c o n f l u e n c e of t h e S o b a t , t h e White N i l e f l o w s a d i s t a n c e o f , s a y ,
840 km, w i t h o u t b e i n g j o i n e d by any i m p o r t a n t t r i b u t a r y e x c e p t by t h e Blue N i l e
a t t h e downstream end o f t h i s r e a c h . The d i s c h a r g e s have f o r some t i m e been measured a t Mogren upstream o f t h e j u n c t i o n of t h e Blue N i l e and n e a r Khartoum. The gauge-discharge measurements f o r t h e p e r i o d 1912-27 a r e shown i n F i g . 21, Appendix E. The gauging s i t e a t Mogren w a s d e s c r i b e d as b e i n g good enough a t low d
s t a g e , b u t n o t good i n a f l o o d . T h i s might e x p l a i n t h e t e r r i b l e s c a t t e r of t h e p o i n t s i n t h e s t a g e - d i s c h a r g e diagram. F o r t h e f i r s t 358 km, t h e r i v e r h a s a waterway o f from 300 t o 500 metres i n w i d t h w i t h numerous i s l a n d s . The mean w i d t h may b e t a k e n a s 425 metres i n low supply when t h e r i v e r i s w i t h i n t h e banks. F o r t h e n e x t 490 km t o t h e t a i l a t Khartoum, t h e mean w i d t h o f t h e water s u r f a c e i s 850 metres i n low s u p p l y . The g e n e r a l d e p t h o f t h e w a t e r a t l o w s t a g e i s 4 metres, and 7 metres i n f l o o d . On e i t h e r s i d e o f t h e waterway o f t h e upper r e a c h i s a low r i d g e swamped i n f l o o d
379
and beyond t h a t i s a deep d e p r e s s i o n , deep i n t h e c e n t r e and r i s i n g t o t h e r i d g e on one s i d e and t o t h e h i g h l a n d and f o r e s t on t h e o t h e r . Each d e p r e s s i o n may be
3 km i n w i d t h where i t i s wide and a few hundred metres where i t i s narrow, so t h a t t h e f l o o d e d v a l l e y may have a w i d t h o f 6 km i n some p l a c e s . The r i d g e s are broken by openings through which t h e water p a s s e s i n and o u t o f t h e marshy d e p r e s s i o n s . These d e p r e s s i o n s are o c v e r e d by a d e n s e growth o f needs. A t B e g e l e i n t h e s i d e d e p r e s s i o n s c o n t r a c t and t h e f o r e s t s approach t h e r i v e r . F i f t y k i l o m e t r e s f u r t h e r t o t h e n o r t h , t h e e x t e n t o f t h e swamps decreases and t h e r i v e r w i d t h v a r i e s from 700 t o 900 metres. Some 30 k m f u r t h e r t o t h e n o r t h t h e sudd g r a s s e s d i s a p p e a r , and though t h e r e i s f l o o d i n g , t h e r e are no swamps. The summer c h a n n e l i n t h e 490 km upstream o f Khartoum i s 850 metres i n w i d t h and t h e f l o o d channel i s 4 . 3 km. The summer d e p t h o f water i s about 4 m. The r a i n f a l l i n t h e 850 km r e a c h o f t h e White N i l e d i e s o u t almost l i n e a r l y w i t h d i s t a n c e from Malakal t o Khartoum. F o r t h e 30-year p e r i o d 1938-1967,
t h e mean r a i n f a l l over
the different stations was: Station No.
Location
Rainfall mm/y r
Station No,
Location
Rainfall m/yr
118
Malakal Kodok Melut Renk Gebelein
8 19
100 91 85 81 79
Kosti/Rebeck Dueim Geteina Jebel A w l i a Khar t oum
403 315 20 2 199 156
117 116 111 106
738 644 541 431
F o r t h e same r i v e r r e a c h , t h e open water e v a p o r a t i o n h a s been e s t i m a t e d a t about 1900 mm/yr a t Malakal t o about 2920 mm/yr a t Khartoum (see C h a p t e r 5 : Evap o r a t i o n ) . From t h e s e f i g u r e s i t i s c l e a r t h a t t h e l o s s p e r y e a r v a r i e s from a b o u t 1080 mm a t Malakal t o 2760 mm a t Khartoum; i f w e take 1 . 9 m as an a v e r a g e l o s s f o r t h e whole r e a c h and t h e a v e r a g e width of t h e water s u r f a c e a t , s a y , lkm,
i t i s t h e r e f o r e n o t d i f f i c u l t t o r e a l i z e t h a t t h e White N i l e from below t h e c o n f l u e n c e o f t h e S o b a t t o j u s t above t h e White N i l e j u n c t i o n looses, on average, 1 . 6 mlrd m3/yr. T h i s f i g u r e d e p a r t s s l i g h t l y from t h e a v e r a g e l o s s f o r t h e p e r i o d 1914-1937, which was 1 . 9 mlrd m3/yr w i t h a s t a n d a r d d e v i a t i o n o f 1 . 2 mlrd m3/yr.
H u r s t r e p o r t e d t h a B m o s t of t h e v a r i a t i o n i n t h e l o s s w a s due t o t h e f a c t
t h a t t h e l o s s w a s t h e d i f f e r e n c e between two much l a r g e r q u a n t i t i e s , t h e r e b y c o n t a i n i n g t h e e r r o r s of b o t h . Not much weight can be a t t a c h e d t o t h e l o s s i n any p a r t i c u l a r y e a r , b u t t h e mean l o s s i s p r o b a b l y c o r r e c t a t h a l f a m i l l i a r d . The J e b e l e l - A u l i a r e s e r v o i r w a s p u t i n t o u s e f o r t h e f i r s t t i m e i n 1937.The e s t i m a t e d l o s s i n t h e p o s t - r e s e r v o i r p e r i o d , 1937-1948, w a s 2.9 mlrd m3/yr. The
a i m and f u n c t i o n o f t h i s s t o r a g e work s h a l l be d i s c u s s e d , however, i n C h a p t e r 9 . I f w e now a d h e r e t o a mean annual flow o f 29.44 mlrd m % r
a t Malakal, t h e n a t u r a l
r i v e r d i s c h a r g e above t h e Blue N i l e j u n c t i o n c a n be t a k e n as 27 t o 28 mlrd m 3 / y r .
380
8.8 8.8.1
THE BLUE NILE BASIN Lake Tana
The Lake Tana a n d i t s c a t c h m e n t have been d e s c r i b e d i n C h a p t e r 2. The c a t c h ment a r e a e x c l u d i n g t h e l a k e , which i n i t s e l f i s 3000 km2, i s 13750 km'.
The
h y d r o l o g i c v a r i a b l e s a r e s o t h a t t h e a v e r a g e r a i n f a l l and e v a p o r a t i o n b a l a n c e e a c h o t h e r a t a b o u t 1300 mm/yr;
t h e run-off
from t h e catchment t o t h e l a k e ,
assuming no change i n s t o r a g e , must t h e n e q u a l t h e l a k e o u t f l o w . T a k i n g t h e annual run-off m3/yr.
c o e f f i c i e n t a s 22%, t h e a n n u a l l a k e o u t f l o w i s a b o u t 3.93 mlrd
T h i s f i g u r e i s n e a r l y t h e same as t h e mean o f t h e o u t f l o w s i n t h e p e r i o d
1920-1933, which w a s 3.85 mlrd m3/yr.
These o u t f l o w s are g i v e n i n T a b l e 8,
Appendix D . For a g i v e n l a k e l e v e l t h e d i s c h a r g e c a n b e r e a d from t h e r a t i n g c u r v e , F i g . 22, Appendix E . T a b l e 8, Appendix D , h a s b e e n u s e d f o r p r e p a r i n g t h e a v e r a g e hydrograph o f Lake Tana o u t f l o w , which is shown i n F i g . 8.37.
c?
ro
300
9
;200 CI
2 z
U
i i 100
Month F i g . 8.37.
The a v e r a g e hydrograph o f t h e B l u e N i l e a t t h e e x i t o f Lake Tana,
Roseires, a n d S e n n a r
6
8.8.2
The B l u e N i l e from Lake Tana t o R o s e i r e s
The B l u e N i l e r e c e i v e s t r i b u t a r i e s a s h o r t d i s t a n c e away from i t s e x i t from Lake Tana. The f i r s t t r i b u t a r y i s c a l l e d C h i m b i l and i s s a i d t o b r i n g as much as 10 m3/sec.
i n f l o o d . Below t h e j u n c t i o n of C h i m b i l i w i t h t h e B l u e N i l e t h e t r i -
b u t a r i e s i n c r e a s e i n s i z e and i m p o r t a n c e as t h e r i v e r e n t e r s t h e canyon i n which
i t r e m a i n s u n t i l w i t h i n a few k i l o m e t r e s from t h e Sudan b o u n d a r y . The B l u e N i l e i n i t s u p p e r r e a c h i s j o i n e d by t h e R i v e r B a s h i l e and R i v e r
Jamma ( s e e t h e map, F i g . 2.18.). The i m p o r t a n t t r i b u t a r i e s o f t h e l o w e r r e a c h e s
381
a r e t h e D i d e s s a , Dabus and Balas. H u r s t a n d h i s co-workers e s t i m a t e d t h e f l o w a t about 2.2 x
lo6
m3/day i n low-flow s e a s o n and a t a b o u t 220 x
lo6
m3/day i n f l o o d
s e a s o n . T h i s means t h a t a f t e r f l o w i n g f o r a b o u t 330 km, t h e d i s c h a r g e becomes a b o u t 10 times as much a s i t s i n i t i a l v a l u e a t t h e e x i t o f t h e l a k e . The Blue
N i l e d i s c h a r g e a t R o s e i r e s , 935 km below t h e e x i t o f Lake Tana, i s a b o u t 7 m i l l i o n m3/day,
e m p h a s i z i n g t h a t t h e c o n t r i b u t i o n of t h e r e a c h below Kutai ( K i l o
330) i s more t h a n t h e c o n t r i b u t i o n of t h e upper r e a c h . I t seems, however, t h a t t h e d i s c h a r g e from Lake Tana up t o R o s e i r e s i n c r e a s e s w i t h d i s t a n c e from t h e lake. The r a t i n g c u r v e of t h e B l u e N i l e a t R o s e i r e s i s shown i n F i g . 2 3 , AppendixE, and t h e monthly and a n n u a l d i s c h a r g e s i n t h e p e r i o d 1912-1973 a r e g i v e n i n T a b l e 9 , Appendix D. The s t a t i s t i c a l d e s c r i p t o r s and t h e s e r i a l c o r r e l a t i o n c o e f f i c i e n t s o f t h e s e d a t a have b e e n c a l c u l a t e d and g i v e n i n T a b l e 8 . 1 4 . From t h i s t a b l e o n e c a n n o t i c e t h a t t h e monthly and t h e a n n u a l d i s c h a r g e s a t R o s e i r e s are v e r y much l e s s d e p e n d e n t t h a n t h e monthly and t h e a n n u a l d i s c h a r g e s a t e i t h e r Mongalla o r M a l a k a l . I t i s o n l y t h e months o f low f l o w , J a n u a r y t h r u ' May, and O c t o b e r t h a t show s i g n i f i c a n t c o r r e l a t i o n a t t h e 95% l e v e l of c o n f i d e n c e . The d i s c h a r g e s of J a n u a r y , F e b r u a r y and March a r e s e e n t o b e s e r i a l l y c o r r e l a t e d up t o l a g 3 , w h e r e a s A p r i l i s s e r i a l l y c o r r e l a t e d a t l a g 1 o n l y and t h e f i r s t
s e r i a l c o e f f i c i e n t is j u s t s i g n i f i c a n t l y d i f f e r e n t from z e r o a t t h e 95% c o n f i d e n c e l e v e l . The May and O c t o b e r series show s e r i a l c o r r e l a t i o n a t l a g 3 and a t l a g 1 or 2 . The months J u n e t h r u ' December, w i t h t h e e x c e p t i o n o f May, and t h e y e a r , are n o t s e r i a l l y c o r r e l a t e d . One c a n g e t some rough i m p r e s s i o n a b o u t t h e l a c k o f d e p e n d e n c e i n t h e s e series by comparing t h e g r a p h i c a l p l o t o f , f o r example, t h e a n n u a l series a t R o s e i r e s ( F i g . 8 . 3 8 . ) t o t h e a n n u a l series a t Malakal o r Mongalla. The dependentcomponent i n t h e series o f J a n u a r y t h r u ' A p r i l c a n b e d e s c r i b e d , however, by a f i r s t - o r d e r a u t o r e g r e s s i v e r e l a t i o n h a v i n g t h e f i r s t c o r r e l a t i o n c o e f f i c i e n t as a p a r a m e t e r ( s e e T a b l e 8 . 1 4 ) . The r e s i d u a l s l e f t a f t e r removing t h e d e p e n d e n t component a p p e a r t o b e u n c o r r e l a t e d a t 95% c o n f i d e n c e l e v e l . The s e r i a l c o r r e l a t i o n c o e f f i c i e n t s of t h e r e s i d u a l s o f t h e s e series are p r e s e n t e d i n T a b l e 8 . 1 5 . The p r o b a b i l i t y d i s t r i b u t i o n f u n c t i o n s t h a t may s e r v e as good f i t t o t h e monthly and a n n u a l d a t a are t h e h e a r s o n Type I11 and
t h e normal f u n c t i o n s . The
f i t o f t h e l a t t e r t o t h e a n n u a l d i s c h a r g e d a t a i s as shown i n F i g . 8 . 3 9 . For a l l t h e d i s c h a r g e series t h e 100 and 200-year d i s c h a r g e s computed from t h e d i s t r i b u t i o n f u n c t i o n b e s t f i t t i n g e a c h series are a s f o l l o w s : Discharge, 106 m 3 J a n . Feb. Mar. Apr. May 100-yr 200-yr
June J u l y
Aug.
Sep.
Oct.
Nov. Dee. Year
1642 1414 1385 1116 1851 3426 10902 22918 19941 13088 4639 2495 73091 1752 1586 1577 1267 2096 3780 11300 23595 20619 13876 4886 2588 75270
w
TABLE 8.14
The b a s i c s t a t i s t i c a l d e s c r i p t o r s and t h e s e r i a l c o r r e l a t i o n c o e f f i c i e n t s o f t h e monthly and annual d i s c h a r g e s o f t h e Blue N i l e a t R o s e i r e s for t h e p e r i o d 1912-1973
I tern
Basic s t a t i s t i c a l descriptor
X,
106 m 3 s , 106 ln3 C
CV
Month of t h e y e a r Jan.
Feb.
Mar.
Apr.
878 268 0.3050 0.7348 4.0851
543 2 35 0.4335 2.1822 8.6791
445 369 246 199 0.5520 0.5378 2.4359 2.2961 9.7202 10.5525
0.5b59 0.4093 0.3560 0.1784 0.3179 0.1772 0.2330 0.2221 0.1424 0.2617 0.0815 0.1939 0.1665 0.0153 0.0572
0.7513 0.5161 0.3503 0.1801 0.1226 0.1404 0,1080 0.1086 0.1483 0.1309 0.1146 0.1227 0.1230 0 .0410 0.0219
0.5126 0.4183 0.3555 0.1091 0.0646 0.0574 0.0394 0.0463 0.1040 0.0960 0.0879 0.2886 0.0445 0.0880 0.1392
May
June
601 1632 34 1 533 0.5679 0.3265 2.1238 1.6445 8.3400 10.0111
July
Aug.
6548 15499 1691 2881 0.2582 0.1859 0.3363 0.2392 3.4673 5.6194
Sep.
Oct.
Nov.
Dec.
Year
12515 2884 0.2305 0.1443 4.1538
6813 2386 0.3502 0.4263 3.4400
2672 748 0.2799 0.4241 3.5667
1470 398 0.2709 0.0479 3.4878
49216 9272 0.1884 0.1747 3.9756
0.1891 0.0527 0.0991 0.1384 0.0237 0.0259 0.0597 0.1337 0.0238 0.1651 0.1826 0.2214 0.0178 6 . 0284 6.1043
c.0237 0.0690 I!.3098 0.1597 0.0514 0.2608 0.0409 0.0815 0.0739 0.0971 0.1670 6.0225 0.0429 6 .OM3 6.1357
q.0613 0.0563 0.0882 0.2219 0.0201 0.0369 0.0206 0 .Of506 0.0126 0.1538 I!.1889 0.1009 0.0307 0.0958 0.0238
0.2171 0.1383 0 . 1 6 0 3 0.0166 0.1329 0.1574 0.1290 0.1441 0.1724 0.0448 0.0616 0.0963 0.1086 0.0237 0.0749 0.0232 0.0140 0.0778 0.1764 0 .0466 6.1365 0.2977 1467 0.1970 0.0065 0.0573 6.1136 0.1566 0.1581 0.0275
Serial correlation coefficient rl
r2 r3 r4 r5 r6 l-7 r8 r9 r10
rll
r12 r13 r14 r, r
,
0.2489 0.0902 0.1394 0.1305 0.0301 0.0989 0.1533 0.1586 0.0659 0.2179 0.2505 0.3045 0.0059 0.0277 0 .0055
0.0269 0.0381 0.3173 0.0754 0.0528 0 . 2903 0.0249 0.0338 0.1250 0.0337 0.0483 6.0710 0. 0094 0.2652 0.0161
0.1702 0.1173 0.0468 0.0898 ,0312 0 .0059 0.1119 0.1735 0.1254 0.2370 0.2221 0.1646 0.0445 0.1590 0.0017 0.1283 0.1117 0.0147 0.0500 0.0876 0.1069 6.0782 0.0515 0 . 1616 6.1430 0.1510 .2122 3026 0 . 1 9 9 1 0.1783
p
c.
c
6.0705
0 .0381
0.0248 0.0987 0.0248 0.1481 0.0557 0.0057 0.0846 0.1179 0.0035 0.3042 0.2180 6.0826 6.0007
0.
01
383
1012'15
0
'20
'25 '30 '35 '40 '45 '50 '55 '60 '65 '70'73 Year
F i g , 8.38. G r a p h i c a l p l o t of t h e annual d i s c h a r g e s e r i e s of R o s e i r e s i n t h e p e r i o d 1912-1973 TABLE 8.15
S e r i a l c o r r e l a t i o n c o e f f i c i e n t s of t h e r e s i d u a l s l e f t from f i t t i n g a u t o r e g r e s s i v e models t o some o f t h e d i s c h a r g e series a t R o s e i r e s Serial correlation coefficients
Month
Jan. Feb. Mar. Apr.
,082 :073 .079 TO16
.067 TO47 _076 .005
.167 .041 .166 .091
T183 T163 TO73 .087
.231 ,111 1085 .017
TO74 _077 .026 .073
.042 . l o 8 TO94 .216 ,031 TO39 .095 .013 TO42 TO20 .054 .029 .081 .131 T156 .160
,127 TO19 ,082 .131
.097 .021 .299 .231
.136 .121 ,131 ,081
T152 ,024 .007 .026
.042
TOO8 .192 .017
80 70 0
E 60
m
0 F
50
0
r U ?!'.U 40
-
s 30
U
0
20
10 0.01
,
I
0.1 0.2 0.5 1
2
I
,
I
I
5
10
,
1
1
1
1
1
I
20 30 40 50 60 70 80
Non exceedance
,
I
90 95
probability, '10
,
I
98 99
99.9
Fig. 8.39. F i t of t h e normal f u n c t i o n to t h e d i s t r i b u t i o n o f t h e annual d i s c h a r g e s a t R o s e i r e s i n t h e p e r i o d 1912-1973
384
The f i r s t phase of t h e c o n s t r u c t i o n of a c o n c r e t e dam on t h e Blue N i l e a t Roseires was completed i n 1966. The primary purpose of t h i s dam i s t o s t o r e water and r e l e a s e i t downstream i n t h e s h o r t a g e season t o supply t h e Gezinah Managil Extension and t h e r i v e r bank pump schemes with water a s may be needed.
