Desing of Storage Tanks for Water Harvesting in Rainfed Areas 24.10.11

8/3/2019 Desing of Storage Tanks for Water Harvesting in Rainfed Areas 24.10.11

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A g r i cu l tu r a l W a t e r M a n a g e m e n t , 1 8 ( 1 9 9 0 ) 1 9 5 - 2 0 7 1 95E l s e v ie r S c i e n c e P u b l i s h e r s B . V ., A m s t e r d a m

D e s i g n o f s t o r a g e t a n k s f o r w a t e r h a r v e s t i n g i nr a i n f e d a r e a sH . N . V e r m a a a n d P . B .S . S a r m a b

aDryland Research Station , Garhshanker, Hoshiarpur, P unjab (India)b W ater Technology Centre, Indian A gricultural R esearch In sti tute, Ne w Delhi (India)( A c c e p t e d 1 4 J a n u a r y 1 9 9 0 )

A B S T R A C TV e r m a , H . N . a n d S a r m a , P.B .S. , 1990 . Des ign of s t o r a g e t a n k s f o r w a t e r h a r v e s t i n g i n r a i n f e d areas .Agric. W ater Manage., 1 8 : 1 9 5 - 2 0 7 .

A p r o c e d u r e w a s d e v e l o p e d t o d e s i g n a t a n k f o r w a t e r h a r v e s t in g a n d c o m p u t e i ts b e n e f i t - c o s t r a t iof o r a r e g i o n i n n o r t h e r n P u n j a b . I t w a s o b s e r v e d t h a t t h e to t a l c o s t o f ta n k p e r u n i t o f c a p a c i t y d e -c r e a s e d w i t h i n c r e a s i n g t a n k c a p a c i ty . T a n k s d e s i g n e d o n t h e b a s is o f se a s o n a l r u n o f f a n d u s e d f o rp r e s o w i n g i r r ig a t io n o f w h e a t, a r e t h e m o s t b e n e f i c i a l w i t h a b e n e f i t - c o s t r a t io r a n g in g f r o m 1 . 6 0 t o4 .56 f o r c a t c h m e n t a r e a s v a r y i n g f r o m 1 t o 100 ha . The p r o b a b i li t y l e v e l o f t h e l o w e s t a s s u r e d r u n o f fc o r r e s p o n d i n g t o t h e l o w e s t a n n u a l c o s t p e r u n i t o f a v a i l a b l e w a t e r i n c r e a s e d w i t h i n c r e a s i n g t a n kc a p a c i t y a n d v a r i e d f r o m 4 0 t o 80%.

I N T R O D U C T I O N

M o s t a r e a s w i t h l o w r a i n f a ll su f f e r f r o m l o w a n d u n s t a b l e c r o p y ie l d s . O f t e no n e l i f e - s a v i n g i r r i g a t i o n t o t h e s e c r o p s a t t h e m o s t c r i t i c a l s t a g e c o u l d s u b -s t a n t i a l l y i m p r o v e y i e l d s .

G e n e r a l l y , s t o r a g e o f r a i n f a ll in t h e s o i l p r o f i le i s c h e a p e r a n d m o r e e f f i c ie n tt h a n s to r a ge o f r u n o f f i n e x c a v a t e d t a n k s . H o w e v e r , t h e m a j o r l im i t a t io n o fs t o ra g e i n t h e s o i l p r o f i l e i s i ts l i m i t e d c a p a c i t y . I n m o s t o f th e r a i n f e d a r e a s,r a i n w a t e r c o n s e r v a t i o n m e a s u r e s c a n n o t c o n s e r v e a ll t h e r a i n w a t e r a n d a c e r-t a in a m o u n t o f r u n o f f i s b o u n d t o o c c u r. T h i s r u n o f f c a n b e c o l le c t e d a n ds t o r e d i n t a n k s f o r a l i f e - s a v i n g i r r i g a ti o n t o r a i n f e d c r o p s .