We s h a l l d e s c r i b e t h i s s t o r a g e work i n more d e t a i l i n t h e next c h a p t e r . 8.8.3
The Blue N i l e from Roseires t o Sennar
The Blue Nile below Roseires i s a mild stream w i t h a s l o p e of about 0.12 x which is about one-tenth t h e s l o p e of t h e t o r r e n t i a l stream which p r e v a i l s a l l t h e way from t h e e x i t of Lake Tana t o R o s e i r e s . There is a gauging s t a t i o n a t Wadi el-Aies,
n e a r Singa, about 180 km downstream of R o s e i r e s , and another a t
Makwar, n e a r Sennar, s o m e 270 km below Roseires. The r a t i n g curves of t h e s e two s t a t i o n s i n t h e given o r d e r a r e shown g r a p h i c a l l y i n F i g s . 24 and 25, AppendixE, respectively. The mean annual r a i n f a l l of t h e 30-yr p e r i o d 1938-67 a t R o s e i r e s , s t a t i o n
llq
i s 785 mm; a t Singa, s t a t i o n 101, 580 mm and a t Sennar, s t a t i o n 97, 463 mm.These d a t a suggest t h e f i g u r e of 600 mm a s an average depth of r a i n i n t h e reach from Roseires t o Sennar. For t h e same reach of t h e Blue N i l e t h e average annual evap o r a t i o n i s about 2450 mm. The n e t l o s s can t h e r e f o r e be e s t i m a t e d a t 1.85 m / y r . I f w e take 1 km a s an average value f o r t h e r i v e r width, t h e evaporation minus p r e c i p i t a t i o n l o s s comes t o 0 . 5 mlrd m3/yr. Hurst and h i s co-workers gave t h e average volume of flow a t Roseires f o r t h e p e r i o d 1912-1950 a s 49.6 x,109 m3/yr and t h a t a t Sennar f o r t h e s a m e p e r i o d as 48.7 x
loy
m3/yr with 0.9 mlrd m 3 / y r
a s t h e t o t a l transmission l o s s e s . They considered t h e l o s s i n a normal y e a r a s 2% and i n a high y e a r a s 4%, both of t h e flow volume a t t h e upstream s t a t i o n , i . e . R o s e i r e s . T h e r e s u l t s of o u r c a l c u l a t i o n f o r t h e mean flow a t t h e two s t a -
t i o n s i n t h e p e r i o d from 1912 up t o and i n c l u d i n g 1973 show t h a t whereas t h e mean a t Roseires was 49216 x
lo6
m3/ yr,
t h a t a t Sennar was 47185 x
These f i g u r e s b r i n g t h e t o t a l l o s s t o 2 mlrd m 3 / y r ,
lo6
m3/yr.
which i s somewhat b i g g e r
than t h e previous r e s u l t s , p o i n t i n g t o t h e p o s s i b i l i t y of i n c r e a s i n g withdrawal of water from t h e r i v e r between Roseires and Sennar. The s t a t i s t i c a l d e s c r i p t o r s and _the s e r i a l c o r r e l a t i o n s of t h e monthly and
annual d a t a a t Sennar a r e given i n Table 8.16. Although t h e mean monthly flows a t Roseires and Sennar are very n e a r l y equal (see F i g . 8 . 3 7 . ) ,
t h e p a i r of
s e r i e s f o r each month a t t h e s e two s t a t i o n s do not always behave s i m i l a r l y . Of t h e Sennar s e r i e s , which show dependence a t l a g 1 w i t h 95% confidence, a r e those of February, September and December; a t l a g 2 , November; l a g 3 , October and l a g 4 , March. For a l l t h e s e s e r i e s , except t h a t of March, one can d e s c r i b e t h e dependence i n t h e s e r i e s by t h e f i r s t - o r d e r l i n e a r Markov model. The c o e f f i c i e n t s of s e r i a l c o r r e l a t i o n of t h e r e s i d u a l series have been computed and
TABLE 8.16
The b a s i c s t a t i s t i c a l d e s c r i p t o r s and t h e s e r i a l c o r r e l a t i o n c o e f f i c i e n t s of t h e monthly and annual d i s c h a r g e s of t h e Blue N i l e a t Sennar f o r t h e p e r i o d 1912-1973
Item Basic s t a t i s t i c a l descriptor
X,
106 m 3 s , 106 m 3
C CV
Month of t h e y e a r Jan.
Feb.
Mar.
Apr.
May
June
July
Aug.
Sep.
Oct.
Nov.
Dec.
Year
682 260 0.3&+7 1.1508 5.4273
449 160 0.3565 1.0651 4.4911
404 170 0.4204 1.1641 5,6132
373 153 0.4094 0.6165 2.9148
573 304 0.5313 1.9219 8.2095
1402 573 0.4083 1.4886 8.6921
5892 1852 0.3143 6.1255 3.9041
15222 2771 0.1820 0.0882 5.1156
12383 3159 0.2551 0.5352 6.4598
6437 2632 0.4089 0.6046 3.8337
2202 1003 0.4554 1.2769 5.8684
1186 476 0.4016 0.9890 5.7073
47185 10012 0.2122 0.5123 5.0149
0.1811 0.0996 0. 1784 0.0421 0.2182 0.0753 0.1118 0.1109 0 .0212 0.1525 6.1070 0.1571 0.0595 0.1780 6.0560
0.3267 0.0450 0.0561 0.2558 6.1209 0.0251 0.0638 0.0303 0.0773 0.0718 0.0516 0.2162 .0719 0.0523 0.0523
0.1574 0.0108 0.0538 0.3774 2031 0.1041 0.1558 0.0492 0.1523 0.0612 0.0366 0.3133 0.0118 .0806 0.0463
0.1699 0.0327 0_.0012 0.2155 .0765 0.0142 0.0669 0.1085 0.1633 0.0398 0.1117 0.0715 0.0559 .2350 0.3877
0.0846 0 . 2 3 3 3 0.0326 0.0599 0 . 1991 0. 0771 0.0713 0.0902 6.0424 6.0880 0.1976 0.1817 0.0517 0.1005 0.0553 6.0205 0.0474 0.1754 0.0160 0.0678 0.0547 0.1207 6.0565 0.1078 0.0155 6.1611 0.2107 6.1245 0.0137 6.2046
0.0274 0.1135 0.1331 0.0552 0.0815 0.1002 0.1756 0.1384 0.0120 0.0276 6.1403 0.1298 0.0776 1453 0.0846
p.0845 0.0529 6.0951 6.1391 6.0497 0.1225 0.0580 0.0652 0.0440 0.0269 0.0153 0.2387 2285 0.1220 0.0294
0.2805 0.0655 0,0979
0.1353 0.0640 0 ,2551 0.0728 0.0250 0,1917 .0240 $. 1433 .OM9 0.0879 2224 0.0716 0.0186 1617 0.0904
0.1973 0,2722 0.2628 0.0144 0.0915 0.1574 0.0790 0.1010 0.0049 0.0910 6.1724 0.0475 0.0237 6.1117 0.0101
0.3017 0.2635 1619 0.0717 0.0793 0.0648 0.0253 0.0916 0.0128 0.0924 6.1445 0.0425 0.0558 1096 0.0244
0.1872 0.0301 0.1176 6.0110 0.0110 0.1281 2.0630 0.0150 0.0374 0.0059 6.1759 0.1837 0,0054 0.1207 0.0368
Serial correlation coefficient rl r2 r3 r4 r5 r6 r7 r8 r9 r10 '11 '12 r13 r14
r15
0
c.
p
p
c.
p.
6.1118
0.0480 0.0425 0.0513 0.1631 0.0179 0.0783
6.1638 0.1748 0.0585 0.0298 0 .0819
0
0
p.
p.
0.
p.
386
l i s t e d i n Table 8.17. The t a b u l a t e d values do n o t j u s t i f y r e j e c t i n g t h e n u l l hypothesis t h a t t h e s e r i a l c o r r e l a t i o n c o e f f i c i e n t s a r e n o t s i g n i f i c a n t l y d i f f e r e n t from zero a t t h e 95% l e v e l of confidence. TABLE 8.17
S e r i a l c o r r e l a t i o n c o e f f i c i e n t s of t h e r e s i d u a l s l e f t from f i t t i n g a u t o r e g r e s s i v e models t o some of t h e discharge s e r i e s a t Sennar Serial correlation coefficient
Month
Feb. Sep. Oct. Nov. Dec.
r1
r2
r3
r4
r5
:075 :021 ,047 ,032 .024
,009 .014 .096 .203 .179
.036 .122 .078 -210 .131
I275 I131 :084 ,048 .120
Tl68 .120 .033 .076 .lo0
r6
r7
I054 T O 3 1 . O O ~,089 .057 .053 -140 .026 .075 TO23
r8
r9
r10
rll
r12
lo07 :178 .063 .098 .074
.076 :054 :llO :020 .053
.057 I033 .lo6 .099 .121
.001 I179 .146 1147 .173
.179 .223 _014 ,008 .096
'13
r14
r16
_040 I064 Llll,022 .029 ,132 .031 :088 .019 .086
I034 :041 .039 .026 .008
The Sennar Dam (Makwar) was b u i l t a c r o s s t h e Blue Nile i n 1925 a few k i l o metres above Sennar. This dam w a s b u i l t e x c l u s i v e l y i n t h e i n t e r e s t of theSudan. I t s e r v e i t w o purposes: i t raises t h e l e v e l s u f f i c i e n t l y high f o r t h e w a t e r t o
flow i n t o t h e main Gezirah c a n a l , and i t s t o r e s water from t h e f l o o d t o be used during t h e p e r i o d January t o A p r i l when t h e r e i s no s u r p l u s water i n t h e Blue Nile over Egypt's requirements. More d e t a i l e d d e s c r i p t i o n of t h i s s t o r a g e w i l l appear i n Chapter 9 . 8.8.4
The Blue N i l e below Sennar t o Khartoum
Below Sennar, t h e Blue Nile flows north-west
f o r a d i s t a n c e of 350 km b e f o r e
i t j o i n s t h e White N i l e a t Khartoum. Between Sennar and Wad Medani t h e Blue N i l e r e c e i v e s t h e Rahad. This t r i b u t a r y rises on t h e E t h i o p i a n P l a t e a u a few k i l o metres w e s t of Lake Tana under t h e name of t h e Sidd. I n i t s course of 750 km t h e r i v e r flows i n t h e Ethiopian P l a i n s a s t h e A i m a and changes i t s name i n t h e Sudan t o t h e Dinder. The drainage b a s i n of t h e Dinder has an a r e a of 160 000 km2. The average a r e a l r a i n f a l l is about 0.80 t o 0.85 m/yr and t h e annual run-off c o e f f i c i e n t reaches 22%. These f i g u r e s suggest a t o t a l run-off
of about 3 mlrd
m3 i n a normal y e a r . Vol. I X of thecNile Basin g i v e s 2.97 mlrd m3 a s an average volume of flow p e r y e a r i n t h e period 1912-1950 and 3.83 mlrd m3 a s t h e flow volume i n a high year (1946). The gauge-discharge measurements f o r e s t a b l i s h i n g t h e r a t i n g curve of t h e Dinder a r e taken a t H i l l e t I d r i s . The d a t a for t h e p e r i o d 1924-1927 a r e shown i n F i g . 26, Appendix E . The Blue Nile below Wad Medani r e c e i v e s t h e Rahad, which SprinRs n e a r l y from t h e same p l a c e a s t h e Dinder. This t r i b u t a r y has a l e n g t h of 800 km and a drainage b a s i n of about 8000 km2. For t h e same r a i n f a l l a s on t h e catchment of t h e
387
Dinder, i . e . 0.80-0.85
m/yr,
and an annual run-off
c o e f f i c i e n t o f , s a y , 161, t h e
t o t a l flow reaching t h e mouth of t h e Rahad comes t o about 1.1 mlrd m3/yr. Vol.
I X of t h e N i l e Basin g i v e s 1.08 mlrd m3 a s t h e y e a r l y average f o r t h e p e r i o d 1912-1950. The r a t i n g curve of t h e Rahad f o r t h e p e r i o d 1922-1927 a t Abu-Haraz, n e a r t h e mouth, i s shown i n F i g . 27, Appendix E. The Dinder and t h e Rahad hardly c a r r y any water i n t h e p e r i o d from January t o May. The hydrograph of each of t h e s e two t r i b u t a r i e s has a more o r l e s s triangul a r shape w i t h a base width of about 200 days. The peak d i s c h a r g e s have been found t o be about 480 and 160 m3/sec.
f o r t h e Dinder and t h e Rahad, respectively.
The mean annual r a i n f a l l v a r i e s from 463 mm a t Sennar, s t a t i o n 97, t o 340 mm a t Managil, s t a t i o n 89 t o 385 a t Wad Medani, s t a t i o n 88, t o 312 a t Ruffa, s t a t i o n 86 t o 254 mm a t Kamlin, s t a t i o n 83, t o 160 mm a t Khartoum s t a t i o n s 78/79.In t h i s reach of 350 km, average depths of 320 mm and 2740 mm/yr can be used f o r r a i n f a l l and evaporation, r e s p e c t i v e l y . Assuming an average width of t h e r i v e r of 800 m, t h e l o s s becomes (2.74
-
0.32) x 350 x
lo6
x 0 . 8 = 0 . 6 8 mlrd m3/yr t o
be rounded t o , s a y , 0.85 mlrd m3/yr t o account f o r some seepage loss. The b a l ance a t t h e mouth of t h e Blue N i l e n e a r Khartoum i s 47.185 0.850 = 50.385 mlrd m 3 / y r .
+ 2.970 + 1.080
-
This f i g u r e i s about 2% less t h a n t h a t given by Hurst
i n Vol. I X of t h e N i l e Basin ( H u r s t , H . E . ,
Black, R.P.,
and Simaika, Y.M.,1959).
The monthly and annual d i s c h a r g e series of t h e Blue N i l e a t Khartoum a r e presented i n Table 11, Appendix D. The d a t a used f o r p r e p a r i n g t h e r a t i n g curve a r e shown i n F i g . 28, Appendix E. The change of t h e gauge s i t e a l t e r n a t i v e l y between B u r i , Soba and Khartoum could be one of t h e reasons r e s p o n s i b l e f o r t h e heavy s c a t t e r of t h e p l o t t e d p o i n t s . The a l r e a d y d e r i v e d annual flow volume a t Khartoum i s n e a r l y i n p e r f e c t agreement with t h e mean annual flow f o r t h e p e r i o d 1912-1973, which was 50.369 mlrd m 3 / y r ( s e e Table 8 . 1 8 ) . E i t h e r f i g u r e , derived o r computed, shows t h a t t h e flow a t Khartoum i s about 6.8% l a r g e r than t h a t a t Sennar. One should not forg e t , however, t h a t t h i s i s an average percentage f o r 62 y e a r s , around which i n d i v i d u a l years f l u c t u a t e p o s i t i v e l y and n e g a t i v e l y . The g r a p h i c a l p l o t of t h e annual series a t Sennar and Khartoum, F i g . 8.40. shows t h a t t h e flow volume i n some y e a r s a t Sennar was equal t o , o r even l a r g e r t h a n , t h a t a t Khartoum. 6
Some of t h e monthly series and t h e annual discharge series a t t h e mouth of t h e Blue N i l e appear t o be s l i g h t l y s i g n i f i c a n t l y d i f f e r e n t from z e r o a t t h e 95% l e v e l of confidence. The remaining series, which a r e t h e May, J u l y , August, September and October s e r i e s , a r e not s i g n i f i c a n t l y d i f f e r e n t from t h e zero a t t h e same l e v e l of confidence. The s e r i e s showing s i g n i f i c a n t dependence a t l a g 1 a r e t h o s e of February, A p r i l , June, December and t h e y e a r . Those a t l a g s 3, 4 , 5 and 6 a r e t h e series of November, March, January and June, r e s p e c t i v e l y . The dependence i n a l l t h e s e series can be adequately d e s c r i b e d by a f i r s t - o r d e r
TABLE 8 . 1 8
w
The b a s i c s t a t i s t i c a l d e s c r i p t o r s and t h e s e r i a l c o r r e l a t i o n c o e f f i c i e n t s of t h e monthly and annual d i s -
W W
charges of t h e Blue N i l e a t Khartoum f o r t h e p e r i o d 1912-1973
Month of t h e y e a r
Item Basic s t a t i s t i c a l descriptor
6,
106 In3
s, 106 El3 C
cz
CV
Jan.
Feb.
Mar.
Apr.
May
740 26 3 0.3551 1.0112 5.5867
455 159 0.3491 1.0912 4.8671
408 157 0.3847 1.3087 6.6164
386 180 0.4657 1.7888 7.8484
0.1420 0.0902 0.1664 0.0247 0.2745 0.0672 0.0450 0.0796 4.0269 0.1097 0.1097 0.1336 0.0280 0.2083 0.0379
0.2836 0.0295 0.0427 0.2222 0.0090 0. 1 2 1 1 0.0100 0.0100 2.0567 0.0066 0.0058 0.1659 0.1313 0.0363 6.0754
0.2268 6.0426 g.1317 0.3582 6.1651 1057 0.1141 6.0155 0.1768 0.0200 0.1111 0.2714 0.0500 0.0710 6.0331
0.2848 0.1060 0.1544 0.0865 0.0540 0.1790 .2050 6.0574 0.1528 6.2076 1588 0.0940 0.1233 6.0238 0.0043 6.2028 0.0458 .0848 0.2177 0 . 1029 0.0896 0.0512 0.2490 0.0508 0.1506 0.0325 0.0117 0.2270 0.0258 0.0420
June
July
Aug.
Sep.
Oct.
Nov.
Dec.
Year
480 1137 225 500 0.4685 0.4394 1.7085 2.2234 7.3397 11.8552
5371 1730 0.3221 0.3456 3.1507
15853 2835 (1.1788 0.5503 4.2886
13945 3243 0.2326 0.1190 4.5216
7657 2932 0.3829 0.5897 4.0881
2487 1061 0.4267 0.1503 5.2131
1264 486 0.3843 0.8370 5.0576
50369 10258 0.2037 0.1919 3.9845
0.2403 0.0230 0.1538 0.0580 6.0206 0.3831 0.0188 6.0390 0.1915 0.1051 0.1125 0.2121 1338 0.2470 0.1809
0.1562 0.0039 .0680 0.1442 0.1104 0.1682 0.1670 0.1587 0.1043 0.0908 0.0584 0.1279 6.0797 6.1616 6.1759
6.1054 0.0913 0.1318 0.0479 0.1228 0.0627 0.0317 0.1298 0.0258 0.0096 0,0254 0.1158 0.2944 0.1556 0.0787
0.1593 0.1411 0.1638 0.1046 0.1783 0.0342 0.1139 1811 0 . 1033 0.0663 0.1801 0.1679 0.0902 0.0071 0.0152
0.1458 0.0216 0.1908 0.1095 0.0287 0 .2068 0.0271 6.1240 0 .0015 0.1174 4.1772 0.0182 0.0033 0.1239 0.1162
0.1345 0.1660 0.2458 0.0630 0.0724 0.0721 0.0171 2.0970 0.0010 0.1105 0.1609 0.0818 0.0037 0.1255 0.0154
0.3182 0.2251 0.1552 6.0411 0.0570 0.0687 0.0226 0.0850 0.0384 0.0725 0.1999 0.0412 0.0238 0.1113 6.0367
0.2521 0.1146 0.0675 0.0950 0.0089 0.1187 0.0746 0.0439 0.0760 .0671 0.1999 0.0913 0 .OX2 0.1343 0.1344
Serial correlation coefficient r1 r2 '3 r4 r5 r6 r7 r8 r9 r10 rll 12 r13 r14 r15
g.
4 g.
g
i.
p
g.
389
80
70
.+0
60
-5
50
2%.- 40
-CI 2 3-30
$2 20
'20 '25 '30 '35 '40 ' 4 5 '50 '55
1912 '15
'60 '65
'70 '73
Year F i g . 8.40. The g r a p h i c a l p l o t of t h e annual series a t Sennar and Khartoum on t h e Blue N i l e i n t h e p e r i o d 1912-1973 a u t o r e g r e s s i v e model f o r which t h e f i r s t s e r i a l c o r r e l a t i o n c o e f f i c i e n t i s a parameter. The r e s i d u a l s l e f t from f i t t i n g t h i s model t o each of t h e dependent s e r i e s when t e s t e d appear not t o be s e r i a l l y c o r r e l a t e d a t t h e chosen l e v e l of confidence. The only exception can be found a t l a g 6 f o r t h e month of June, which i s s i g n i f i c a n t l y d i f f e r e n t from zero. The s e r i a l c o r r e l a t i o n c o e f f i c i e n t s of t h e r e s i d u a l series a r e given i n Table 8.19.
TABLE 8.19
S e r i a l c o r r e l a t i o n c o e f f i c i e n t s of t h e r e s i d u a l s l e f t from f i t t i n g a u t o r e g r e s s i v e models t o some of t h e d i s c h a r g e s e r i e s a t Khartoum -
Month
Serial correlation coefficient
&
Year
rl
r2
r3
r4
r5
r
r7
6
r8
r9
r10
rll
=12
r13 ~~~
Jan. Feb Mar. Apr June Nov Dec . Year
.
.
.
-057 .lo7 .053 TO14 -.020 .016 TO42 .033
.095 .030 TO31 ,045 .089 .155 .154 .087
,071 lo81 :037 .238 .066 1278 .077 :183 .171 :113 .229 .080 .135 1091 .062 .079
-162 1158 1202 ,087 .084 .095
I058 .031 TO89 1108 .396 .067 .080 .066 .013 . l l O
TO40 TOO9 TO46 I090 .067 .005 YO33 .058
The monthly series of t h e low-flow
.116 .051 TO04 _029 .063 .lo6 _062 .031
TO91 .058 ,191 .073 :199 .018 ,003 .031
.120 I 0 9 1 .003 :042 .046 ,123 -.209 L054 .137 .087 .099 ,123 -.115 _222 .017 .183
.064 .116 .251 .183 .228 TO35 .117 .133
.061 .077 .012 :053 .110 ,016 .002 .003
r14
r15
1194 .063 1080 .082 T133 1091 1080 .098
.033 .003 .022 -.088 .062 0 16 -0 15 .049
~~
~~
. .
season a r e d i s t r i b u t e d l i k e a lognormal,
whereas t h e monthly series of t h e high-flow season are d i s t r i b u t e d more o r less l i k e a Pearson Type I 1 1 d i s t r i b u t i o n . The s e r i e s of annual discharges is n e a r l y normally d i s t r i b u t e d (see F i g . 8.41.).
390
Non exceedance
probability
Fig. 8.41. F i t of t h e normal f u n c t i o n t o t h e d i s t r i b u t i o n of t h e annual d i s charges a t Khartoum on t h e Blue Nile i n t h e p e r i o d 1912-1973
For a l l d i s c h a r g e s e r i e s a t Khartoum, t h e 100 and 200-year d i s c h a r g e s computed from t h e d i s t r i b u t i o n f u n c t i o n b e s t f i t t i n g each s e r i e s a r e a s f o l l o w s : Discharge 106
,3
100-yr 200-yr
J a n . Feb. Mar. Apr. May 1537 946 912 1670 1041 998
June J u l y
Aug.
Sep.
Oct.
Nov. Dec. Year
1016 1255 2995 9835 21268 21482 15720 5797 2686 74424 1133 1396 3367 10388 21693 22658 16834 6328 2895 77194
THE MAIN NILE BELOW KHARTOUM TO JUST ABOVE THE JUNCTION OF THE ATBARA
8.9 8.9.1
Tamaniat d i s c h a r g e s
Regular d i s c h a r g e measurement of t h e MaixPNile began a t Tamaniat i n 1912. This s t a t i o n i s s i t u a t e d 41 km below t h e confluence of t h e Blue and White N i l e s a t Khartoum. Since 1934 t h e f l o o d measurements have been taken a t Shambat, which
is 6 km f u r t h e r below. The gauge-discharge measurements f o r t h e p e r i o d 1912-1973 a r e p l o t t e d i n F i g . 29, Appendix E. The monthly and annual discharges f o r t h e same p e r i o d a r e given i n Table 12, Appendix D. These d i s c h a r g e d a t a have been s t a t i s t i c a l l y analyzed and t h e r e s u l t s presented i n Table 8.20.
TABLE 8 . 2 0
The b a s i c s t a t i s t i c a l d e s c r i p t o r s and t h e s e r i a l c o r r e l a t i o n c o e f f i c i e n t s of t h e monthly and annual d i s c h a r g e s of t h e Main N i l e a t Tamaniat f o r t h e p e r i o d 1912-1973
I tem
B a s i c s t a t ist i c a l des c r i D t o r
i,106 8 ,
In3 106 m 3
C CV
Month of t h e y e a r Jan.
Feb.
Mar.
Apr.
3219 832 0.2585 0.6503 3.8837
2387 639 0.2678 1.3988 6.1783
2397 6 50 0.2710 0.3981 3.2296
2352 903 0.3840 0.6738 2.7410
0.2789 0.1997 1381 0.0930 0.0624 0,0287 0.0683 0.0225 0.0129 0.0401 0.1861 0.0024 0.0498 0.0432 0.1759
0.3731 0.1855 0.2437 p.0707 0.0560 0.1120 0.0351 0.0119 0.0617 0.0181 0.0932 0.0178 .0820 0.0626 0.1269
0.6303 0.4002 0.4779 0.3513 0.2665 0.3933 0.4059 0.3795 0.4047 0.3090 0.2545 0.2720 0.169 1 0.1381 0.2128
0.7757 0.6804 0.6463 0.5397 0.5138 0.5044 0.5175 0.4840 0.4621 0.3999 0.3479 0.3387 0.2713 0.2433 0.2760.
June
July
Aug.
Sep.
Oct.
Nov.
Dec.
Year
2328 893 0.3838 0.9868 3.2408
2889 724 0.2506 1.2860 5.1900
6645 1586 0.2388 0.5751 3.8961
16585 3103 0.1871 0.8162 4.9800
16339 3379 0.2068 0.0114 3.6400
10906 3136 0.2075 0.8752 4.8392
5406 1366 0.2527 1.2665 5.7600
4031 822 0.2040 0.5521 4.1110
75575 11341 0.1506 0.4132 4.3247
0.6064 0.4715 0.4051 0.2572 0.3301 0.2965 0.3474 0.3770 0.2576 0.2468 0.1141 0.0379 0.0158 0.0130 0.0820
0.3581 0.1138 0.0609 6.2451 6.1449 0.0493 0.0265 0.0586 0.1402 0.0570 0.0340 0.0437 .2559 0.2786 6.1287
0.2470 0.0055 6.1191 6.1707 6.1613 0.0942 0.0651 0.1061 6.0450 0.0629 6.0068 0688 0.1081 0.1969 6.2367
6.0886 0,1776 0.0279 0.0265 0.0229 0.0898 0.0081 0.0645 0.0182 0.0757 0.0801 0.0810 0.2072 0.1799 0.1238
0.2839 0.1672 0 . 1653 0.1067 0.0629 6.0372 0.0642 0.0469 0.0724 0.0561 0.1577 0.1733 6.0084 0.0222 0.0803
0.1636 0.0327 0.1862
0.1431 0.1943 0.1992 0.0858 0.0175 0.0775 0.0417 0.0627 0.0030 0.1366 0.0918 0.0260 0.0523 0.0822 0.1304
0.3896 0.2659 0.1655 0.0791 6.0690 q.0987 0.1471 0.0874 0.0576 0 .0174 0.1027 0.0981 0 .0638 0.0462 0.0631
0.2998 0.0869 0.0043 4.2272 0.1608 .0685 0.1356 0.1054 0.0829 0.0312 6.1376 0.0995 0.0982 0.1396 0.0412
May
Serial correlation coefficient rl
r2
r3
r4 r5 6 ' r7
r8
r9 r10
rll
I12 r13 r14 r15
0.
p
q
c.
6.1114
6.0526 0. 1676 0.0410 0.1441 6.0074 6.0900 6.1566 0.0419 0.0822 0.0755 0.0353
q
392
The discharge of t h e Main N i l e a t Tamaniat is equal t o t h e sum of t h e d i s charges of t h e Blue N i l e and t h e White N i l e , both a t Khartoum. Since t h e l a t t e r has been e s t i m a t e d from t h e Malakal d i s c h a r g e s , one can say t h a t t h e d i s c h a r g e
a t Tamaniat i s e q u a l t o t h e d i s c h a r g e of t h e Blue N i l e a t Khartoum p l u s t h a t of t h e White N i l e a t Malakal minus t h e l o s s e s from Malakal t o Khartoum. Because of t h e s m a l l d i s t a n c e between Khartoum and Tamaniat, 40 km, t h e conveyance l o s s
between t h e s e two s t a t i o n s can be ignored without any marked e f f e c t on t h e f i n a l r e s u l t . Since t h e outflow of t h e Blue N i l e a t mouth is n e a r l y twice a s much a s t h e flow o f t h e White N i l e a t Khartoum, i t i s then n o t s t r a n g e t h a t t h e d i s charge s e r i e s a t Tamaniat, e s p e c i a l l y t h e annual series, a r e more a f f e c t e d by t h e Blue N i l e d i s c h a r g e s than by t h e White N i l e ones. T h i s can be made e v i d e n t by comparing t h e p l o t i n Fig. 8.42. with t h o s e i n Figs. 8.40. and 8.35.
I0
5 60
m
E 50
m
s! 40 al
01
b
30
5 20 ul
i 10 l
L ~ ~ l - ~a 1 n o *l S t 8
1912'15
'20
'25
i
n I s I 1 1
l l l l a
n n I I .a* l l l l
I I 1 l a l l l l l
'
'30 '35 '40 '45 '50 '55 '60 '65 ' 7 0 ' 1 3 Year
Fig. 8.42. Graphical p l o t of t h e annual discharge s e r i e s a t Tamaniat on t h e Main N i l e i n t h e p e r i o d 1912-1973 I n t h e above discussion; no account has been taken f o r t h e e f f e c t of t h e t i m e l a g between one s t a t i o n and t h e o t h e r , t h e e f f e c t of t h e backwater produced by t h e j u n c t i o n of t h e two N i l e s and of t h e s t o r a g e i n r e s e r v o i r s , t h e e f f e c t of r e s e r v o i r r e g u l a t i o n and t h e s t o r a g e losses and inaccuracy i n measuring t h e water s t a g e and d i s c h a r g e . The o v e r a l l e f f e c t of these items can be seen from t h e r e s u l t s of t h e r e g r e s 6
s i o n and c o r r e l a t i o n a n a l y s i s of t h e d i f f e r e n c e i n t h e flow between Tamaniat on t h e Main N i l e and Khartoum on t h e Blue N i l e ,
Y, and t h e flow a t Malakal on t h e
White N i l e , X. The a n a l y s i s was performed on t h e monthly and t h e annual series. The X ' s a r e r e a d i l y a v a i l a b l e i n Table 7 , Appendix D, whereas t h e Y's a r e t h e d i f f e r e n c e s between Tables 12 and 11, Appendix D.
39 3
Let Y = a + b X
(8.7)
where a and b a r e t h e r e g r e s s i o n c o n s t a n t and c o e f f i c i e n t , r e s p e c t i v e l y . The c o r r e l a t i o n c o e f f i c i e n t rxy can be computed from t h e formula
r
XY
=
-
{ n EX'
n CXY
(CX)'
-
ZXCY n EYz
-
(8.8)
(EY)~ }3
The values of a , b , and rXYf o r t h e 13 Series have been c a l c u l a t e d and put i n Table 8.21. The c o e f f i c i e n t of l i n e a r c o r r e l a t i o n is a t a f a i r l y high value i n January and February then f l u c t u a t e s between moderate t o f a i r l y s t o n g t i l l J u l y . The high f l o o d discharges of t h e Blue N i l e are probably t h e reason behind t h e almost z e r o t o poor c o r r e l a t i o n during August, and September and October, res1
p e c t i v e l y . From November onwards t i l l t h e end of t h e y e a r , t h e c o r r e l a t i o n b e t ween X and Y improves considerably.
TABLE 8 . 2 1
Regression of t h e flow d i f f e r e n c e between Tamaniat and Khartoum on t h e flow a t Malakal
Month Year January February March April may June July August September October November December Year
a, 106 m 3
b
969.375 756.845 -1214.280 -275.977 -1551.564 1190.822 1419.763 -1449.095 257.030 1658.424 903.017 1113.904 792.214
0 . 6 18756 0.675871 0.454712 1.512053 2.06 1950 1.45989 1 1.75955 6 , Z O 1233 0.855047 0.467589 0.608749 0.524 161 0.834794
Regression Equation
&
Y Y Y Y Y Y Y Y Y Y Y Y Y
= =
=
=
= = = =
=
= = = =
969.375 756.845 1214.280 - 275.977 -1551.564 -1190.822 -1419.763 1449.095 - 257.030 1658.424 903.017 1113.904 792.214
+ 0.618756 X + 0.675871 X
+
0.454712 1.512053 + 2.061950 + 1.459891 + 1.075955 - 0.201233 + 0.855047 + 0.467589 + 0.608749 + 0.524161 + 0.834794
+
X X X X
X
X
X X
X X X
-
rXY 0.84493 0.84838 0.57058 0.73911 0.84765 0.79768 0.60137 0.06804 0.33322 0.38881 0.71579 0.82904 0.91140
X and Y a r e expressed i n m i l i i o n m3
The s t r o n g e s t c o r r e l a t i o n between X and Y can be found, a s can be expected, i n t h e annual series. The r e g r e s s i o n l i n e for t h e s e d a t a i s shown i n F i g . 8.43. The e s t i m a t e of t h e mean d i f f e r e n c e of flow between Tamaniat and Khartoum is 25367
2
4310 or from 21057 t o 29677 m i l l i o n m3/yr (95% confidence l e v e l ) . As o u r
e s t i m a t e of t h e annual flow a t Khartoum on t h e White N i l e j u s t above t h e conf l u e n c e of t h e Blue Nile i s a t 27 t o 28 mlrd m3 ( s e e s e c t i o n 8 . 7 . 3 ) , one is not
394
44 m’
E 42
cn
2 40
>. 38
5 36
0
c
;34
L
Y I
32
c
_0 30 C
28
5 26
t-
-2 2 4 -+ 2 2 I?
C
20 18 16 16 18 20 22 24 26 28 30 3 2 34 36 38 40 42 44 46 48 50 52 Annual
f l o w at
Moloka, X ,
lo9
rn3.
Fig. 8.43. Simple l i n e a r r e g r e s s i o n of t h e d i f f e r e n c e i n a n n u a l f l o w between Tamaniat a n d Khartoum, Y, and t h e a n n u a l f l o w a t M a l a k a l , X, f o r t h e p e r i o d 1912-1973 j u s t i f i e d i n r e j e c t i n g t h e h y p o t h e s i s t h a t t h e two estimates are n o t s i g n i f i c a n t l y d i f f e r e n t one from t h e o t h e r a t t h e g i v e n l e v e l of c o n f i d e n c e . The r e s u l t s p r e s e n t e d i n T a b l e 8 . 2 0 i n d i c a t e t h a t a l l t h e d i s c h a r g e series a t T a m a n i a t , e x c e p t t h o s e o f August, O c t o b e r and November, are s e r i a l l y c o r r e l a t e d . The dependent component i n t h e series w i t h c o r r e l a t i o n c a n b e d e s c r i b e d by an a u t o r e g r e s s i v e model o f t h e f i r s t or s e c o n d o r d e r . The series of t h e r e s i d u a l s have been t e s t e d a n d f o u n d n o t s e r i a l l y c o r r e l a t e d . T h i s c a n b e s e e n from t h e r e s u l t s included i n Table 8.22. The p r o b a b i l i t y d i s t r i b u t i o n f u n c t i o n s t h a t h a v e been found a s good f i t t o t h e d i s c h a r g e s a t Tamaniat a r e t h e P e a r s o n T y p e y I I I and t h e 2-parameter
lognormal.
F i g . 8.44.shows t h e f i t o f t h e l a t t e r t o t h e a n n u a l volumes o f f l o w d u r i n g 19121973. The 100- and 200-yr d i s c h a r g e s o b t a i n e d from t h e t h e o r e t i c a l f u n c t i o n s f o r a l l t h e 1 2 mont,hs a n d t h e y e a r are as f o l l o w s : Discharge 106 ,3 J a n . Feb. Mar. Apr. 100-yr 200-yr
May
June J u l y
Aug.
Sep.
Oct.
Nov. D e c .
Year
5540 4477 4097 4902 5027 5213 10983 21953 24212 20110 9757 6267105402 5863 4833 4314 5262 5443 5600 11577 22263 25043 21506 10474 6572109144
395
TABLE 8.22
S e r i a l c o r r e l a t i o n c o e f f i c i e n t s of t h e r e s i d u a l s l e f t from f i t t i n g a u t o r e g r e s s i v e models t o some of t h e d i s c h a r g e series a t Tamaniat
Month
Serial correlation coefficient
83
Year
rl
r2
r3
r4
r5
r6
*7
r8
r9
'10
rll
r12
'13
'14
r15
~~
.011 :084
Jan. Feb Mar. Apr
. .013 .014 . --168
-.089 ,003 .034 . -.060 . -.055 .037
May June July Sep Dec Year
:028 .254 TO55 :027 :058 .004 .098 .137 .061
.120 .213 .294 .204 .185 .121 TO94 .150 .129 .032
--
1218 .153 .025 .098 Tl60 T287 1070
:lo3 :060
.016 .099 .111 .163 .055 .121 .201 .087 .096 .181 1128 :lo9 ,121 .163 :073 T123 :004 .026 T151 .047 .039
1133 .030 .098 .036 :052 .085 .030 2113 2114 .lo6
,044 .003 .064 .014 .173 .041
.lo3
TO76 TO28 .093 .112
.OOO
-.128 .058
TO29 I065 .037 -.038 .040 .050
1036 1163 .026 TO12 :067 ,129 .056 TO62 .038 :015 .117 TO52 Toll , 0 3 1 .126 TO57 .156 ,010 .008 1039 .015 ,069 .044 _171 .036 :057 .lo8 .094 .006 1176 .219 TO98 :047 :167 .129 .027 .006 .156 . l l O .068
TO59 .137 TO62 .141 TO78 .143 TO61 .138 1028 ,062 _151 :056 .098 .083 _021 .025 :074 _049 .132 .005
2 00
-300
E rn 80
SI
60
& 50 L
," 40
: U
-
30
:20 20 C C
4
i. n "
.01
.05.1 .2 .5 1
2
10
5
Non
20 30 40 50 60 70 80
exceedance
probability
9 0 95
98 9999.5
F i g . 8.44. F i t of t h e 2-parameter lognormal f u n c t i o n to t h e d i s t r i b u t i o n of t h e annual d i s c h a r g e s of t h e Main N i l e a t Tamaniat i n t h e p e r i o d 1912-1973 6
8.9.2
Hassanab d i s c h a r g e s
The Main N i l e flows a d i s t a n c e of 277 km below Tamaniat b e f o r e i t r e a c h e s Hassanab s t a t i o n , which i s l o c a t e d about 5 km above t h e j u n c t i o n of t h e Atbara w i t h t h e N i l e . I n t h i s r e a c h of t h e Main N i l e t h e r e i s no g a i n a t a l l . The annual r a i n f a l l d e c r e a s e s from about 160 mm/yr a t Khartoum t o about 65 mm/yr a t Atbara and Zeidab. The p o t e n t i a l e v a p o r a t i o n i n a l l t h i s r e a c h h a s an average of 8 =/day
o r 2920 mm/yr.
The n e t loss can be taken a s 2800 mm/yr.
Assuming t h e
396 average width of t h e a r e a from which t h e l o s s e s t a k e p l a c e a s 1 . 5 km, t h e annual
lo6
l o s s should then be i n t h e o r d e r of 277 x 1.5 x 2.8 x a y e a r . T h i s i s about t h e same a s 1 . 2 mlrd m3/yr Vol. I X of t h e N i l e Basin ( H u r s t , H . E . ,
or about 1.16 mlrd m3
given f o r t h e p e r i o d 1912-52 i n and Simaika, Y . M . ,
Black, R . P . ,
1959).
The gauge-discharge measurements f o r t h e p e r i o d 1924-1927 a t Hassanab a r e shown g r a p h i c a l l y i n F i g . 30, Appendix E . The monthly and annual d i s c h a r g e s i n t h e p e r i o d 1912-1973 a r e given i n Table 1 3 , Appendix D , and t h e r e s u l t s of t h e i r s t a t i s t i c a l a n a l y s i s a r e i n Table 8 . 2 3 . From t h i s t a b l e and from Table 8 . 2 0 , one can compute t h e monthly and y e a r l y volumes r e p r e s e n t i n g t h e change i n t h e c o n t e n t s of t h e r i v e r trough p l u s t h e l o s s e s o r g a i n s averaged o v e r t h e same p e r i o d , i . e . 1912-1973. The r e s u l t s of computation a r e given below t o g e t h e r w i t h t h e f i g u r e s averaged f o r t h e p e r i o d 1912-1952,
f o r comparison.
Change of c o n t e n t s p l u s g a i n s o r l o s s e s , Period
lo6
m3,
averages f o r
J a n . Feb. Mar. Apr. May June J u l y Aug. Sep. O c t . Nov. Dec. Year
1912-52 -50* 0 1912-73 -51 -23
40 32
70 69
150 280 125 295
650 698
600 603
100 93
-400 -260 -30 -331 -192 -58
1200 1299
These r e s u l t s show c l e a r l y t h a t t h e sum of g a i n s i s s m a l l e r t h a n t h e sum of l o s s e s . The n e t y e a r l y loss is about 1 . 2 t o 1 . 3 mlrd m 3 / y r .
The g a i n between
Tamaniat and Hassanab t a k e s p l a c e i n t h e p e r i o d from October t o February, whereas t h e l o s s t a k e s p l a c e i n t h e remaining months. The change i n t h e flow from Tamaniat t o Hassanab caused by t h e g a i n s , l o s s e s and change i n r i v e r trough c o n t e n t s h a s a f f e c t e d t h e s t r u c t u r e o f some of t h e d i s c h a r g e s e r i e s . Compared t o Tamaniat, t h e J u l y , September and t h e y e a r l y s e r i e s of Hassanab a r e s e r i a l l y u n c o r r e l a t e d . The low-flow
s e r i e s , i . e . January
t h r u ' June and December remain, as do t h o s e of Tamaniat, s e r i a l l y c o r r e l a t e d . Like t h e s e r i e s of t h e upstream s t a t i o n s , t h e dependence i n t h e c o r r e l a t e d d i s charge s e r i e s of Hassanab can be d e s c r i b e d by an a u t o r e g r e s s i v e model. The resid u a l s e r i e s have been t e s t e d and found n o t t o be c o r r e l a t e d ( c o n f i d e n c e l e v e l = 957,). Table 8 . 2 4 g i v e s t h e v a l u e s of t h e s e r i a l c o e f f i c i e n t s of t h e r e s i d u a l s e r i e s . The m a j o r i t y of t h e d i s c h a r g e series a t Hassanab can be w e l l f i t t e d by t h e Pearson Type I 1 1 f u n c t i o n and t h e rest by t h e normal and lognormal f u n c t i o n s 6
The 100-yr and t h e 200-yr d i s c h a r g e s o b t a i n e d from t h e t h e o r e t i c a l d i s t r i b u t i o n s of a l l t h e s e r i e s a r e a s f o l l o w s : Discharge 106 ,3 J a n . Feb. Mar. Apr. May 100-yr 200-yr
f
June J u l y
Aug.
Sep.
Oct.
Nov. Dec.