U s e o f w a t e r h a r v e s t i n g ta n k s i n r a in f e d a re a s h a s c o m e i n t o v o g u e o n l yr e c e n tl y ( P a l m e r e t a l. , 1 9 8 2 ; H e l w e g a n d S h a r m a , 1 9 8 3 ) a n d n o t m u c h l it -e r a tu r e i s a v a i l a b l e o n t h e d e s i g n o f t a n k s . H e n c e , a s u it a b l e t e c h n o l o g y i s n o ta v a i l a b l e t o t h e f a r m e r s o f d i f f e r e n t r a i n f e d a r e as . A p r o p e r d e s i g n o f s to r a g et a n k s f o r r a i n f e d a g r i c u l tu r e i n v o l v e s :( a ) h y d r o l o g ic a l a n a l y s is i n c l u d i n g p r o b a b i l i ty o f o c c u r r e n c e o f r u n o ff ;

0 3 7 8 - 3 7 7 4 / 9 0 / $ 0 3 . 5 0 1 9 90 - - E l s e v i e r S c i e n c e P u b l i s h e rs B .V .


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196 H . N . V E R M A A N D P . B .S . S A R M A

( b ) h y d r a u l i c d e s i g n t o d e t e r m i n e p h y s i c a l si ze s o f th e t a n k c o n s i d e r i n g s e e-p a g e a n d e v a p o r a t i o n l os se s;( c ) p o l i c i e s f o r u t i l i s in g t h e s t o r e d w a t e r i n te r m s o f ( 1 ) t im i n g , ( 2 ) q u a n t i t y ,( 3 ) c r o p t o b e i rr i g a te d , a n d ( 4 ) s e l e c ti o n o f i r ri g a t i o n sy s t em ;

( d ) e c o n o m i c v i a b il it y o f t h e s c h e m e .T h e r e s u lt s o f i n v e s t i g a ti o n s c a r r i e d o u t i n I n d i a r e v e a l e d t h a t w a t e r c o l-l e c t e d i n t a n k s s h o u l d b e u t i l i z e d f o r a s i n g l e l i fe - s a v i n g i r r i g a t i o n w i t h a l i m -i t e d q u a n t i t y o f w a t e r t o t h e m o s t r e s p o n s i v e c r o p a t i ts m o s t c r i t ic a l st ag e too b t a i n t h e m a x i m u m b e n e f i t f r o m a w a t e r h a r v e s t i n g s y s t e m ( V e r m a , 1 98 7 ) .A s t o r a g e ta n k f o r t h i s p u r p o s e s h o u l d b e d e s i g n e d s u c h t h a t t h e l o w e s t po s -s i b le u n i t c o s t o f s t o r e d w a t e r i s o b t a i n e d .T h e d e s i g n o f w a t e r h a r v e s t i n g t a n k s a n d s u p p l e m e n t a l i r r ig a t i o n s y s te m sis h ig h l y l o c a t i o n - s p e c i f i c a n d , t h u s , m a k e s i t v e r y d i f f i c u l t t o d e v e l o p a g e n -e r a l m o d e l w h i c h c a n b e u s e d f o r al l a r e a s . I n th i s s t u d y , a p r o c e d u r e f o r t h ed e s i g n o f w a t e r h a r v e s t i n g t a n k s f o r r a i n f e d f a r m i n g i n n o r t h e r n P u n j a b , I n -d i a , w a s d e v e l o p e d w h i c h c a n b e m o d i f i e d a n d u s e d f o r o t h e r r a i n f e d ar e a s.MATERIALS AND METHODS