Year
5320 4386 4024 4511 4587 4351 10132 20522 22890 18523 9400 5913 98210 5581 4700 4236 4799 4916 4606 10692 20803 23498 19417 9947 6121 100931
a l l t h e minus s i g n s h e r e mean g a i n
TABLE 8.23
The b a s i c s t a t i s t i c a l d e s c r i p t o r s and t h e s e r i a l c o r r e l a t i o n c o e f f i c i e n t s of t h e monthly and annual d i s charges o f t h e Main N i l e a t Hassanab for t h e p e r i o d 1912-1973
Item
Month o f t h e y e a r
Basic s t a t i s t i c a l descriDtor
X,
Jan.
Feb. ~~
~~~
Mar. ~
Apr. ~~
May ~
June ~~
July ~
~
C CV Serial correlation coefficient
r--
Sep.
0.2873 0.0539 0.1465 0.1117 0.0354 0.0152 0.0901 0.0367 0.0860 0500 . 0.1967 0.0888 0.0625 0.0731 0.0971
5.
0.3053 0.1360 0.2461 6.0916 0.1068 0.2651 0.0670 0.2068 0.2060 5.0066 0.0689 0 .0308 0.0897 0.0823 0.1244
0.5758 0.3374 0.4184 0.3218 0.3637 0.4761 0.4200 0.4044 0.3867 0.2597 0.2527 0.2850 0.1510 0.1189 0.1701
0.7643 0.6791 0.6500 0.5617 46114 0.6035 0.5762 0.5329 0.4821 0.4157 0,3772 0.3661 0.3019 0.2869 0.3064
0.6911 0.5679 0.4898 0.3491 0.4237 0.4329 0.4501 0.4135 0.3026 0.2592 0.1445 0.0960 0.0921 0.1035 0.1404
0.3636 0.3156 0 .0960 0.2080 0.0035 0.0428 0.2141 0.2329 0.2518 0.0612 6.0823 0.0887 4.1614 0.1369 0.1285
0.1626 5.0495 0.1053 0.1198 0.1018 0.1686 0.1506 0.1119 6.1292 0.0737 0.0151 0.1835 0.0638 0.1422 0.2713
Oct.
Nov.
Dec.
Year
~~
16432 5947 15982 2203 2594 2365 2283 3270 2410 3039 1533 2602 840 621 632 847 784 638 0.2399 0.2648 0.2674 0.3709 0.3814 0.2393 0,2578 0.1628 0.1849 0.3969 1.1142 0.4132 0.4204 0.7249 0.7058 0.5622 0.7796 0.2655 3.5739 5.8476 4.0487 2.4931 2.6125 3.1923 4.1176 4.0537 3.7169
lo6 m 3
s , 106 m 3
1' 2' 3' r4 r5 r6 r7 r8 r9 r10 '11 r12 r13 r14
Aug.
z. 0772
0.0206 0.0752 0 .0356 0.0385 0.1348 0.0465 0.0553 6.0467 6.0145 6.0609 0.1280 6.0750 0.0216 0.0099
0.1237 0.1007 0.1229 0.1156 0.0694 6.0984 0.0332 0.1550 0.1062 6.0396 6.1459 0.2487 0.0161 0.1102 0.1608
74276 5598 4089 11237 1320 757 9931 2864 0.2548 0.2358 0.1851 0.1337 0.2945 0.7769 0.1384 0.1379 3.5484 4.4776 3.6457 4.1719
0.1608 0.0179 0.2022 0.1083 0.0384 0.1996 0.0365 0.1493 0.0253 6.0730 6.2051 0.0987 0.1107 0.0288 0.0324
0.1173 0.0632 0.1232 0.0942 0.1448 0.0012 0.0316 0.1315 0.0085 0.1245 6.0811 0.0329 0.1170 0.0214 0.1623
0.2747 0.1578 0.0630 2.0960 0.0378 1935 0.1105 6.0034 0.0088 0.0280 6.1206 0.0955 0.0670 0.0483 0.0029
5.
0.2194 0.0144 0.0220 2.2246 0.1460 .0098 1094 0.1353 0.1150 6.0711 6.0961 0.1633 0.0008 0.0228 0.0048
p
0.
TABLE 8.24
S e r i a l c o r r e l a t i o n c o e f f i c i e n t s of t h e r e s i d u a l s l e f t from f i t t i n g a u t o r e g r e s s i v e models t o some of t h e discharge s e r i e s a t Hassanab Serial correlation coefficient
Month rl
Jan. Feb. Mar. Apr. May June Dec.
.052 :020 ,012 .171 l125 .088 .037
8.10
2 '
'3
1035 ,060 ,171 .028 .022 .191 .092
.115 .212 .240 .169 .165 .073 .052
r4
6 '
r5
1151 1018 TO39
.161 TO32 1165 ,215
.025 .033 .158 .152 .071 .159 .035
1z
.128 .237 .130 _096 ,020 .175
=7
r8
1109 .044 .071 .052 .lo5 :131 .073
1019 .015 .086 .057 .138 _095 .007
r9
r10
rll
r12
r13
'14
r15
.006 ,068 1199 . l o 3 1034 1059 .076
.002 j075 .116 .080 _045 .079 ,140
.183 ,060 .033 ,154 .024
,020 .029 .011 ,001 _ l o 7 ,042 .018 _070 .041 .178
.278 .lo2 1026 .035 .lo2
:035 .047 1010 .087 .026
1029 .020 1004 ,107 .053
.112 .127 .048 .128 .027
-
THE ATBARA R I V E R
The t o t a l s u r f a c e a r e a of t h e drainage b a s i n of t h e Atbara i s about 100 000 km2, of which about 68000 km2 comprise t h e b a s i n of t h e S e t i t , which i s t h e major t r i b u t a r y of t h e Atbara. The rest of t h e a r e a belongs t o t h e lower Atbara below t h e j u n c t i o n of t h e S e t i t . I t i s t h e r a i n f a l l on t h e catchment a r e a of t h e S e t i t t h a t is r e s p o n s i b l e f o r t h e major p a r t of t h e flow i n Atbara. The annual mean r a i n f a l l on t h e catchments of t h e S e t i t and Lower Atbara can be taken a s 800 mm/yr and 300 mm/yr r e s p e c t i v e l y . The annual run-off two catchments
i n t h e i r o r d e r can be taken a s 0.20 and 0.10 r e s p e c t i v e l y . These
f i g u r e s produce an annual run-off 11.84 x
lo9
coefficients for these
of 68 x
lo9
x 0.8 x 0.2
+
32 x lo9 :0 . 3 x 0 . 1 =
m3/yr.
The gauge-discharge measurements of t h e Atbara a t mouth a r e shown g r a p h i c a l l y i n F i g . 31, Appendix E . The monthly and annual d i s c h a r g e s i n t h e p e r i o d 1912-73 a r e given i n Table 14, Appendix D , and t h e r e s u l t s of t h e s t a t i s t i c a l a n a l y s i s of t h e s e discharge d a t a a r e included i n Table 8.25. The mean flow a t Atbara, near t h e mouth of t h e r i v e r , f o r t h e p e r i o d considered was 11.88 mlrd m3/yr. This volume of flow d i s t r i b u t e s i t s e l f o v e r t h e months, i n a normal y e a r , a s shown i n Fig. 8.45. The f i r s t f i v e months of t h e y e a r a r e p r a c t i c a l l y d r y . The e f f e c t i v e base width of t h e hydrograph i s from June up t o and i n c l u d i n g December. The flood season covers August and September and t h e remaining months r e p r e s e n t t h e low-flow season. The Atbara, i n t h i s r e s p & t ,
resembles t h e Blue N i l e ; both
a r e t o r r e n t i a l streams. This f e a t u r e l e a d s us t o review t h e r e s u l t s i n Table 8.25 c a r e f u l l y . The p e r i o d from January up t o and i n c l u d i n g May i s p r a c t i c a l l y dry. There a r e a few y e a r s i n which t h e r i v e r can b r i n g some l i t t l e water i n t h i s period. This does n o t , however, improve t h e long-term mean s e n s i b l y . I n s t e a d i t produces a considerable s c a t t e r l e a d i n g t o a high c o e f f i c i e n t of v a r i a t i o n and r a t h e r meaningless c o e f f i c i e n t s of skew and k u r t o s i s .
TABLE 8.25
The b a s i c s t a t i s t i c a l d e s c r i p t o r s and t h e s e r i a l c o r r e l a t i o n c o e f f i c i e n t s of t h e monthly and annual d i s charges of t h e River Atbara a t A t b a r a , n e a r mouth, f o r t h e p e r i o d 1912-1973
I tem
Basic s t a t i s t i c a l descriptor
X, S ,
106 m 3 106 m3
c
CV
f:onth of t h e y e a r Jan.
Feb.
Mar.
Apr.
May
7.7 1.1 3.6 31.8 4.1 18.8 1 . 5 19 2.7352 I g e 7 3.6348 5.2828 4.1288 2.2993 5.0538 5.7714 5.7813 4.6593 10.1766 32.8253 40.6174 36.8470 24.6862 20.3
Y
7.2
June
July
Aug.
Sep.
Oct.
Nov.
73.2 1616 812 101 1.3748 0.5026 1.5918 1.9888 4.9233 9.7805
5582 1842 0.3299 1.4805 7.1945
3496 1421 0.4065 0.4947 3.6017
812 415 0.5115 0.8970 4.6077
176 112 0.6332 1.1367 5.7258
6.2172 1892 0.1145 0.1876 0.0185 0.1052 0.1247 0.0462 0.0659 0.1261 0.0823 0.2258 0.0834 .0059 0.1900
0.2240 0 .0149 0.1476 0.0483 0.0479 0.1144 0.1125 0.2373 0.1087 0.0488 0.1270 0.0459 .0521 0.1179 0.0508
0.2212 0.0159 0.0318 0 .0637 0.2231 0.0124 0.0920 0.0740 0.1521 0.1084 1111 0.0197 0.0700 .1699 0.1220
0 .4174 0.2319 0.1443 0.0701 0.1295 0.1486 0.0979 0.0196 0.0256 0.1576 4.1224 0.1538 0.1043 0.3209 0.3084
0.2469 0.3424 0.2688 0.1054 0,2966 0.0774 0.0165 0.1580 4.1589 0.0237 c.2330 0.2523 0.1108 6.2930 6.1448
Dec. 57.6 47.2
0.8186 0.6843 2.9403
Year 11885 3913 0.3292 1.0933 5.6846
Serial correlation coefficient rl r2 r3 r4 l-5
r7 '8
r9 l-10 rll r12 r13 r14
r- -
0.3363 0.3139 0.3972 0.1523 0.3116 0.1995 0.1824 0.3030 0.0171 0.0152 0.1243 0.0051 0.0919 0.0372 0.0204
0.1957 0,1248 0.1756 0.0069 0.1404 0.0640 0.0740 0.3010 6.0327 0.0074 0.0759 0.0728 0.0434 0.0470 0.0523
0.0886 0.0784 0.1637 0.0072 0.1384 0.0435 0.1557 0.0752 0.0816 0 .0621 0905 .0892 0. 0812 0916 0.0843
4. 0
0.
c.
0.4284 0388 0.0401 0.0414 6.0428 .0442 0.0311 0.0084 0.1007 0.0593 .0111 0 .0120 0 .0130 0.0140 0.0151
p
4
0,3328 .0122 0 .0358 0.0186 0.0701 .0700 0.0477 0.0304 6.0125 0.0130 4.0338 0.0134 .0376 0.0396 0.0417
p
p
p
p.
p
6.2936 0.0429 0.0944 0 . 0504 0.0793 0.0079 0.0311 0.2671 0.2832 0.1466
4 .om1 0.1616 0.0231 0.0985 6.0406
p.
0.4918 0.3979 0.3274 0.2067 0.2861 0.1414 0.1690 0.1940 0.0322 0.1379 0 . 2048
0.1300
0.0515 0.2120 0.1217
0.0342 0 . 0400 0.0062 0.0427 0.0956 0 . 0772 0 .0590 0 . 1909 0.1857 0.0571 0.1697 0695
0.
2.0464
0.1643 6.0702
400
Month Fig. 8.45. The average hydrograph of t h e Atbara f o r t h e p e r i o d 1912-1973 a t Atbara, n e a r mouth
W e s h a l l , t h e r e f o r e , consider t h e monthly series from June t o December only and t h e annual series. Of t h e s e series t h o s e belonging t o June, October, November and December a r e t h e ones whose i n d i v i d u a l s appear t o be s e r i a l l y c o r r e l a t e d . F i r s t - o r d e r a u t o r e g r e s s i v e models have been f i t t e d t o t h e h i s t o r i c d a t a of t h e s e s e r i e s and t h e r e s i d u a l s examined s t a t i s t i c a l l y . The s e r i a l c o r r e l a t i o n s of t h e r e s i d u a l s e r i e s a r e given i n Table 8.26. They show t h a t they a r e not s i g n i f i c a n t l y d i f f e r e n t from zero with 95% confidence, except f o r November a t l a g s 2 and 4, and f o r December a t l a g 8 .
TABLE 8.26
S e r i a l c o r r e l a t i o n c o e f f i c i e n t s of t h e r e s i d u a l s l e f t from f i t t i n g a u t o r e g r e s s i v e models t o some of t h e d i s c h a r g e s e r i e s a t Atbara
~
Serial correlation coefficient
Month rl
July Oct. Nov. Dec.
r2
:136 :058 :188 .003 :067 .121 .166
r3 .023 TO97 .189 .232
r4
r5
r6
:217 .121 I 0 3 1 .019 .097 .154 .OOO .021 .153 .250 .010
.fi
r7
r8
r9
'10
'11
TO97 .165&1023 .162 T201 TO76 .010 :132 .078 :058 1031 .204 _158 :060 :150 .042 .115 .029 .049
.=
r12 .049 :069 :154 .161
r13
r14
r15
1170 1016 I087 :021 :134 :228 .005 :193 .039 .091 .124 .063
The p r o b a b i l i t y d i s t r i b u t i o n f u n c t i o n s t h a t s e r v e a s Rood f i t t o t h e d i s charge d a t a of t h e Atbara a r e t h e 2-parameter lognormal and t h e Pearson I 1 1 f u n c t i o n s . The f i t of t h e former t o t h e annual flow volumes i n t h e p e r i o d 19121973 i s shown i n F i g . 8.46.
40 1
40
-
30 m
I
,
I
,
I
,
I
0
.20 -
E
m
0
-
10
c
$ 8 ;
5 6 -
sul 5 -
-0
4 -
$ 3 U
2 -
1
I
,
,
I
,
I
1
1
I
l
l
,
,
I
,
1
s
The computed 100-yr and 200-yr discharges f o r t h e months from June up t o and i n c l u d i n g December and f o r t h e y e a r a r e a s f o l l o w s : Discharge 106 m3 100-yr 200-yr 8.11
8.11.1
June
July
Aug.
Sep.
Oct.
Nov.
Dec.
Year
415 476
4543 5106
11697 12747
7311 7817
2039 2223
524 571
191 210
23961 25874
THE MAIN NILE BELOW THE MOUTH OF THE ATBARA
The discharge of t h e Main N i l e a t Dongola
The monthly and annual discharges of t h e Main N i l e a t Dongola f o r t h e period 1912-1973 is given i n Table 15, Appendix D. The Main N i l e flows a d i s t a n c e of about 760 km below t h e c o d l u e n c e of t h e Atbara b e f o r e i t reaches Dongola. A p i c t u r e of t h e average width of t h i s reach of t h e r i v e r during t h e high-flow season can be seen from F i g . 2.23. The o v e r a l l average width between high and low flow seasons can be taken a s 400 m . This r i v e r reach runs i n a r e a l a r i d zone with about 8 mm/day f r e e water evaporation. This f i g u r e l e a d s us t o t h e conclusion t h a t t h e annual flow reaching Dongola i s about 0 . 8 8 mlrd m3/yr
less
than t h a t flowing j u s t below t h e Atbara j u n c t i o n . From Table 8.27, which cont a i n s t h e v a l u e s of t h e b a s i c s t a t i s t i c a l d e s c r i p t o r s and t h e s e r i a l c o r r e l a t i o n c o e f f i c i e n t s of t h e monthly and annual discharges a t Mongalla, one can f i n d t h e
40 3
annual flow volume averaged over t h e p e r i o d 1912-1973 a t 85.57 mlrd m3. This f i g u r e i s 0 . 5 8 mlrd m3/yr
less t h a n t h e sum of t h e mean flows a t Hassanab and
Atbara f o r t h e same p e r i o d . F i g . 8.47. shows a g r a p h i c a l p l o t of t h e annual flow volumes a t t h e mouth of t h e Atbara, a t Hassanab and a t Dongola. Because of t h e l a r g e n e s s of t h e flow a t Hassanab i n p r o p o r t i o n t o t h e flow a t t h e Atbara mouth, almost 6 t o 1, i t i s q u i t e u n d e r s t a n d a b l e t h a t t h e p l o t of t h e flow series a t Dongola i s very much p a r a l l e l t o t h a t of Hassanab and n o t of t h e Atbara.
1912'15 '20 '25 '30 '35 '40 '45 '50 '55
'60 '65 '70'73
Year F i g . 8.47. Graphical p l o t of t h e annual flow volume series a t A t b a r a , Hassanab and Dongola i n t h e p e r i o d 1912-1973
The s c a t t e r diagram of t h e sum of t h e flows a t Hassanab and a t t h e mouth of t h e Atbara, Y, a g a i n s t t h e flow a t Dongola, X , can b e seen from F i g . 8 . 4 8 . The r e g r e s s i o n of Y on X can be r e p r e s e n t e d by t h e e q u a t i o n
Y = 3.4125 + 0.96703 X
i n which Y and X a r e given i n mlrd m3/yr.
(8.9) The c o r r e l a t i o n c o e f f i c i e n t , r
b e i n g equal t o 0.96632, i s c e r t a i n l y s t r o n g .
XY'
From t h e v a l u e s of t h e s p i a l c o r r e l a t i o n c o e f f i c i e n t s l i s t e d i n Table 8 . 2 7 , one can observe t h a t a l l t h e s e r i e s , except t h o s e of t h e h i g h f l o o d d i s c h a r g e s , August and September, a r e s i g n i f i c a n t l y c o r r e l a t e d , a t l e a s t w i t h l a g 1. The f i r s t c o r r e l a t i o n c o e f f i c i e n t shows, i n g e n e r a l , a c o n s i d e r a b l e rise with d e c r e a s i n g mean d i s c h a r g e . The d e t e r m i n i s t i c component i n t h e s e r i a l l y c o r r e l a t e d d i s c h a r g e series can be f i t t e d each by a f i r s t - o r d e r a u t o r e g r e s s i v e scheme with t h e f i r s t s e r i a l c o e f f i c i e n t a s a parameter. The s t o c h a s t i c o r t h e r e s i d u a l component, which was l e f t from t h e f i t , was t e s t e d and t h e n u l l h y p o t h e s i s t h a t
404
Annual
flow
at
Dongola,
lo9
m3
Fig. 8.48. Simple l i n e a r r e g r e s s i o n between t h e sum of t h e f l o w s a t Hassanab and A t b a r a mouth, Y, and t h e f l o w a t Dongola, X, f o r t h e p e r i o d 1912-1973 that its s e r i a l
c o r r e l a t i o n c o e f f i c i e n t s a r e n o t s i g n i f i c a n t l y d i f f e r e n t from
z e r o (95% c o n f i d e n c e i e v e l ) c o u l d n o t b e r e j e c t e d . The o n l y e x c e p t i o n w a s t h e r e s i d u a l s e r i e s o f J u n e where t h e c o e f f i c i e n t a t l a g s 3 a n d 4 d i f f e r s s l i g h t l y from z e r o a t 95% l e v e l of c o n f i d e n c e . The s e r i a l c o r r e l a t i o n s of t h e r e s i d u a l
series are l i s t e d i n T a b l e 8 . 2 8 . The p r o b a b i l i t y d i s t r i b u t i o n f u n c t i o n s which a r e good f i t t o t h e monthly and a n n u a l d i s c h a r g e series a r e t h e P e a r s o n 111, t h e 2 - p a r a m e t e r l o g n o r m a l and t h e normal f u n c t i o n s . The f i t o f t h e P e a r s o n Type I 1 1 f u n c t i o n t o t h e d i s t r i b u t i o n o f t h e a n n u a l flow volumes a t Dongola i s shown g r a p h i c a l l y i n F i g . 8 . 4 9 . The t h e o r e t i c a l l y computed 100-yr and 200-yr d i s c h a r g e s are as f o l l o w s : P
Discharge 106 ,3 J a n . Feb. Mar. Apr. May 100-yr 200-yr
J u n e J u l y Aug.
Sep.
Oct.
Nov.
Dec.
Year
5517 4708 4685 4389 4622 4486 9524 27322 30690 22923 12103 6814 117755 5788 5079 4806 4688 5005 4846 9 8 1 1 28053 31600 24027 12845 7114 121617
405
TABLE 8 . 2 8
S e r i a l c o r r e l a t i o n c o e f f i c i e n t s of t h e r e s i d u a l s l e f t from f i t t i n g a u t o r e g r e s s i v e models t o some of t h e d i s c h a r g e series of t h e N i l e a t Dongola
Serial correlation coefficient
Jan. Feb. Mar. Apr. May June July Oct. Nov. Dec.