T h e r a i n f e d a r e a o f G a r h s h a n k e r T e h si l o f n o r t h e r n P u n j a b , w h i c h is a ty p -i c al r e p r e s e n t a t i v e o f t h e w e l l - k n o w n r a i n f e d b e l t c a l l ed Kandi, w a s s e l e c t e df o r t h e s t u d y . T h e m a j o r c r o p p e d a r e a o f t h e Kandi b e l t l ie s o n t h e n a r r o w f ia ts t ri p b e l o w t h e H i m a l a y a n f o o t h il ls a n d h a s a m i l d u n d u l a t i n g t o p o g r a p h yw i t h e r o d i b l e a n d l o w w a t e r r e t e n t i v e s oi ls . T h e m a j o r i t y o f so il s o f t h is b e l th a v e s a n d y l o a m a n d l o a m y s a n d t e x t u r e s . W h i l e i n l o a m y s a n d s o i l s i t i sp o s s i b l e t o u t i l i s e r a i n w a t e r c o m p l e t e l y b y w a t e r c o n s e r v a t i o n m e a s u r e s , i ns a n d y l o a m s o il s r u n o f f i s b o u n d t o o c c u r , e v e n w i t h s u c h m e a s u r e s . H e n c e , ap r o p e r l y d e s i g n e d w a t e r h a r v e s t i n g t a n k s y s t e m t o s t o re t h e r u n o f f t o p r o v i d ea l i f e - sav ing i r r iga t ion i s feas ib le in th i s a r ea .L o c a l m a i z e f o l l o w e d b y d w a r f w h e a t w a s i d e n t i f i e d a s t h e b e s t c r o p s e-q u e n c e i n s a n d y l o a m s o il s o f t h i s r e g i o n ( S i n g h e t al ., 1 9 8 3 ) . F u l l b e n e f i t o fi r r ig a t i o n c a n b e o b t a i n e d i f i r r ig a t i o n i s a p p l i e d d u r i n g a l o n g d r y s p el l w h i c hc o i n c i d e s w i t h t h e c r i ti c a l g r o w t h s ta g e o f t h e c r o p . H o w e v e r , i f r a in f a l l o c c u rsi m m e d i a t e l y a f t e r s u c h a n i rr i g a t io n t h e i m p a c t ( b e n e f i t ) o f i r ri g a t io n w o u l db e o n l y p a r t i a l. A n a l y s i s o f d r y s p e l ls f o r t h i s a r e a i n d i c a t e s t h a t t h e n e e d f o ra p r e - s o w i n g i r r i g a t i o n t o t h e w h e a t c r o p i s 9 6 . 3 % w h i c h i s a s s o c i a t e d w i t hf u ll b e n e f i t ( c e n t p e r c e n t r e s p o n s e ) . O n t h e o t h e r h a n d t h e n e e d f o r a n i r ri -g a t i o n t o t h e m a i z e c r o p a t i t s m o s t c r i t i c a l s t a g e ( t a s s e l i n g - s i l k i n g ) i n t h e3 5 t h w e e k is 5 5 . 5 % a n d i s a s s o c i a t e d w i t h f u ll b e n e f i t s i n o n l y 8 0 % o f t h ei r r ig a t i o n e v e n t s ( V e r m a a n d S a r m a , 1 9 8 9 ) . T h e r e su l ts o f i n v e s t ig a t i o n s c a r-r i e d o u t b y V e r m a ( 1 9 8 7 ) o n r a i n w a t e r m a n a g e m e n t i n t hi s ar e a i n d i c a t e dt h e f o l l o w i n g f a ct s:( 1 ) T h e a v e r a g e r e s p o n s e t o a 5 0 m m i r ri g a t i o n w h e n a p p l i e d a t t a s s e l i n g -