.050 .067 .067 1052 :119 :044 .024 ,037 ,004 .018
.083 .086 1208 ,037 TO38 1137 1012 TO62 1092 I120 .045 1066 .003 .079 .135 ,220 ,070 .061 .053 .006 .001 ,062 ,208 .046 ,042 7112 .161
TO69 Tl6l T155 TO36 .019 .040 .033 .169 .212
.154 .148 .154 ,250 .222 .232 .243 .154
,081 .042 1109 .326 .003 :077
.065 .050 .118 .155 .081 ,082 .173 .d87 .033 .026 .239 .080 .159 .076 TO79 .070 TO20 .008
.087 TO16 .047 ,096 .056 .039 ,076 .092 .015 TO50 ,051 .154 .038 .077 T O 4 8 .060 .162 :061 .111 _005 TO26 .128 .153 .lo1 .095 ,009 .063 . l o 5 .068 ,107 . 0 1 1 .082 TO36 . l o 4 . l o 8 TO90 .026 TO53 .113 T122
.lo4 .118 .027 .041 .138 .098 .021 .115
,045 .027 ,012 ,127 .166
,076
1050 .174 TO98 .022 :096 .063 .094 .094 T l l l
120
110
100 0
rn
90
0
&
80
L
0
II
s1
0
-0
70
C
2
60
50
0
40 Non
- exceedance
.114
.039 .094
p r o b a b i l i t y , '10
Fig. 8.49. F i t o f t h e Pearson Type I 1 1 f u n c t i o n t o t h e d i s t r i b u t i o n of t h e annual d i s c h a r g e s i n t h e Main N i l e a t Dongola f o r t h e p e r i o d 1912-1973
.077 .093 T147
.006
.096 .135
406
8.11.2
The d i s c h a r g e of t h e Main N i l e a t Aswan
Downstream of Dongola t h e Main N i l e flows t o t h e n o r t h then bends t o t h e e a s t and t o t h e w e s t and once more t o t h e e a s t t h e n n o r t h - e a s t t o Wadi H a l f a . T h i s reach i s about 450 km i n l e n g t h . Discharge measurement a t Wadi H a l f a began i n 1911, and w i t h a gap d u r i n g t h e F i r s t World War, c o n t i n u e d
u n t i l 1931. The
gauge-discharge measurements, 1911-1927, which were used f o r c o n s t r u c t i n g t h e r a t i n g curve f o r t h i s s t a t i o n a r e shown i n F i g . 32, Appendix E . The measuring s i t e a t Wadi H a l f a was inundated by t h e backwater caused by t h e second heighteni n g of t h e Aswan Dam about 1934. The s i t e was moved t o K a j i n a r t i , which i s about 50 km s o u t h of H a l f a . The l o s s i n t h e reach below t h e j u n c t i o n of t h e Atbara w i t h t h e Main N i l e and Wadi H a l f a i s about 1210 km x 0 . 4 km x 2.7 m , or about 1 . 3 mlrd m 3 / y r .
Vol. I X
of t h e N i l e Basin g i v e s 8 6 . 1 mlrd m 3 / y r as t h e sum of t h e flows a t Hassanab and t h e Atbara mouth. The flow i n t h e Main N i l e a t K a j i n a r i t y or H a l f a was 85.3 mlrd
m3/yr.
These two f i g u r e s a r e t h e averages f o r t h e p e r i o d 1912-1952. T h i s means
t h a t t h e l o s s averaged over t h e same p e r i o d i s 0 . 8 mlrd m 3 / y r . T h i s i s r e l a t i v e l y much l e s s t h a n t h e 1 . 3 mlrd m3/yr
a l r e a d y given by u s . One s h o u l d n o t f o r g e t ,
however, t h a t t h e a b s o l u t e f i g u r e i s i n i t s e l f q u i t e s m a l l , and having i t e s t i mated as t h e d i f f e r e n c e between two much l a r g e r q u a n t i t i e s s t r o n g l y i n f l u e n c e s
i t s accuracy. The Main N i l e flows about 345 km i n a r e a l a r i d zone, w i t h r a i n f a l l of less than 10 m m / y r ,
b e f o r e r e a c h i n g Aswan. The r a t i n g c u r v e p o i n t s measured i n t h e
p e r i o d 1918-1927 a t Khannaq s t a t i o n a r e shown i n F i g . 33, Appendix E.The monthly and annual d i s c h a r g e s a r e l i s t e d i n Table 1 6 , Appendix D . These d a t a d i f f e r from t h e corresponding ones a t Dongola by t h e conveyance l o s s b e t w e e n t h e two s t a t i o n s and t h e s t o r a g e l o s s e s a t Aswan. The t o t a l loss i n a y e a r averaged over t h e p e r i o d 1912-1973 was 3.368 mlrd m 3 , or about 3.94% of t h e annual flow a t D o n g o l a . The c l o s e resemblance between t h e d i s c h a r g e s a t Aswan and Dongola can be s e e n , f o r example, from a comparison between F i g . 8.50.and F i g . 8 - 4 7 . , both showing t h e p l o t of t h e annual flow volumes. The monthly and annual d i s c h a r g e series a t Aswan, l i k e a l l main s t a t i o n s on t h e N i l e , have been analyzed s t a t i s t i c a l l y and t h e r e s u l t s o b t a i n e d a r e presen5
t e d i n Table 8 . 2 9 . Examination of t h e s e r e s u l t s shows t h a t a l l d i s c h a r g e series a t Aswan, monthly and y e a r l y , undergo some s i g n i f i c a n t dependence i n t h e i r s t r u c t u r e . The dependent component i n each s e r i e s can be d e s c r i b e d by a f i r s t - o r d e r a u t o r e g r e s s i v e e q u a t i o n i n which t h e f i r s t s e r i a l c o r r e l a t i o n c o e f f i c i e n t i s a parameter. The r e s i d u a l s l e f t a f t e r removing t h e dependent component from each d i s c h a r g e s e r i e s have been examined and t h e h y p o t h e s i s t h a t t h e s e r i a l c o r r e l a t i o n between t h e i t e m s of t h e r e s i d u a l series i s n o t s i g n i f i c a n t ( a t 95% c o n f i dence l e v e l ) could n o t be r e j e c t e d f o r most of t h e d i s c h a r g e series. I t i s
407
120
3
0
110
..- 19000 -2 80 :rn' 70 O
3
3
E 60
3
< z 50 1912'15
'20 '25
'30 '35 '40 '45 '50 '55 '60 '65 '70'73 Year
F i g . 8.50. G r a p h i c a l p l o t o f t h e a n n u a l f l o w volume series a t Aswan on t h e Main N i l e i n t h e p e r i o d 1912-1973 p o s s i b l e t h a t t h e dependence i n some o f t h e r e m a i n i n g series n e e d s t o b e d e s c r i b e d by h i g h e r - o r d e r a u t o r e g r e s s i v e e q u a t i o n s so as t o r e n d e r t h e r e s i d u a l components t o b e u n c o r r e l a t e d . The s e r i a l c o r r e l a t i o n c o e f f i c i e n t s of t h e resid u a l series c a n b e f o u n d i n T a b l e 8.30. The a v e r a g e h y d r o g r a p h a t Aswan f o r t h e p e r i o d 1912-1973 i s shown i n F i g . 8 . 5 1 T h i s h y d r o g r a p h does n o t r e p r e s e n t , i n t h e f i r s t p l a c e , t h e n a t u r a l c o n d i t i o n . The low-flow s u p p l y o f t h e r i v e r i n t h e p e r i o d from November up t o and i n c l u d i n g J u n e i s somewhat m o d i f i e d by t h e e x i s t i n g s t o r a g e works on t h e N i l e . The s t o r a g e r e s e r v o i r a t Aswan a l o n e h a s r e s u l t e d i n t h e i n c r e a s e i n t h e a v e r a g e volume of f l o w d u r i n g t h e mentioned s e a s o n from 26.12 m l r d m3 a t Dongola t o 27.69 mlrd m 3
a t Aswan, i . e . a n e t g a i n of 1 . 5 7 m l r d m 3 . A d e t a i l e d d i s c u s s i o n of t h e s t o r a g e on t h e N i l e s h a l l b e p r e s e n t e d , hwoever, i n t h e n e x t c h a p t e r . The p r o b a b i l i t y f u n c t i o n s t h a t s e r v e a s good f i t t o t h e monthly d i s c h a r g e d a t a a r e t h e P e a r s o n Type I 1 1 and t h e % p a r a m e t e r l o g n o r m a l . The normal f u n c t i o n
is a v e r y good f i t to t h e d i s t r i b u t i o n o f t h e a n n u a l d i s c h a r g e series. T h i s c a n b e s e e n from F i g .
8 . 5 2 . The monthly and a n n u a l f l o w s w i t h 100 and 200 y e a r s
r e t u r n p e r i o d have been computed from t h e s e d i s t r i b u t i o n f u n c t i o n s and a r e as g i v e n below:
Discharge 106 ,3 J a n . Feb. Mar. Apr. May 100-yr 200-yr
J u n e J u l y Aug.
Sep.
Oct.
Nov.
Dec.
Year
5546 4898 5162 4659 6337 8018 8355 26416 28792 21265 13145 7624 117618 5809 5208 5587 5025 7 0 3 1 9056 8816 27114 29144 21696 13640 8001 121416
The above a n a l y s i s o f t h e d i s c h a r g e s a t Aswan i s b a s e d on t h e a s s u m p t i o n t h a t t h e y a r e homogeneous. The v a l i d i t y o f t h i s a s s u m p t i o n seems q u e s t i o n a b l e .
9.
TABLE 8 . 2 9
0 0
The b a s i c s t a t i s t i c a l descriptors and the s e r i a l correlation c o e f f i c i e n t s of the monthly and annual discharges of the N i l e a t Aswan, for the period 1912-1973
~
I tem
Basic s t a t i s t i c a l descriptor
?, 106 m3
s , 106 m3 C
CV
Month of the year June
July
Aug.
Sep.
Oct.
NOV.
Dec.
Year
2399 1610 0.6713 0.7765 5.1028
4670 1420 0.3040 0,3694 3.0563
16623 5168 0.3109 0.5681 2.9880
19466 6388 0.3282 0.1680 4.1945
13742 4311 0.3137 0.7856 4.0836
7376 2241 0.3038 0.3376 3.2447
4788 1020 0.2131 0.6138 4.1724
82202 15224 0.1852 0.1610 3.1683
0.4750 0.2813 0 $0503 0.0802 0.0975 0.2759 0.1627 0.1331 0 ,0604 I,.0417 0.0129 0.0877 0.1041 0.2234 0.2515
0.4028 0.6773 0.4415 0.5779 0.3200 0.4575 0.2886 0.3050 0.1232 0.2561 2.1201 2.0435 0.0411 0.0300 .1222 0.0605 0 . 1825 0.1917 0.1010 0.1563 0.1659 0.1708 0.0629 0.1021 0.2363 0.1945 0.1882 0.1713 0.1817 0.0873
0.5825 0.2422 0.3128 0.4198 0 -2059 0.2150 0.3146 0.2193 0.1412 0.1217 5.0769 6.1717 0.1336 0.0817 0.0609 0.0754 0.0023 0.1121 0.0169 0.1147 0.1706 2.0695 0.0579 0.0423 0 , 0969 6.0967 0.0354 0.1163 0.0451 0.0575 0,1198 0.1707 0.1302 2.0512 0.1166 0.0295 0.0742 0.0208 2 -0927 0.1647 0.1612 0.0592 0.1277 0.0213 0.1157
0.4838 0.2979 0.1679 0.0033 0 .0239 0.0362 0,. 1360 0.0973 0 , 1543 0.0974 0 . 1478 0.0282 0.1212 0,2148 0,1268
Jan.
Feb.
Mar.
APr.
3578 740 0.2069 0.4676 4.3987
2704 777 0.2874 0.6999 3.8095
2480 868 0.3501 1.0892 3.7018
2153 861 0.3998 0.8272 2.9905
2214 1261 0.5695 1.3802 4.0417
0.2623 0.0012 0.0265 2332 0.1661 0.y 8 8 0.1844 0.0753 0.1196 0.0425 6.2115 0.0185 0.0043 0.0589 0.1374
0.4792 0.3085 0.3268 0.0584 0.1387 0.1851 0.1207 0.1917 0.1302 0.0568 0.2149 0.0536 0.0352 0.0422 0.1113
0.6342 0.5246 0.4820 0.3157 0.3027 0.3187 0.2627 0.2659 0.1591 0.0497 0.0226 0.0232 6.0005 8.0104 0.0873
0.7779 0.6737 0.6242 0,5071 0.4893 0.4737 0.4597 0.4613 0.3885 0.3437 0,3166 0.2938 0 .a389 0.2256 0.2340
0.8389 0.7964 0.7537 0.7179 0.6772 0.6410 0.5665 0.4976 0.5169 0.4611 0.4455 0.3325 0.4047 0.2246 0.3662 0.2296 0.2990 0.1062 0.2463 0.0775 0.2038 0.0079 0.1759 0.0198 0.1637 0 -0268 0.1439 0.0524 0.1496 0.0045
May
S e r i a l correlation coefficient rl
r2 r3 r4 r5 '6 r7 r8 r9 r10 rll r12 r13 r14 1. c
!.
p
409 TABLE 8 . 3 0
S e r i a l c o r r e l a t i o n c o e f f i c i e n t s of t h e r e s i d u a l s l e f t from f i t t i n g f i r s t - o r d e r a u t o r e g r e s s i v e models t o t h e d i s c h a r g e a e r i e s of t h e Main N i l e a t Aswan
Month
Serial correlation coefficient
0
Year Jan * Feb. Mar. Apr May June July Aug Sep. Oct. Nov.
.
.
DeC
.
Year
I1
.*
r2
448 .%
;126 1117 .221 .114 T255 TO40 -. 1 4 3 .046 -.229 .083 -.049 - 1 2 3 -.074 .% -.110 .197 -. 0 4 3 . 1 3 4 -.026 .185 -.051 -154 .041 .114 4
'3
r4
IS
'6
.=1067
-
r8
.=.=.=.=1040
-076 1176 -004 .009 .349 .268 .063 .135 .239 .150 .042 .178 -195 .138 .278 .209 :339 .087 .118 -080 .024 -254 .141 .203 .029 :175 .173 1007 .086 .202 ,060 .161 . 1 4 1 .231 ,083 .151 ,031 .163 :170 .059 .036 .096 .015 .034 .082
-_
'7
.=
'9
,247 ,057 :OS7 .OS8 .006 .069 ,066 .061 .141 .003 :043 ,021 ,023 .079 :143 ,032 ,038 . O W ,029 .095 ,090 .037 :lo2 .044 :137 -032 ,049 :070 . l o 2 OS9 .093 .040 1070
.382 .240 -106 .097 .1S8 .002 .161 .130 ,018 .165 ,160 .094 ,033 -.188 ,141 -.034 .058
'10
.
'11
'12
:252 ,203 :208 .030
.a21 .071 .lo4 .061
:008
-050
'13
'14
,189 ,178 ,007 ,001 ,027 ,080 .010 ,129 :004 ,056 ,010 :138 ,054 ,112 ,194 ,044 ,227 ,015 -031 -103 .043 I124
_045 .086 :059 - 1 7 3 ,151 .113 :115 .182 :041 ,054 ,129 .061 ,159 .055 ,068 :086 .134 . l o 6 .096 .117
'15 .189
.OW
.147 .136 .lo7 ,155 _134 :053 :041 .069
~~
The u n d e r l i n e d v a l u e s are t h o s e s e r i a l c o e f f i c i e n t s which are s i g n i f i c a n t l y d i f f e r e n t from z e r o a t 95% c o n f i d e n c e l e v e l
F i g . 8.51. 1912-1973
The a v e r a g e hydrograph o f t h e Main N i l e a t ASWM
.OX
,092 .024
f o r t h e period
4 10
120
110
100 m
m
90
0 $ 80 L
0
r d U
< 70
-
0 3
Q
60
50
.01
.05 .1 .2 .5 1
2
5
10
Non
20 30 40 50 60 70 80
- cxceedancc
probability, -1.
90 95
98 99 99.5
F i g . 8.52. F i t of t h e normal f u n c t i o n t o t h e d i s t r i b u t i o n of t h e annual d i s charges i n t h e Main N i l e a t Aswan f o r t h e p e r i o d 1912-1973 A mathematical model of t h e River N i l e from i t s e x i t a t Lake Albert t o i t s
e n t r a n c e i n t o Lake Nasser upstream Aswan has very r e c e n t l y been designed and c a l i b r a t e d (Fahmy, A . ,
Panattoni, L.,
and Todini, E., 1982).
The d i f f e r e n t components of t h e N i l e system w e r e a l l modelled using t h e soc a l l e d Constrained Linear Systems model (CLS). This type w a s chosen as a consequence of an a n a l y s i s of t h e purposes of t h e model and t h e amount and n a t u r e of t h e a v a i l a b l e d a t a . The f i r s t component covers t h e reach from Pakwach ( e x i t of t h e N i l e a t Lake A l b e r t ) t o Mongalla; t h d s e c o n d covers t h e reach from Mongalla t o Malakal and H i l l e t Doleib on t h e White Nile and t h e t h i r d , which is divided i n t o two p a r t s , covers t h e River Sobat. The f o u r t h component o r submodel covers t h e reach from Malakal (downstream t h e mouth of t h e Sobat) t o Mogren; t h e f i f t h d e a l s with t h e Blue N i l e , and t h e s i x t h covers t h e reach from Mogren t o Wadi Halfa. A f t e r t h e i n d i v i d u a l submodels w e r e i d e n t i f i e d , they w e r e assembled t o g i v e a model of t h e e n t i r e system from Lake A l b e r t t o Lake Nasser. A f o r t r a n programme was used t o s i m u l a t e t h e behaviour o f a l l t h e system and t h e d i f f e r e n t reaches.
411 The model has been used by i t s d e s i g n e r s f o r computing t h e discharges a t Wadi Halfa a f t e r feeding i t with t h e discharges a t Pakwach i n t h e period from 1953 t o 1972. The comparison between t h e observed and computed hydrographs a t Wadi Halfa show t h a t t h e r e s i d u a l s have a mean and a s t a n d a r d e r r o r o f 2.55 and 28.6 m i l l i o n m3/day, r e s p e c t i v e l y . The l a t t e r f i g u r e corresponds t o 12.2% of t h e d a i l y discharge a t Wadi Halfa. 8.11.3
From Aswan t o t h e Mediterranean Sea
The water t h a t had been l e a v i n g t h e o l d Aswan Dam every y e a r used t o flow i n t h e Nile and i t s branches on i t s way t o t h e Mediterranean Sea. A c e r t a i n p a r t of t h i s flow had been used f o r l a n d i r r i g a t i o n and f o r domestic purposes and t h e
rest had been discharged i n t o t h e s e a . The annual q u a n t i t y passing downstream of Aswan Dam i n such a normal y e a r a s 1947 was 8 4 . 3 mlrd m3. Of t h i s amount, 6 . 6 mlrd m 3 were used f o r b a s i n i r r i g a t i o n , 38.1 mlrd m 3 f o r p e r e n n i a l i r r i g a t i o n and less than 1 mlrd m 3 f o r domestic and i n d u s t r i a l purposes. The rest, almost 39 mlrd m 3 was thrown i n t o t h e s e a . The consumption of water i n Egypt t h a t y e a r , which reached 45.7 mlrd m3, was very n e a r t o t h e f u l l s h a r e of Egypt i n t h e N i l e water a t t h a t t i m e . This was l i m i t e d t o 48 mlrd m3/yr.
I r r i g a t i o n was, and s t i l l
i s , accomplished v i a an i n t r i c a t e c a n a l system. The g e n e r a l l a y o u t of t h e main
c a n a l s i s shown i n F i g . 8.53. From Aswan t o C a i r o t h e r e i s hardly any r a i n f a l l worth mentioning and t h e discharge downstream of Aswan is p r a c t i c a l l y t h e only s o u r c e of water. I n t h i s r e a c h , gauge d i s c h a r g e measurements a r e taken more o r less r e g u l a r l y a t a number of p l a c e s . Examples of t h e s e a r e given f o r Hawatka s t a t i o n n e a r Assiut i n 19261927, F i g . 34, Appendix E , and f o r Beleida s t a t i o n n e a r Koraimat i n 1920-1922, F i g . 35, Appendix E. Before t h e c o n s t r u c t i o n and o p e r a t i o n of t h e s t o r a g e works on t h e N i l e , a g r i c u l t u r e i n Egypt depended almost e n t i r e l y on t h e n a t u r a l supply of t h e r i v e r . The annual inundation of t h e N i l e Valley i n t h e l a t e summer g e n e r a l l y supplied enough moisture t o t h e s o i l t o e n s u r e f a i r crops i n t h e f a l l and w i n t e r . No wonder, t h e n , t h a t t h e most important annual event i n Egypt was t h e Nile flood and, t h e r e f o r e , r e c o r d s were engraved on t h e c l i f f w a l l s i n v a r i o u s p l a c e s , 9
notably a t Semna, a s e c t i o n of . t h e second c a t a r a c t ( s e e map, F i g . 2.25.). Several s e c t i o n s of n i l o m e t e r s , a t v a r i o u s p o i n t s along t h e stream channel, have been discovered, and t h e i r i n s c r i p t i o n s have been deciphered and c o r r e l a t e d . C.S.
J a r v i s combined t h e d i s c o v e r i e s and memoires of S i r H . Lyons, M . LeGrain,
P r i n c e O m a r Toussoun, Aboul Mehasin and many o t h e r s who kept themselves busy with t h e N i l e w a t e r l e v e l s . This l e d J a r v i s t o p u b l i s h h i s marvellous paper on t h e flood-stage records of t h e River N i l e i n Egypt ( J a r v i s , C . S . ,
1935). From
4 12
t h i s paper we have copied t h e maximum and minimum annual l e v e l s of t h e r i v e r a t t h e Roda n i l o m e t e r , Cairo, i n t h e p e r i o d from 622 up t o 1933 and p r e s e n t e d i t
h e r e as Fig. 8.54.
MEDITERANEAN
SEA
lsmailia Canal
Western N a g Hammadi Canal
Kallabiya
Canal
u
1 1, Y--
Assiut
Barrage
Eastern N a g H a m m a d i Canal
Asfoun C a n a l
Esna
Barrage
Old Aswan D a m H i g h Dam
i'
Fig. 8.53. The d i s c u s s i o n s on t h i s paper by Hurst and K.O. Ghaleb, both w e l l equipped w i t h o u t s t a n d i n g information gained from a c t u a l experience w i t h t h e r i v e r , have pointed t o a number of sources of error i n t h e s e d a t a . With f u l l r e c o g n i t i o n of t h e s e e r r o r s and o t h e r p o s s i b l e d i s c r e p a n c i e s i n t h e d a t a , one cannot deny t h e value of t h e l e n g t h of t h i s series and i t s f a i r completeness, e s p e c i a l l y from about 622 t o about 1450 A.D.
413
From t h e s e d a t a J a r v i s found t h e rise i n t h e N i l e f l o o d l e v e l a s c r i b a b l e t o sedimentation t o range from 0.10 t o 0.15 m p e r c e n t u r y . J . C .
Stevens repeated
t h i s c a l c u l a t i o n u s i n g t h e simple 10-yr averages and t h e p r o g r e s s i v e 50-yr averages ( J a r v i s , C.S.,
1935). H e concluded t h a t t h e average r i s e was p r a c t i -
c a l l y 4 inches (0.10 m) p e r century f o r both t h e maximum and minimum s t a g e s . He f u r t h e r concluded t h a t no p e r i o d i c c y c l e w a s i n evidence, but high c y c l e s a l t e r n a t e d w i t h low c y c l e s of i r r e g u l a r d u r a t i o n . From t h a t n e a r l y continuous record and from t h e s p o r a d i c records of i s o l a t e d p e r i o d s running back over 5000 y e a r s , t h e r e appears t o have been l i t t l e o r no c l i m a t i c changes t h a t can be d e t e c t e d . The conclusions drawn by T . H . Means from h i s d i s c u s s i o n on t h e same paper were almost i d e n t i c a l t o t h o s e given by Stevens which have a l r e a d y been presented ( J a r v i s , C.S.,
1935).
We s h a l l p r e s e n t t h e change i n t h e r i v e r bed a s a r e s u l t of t h e c o n s t r u c t i o n of t h e High Aswan Dam i n Chapter 9 . The evaporation l o s s between Aswan and C a i r o i n t h e pre-High Dam p e r i o d could be f i g u r e d o u t approximately a s t h e l e n g t h of t h e reach, 900 km, times t h e average weighted width, 500 m , t i m e s t h e annual evaporation depth, 2 m . This gives 0 . 9 mlrd m3/yr t o be rounded o f f t o 1 . 5 mlrd m 3 / y r t o account f o r t h e l o s s e s from t h e network of i r r i g a t i o n c a n a l s . Whether t h e r e s u l t a n t of t h e seepage from t h e r i v e r and t h e r e t u r n flow t o i t could be considered as a n e t gain
or n e t l o s s i s n o t p r e c i s e l y known. The r e l a t i o n s h i p between both t h e l o s s or g a i n and t h e f a c t o r s a f f e c t i n g i t i n t h e post-High Dam p e r i o d , w a s s t u d i e d by Saleeb, S.I. (1977). To implement t h i s study, t h e r i v e r reach from downstream of Aswan t o C a i r o w a s divided i n t o four reaches ( s e e map, F i g . 8 . 5 3 . ) . The outflow from an upstream reach i s cons i d e r e d a s inflow t o t h e next downstream r e a c h , and so on. Taking t h e town of Assiut a s t h e c e n t r e of g r a v i t y of t h e reach from Aswan t o C a i r o , t h e l o s s or g a i n w a s found t o be a f f e c t e d by t h e d i s c h a r g e j u s t downstream of Aswan, themean a i r temperature and humidity a t Assiut 5 days lagging behind Aswan and t h e groundwater l e v e l a t A s s i u t . The r e g r e s s i o n models f o r t h e d a i l y and monthlyloss or g a i n have been developed and t h e optimum monthly discharges found. These d i s -
charges are included i n Table 8.31. The monthly means of t h e 6-yr p e r i o d 1968-73 and of t h e 56-yr p e r i o d 19f2-67 a r e a l s o included i n t h i s t a b l e f o r t h e purpose of comparison. The optimum d i s c h a r g e s have been found on t h e grounds t h a t they correspond t o t o t a l l o s s o r g a i n equal t o z e r o . A s h o r t d i s t a n c e below C a i r o , t h e r i v e r b i f u r c a t e s i n t o i t s t w o branches:
D a m i e t t a and R o s e t t a . These branches are t h e main source of w a t e r feeding t h e i r r i g a t i o n c a n a l s i n Lower Egypt. They were a l s o used i n t h e pre-High Dam period t o convey t h e excess f l o o d water t o t h e Mediterranean Sea. This i s no longer t h e
case a f t e r e x e r c i s i n g f u l l c o n t r o l on t h e N i l e water by means of theHighAswanDam
4 14
415 In F i g . 8 . 5 4 . , t h e v a r i o u s p l o t t e d p o i n t s may be i d e n t i f i e d a s follows: Applying t o a l l t h e records:
= f i v e - y e a r average f o r t h e d a t a shown by d o t s , sup-
V
X
*
plemented by - o r (-) when t h e l o c a t i o n of t h e dot i s n o t given; = ten-year average f o r d a t a shown by d o t s , suppleo r (-) when t h e l o c a t i o n of t h e dot i s mented by n o t given; and, = one hundred-year average
-
Applying t o t h e Roda Gauge a t Cairo: r e c o r d s compiled by Omar Toussoun+ covering t h e 1300-yr p e r i o d from 622 t o 1921 A.D.; confirmation from t e x t u a l n o t e s f o r t h e r e c o r d s compiled by Omar Toussoun: when - i s l a c k i n g , an agreemept is i n d i c a t e d between a l l t h r e e s o u r c e s ; r e c o r d s compiled by Aboul Mehasin++, covering t h e p e r i o d , 20 t o 855 of t h e Hegira, o r 641 t o 1451 A.D., a t o t a l of 811 y e a r s . A small + i n d i c a t e s e x t r a d a t a , r e p r e s e n t i n g 25 s u r p l u s y e a r s of Mohammedan reckoning; r e c o r d s from n o t e s compiled by Ibn Iyas and others+++, f o r t h e period 769 t o 1878 A.D. r e c o r d s by Lyons'; and r e c o r d s i n d i c a t i n g "wafa", or t h e s t a g e t h a t a s s u r e s p l e n t y , a t which t h e c a n a l s w e r e opened t o supply t h e b a s i n s ; t h e maximum f l o o d s t a g e , o r d i n a r i l y , w a s somewhat h i g h e r . Applying t o t h e gauge downstream from Aswan Dam:
A = maximum annual r i v e r s t a g e s a t t h e Aswan gauge above t h e assumed datum, 71.0 m, or 232.9 f t , above mean Mediterranean Sea l e v e l ;
w = maximum ten-day average gauge h e i g h t s , and, t h e r e f o r e , somewhat below t h e a c t u a l maximum.
Applying t o t h e E l - L e i s i
* =
gauge, 37 miles upstream from Cairo:
maximum ten-day average gauge h e i g h t s and, t h e r e f o r e , somewhat below t h e a c t u a l minimum.
The gauge h e i g h t s p l o t t e d a s o r d i n a t e s i n F i g . 8 . 5 4 . a r e readings from t h e Roda gauge on t h e N i l e River a t Cairo. The e x c e p t i o n s , marked A o r J , r e f e r t o r e a d i n g s of t h e Aswan gauge and t h o s e marked 0 r e f e r t o readings of t h e E l - L e i s i gauge, 37 m i l e s above C a i r o . I n a l l c a s e s t h e r e a d i n g s a r e t h e mean Mediterranean Sea l e v e l e l e v a t i o n s a t Alexandria,.
Scale of Meters 0
10
20
I . ~ . ' . 25 ' . I ' " . 'I ~.50l ' ' . , ! - ' 75, ' * : - ' " I Scale of Feet Scale of Cubits (Egyptian) 10 0 I . . ... . . ! . .'!. :. * * :. . .'.. 30 20 0 10 Scale of Feet
30
0
.I..
1 Cubit
=
-
6'.
9
I
23.9417 Digits, 10 Digits
++
100
- - - :, 20
I
50
40
: . ' . ' -125: ~ " * . 150 '"''*.I 154 T.'...'.
40
- . ...
30 I
.-I
5051 L
8.5875 Inches
++
LOC. C i t . , 1923, Vol. 4 . ; loc. cit., +Memoirs, I n s t . of Egypt, 1925 Vol. 9 . ; 1923 and 1925, Vol. 4 and 9 . ; *"The Nile Flood i n 1905", by Capt. H.G. Lyons
4 16
TABLE 8 . 3 1
Comparison between t h e a c t u a l and t h e proposed optimum discharges downstream of Aswan Dam
Month &
Average f o r t h e p e r i o d s , mlrd m 3
Year
1912-67 (56 y r s )
1968-73 (6 y r s )
January February March April May June July August September October November December Year
36 25 25 79 2335 1968 1894 1959 4448 17743 21095 14808 7777 4948 85149
3142 3870 3836 3877 5200 6507 6 742 6167 4265 3788 3634 3296 54324
Optimum d i s c h a r g e from m u l t i p l e r e g r e s s i o n model mlrd m3 2713 3646 3900 3588 5 180 6552 6699 5943 389 1 3419 3069 2880 51480
REFERENCES Ahmed, A . A . , 1960. Recent developments i n N i l e c o n t r o l . Proceedings I n s t i t u t i o n of C i v i l Engineers, London, Vol. 17, Paper No. 6102: 137-180. Berg, C . L . , 1953. D e t a i l e d a n a l y s i s of a discharge measurement on t h e V i c t o r i a N i l e . Proceedings I n s t i t u t i o n of C i v i l Engineers, London, P a r t 111, Vol. 2, Paper No. 5935: 609-613. C a i r o University/MIT, 1977. The River N i l e p r o j e c t ; s t o c h a s t i c modelling of t h e N i l e inflows t o Lake Nasser. Cairo University/MIT t e c h n o l o g i c a l planning programme, C a i r o , Egypt. Fahmy, A . , P a n a t t o n i , L . , and Todini, E . , 1982. A mathematical model of t h e River N i l e . Engineering a p p l i c a t i o n s of computational h y d r a u l i c s (Abbott, M.B., and Cunge, A . J . : e d i t o r s ) , Pitman, London: 111-130. H u r s t , H . E . , and P h i l i p s , P . , 1932. The N i l e Basin, Vol. 11, measured discharges of t h e N i l e and i t s t r i b u t a r i e s , P h y s i c a l Department Paper N o . 28, Government P r e s s , C a i r o , Egypt. Hurst, H . E . , and P h i l i p s , P . , 1933. The N i l e Basin, Vol. I V , ten-day mean and monthly mean discharges of t h e N i l e and i t s t r i b u t a r i e s . Physical Department Paper No. 30, Government P r e s s , Cairo, Egypt. Hurst, H.E., and P h i l i p s , P . , 1938. The N i l e Basin, Vol. V , t h e hydrology of t h e Lake P l a t e a u and t h e Bahr e l - J e b e l . P h y s i c a l Department Paper No. 35, S c h n i d l e r ' s P r e s s , C a i r o , 235 pp. Hurst, H . E . , Black, R.P., and Simaika, Y . M . , 1951. The N i l e Basin, Vol. V I I , t h e f u t u r e c o n s e r v a t i o n of t h e N i l e . PhysiSal Department Paper No. 51, Eastern P r e s s ( r e p r i n t e d ) , C a i r o , 157 pp (with appendices). Hurst, H . E . , 1950. The Nile Basin, Vol. V I I I , t h e hydrology of t h e Sobat and White Nile and t h e topography of t h e Blue Nile and Atbara, Physical Dapartment, Paper No. 55, Government P r e s s , C a i r o , 125 pp. Hurst, H . E . , Black, R.P., and Simaika, Y . M . , 1959. The N i l e Basin, Vol. I X , t h e hydrology of t h e Blue Nile and Atbara and of t h e Main N i l e t o Aswan with Some r e f e r e n c e t o p r o j e c t s . N i l e Control Department, Paper No. 12, General Organiz a t i o n f o r Government P r i n t i n g O f f i c e s , Cairo, 206 pp. Hurst, H . B . , Black, R.P., and Simaika, Y . M . , 1966. The N i l e Basin, Vol. X , t h e major N i l e p r o j e c t s . Nile Control Department, Paper No. 23, General Organizat i o n f o r Government P r i n t i n g O f f i c e s , C a i r o , 217 pp.
4 17
J a r v i s , C . S . , 1935. Flood-stage records of t h e River N i l e , Transactions ASCE, Paper No. 1944 (with d i s c u s s i o n s by H.P. G i l l e t e , R . W . Davenport, H . E . Hurst, T . H . Means, J . W . Breadsley, J . C . Stevens, J . W . Shuman, K.O. Ghaleb and C . S . J a r v i s ) : 1012-1071. J o n g l e i Area Executive Organ for Development P r o j e c t s , 1975. J o n g l e i P r o j e c t , r e p o r t on phase I , Tamaddun P r e s s , Khartoum, 100 pp. King, J . W . , 1975. S o l a r phenomena, weather m d c l i m a t e . European Space Agency, ESA B u l l e t i n No. 3, Neuilly-sQr-Seine, France: 24, 49-51. K i t e , G.W., 19Sl. Recent changes i n l e v e l of Lake V i c t o r i a , B u l l e t i n of hydrol o g i c a l s c i e n c e s , V o l . 26, No. 3: 233-243. Rodriguez, I . and Yevjevich, V . , 1967. Sunspots and hydrologic t i r e a e r i e s . Proceedings of t h e I n t e r n a t i o n a l Hydrology Symposium, F t . C o l l i n s , Colorado, V o l . I : 397-405. Saleeb, I . S . , 1977. River N i l e r a t e r s y s t e m a n a l y s i s u s i n g d i g i t a l computers. Proceedings of t h e I n t e r n a t i o n a l Conference on Computer A p p l i c a t i o n s i n Developing C o u n t r i e s . AIT, Bangkok, Vol. 11: 777-791. Shahin, Y . , 1983. E f f e c t of s t o r a g e works i n t h e N i l e River system on t h e hog e n e i t y i n t h e annual flow series. Proceedings of t h e Symposium on K a t e r Resources, Varna, Bulgaria: 11-23. W,1974. Hydrometeorological survey o f t h e catchments of Lakes V i c t o r i a , Kyoga and A l b e r t , Vol. I , P a r t s I 0 11: meteorology and hydrology of t h e b a s i n , WHO, Geneva. WHO, 1974. Hydrometeorological survey of t h e Catchments of Lake8 V i c t o r i a . Kyoga and A l b e r t , V o l . 111: p r e l i m i n a r y r e p o r t s on t h e index catchments, .yy). Geneva. WMO, 1974. Hydrometeorological survey of t h e catchments of Lakes V i c t o r i a , Kyoga and A l b e r t , Vol. I V : h y d r o l o g i c a l s t u d i e s of s e l e c t e d r i v e r b a s i n s , WMO, Geneva.
4 19
Chapter 9 WATER STORAGE AND CONSERVATION INTRODUCTION
While analyzing and d i s c u s s i n g t h e stream flow i n t h e River N i l e system, Chapter 8, a b r i e f mention was made t o water s t o r a g e and conservation works. This c h a p t e r w i l l d e a l with a somewhat d e t a i l e d d e s c r i p t i o n of such works; those completed and f u n c t i o n i n g and those which a r e underway. The f i r s t s t o r a g e work b u i l t , a t l e a s t i n contemporary h i s t o r y , i s t h e o l d dam a t Aswan, Egypt (1898-1902).
The o p e r a t i o n of t h i s work was based on s t o r i n g
a s m a l l volume of f l o o d water and r e l e a s i n g i t i n t h e next low-flow season t o t h e downstream reach of t h e r i v e r t o improve t h e n a t u r a l discharge. Both s t o r a g e and r e l e a s e were accomplished i n t h e same water y e a r . One may thus d e s c r i b e t h e r e s e r v o i r s formed by t h e o l d Aswan Dam and i t s heightenings a s annual s t o r a g e works. Subsequent s t o r a g e works have been designed and operated on t h e same b a s i s . The only exception is t h e High Dam a t Aswan, Egypt (1956-1964) which was designed according t o t h e theorem of long-term o r century s t o r a g e . The p r i n c i p a l l i n e s of t h e annual and long-term s t o r a g e theorems a r e b r i e f l y reviewed and d i s cussed i n t h e next s e c t i o n s . 9.1
WATER STORAGE I N THE NILE BASIN
9.1.1
Annual s t o r a g e
The method used t o determine t h e l i v e - s t o r a g e c a p a c i t y of a r e s e r v o i r was developed i n 1882 by Rippl. Since t h i s method can, p r e f e r a b l y , be worked o u t g r a p h i c a l l y , i t i s commonly r e f e r r e d t o a s t h e Rippl-diagram o r mass curve method. 9.1.1.1
Simple c a s e of seasonal s t o r a g e where demand and supply a r e equal and t h e r e a r e no s t o r a g e l o s s e s
Suppose t h a t t h e d a t a i n cobumns 2 and 4 of Table 9 . 1 r e p r e s e n t t h e monthly n a t u r a l flow reaching a r e s e r v o i r and t h e demand downstream of i t , r e s p e c t i v e l y . The graphic p l o t of t h e s e data v e r s u s t h e t i m e i n months shows c l e a r l y t h a t t h e r e a r e months of excess ( n a t u r a l supply > demand) and months of d e f i c i t ( n a t u r a l supply < demand). Fig. 9 . l a . shows t h a t t h e season from August t o January i s a p e r i o d of excess whereas t h e r e s t of t h e y e a r , except February, i s one of d e f i c i t . February is t h e only p e r i o d without excess or d e f i c i t . This case i m p l i e s t h a t t h e r e s e r v o i r must be f i l l e d i n t h e p e r i o d of e x c e s s , n e i t h e r f i l l e d nor emptied i n February, and emptied from March t o J u l y t o compensate f o r
420
the d e f i c i t of t h e n a t u r a l supply i n t h i s p e r i o d . The Rippl-diagram method o f f e r s a simple tool f o r determining t h e r e q u i r e d r e s e r v o i r c a p a c i t y . The mass curves of t h e supply and demand can be o b t a i n e d by p l o t t i n g t h e f i g u r e s i n columns 3 and 5 , Table 9 . 1 , v e r s u s t i m e . The d i f f e r e n c e between t h e o r d i n a t e s of t h e two mass curves a t any i n s t a n t equals t h e volume of r e s e r v o i r c o n t e n t s a t t h a t p a r t i c u l a r i n s t a n t . I n t h e p r e s e n t example t h e maxi-
mum s t o r a g e i s a t t a i n e d a t t h e end of January and remains c o n s t a n t t i l l t h e end of February, a f t e r which t h e r e s e r v o i r i s emptied t i l l t h e end of t h e y e a r . The same r e s u l t can be seen i n t h e l a s t column i n Table 9 . 1 . TABLE 9 . 1
Monthly and cumulative supply and demand (Shahin, M., 1971)
Month August September October November December January February March April May June July
Monthly supply, 106 m 3
Cumulative supply, 106 m 3
Monthly demand, 106 m 3
Cumulative demand,
Cumulative supply-demand, 106 m 3
80 70 35 25
80 150 185 2 10 230 245 255 265 275 2 85 290 300
30 15 15 10 05 05 10 20 30 40 50
30 45
50 105 125 140 155 165 165
20
15 10 10 10 10 05 10
106 m 3
60
70 75 80 90 110 140 180 2 30 300
70
155 135 105 60 0
Sometimes i t could be m o r e convenient to draw t h e s o - c a l l e d " d i f f e r e n t i a l mass curve", which i s simply a g r a p h i c a l p l o t of t h e o r d i n a t e d i f f e r e n c e between t h e two mass curves versus t i m e , using a h o r i z o n t a l datum. This curve f a c i l i -
t a t e s t h e reading of t h e r e s e r v o i r c o n t e n t s a t any t i m e during t h e year ( s e e Fig. 9 . l b . ) .
In t h i s example, t h e maximum c a p a c i t y of t h e r e s e r v o i r a s read from
t h e l a s t column of Table 9 . 1 i s 165 x
lo6
m3.