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D E S I G N O F S T O R A G E T A N K S 1 9 7

s i lk i n g s ta g e i s 4 0 0 k g h a - ~ o f m a i z e g r a i n , w h e r e a s w i t h a p r e s o w i n g ir r ig a -t i o n i t is 7 7 0 k g h a - J o f w h e a t g r a i n a n d 1 3 00 k g / h a o f w h e a t s tr aw . I n m o n -e t a r y te r m s t h e a n n u a l r e t u r n o f a n i r r ig a t i o n is R s 5 2 8 h a - ~ f o r m a i z e a n dR s 1 6 6 8 h a - ~ f o r w h e a t a t 1 9 8 7 p r i c es . T h e a n n u a l n e t r e tu r n , b a s e d o n 4 0y e a r s o f e f fe c t iv e l if e o f t h e t a n k a n d a n a n n u a l p r i c e g r o w t h r a t e o f 5 a n d 6 % ,is R s 3 3 8 0 h a - l a n d R s 9 6 4 h a - 1 f o r w h e a t a n d m a i z e , r e s p ec t iv e l y.

( 2 ) F o r o n l y o n e i r r ig a t i o n o f sh o r t d u r a t i o n ( 1 0 d a y s ) p e r y e a r f r o m t a n kw a t e r, t h e f u r r o w i r r i g a t io n m e t h o d is m o s t s u i ta b le . F o r l i ft in g a n d c o n v e y -a n c e o f w a t e r, p o r t a b l e d i e se l e n g i n e p u m p i n g se ts ( 5 to 1 0 h p ) w i t h P V Cp i p e s a re m o s t s u i t a b le a n d e c o n o m i c a l . T h e a n n u a l c o s t o f s u c h a n i r r ig a t io ns y s t e m ( e x c l u d i n g c o s t o f t a n k ) is R s 3 4 0 h a - ~ .( 3 ) T h e s t u d y a re a r e c e i v es a n a v e r a g e a n n u a l r a i n fa l l o f 88 9 m m o f w h i c h7 03 m m f alls d u r i n g t h e K h a r i f s e a s o n ( J u n e - S e p t e m b e r ) a n d 186 m m d u r-i n g t h e R a b i s e a so n ( O c t o b e r - A p r i l ) . T r a d i t i o n a l r a in f e d f a rm i n g in s a n d yl o a m p r o d u c e s a n a v e ra g e o f 1 98 m m r u n o f f f r o m J u n e t o S e p t e m b e r , w h i c hr e d u c e s t o 1 0 0 m m w h e n w a t e r c o n s e r v a t i o n m e a s u r e s , c o n t o u r c u l t i v a t i o na n d r id g e a n d f u r r o w f o r m a t i o n a r e u s e d . T h e l o w e s t a s su r e d r u n o f f f a t d i f-f e r e n t p r o b a b i l i t y l ev e ls , o n a c r o p p e d ( m a i z e ) s a n d y l o a m c a t c h m e n t w i t ht h e a b o v e s o il a n d w a t e r c o n s e r v a t i o n m e a s u r e s , w a s e s t im a t e d u s in g a n i n -c o m p l e t e g a m m a d i s t r i b u t io n ( T a b le 1 ) ( V e r m a a n d S a r m a , 1 9 8 8 ).

T h e f o ll o w i n g c a se s o f t a n k d e s i g n w e r e c o n s i d e r e d i n t h i s s t ud y :Case 1 : B a s e d o n p e r i o d i c a l ( w e e k s 2 4 to 3 5 ) r u n o f f ( T3 5 ) f o r ir r i g a t io n o f

m a i z e i n t h e 3 5 t h w e e k f o r 5 5 .5 % o f t h e t i m e , a n d p r e s o w i n g i rr ig a -t i o n t o w h e a t i n t h e 4 4 t h w e e k o f t h e r e m a i n i n g t im e .

Case 2 : B a s e d o n s e a s o n a l ( w e e k s 2 4 to 3 9 ) r u n o f f ( T 39 ) f o r p r e s o w i n g i rr i-g a t io n o f w h e a t i n t h e 4 4 t h w e e k .T A B L E 1

Lowest assured periodical and seasonal runof f for a sandy loam catchme nt with i mproved moistureconservation measures, and maize cultivationProbability Periodical runoff (m m) Seasonal runoff ( mm )level (%) Weeks 24-35 Weeks 36-39 Weeks 24-39

10 188.4 27.7 191.920 141.3 13.7 147.630 107.7 6.1 119.040 87.5 1.7 100.050 67.3 - 82.860 53.2 - 68.170 39.4 - 54.380 26.9 - 40.990 14.8 - 26.7

100 4.3 - 11.7


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198 H.N. VERMA AND P.B.S. SARMA

Design of tanksIn this study, it is assumed that complete runoff resulting from the catch-ment treated with in-situ water conservation procedures, enters the tanks, and

suitable inlet and spillway structures are provided. The most suitable shapecons idered is the inverted truncated pyramid with a square base. A side slopeof 1 : 1 and a ma ximum water depth of 5 m were considered in this study. Thewatershed areas consist of small cropped fields suitably treated for soil andwater conservation and efficient disposal of excess runoff. The main criteriafor determining the volume of tanks was taken as the lowest assured runoffoccurring during 24th to 35th week in the case of irrigation to maize andduring the 24th to 39th week in the case of presowing irrigation to wheat.Thus the tanks should be completely filled during the 35th and 39th week inthe first and the second case, respectively.Seepage is the main problem in the case study area because it is as high as520 1 m -2 day -1 in newly excavated unl ined tanks (Verma, 1981 ). Hence ,lining is essential. After detailed consideration of installation and mainte-nance costs, and durability, the combination of polyethylene sheet (200-250~tm thickness) burried under a 0.2 m thick soil layer at the tank bot tom andlining of 75 mm thick brick in cement at the sides of the tanks was repor tedto be the most suitable for the case study area (Verma, 1981 ). The liningreduced the average seepage losses to 12.71 1 m -2 d ay- 1, with the assumptionthat the change in seepage rate with water depth is negligible.The tank capacity for silt accumulation is taken to be 10 m 3 ha-1 for 5years, i.e., at the rate of 2 m 3 ha -1 yr -1 (2.5 t ha -1 yr -1 ). It is a ssumed thatafter every 5 years the extra silt will be removed by the farmers. Daily panevaporation data for 22 years (1961-1981 ) were collected from the nearestmeteorological station, Chandigarh. Average weekly evaporation from thetanks was com put ed with a pan coefficient of 0.7.Cost of tanks

The total cost of a tank comprises its construction cost (escava-tion +li ning), cost of inlet and spillway structures, operation and mainte-nance costs and the shadow price (productivity cost) of land occupied by thetank. From available information on tank construction and maintenance inthe case study area (Anonymous, 1975-1983) it was observed that the costof inlet and spillway structure constitutes 8% of the construc tion cost and themaintenance cost is about 2% of the construction cost. Hence, in this study,10% of the construction cost is taken as "other costs" to account for construc-tion of inlet and spillway and maintenance cost.

The schedule of Rates of the Public Works Department, Government ofPunjab, 1983 updated to 1987, was used to analyse excavation costs for 5 m


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DESIGN OF STORAGZ TANKS 19 9