The same f i g u r e can be o b t a i n e d
e i t h e r from t h e maximum o r d i n a t e d i f f e r e n c e between t h e two curves or simply a s t h e maximum o r d i n a t e of t h e d i f f e r e n t i a l mass curve. b
9.1.1.2
C a s e of supply g r e a t e r than demand
I f , i n t h i s c a s e , w e use t h e same mode of computation as i n s e c t i o n 9.1.1.1, a c e r t a i n amount of water w i l l remain u n u t i l i z e d up t o t h e end of t h e y e a r (see Fig. 9 . 2 a . ) . This amount is simply t h e excess of t h e y e a r l y supply over t h e y e a r l y demand. Were i t assumed t h a t t h e r e s e r v o i r must be empty by t h e end of t h e y e a r , t h e l a s t o r d i n a t e s of t h e supply and demand m a s s curves must then be e q u a l , The method of computation can be modified a s follows: w i t h r e f e r e n c e t o
42 1
Fig. 9.2a.,
i n s t e a d of s t a r t i n g t h e computation o f t h e m a s s demand from t h e
b e g i n n i n g of t h e y e a r a t 0 , t h e order of t h e p r o c e d u r e is r e v e r s e d , and t h e comp u t a t i o n s t a r t s from p o i n t B c o r r e s p o n d i n g t o t h e end o f t h e y e a r . The mass demand i s now drawn from r i g h t t o l e f t u n t i l i t i n t e r s e c t s t h e mass s u p p l y a t C . From C down t o 0 t h e mass demand s h o u l d c o i n c i d e w i t h t h e mass s u p p l y . I n o t h e r words, t h e r e s e r v o i r h a s t o b e k e p t empty i n t h e p e r i o d OC and f i l l i n g s t a r t s o n l y on t h e d a t e c o r r e s p o n d i n g t o C . T h i s method r e s u l t s i n a maximum r e s e r v o i r c a p a c i t y R2 smaller t h a n R1 ( t h e d i f f e r e n c e b e i n g e q u a l t o t h a t between t h e mass s u p p l y and t h e mass demand). A s t h e d i s c h a r g e o f a r i v e r c a n h a r d l y be f o r e c a s t a c c u r a t e l y , one may, i n s t e a d o f waiting till t h e t i m e r e p r e s e n t e d by p o i n t C , b e g i n t h e f i l l i n g a t 0 apd c o n t i n u e till t h e r e q u i r e d amount i s s t o r e d . A f t e r t h a t , t h e r e s e r v o i r h a s t o b e m a i n t a i n e d f u l l , u n t i l demand e x c e e d s t h e n a t u r a l supply. Another way t o accomplish t h e same p u r p o s e i s shown i n F i g . 9.2b. The diagram shown c a n b e c o n s t r u c t e d by f i r s t p l o t t i n g t h e mass s u p p l y c u r v e . The o r d i n a t e s of t h e mass demand a r e computed and t h e r e s e r v o i r s i z e , R2, found. The mass demand i s t h e n drawn s t a r t i n g from t h e o r i g i n . On t h i s diagram t h e o r d i n a t e DE, e q u a l t o R2, c a n b e found. From E a c u r v e p a r a l l e l t o t h e mass s u p p l y c u r v e i s drawn, as i s a c u r v e p a r a l l e l t o t h e m a s s demand c u r v e s t a r t i n g from p o i n t B. The t w o m a s s c u r v e s i n t e r s e c t a t p o i n t G and t h e l i n e OEGB s h o u l d r e p r e s e n t t h e
m a s s curve o f t h e outflow. This curve can be divided i n t o t h r e e d i s t i n c t p a r t s : i n which t h e o u t f l o w e q u a l s t h e demand and t h e f i l l i n g o f t h e r e s e r v o i r
0-E,
t a k e s p l a c e ; E-G,
i n which t h e o u t f l o w exceeds t h e demand t h e t h e r e s e r v o i r con-
t e n t s are k e p t c o n s t a n t , and G-B, i n which t h e o u t f l o w e q u a l s t h e demand t h e and t h e r e s e r v o i r emptying t a k e s p l a c e .
. 90
5 c
W
80
; .70
6 100
5 200
m
“0E
m, E
60
U
50
C
.,
--
!!j40
Mass suppl
7-
‘izoo Q
0
5 30
1
ln
--
.-Y 100
0 20 10
0
300
0
0 3
8 9101112 1 2 3 4 5 6 7 Month
Fig. 9 . l a . Hydrographs of n a t u r a l s u p p l y and demand
s V
I
I
/#S.
./
-e--
o
Mass demand
I
8 9101112 1 2 3 4 5 6 7 Month
Mass c u r v e s o f supply Fig, 9.lb. and demand. D i f f e r e n t i a l mass c u r v e and r e s e r v o i r c a p a c i t y , a l l f o r e q u a l c u m u l a t i v e s u p p l y and demand
42 2
400
B
300
B'
'cl
2
x
/ \ Mass oYg'9;o'11'1;1
demand
'2'3'&'7' Month
8 9 1011 12 1 2 3 4 5 6 7 Month
Fig. 9 . 2 a . Mass curves of supply. demand and outflow and minimum r e s e r v o i r s i z e f o r t h e c a s e of cumu l a t i v e supply g r e a t e r than cumulat i v e demand
9.1.1.3
Fig. 9.2b. A possible solution f o r t h e minimum r e s e r v o i r c a p a c i t y i n t h e c a s e of cumulative supply g r e a t e r t h a n cumulative demand ( n o t i c e t h e change i n mass outflow than i n 9 . 2 a . )
Case of supply equal t o , o r g r e a t e r t h a n , demand, i n i t i a l s t o r a g e required
In t h e above cases supply and demand have been arranged so t h a t t h e o r d i n a t e of t h e mass supply a t any t i m e i s equal t o , o r g r e a t e r t h a n , t h e o r d i n a t e of t h e
mass demand. This i s , however, n o t always t h e case and t h e two mass curves o f t e n i n t e r s e c t a t least once. The s t o r a g e c a l c u l a t i o n h e r e can b e t t e r be e x p l a i n e d using t h e d a t a i n Table 9 . 2 (Shahin, Y . ,
TABLE 9 . 2
1971).
Monthly and cumulative supply and demand and d i f f e r e n c e between mass supply and demand w i t h and without i n i t i a l s t o r a g e
Month
August September October November December January February March April May June July
Cumulative supply minus Monthly Cumulative Monthly Cumulative cumulative demand, lo6 m3 supply, supply, demand, demand, lo6 m 3 lo6 m 3 lo6 m3 lo6 m 3 without i n i t i a l with i n i t i a l storage storage 10 15
30 60
80 50
20 10 10 05 05 05
10 25 55 115 195 245 265 2 75 2 85 290 295 300
45 35
30
15 10 05 10 15 25
30
35 45
45 80 110 125 135 140 150 165 190 220 255 300
-35
-55 -55 10 60 105 115 110 95 70 40 0
-
20 0
0 45 115 160 170 165 150 125 95 55
413
W e s t a r t by p l o t t i n g t h e supply and demand hydrographe a8 shown in Fig. 9.3..
This p l o t shows one p e r i o d o f e x c e s s , whereas t h e d e f i c i t is s p l i t i n t o two s h o r t e r p e r i o d s ; one a t t h e beginning o f t h e y e a r and t h e o t h e r from March t i l l t h e end of t h e year. Since the y e a r l y demand and supply are e q u a l , t h e volumes of e x c e s s and d e f i c i t m u 6 be equal too. As b e f o r e , w e s t a r t by drawing t h e m p s s supply and demand curves. F r o m Fig.
9.3b. and Table 9 . 2 one can see t h a t t h e r e q u i r e d demand cannot be f u l f i l l e d ,
u n l e s s a c e r t a i n i n i t i a l s t o r a g e is provided. Furthermore, t h e m a x i m u m negative cumulative d i f f e r e n c e which Table 9 . 2 g i v e s a s 55 x lo6 m3 should be considered a s t h e minimum volume needed f o r t h e i n i t i a l s t o r a g e . To a d j u e t t h e mass supply curve t o t h i s s i t u a t i o n one need8 t o add an amount of 55 x lo6 m3 t o i t s o r d i n a t e s . F i g . 9.3b.,
a s w e l l a s Table 9 . 2 , show t h a t t h e m a x i m u m d i f f e r e n c e bet-
ween t h e a d j u s t e d mass supply and t h e a d j u s t e d mass demand is 170 x lo6 m 3 . This r e p r e s e n t s t h e f u l l c a p a c i t y r e q u i r e d f o r t h e r e s e r v o i r . The same volume can be fouad from t h e d i f f e r e n t i a l mass curve and from t h e last column in Table 9 . 2 . The mass diagram can b e s p l i t i n t o f o u r p a r t s . The p a r t i a l l y f u l l r e s e r v o i r (55 x
lo6
m3) in Augu6t i s d e p l e t e d g r a d u a l l y and becomes e r p t y a t t h o end of
September. The demand is e q u a l t o t h e n a t u r a l supply in October and so t h e r e s e r v o i r r e m a i n s empty. In November t h e supply exceeds t h e demand and t h e storage continues t i l l t h e r e s e r v o i r becomes completely f u l l a t t h e end of February (volume of c o n t e n t s = 170 m i l l i o n m 3 ) .
From t h e beginning of March t i l l t h e end
of t h e y e a r t h e r e s e r v o i r is p a r t l y emptied till t h e volume o f c o n t e n t s reaches t h e i n i t i a l s t o r a g e ( 5 5 x lo6 m3) by t h e end of J u l y .
1 100 - 1
5
6 E
E
90 80
:100
70
0
0
s. 6500
(D
400
I
.....,..,.
Adjusted mass supply
u
P 40 0
5ul
30
20 10
0"
' " '
I
'
" '
'
J
8 91011 12 1 2 3 4 5 6 7 Month
Fig. 9.311. Hydrographs or n a t u r a l supply and demand
8 91011121 2 3 4 5 6 7 Month Fie. 9 . a . Mass curves and d i f f e r e n t i a l mass curves. I n i t i a l s t o r a g e is needed
424
9.1.1.4
C a s e of a sequence of y e a r s , t h e cumulative supply i n each y e a r being
e q u a l t o , or g r e a t e r t h a n , t h e cumulative demand I t i s common i n r e s e r v o i r o p e r a t i o n t o c o n s i d e r a sequence of y e a r s r a t h e r than a s i n g l e year. The demand i s f i x e d according t o t h e purpose f o r which w a t e r
i s used: i r r i g a t i o n , h y d r o - e l e c t r i c power development, f l o o d c o n t r o l , e t c . , and f o r a t l e a s t some t i m e , i t remains t h e same, o r n e a r l y t h e same, each y e a r . The quota f o r Egypt i n t h e pre-High Aswan Dam c o n d i t i o n was l i m i t e d t o 48 mlrd m3/yr (1929-1964).
This quota has been i n c r e a s e d t o 55.5 mlrd m3/yr i n t h e post-dam
c o n d i t i o n which began i n 1965. From 1869-1870 up t o 1979-1980 t h e annual supply a t Aswan always exceeded e i t h e r f i g u r e , except in t h e water year 1913-1914 when the supply f e l l t o about 42 mlrd m3.
A number of methods f o r determining t h e " s a f e y i e l d " given a r e s e r v o i r was reviewed by B e r n i e r J .
(1966).He defined t h e s a f e y i e l d a s t h a t y i e l d correspon-
ding t o a c e r t a i n p r o b a b i l i t y of f a i l u r e i n f i l l i n g t h e r e s e r v o i r . The method he developed f o r computing t h e s a i d p r o b a b i l i t y is based on t h e theory of t h e Markov processes which allow t a k i n g i n t o account t h e dependence between t h e i n p u t s t o t h e r e s e r v o i r . For a more e x t e n s i v e p r e s e n t a t i o n of t h e a v a i l a b l e techniques and methods r e l a t e d t o r e s e r v o i r c a p a c i t y and y i e l d , t h e r e a d e r is r e f e r r e d t o t h e work of McMahon, T . ,
and Mein, R. (1978). F i g . 9.4.
shows a s h o r t
sequence composed of t h r e e y e a r s . In t h e f i r s t y e a r supply e q u a l s demand and t h e r e s e r v o i r c a p a c i t y needed t o guarantee t h e f i x e d demand i s 165 x f i g u r e can be obtained a s d e s c r i b e d i n s e c t i o n 9 . 1 . 1 . 1 . same r e s e r v o i r c a p a c i t y , i . e . 165 x t h e n e t supply i n t h i s year (380 x
(300 x
lo6
lo6 m3 is necessary lo6 m3) is, however,
lo6
m3.
This
In t h e second y e a r t h e t o s a t i s f y demand. Since i n excess of t h e demand
m3), t h e r e s e r v o i r a t t h e end of t h e water year i s not t o t a l l y empty.
I n s t e a d , t h e r e remains a volume of c o n t e n t s of 80 x
lo6
m 3 . I f the reservoir
should be empty by t h e end of t h e y e a r , t h e procedure described i n s e c t i o n 9 . 1 . 1 . 2 ( s e e Fig. 9.2b.) has t o be used. A s a r e s u l t of t h e excess of t h e n e t supply over t h e demand, t h e outflow curve does n o t c o i n c i d e i n its f u l l l e n g t h with t h a t of t h e demand. The outflow curve h e r e c o n s i s t s of t w o e x t e r n a l arms each similar t o t h e corresponding p a r t s of t h e demand curve, and of a c e n t r a l p a r t which is p a r a l l e l t o t h e supply curve apd i n which t h e volume of t h e reserv o i r c o n t e n t s is maintained c o n s t a n t . The t h i r d y e a r has a cumulative supply of 420 x
lo6
m 3 a t i t s end which is g r e a t e r t h a n t h e f i x e d demand (300 x lo6 m 3 ) .
Moreover, t h e monthly values of t h e supply and t h e demand a r e so t h a t a new r e s e r v o i r c a p a c i t y R2 s m a l l e r than R
1
would be adequate t o guarantee t h e
r e q u i r e d demand. I f t h e supply could be p r e c i s e l y f o r e c a s t e d one needs t o f i l l (115 x lo6 m 3 ) . The r e s e r v o i r c o n t e n t s should then 2 be kept c o n s t a n t a t t h i s volume t i l l t h e n a t u r a l supply begins t o f a i l t o the r e s e r v o i r p a r t l y up t o R
s a t i a f y t h e demand. From t h i s moment onwards t h e r e s e r v o i r c o n t e n t s a r e depleted
425
gradually t i l l t h e end of t h e y e a r . The above-described case is n o t r e a l l y t o o d i f f e r e n t from t h a t of t h e annual s t o r a g e on t h e Main N i l e or its t r i b u t a r i e s . The annual supply is, i n g e n e r a l , much g r e a t e r than t h e annual demand. Nevertheless t h e d a i l y supply is, i n some months, less than t h e d a i l y demand and i n o t h e r months, more. The s t o r a g e capac i t i e s a r e f a i r l y s m a l l , so most of t h e f l o o d water i s r e l e a s e d downstream without being used, and a s m a l l amount only is s t o r e d t o h e l p improve t h e n a t u r a l supply d u r i n g t h e low-flow season to meet with t h e demand.
' 8
1012 2 4 6 8 1 0 1 2 2 4 6 8 1 0 1 2 2 4 6
+Year
nP 1 -+-Year
Month n-0 2
+Year nS! 3 -4
Fig. 9.4. Reservoir c a p a c i t y and o p e r a t i o n f o r a sequence of y e a r s i n which t h e y e a r l y n e t supply e q u a l s or exceeds t h e y e a r l y demand 9.1.2 9.1.2.1
Annual s t o r a g e works on t h e N i l e and i t s t r i b u t a r i e s The o l d Aswan Dam
Up t o t h e beginning of t h e beginning of t h e t w e n t i e t h century t h e amount of water t h a t could be used f o r i r r i g a t i n g t h e summer crops i n Egypt was almost l i m i t e d t o t h e n a t u r a l supply of t h e r i v e r . Such an amount i n a low-flow y e a r was hardly s u f f i c i e n t f o r i r p i g a t i n g 1 . 5 m i l l i o n a c r e s . Under t h e t h r u s t of i n c r e a s i n g population i t was decided t o extend p e r e n n i a l i r r i g a t i o n to a v a s t area. To f u l f i l l t h e necessary i r r i g a t i o n requirement t h e Aswan Dam was f i r s t b u i l t i n 1898-1902 t o s t o r e j u s t 1 mlrd m 3 of t h e f l o o d water and t o use i t t o g e t h e r w i t h t h e n a t u r a l supply of t h e r i v e r i n t h e followi n g low-flow season.
The dam was f i r s t heightened i n 1912 and thereupon t h e
s t o r a g e c a p a c i t y of t h e r e s e r v o i r i n c r e a s e d t o about 2 . 3 mlrd m 3 . The second heightening of t h e dam took p l a c e i n 1934, which brought t h e c a p a c i t y t o a
426
f i g u r e between 5 . 0 and 5 . 1 mlrd m 3 ( n o t i c e t h a t i n t h e p e r i o d 1902-1964 t h e mean annual s u p p l y w a s a b o u t 84 mlrd m3 and t h e a n n u a l demand 48 mlrd m3). I n 1944 t h e i d e a o f a t h i r d h e i g h t e n i n g o f t h e dam by a b o u t 11 metres, which would have i n c r e a s e d t h e r e s e r v o i r c a p a c i t y up t o 10 mlrd m 3 , w a s proposed. The i d e a w a s g i v e n up owing t o t h e danger of s i l t d e p o s i t i o n i n t h e r e s e r v o i r when used f o r f l o o d p r o t e c t i o n and t o t h e d i f f i c u l t y of f i l l i n g t h e e n l a r g e d reserv o i r w i t h n o n - s i l t y water, and a l s o t o r e l u c t a n c e t o c o m p l i c a t e t h e s t r u c t u r e any f u r t h e r ( H u r s t , H.E., B l a c k , R.P., and Simaika, Y.M., 1959). The c r o s s - s e c t i o n o f t h e dam and i t s two h e i g h t e n i n g s i s shown i n F i g . 1, Appendix F. Some o f t h e t e c h n i c a l d a t a r e l e v a n t t o t h e dam and t h e s t o r a g e r e s e r v o i r are i n c l u d e d i n t h e same Appendix. The r u l e c u r v e i l l u s t r a t i n g t h e u s u a l o p e r a t i o n o f t h e r e s e r v o i r is shown i n F i g . 9.5. f o r 1953. Towards t h e e n d
of J u l y the r e s e r v o i r w a s r a t h e r empty and i t s l e v e l had f a l l e n t o a b o u t 98 where i t remained f o r a s h o r t t i m e u n t i l t h e f l o o d wave r a i s e d t h e l e v e l t o something between 1 0 1 and 103 ( n o t i c e t h a t t h e water l e v e l i n t h e N i l e down-
stream t h e dam had i n c r e a s e d by a b o u t 6 m). The r e s e r v o i r l e v e l f e l l t o about 99
a t t h e end o f September and by t h e t e n t h o f October t h e normal f i l l i n g u s u a l l y began and was complete a t l e v e l 121 sometime i n December. I t remained a p p r o x i mately a t t h a t l e v e l u n t i l t h e s u p p l y r e a c h i n g t h e r e s e r v o i r f a i l e d t o cope w i t h t h e r e q u i r e m e n t s f o r i r r i g a t i n g t h e summer c r o p s . T h i s u s u a l l y happened a t t h e b e g i n n i n g o f F e b r u a r y . Water from t h e r e s e r v o i r w a s t h e n used t o supplement t h e s u p p l y and t h e l e v e l f e l l u n t i l t h e r e s e r v o i r was a l m o s t empty towards t h e e n d o f J u l y . I t w a s customary t o b e g i n the f i l l i n g p h a s e when t h e r i v e r s t a g e downstream Aswan w a s 90.5 m above sea l e v e l o r when t h e f a l l i n g limb of t h e s u p p l y hydrograph r e a c h e d 530 m i l l i o n m3/day.
T h i s always took p l a c e between t h e t e n t h and
t h e 2 0 t h o f October e v e r y y e a r . These f i g u r e s were t h e outcome of e x p e r i m e n t s which showed t h a t a t l e v e l 90.50 t h e suspended matter i n t h e N i l e w a t e r had p r a c t i c a l l y no i n f l u e n c e on t h e s t o r a g e c a p a c i t y o f t h e r e s e r v o i r . The r e s u l t o f t h e e x p e r i m e n t s f o r t h e p e r i o d 1914-1927 i s shown i n F i g . 9 . 6 . The f i n a l programme of t h e f i l l i n g phase had t o be planned as soon as t h e r e a d i n g of t h e l o c a l gauge a t Atbara r e a c h e d 1 4 . 0 0 , which meant t h a t t h e l e v e l chosen a t Aswan would be r e a c h e d t h e r e a b o u t 8 days l a t e r ( d i s t a n c e = 1555 k m P
and v e l o c i t y of p r o p a g a t i o n o f f l o o d = 2 . 2 5 m/sec.). The r e s e r v o i r was used a number o f times as an emergency f l o o d e s c a p e t o r e d u c e t h e danger o f b r e a c h i n g t h e r i v e r banks i n Middle and Lower Egypt. T h e volume of s i l t y water impounded i n t h e r e s e r v o i r d u r i n g t h e d i s a s t r o u s f l o o d of 1954 was a b o u t 3 mlrd m 3 w i t h a s i l t c o n t e n t of a b o u t 9 m i l l i o n m 3 .