d e e p t a n k s . A l i n e a r r e l a t i o n s h i p w a s d e v e l o p e d f o r t h e e x c a v a t i o n c o s t ( C~ )o f t a n k s w i t h t h e p r o p o s e d s h a p e a n d b o t t o m w i d t h ( B ) , w h i c h c an b e gi ve nb y C~ - - 0 . 0 0 4 3 B + 3 . 3 5 17 R s m - 3. T h e c o s t o f l i n in g o f th e b o t t o m w i t h p o l -y e t h y l e n e s h e e t ( C 2 ) a n d c o s t o f si d e l in i n g w i t h b r ic k i n c e m e n t ( C 3 ) a tp r i c e s o f 1 9 8 6 - 1 9 8 7 a r e R s 5 . 7 6 m - 2 a n d R s 2 2 . 7 5 m -2 , r e s p e c ti v e l y .T h e e x i s t i n g l e a se r a te o f la n d i s R s 1 3 0 0 h a - ~y r - ~f o r le v e l l e d a n d t e r r a c e dl a n d , a n d R s 7 0 0 h a - ~ y r - ~ f o r u n d u l a t i n g l a n d . C o n s i d e r i n g t h e l o w p r o f i tsi n r a i n f e d f a r m i n g t h e l o w v a l u e o f t h e l e a s i n g ra t e o f l a n d is t a k e n a s t h ep r o d u c t i o n c o s t o f l a n d i n t h e s t u d y a r ea . T h e l a n d a r e a o c c u p i e d b y t h e t a n kd e p e n d s o n t h e to p o g r a p h y a n d s o il t y p e a n d t h e s u r r o u n d i n g s . I n th i s s tu d y1 50 % o f th e t o p s u r fa c e a r e a o f t h e t a n k is ta k e n a s t h e a r e a o c c u p i e d u n d e rt h e t a n k , w h i c h i n c l u d e s s id e b u n d s .

A n n u a l c o s t s w e r e c o m p u t e d f o r d i f f e r e n t s iz es o f t a n k s b a s e d o n t i m e o fi r r i g a t i o n , p r o b a b i l i t y l e v e l a n d c a t c h m e n t a r e a . T h e i n t e r e s t r a t e o n i n v e s t -m e n t w a s t a k e n a s 1 0% p e r y e a r . A n e f f e c t i v e l if e o f 4 0 y e a rs f o r l i n e d ta n k sw a s t a k e n c o n s i d e r i n g t h e l o c al s i t u a t i o n s a n d w e a t h e r i n g o f li n in g .Ava i lable wa ter fo r i rr igat ion

I f a t a n k i s d e s i g n e d a t a l o w e r p r o b a b i l i t y l e v e l o f a s s u r e d r u n o f f , i t w i llh a v e a l a r g e r c a p a c i t y a n d l o w e r c h a n c e o f b e i n g f il le d u p t o i ts f u ll c a p a c i t y .O n t h e o t h e r h a n d , a t a n k d e s i g n e d o n t h e b a s is o f a h i g h e r p r o b a b i li t y le v e lo f a s s u r e d r u n o f f w i ll h a v e a l o w s t or a g e c a p a c i t y b u t c h a n c e s o f it s b e in gf il le d t o f u ll c a p a c i t y w i ll b e g r e a t e r a n d t h u s t h e e x p e c t e d s t o r a g e p e r u n i tv o l u m e o f t a n k w i l l b e h i g h e r . I n t h i s s tu d y , p r o b a b i l i t y le v e ls ( P i ) o f a s s u r e dr u n o f f a r e c o n s i d e r e d a t t e n e q u a l i n t e r v a l s , i. e. , i = 1 0, 2 0 , 3 0 , 40 , 5 0 , 6 0, 7 0 ,8 0 , 9 0, 1 00 a n d t h e c o r r e s p o n d i n g a s s u r e d r u n o f f v a l u e X i w a s u s e d f o r t h ed e s i g n . T h u s , e a c h X ~ v a l u e h a s a p r o b a b i l i t y o f 0 .1 . A t a n k d e s i g n e d o n t h eb a sis o f a r u n o f f a m o u n t o f Xr, a t a p r o b a b i l i t y l e v e l P r h a s a c a p a c i t y VTr( a f te r d e d u c t i n g lo s se s d u r i n g t h e p e r i o d o f c o ll e c t io n a n d s t o ra g e ) a n d w i llh o l d a v o l u m e o f Vs~ a t t h e t i m e o f i rr i g a t io n . T h e p r o b a b i l i t y o f s t o r i n g a na m o u n t o f Vs~ a t t h e t i m e o f i r r ig a t i o n is o n l y r b e c a u s e a t l o w e r p r o b a b i l i t yl e v e l s ( i~ < r ) i t w i l l c o l l e c t r u n o f f o n l y u p t o i t s f i x e d c a p a c i t y V T~ a n d t h e r e s to f t h e r u n o f f w i ll b e w a s te d . B u t a t a h i g h e r p r o b a b i li t y ( i > r ) t h e r u n o f fa m o u n t w i ll be le ss t h a n X~ a n d t h e t a n k v o l u m e V T r, b e i n g t h e s a m e , w i ll b ep a r t i a l l y f i l l ed an d V~, ( i > r ) w i l l a l way s b e l e s s t h a n V~r. In t h i s cas e t h e ex -p e c t e d v o l u m e o f s t o r e d w a t e r a t t h e t i m e o f ir r ig a t io n (E V O Lr) c a n b e g i v e na s