42 7
. 95
d
94 93 VI t 92 F 91 €. 9 0 lJi
lJ
-
ASWAN
(D.S. Dam)
-
Jan. Feb.Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov.Dec. Month Fig. 9 . 5 . The s t a g e hydrograph of t h e N i l e downstream of Aswan and t h e r u l e curve of t h e r e s e r v o i r , both f o r 1953 (Hurst, H.E., Black, R.P., and Simaika, Y.M., 1959) Some of t h e r e s u l t s obtained from i n v e s t i g a t i n g t h e s i l t regime i n t h e N i l e , e s p e c i a l l y a t K a j n a r t i , Wadi Halfa and Gaafra ( c l o s e s t t o Aswan), have been publ i s h e d , among o t h e r s , by Hurst and co-workers i n Vol. IX of t h e Nile Basin (Hurst, H . E . ,
Black, R.P., and Simaika, Y.M., 1959), by Simaika and El-Sherbini
(1957) and by Simaika alone (1961). The main r e s u l t obtained from t h a t l a s t i n v e s t i g a t i o n i s t h a t t h e s i l t c o n c e n t r a t i o n , expressed i n p a r t s p e r mi1lio.n p e r weight, was approximately 400, P O O , 2 5 0 0 , 1000 and 300 f o r J u l y , August, September, October and November, a l l measured a t Wadi Halfa. The measurements a t Gaafra seemed t o be i n f l u e n c e d by t h e o p e r a t i o n of t h e Aswan r e s e r v o i r . Longterm averages a t Gaafra were, however, 300, 3000, 2 0 0 0 , 700 and 170 p a r t s per m i l l i o n by weight f o r t h e months J u l y up t o and i n c l u d i n g November s u c c e s s i v e l y . I t was f u r t h e r r e p o r t e d t h a t t h e amount of t h e suspended sediment i n t h e r i s i n g s t a g e of t h e f l o o d was much h i g h e r than i n t h e f a l l i n g s t a g e . With t h e advancement of t h e f l o o d wave i n time from t h e end of J u l y towards t h e end of October a s i g n i f i c a n t i n c r e a s e i n t h e percentage of t h e c o a r s e f r a c t i o n (sand) and a
428
c o r r e s p o n d i n g d e c r e a s e i n t h e f i n e f r a c t i o n ( s i l t and c l a y ) had b e e n n o t i c e d . The d e t a i l e d r e s u l t s o b t a i n e d from i n v e s t i g a t i n g t h e s i l t i n t h e N i l e have been employed i n t h e e s t i m a t i o n of t h e c a p a c i t y t o b e a l l o t t e d t o dead storage ( s t o r age of s e d i m e n t s ) i n t h e r e s e r v o i r c r e a t e d by t h e Aswan High Dam.
93.5
r
I
I
I
I
I
6.0 2.5 5.0 7.5 10.0 12.5 15.0 17.5 20.0 22.5 Silt load,
m3
Water l e v e l a t Aswan and t h e s i l t l o a d i n t h e r i v e r w a t e r
F i g . 9.6. 9.1.2.2
lo6
The Sennar (Makwar) Dam
The dam c o n s t r u c t i o n w a s completed i n 1925. I t i s b u i l t on t h e Blue N i l e some 350 k i l o m e t r e s s o u t h - e a s t of Khartoum f o r t h e b e n e f i t of t h e Sudan. I t s purpose i s t o s t o r e p a r t o f t h e Blue N i l e w a t e r f o r i r r i g a t i n g t h e c o t t o n r a i s e d i n t h e
Gezirah a r e a and t o r a i s e t h e water i n t a k e d p s t r e a m o f t h e G e z i r a h c a n a l up t o t h e r e q u i r e d l e v e l . The r e s e r v o i r c a p a c i t y c r e a t e d by t h e Sennar Dam i s a b o u t 0 . 8 m l r d m3.
The c r o s s - s e c t i o n of t h e dam and some of t h e r e l a t e d t e c h n i c a l d a t a
are p r e s e n t e d i n F i g . 2 , Appendix F . The Sennar r e s e r v o i r h a s been d e s i g n e d t o o p e r a t e i n such a manner t h a t duri n g t h e low s t a g e s of t h e r i v e r , i . e . from J a n u a r y t o J u l y , t h e d i s c h a r g e downs t r e a m o f t h e dam remains t h e same as i t would have been had t h e r e been no dam
a t a l l . T h i s means t h a t t h e d i s c h a r g e p a s s i n g through t h e s l u i c e s i n t h i s p e r i o d i s about e q u a l t o t h a t which e n t e r s t h e r e s e r v o i r from t h e upstream; b u t a s t h e
429
G e z i r a h c a n a l s t i l l c o n t i n u e s t o draw i t s f u l l y s u p p l y , t h e amount o f water l e a v i n g t h e r e s e r v o i r d u r i n g i t s emptying p e r i o d must b e g r e a t e r t h a n t h e amount t h a t e n t e r s i t . Therefore, t h e r e s e r v o i r l e v e l during t h e s a i d period drops g r a d u a l l y w i t h t i m e ( s e e F i g . 9 . 7 . , El-Zein Sagheyroon, S . S . ,
425
420 d
v; 415 0
410
aJ
2
-J
405
I
:ents
it‘n
400 I
R.L.LZ1.70
1965).
1
u -----_____-----
sluices River level downstream of the reservoir I
I
I
Jan. Feb. Mar, Apr. May Jun. Jul. Aug. Sep. O c t . Nov. Dec. Month
Fig. 9.7.
O p e r a t i o n o f t h e S e n n a r r e s e r v o i r and t h e downstream w a t e r l e v e l
B e a r i n g i n mind t h a t i r r i g a t i o n o f the G e z i r a h l a n d m u s t b e g i n a t t h e e n d o f J u l y , t h e f i r s t f i l l i n g i s a c c o m p l i s h e d i n t h e s e c o n d h a l f o f J u l y so t h a t on t h e t h e f i r s t o f August t h e l e v e l u p s t r e a m o f t h e dam i s r a i s e d t o 417.20 t o e n a b l e t h e c a n a l t o draw i t s f u l l s h a r e from t h e r i v e r . The c r i t e r i o n t o s t a r t f i l l i n g t h e r e s e r v o i r i s t h a t t h e d a i l y n a t u r a l f l o w of t h e B l u e N i l e e q u a l s o r e x c e e d s 160 m i l l i o n m3/day. On t h e f i r s t of August t h e river flow becomes b i g enough t o p e r m i t f u r t h e r r a i s i n g of w a t e r t o l e v e l s h i g h e r t h a n 4 1 7 . 2 0 . However, t h i s cann o t be done u n t i l t h e N i l e w a t e r i s s u f f i c i e n t l y c l e a r o f s i l t , t o e n s u r e t h a t s o l i d s a r e n o t d e p o s i t e d i n t h e r e s e r v o i r . Sediment d e p o s i t i o n below t h e l e v e l o f 417.20 is n o t , however, too h a r m f u l as t h e c o r r e s p o n d i n g r e s e r v o i r c o n t e n t s m e r e l y act as a water c u s h i o n a n d have n o t h i n g t o do w i t h l i v e s t o r a g e . To a v o i d accumulation of silt d e p o s i t s a t h i g h e r l e v e l s t h e r e s e r v o i r l e v e l s h o u l d remain
a t 417.20 t i l l t h e B l u e N i l e water i s s u f f i c i e n t l y f r e e of s i l t , u s u a l l y a r o u n d mid-October,
A t t h i s t i m e t h e se’cond f i l l i n g of t h e S e n n a r r e s e r v o i r s t a r t s and
i s c o n s i d e r e d c o m p l e t e when t h e l e v e l i s 4 2 1 . 7 0 , a t t h e end of November. The r e s e r v o i r l e v e l i s k e p t a t , o r h i g h e r t h a n , 417.20 a s l o n g a s t h e G e z i r a h l a n d h a s t o b e i r r i g a t e d and t h e G e z i r a h c a n a l h a s t o draw i t s f u l l s h a r e . T h i s c o n t i n u e s t i l l t h e f i r s t of A p r i l , a f t e r which t h e l e v e l i s a l l o w e d t o f a l l below 4 1 7 . 2 0 , The c a n a l t h e n s e r v e s i n s u p p l y i n g water f o r d o m e s t i c p u r p o s e s . The f a l l goes on u n t i l t h e r e s e r v o i r a t t a i n s i t s minimum l e v e l between t h e end o f May a n d mid-July
(see Fig. 9.7.),
and t h i s d a t e can t h e r e f o r e be taken as t h e
430
end of t h e emptying phase of t h e o p e r a t i o n c y c l e . 9.1.2.3
The J e b e l el-Aulia Dam
This dam was c o n s t r u c t e d i n 1937 on t h e White N i l e some 44 k i l o m e t r e s s o u t h of Khartoum t o s t o r e water f o r t h e b e n e f i t of Egypt. The s t o r a g e c a p a c i t y of t h e r e s e r v o i r a t t h e t i m e t h e dam was completed was 3 . 5 mlrd m 3 . Due t o t h e continuous s i l t i n g up of t h e r e s e r v o i r b a s i n t h e l i v e c a p a c i t y has shrunk g r a d u a l l y t o 2.2 mlrd m3 a t Aswan by 1960. The c r o s s - s e c t i o n of t h e J e b e l el-Aulia Dam and some t e c h n i c a l l y - r e l a t e d d a t a a r e given i n Fig. 3, Appendix F. The o p e r a t i o n of t h e J e b e l el-Aulia r e s e r v o i r depended t o a l a r g e e x t e n t on conditions a t Aswan. A t t h e beginning of February both r e s e r v o i r s used t o be f u l l . Since demand f o r i r r i g a t i o n i n Egypt a t t h a t t i m e exceeded n a t u r a l supply, t h e emptying of t h e Aswan r e s e r v o i r used t o begin i n February; when i t reached a c e r t a i n l e v e l , t h e emptying of t h e J e b e l el-Aulia r e s e r v o i r s t a r t e d . By t h e t i m e t h e r e l e a s e d w a t e r a r r i v e d a t Aswan, t h e volume of t h e r e s e r v o i r c o n t e n t s
had f u r t h e r decreased, so t h a t enough room was a v a i l a b l e f o r r e c e i v i n g t h e r e l e a s e d w a t e r as w e l l as t h e n a t u r a l r i v e r supply. The volume of r e s e r v o i r c o n t e n t s is t h a t volume of water o v e r l y i n g t h e surf a c e l e v e l of t h e n a t u r a l r i v e r . For a c e r t a i n l e v e l i n a r e s e r v o i r , t h e cont e n t s drop a s t h e l e v e l of t h e n a t u r a l r i v e r rises, and vice-versa.
The estimate
of t h e r e s e r v o i r c o n t e n t s t h e r e f o r e depended on t h e upstream gauge l e v e l (Wadi Halfa i n t h e case of the Aswan Dam). curves i n t e r m s of t h e r e s e r v o i r
F i g . 9.8. shows t h e r e s e r v o i r content
l e v e l and t h e gauge reading a t Halfa.
The emptying phase of o p e r a t i o n of t h e J e b e l el-Aulia r e s e r v o i r s t a r t e d b e t ween t h e f i r s t of February and t h e f i r s t of March according t o t h e n a t u r a l i n come o f t h e r i v e r . This phase used t o l a s t about two-and-a-half
months, while
t h a t of t h e Aswan r e s e r v o i r continued u n t i l t h e end of J u l y , when t h e n a t u r a l income began t o s u r p a s s t h e q u a n t i t y of water r e q u i r e d . S i m i l a r t o t h e Sennar r e s e r v o i r , t h e J e b e l el-Aulia was f i l l e d i n two s t a g e s ; t h e f i r s t from t h e end of J u l y t i l l t h e t w e n t i e t h of August, when t h e r e s e r v o i r l e v e l reached 376.50 m above mean s e a l e v e l and t h e second s t a g e from t h e f i r s t of September and continued t i l l t h e l e v e l reached 377.20. The d i f f e r e n c e between t h e two l e v e l s , i . e . 376.50 and 377.20, l e f t during t h e break between t h e two f i l l i n g s t a g e s corresponds t o 1 mlrd m 3 .
I t was meant t o a c t as a s a f e t y valve when t h e Blue
Nile showed an extremely high f l o o d . I f t h i s occurred, a l l t h e s l u i c e s of t h e dam were f u l l y opened and p a r t of t h e f l o o d water flowed backwards i n t o t h e White N i l e .
431
0
1
2 3 4 5 Reservoir contents, log,?
6
7
F i g . 9.8. A s w a n ' r e s e r v o i r c o n t e n t s and t h e i r r e l a t i o n t o H a l f a gauge r e a d i n g and t h e r e s e r v o i r l e v e l ( H u r s t , H.E., B l a c k , R.P., and Simaika, Y.M., 1959) I n t h e pre-High Aswan Dam p e r i o d , t h e o l d Aswan and t h e J e b e l e l - A u l i a reserv o i r s used t o c o n t r i b u t e about 7 . 8 m l r d m3/yr a t Aswan t o t h e s u p p l y i n t h e lowflow s e a s o n , February t o J u l y .
9.1.2.4
Khashm e l - G i r b a Dam
The main o b j e c t i v e o f t h e r e s e r v o i r c r e a t e d by t h i s dam is t o r e g u l a t e some o f t h e A t b a r a water i n o r d e r t o s u p p l y t h e i r r i g a t i o n canals o f t h e A t b a r a scheme w i t h t h e n e c e s s a r y flow. I n t h e t e c h n i c a l d a t a i n F i g . 4, Appendix F, i t
is mentioned t h a t t h e i n i t i a l
s t o r a g e c a p a c i t y was 1 . 3 mlrd reduced t o about
0.95 m l r d m 3 by 1971, w i t h &he p o s s i b i l i t y o f a f u r t h e r r e d u c t i o n by 40 m i l l i o n
m3 e a c h y e a r . The f l o o d o f t h e A t b a r a b e g i n s i n t h e l a t t e r h a l f of J u n e , depending on t h e
d a t e of f a l l of t h e t o r r e n t i a l r a i n s . The f i r s t f i l l i n g of t h e r e s e r v o i r up t o l e v e l 462.00 m above mean sea l e v e l b e g i n s i n t h e p e r i o d from t h e f i r s t o f J u l y
t o t h e t e n t h o f J u l y e v e r y y e a r i n o r d e r t o o p e r a t e t h e t u r b i n e pumps t o l i f t
water t o t h e main c a n a l . T h i s c a n a l b r a n c h e s a t k i l o 26 o f f t o t h r e e branch c a n a l s and a t k i l o 14 t h e r e i s a pumping s t a t i o n f o r d i r e c t i r r i g a t i o n of some
4 32
land. I t may b e of i n t e r e s t t o mention t h a t a l l t h e land i r r i g a t e d by t h e s e c a n a l s i s d e s t i n e d f o r t h e people of Wadi Halfa and d i s t r i c t who were d i s p l a c e d by t h e High Aswan Dam p r o j e c t i n Egypt. During t h e f i r s t p a r t of t h e f i l l i n g phase t h e main s l u i c e s of t h e dam a r e l e f t open t o l e t t h e s i l t - l a d e n water flow t o downstream. This goes on t i l l t h e end of August o r when t h e r i v e r d i s c h a r g e n e a r t h e dam s i t e f a l l s t o 110 x
lo6
m3/day. The second p a r t of t h e f i l l i n g phase then begins and continues t i l l t h e beginning of October when t h e r e s e r v o i r l e v e l reaches 473.20. During and a f t e r t h i s p e r i o d a l l t h e e x c e s s w a t e r is allowed t o flow downstream u n t i l t h e f l o o d c e a s e s . In t h e second p a r t of t h e f i l l i n g phase t h e main s l u i c e s of t h e dam a r e shut o f f . The s t o r e d w a t e r s e r v e s t o supply t h e main c a n a l from t h e beginning of November t i l l t h e end of May. 9.1.2.5
The Roseires Dam
The s t o r a g e r e s e r v o i r formed by t h i s dam w a s designed t o r e t a i n water up t o l e v e l 480 m above mean s e a l e v e l i n i t s f i r s t phase and up t o 490 m above mean s e a l e v e l i n i t s second phase. These two l e v e l s correspond to volumes of 3 mlrd
m3 and 6 . 8 mlrd m3, r e s p e c t i v e l y . The primary purpose of t h e Roseires Dam i s t o s t o r e water and t o p a s s i t downstream when r e q u i r e d by t h e Cezirah, Managil e x t e n s i o n and t h e r i v e r bank pump schemes (see F i g . 2 . 2 2 . ) ,
a l l f o r t h e b e n e f i t of t h e Sudan.
The Roseires r e s e r v o i r i s operated i n conjunction w i t h Sennar w i t h t h e purpose of s a t i s f y i n g t h e i r r i g a t i o n requirements upstream and downstream of t h e dam, and g e n e r a t i n g t h e maximum p o s s i b l e power. The f i l l i n g of t h e r e s e r v o i r during t h e r i s i n g f l o o d i n c l u d i n g t h e peak, when t h e s i l t c o n t e n t i s a t its max-
i m u m , i s avoided and f i l l i n g i s delayed t o t h e l a t e s t p o s s i b l e t i m e during t h e f a l l i n g f l o o d . Therefore t h e f i l l i n g d a t e i s enforced by e i t h e r t h e f i r s t of September, i f t h e flow h a s never r i s e n above 325 m i l l i o n m3/day,
or by t h e day
l a t e r than t h e f i r s t of September when t h e d i s c h a r g e has f a l l e n t o 325 m i l l i o n m3/day. Cross-sections
and some of t h e t e c h n i c a l d e t a i l s of t h e dam are given i n Fig..
5 , Appendix F .
9.1.3
Over-Annual s t o r a g e
Consider t h e s u c c e s s i o n of mass inflow curves shown i n F i g . 9.9. Assume t h a t l i n e AB has a s l o p e Q t h a t should n o t be exceeded t o avoid t h e flooding of t h e
area downstream of t h e s t o r a g e r e s e r v o i r . For t h i s purpose t h e maximum s i z e R2 needs t o be empty before t h e a r r i v a l of t h e f l o o d i n t h e second y e a r t o s t o r e
a l l inflow discharge i n excess of Q. From t h e same f i g u r e i t is c l e a r t h a t i n
433
y e a r number 3 one needs a n empty s p a c e R3 < R2 f o r r e g u l a t i n g t h e flow from t h e r e s e r v o i r t o t h e downstream. The assumed Q, however, i m p l i e s t h a t t h e r e s e r v o i r cannot be emptied completely b e f o r e t h e f l o o d i n t h e f o u r t h y e a r comes. A reserv o i r as s u c h is s a i d t o h a v e an o v e r - y e a r or over-annual s t o r a g e . T h i s t y p e of s t o r a g e work does n o t e x i s t i n t h e N i l e B a s i n . A l l t h e s t o r a g e works t h e r e b e l o n g t o e i t h e r t h e annual s t o r a g e , o r t h e long-term s t o r a g e t y p e s .
C Long term storage
2
1
Fig. 9.9. 9.1.4
3 Year No.
4
5
Over-annual and long-term r e s e r v o i r o p e r a t i o n
Long-term s t o r a g e
A s s u m e t h a t t h e i n f l o w and demand mass c u r v e s i n F i g . 9 . 9 . ,
i n s t e a d of b e i n g
f o r f i v e y e a r s o n l y , r e p r e s e n t a much l o n g e r s e q u e n c e . L e t t h e l i n e c o n n e c t i n g
OC have a s l o p e e q u a l t o t h e mean n e t i n f l o w of t h e s e q u e n c e , ;e
r e q u i r e d t o p r o v i d e t h e downstream w i t h demand
6
4.
The s t o r a g e
is R i . T h i s i s known as long-
t e r m s t o r a g e and sometimes a s c e n t u r y s t o r a g e . 9.1.4.1
Design o f r e s e r v o i r c a p a c i t y
The Rippl-diagram
method c a n also b e used f o r d e t e r m i n i n g t h e c a p a c i t y of a
long-term s t o r a g e r e s e r v o i r . L e t t h e a n n u a l volumes of i n f l o w t o such a reserv o i r b e X 1, X2, r a t e ) and
.....
X
and t h e a n n u a l demand b e a i , where 0 < 1 (a = d r a f t
% i s t h e long-term mean i n f l o w .
4 34
According t o Kottegoda, N. (1980), one needs t o determine t h e e a r l i e s t y e a r , j , which s a t i s f i e s t h e c o n d i t i o n
X. 2 a J
X
,
> X.
J+1
j = l , 2
,..., n
- 1
(9.1)
Suppose t h i s corresponds t o year j = K1,
i t i s suggested t h a t two computa-
t i o n s have t o be made: i)
The l e n g t h 1 of t h e p e r i o d i n which t h e r e s e r v o i r l e v e l l i e s below t h e l e v e l a t t i m e K1.
From a l l such d e p l e t i o n p e r i o d s s e l e c t t h e maximum l1 f o r which
t h e following c o n s t r a i n t is s a t i s f i e d
ii) The d e f i c i t d given by t h e e q u a t i o n
d =
-
max (aXm l