100EVOLr'~-PrVsr"]- E (P i Vsi )i= r+ 10

w h e re P~-- O . 1 fo r i = r + 1O , r + 2 0 , r + 3 0 ,. .. , r + 1 0 0.


6/13

2 0 0 H . N . V E R M A A N D P .B .S . S A R M A

R E A D W K~ 0 R ~ .A O T R I~PUTE SILT VOLUME (VS ) REAO T R I Ivs: ZA~I 0 RAO E R

L A D S R S , $ R B ~ C ~ . / C ) C L

B , , lOMPUI'E TANg EqMENSLONS AND i : SUBROUTINESTORAGE CAp~,CITy {VNP} FOR NRIRUNOFF [TRI (I P) ] CONSiOERLNGWEEKLY RUNOFF, EVAPORATION ANDSEEPAGE (FROM WEEK NO 2L,-~S)AVAILABLE wATER IN }S th wEEK=VNPL WRITE IA,IPaOTA, ATOP ~TSlOA,

OF, OS,TVOL, VNPC

THE TANK OCOST, TCOST, UCOSTEACOST, PLCO~T, TACOS T

[ : . . . . . . . . . . . . I ) . . . . . . .R E S ~ O U A L S T O R A G E RSR =VNP~N TANK, RSR ;C

O N ~ SUBROUTINECOMPUTE AVERAGE AVAILABLE WATER ~ NRI{EVOL {tP{] IN ~GID wEEK~ CONSIDERING TRII,SEEPAGE ANO EVAPORATION LOSSES I WRITE EVOL ([P)IRRI@ (IP). . . . ~ % ; o , ~ 0 ( . P ) }

lE ~UBROUTINECOMPUTE AVAILABLE WATER IN leith WEEK (VOL) IN THE N R ISANE TANK {HAVING CAPACITY =VNP} FORHIGHER PROBABILITY (NP) OF RUNOFF TRK(NP))

COv~puTE AVERAGE AVAILABLE WATER NRICEVOL(NP)} 4~,{h WEEK, CONSIOERING TRI~SEEPAGE AND EVAPORATION LOSSES L WRITE EVOL (NP)

~RRG(NP}

F i g. 1 . F l o w c h a r t o f c o m p u t e r p r o g r a m m e A f o r t a n k d e s i g n o n p e r i o d i c a l r u n o f f b a s i s a n dc o m p u t a t i o n o f i ts c o s ts a n d a v a i l a b l e w a t e r a t th e t i m e o f i rr i ga t io n .


7/13

D E S I G N O F S T O R A G E T A N K S 2 0 1

Computational procedureT w o c o m p u t e r p r o g r a m m e s w e r e d e v e l o p e d t o d e s ig n th e st or a ge t a n k s a n d

e v a l u a t e t h e i r c o s t s a n d a m o u n t o f w a t e r a v a i l a b l e a t t h e t i m e o f ir r i g at io n .T h e f ir st p r o g r a m m e ( A ) i s f o r c a se 1 a n d t h e s e c o n d p r o g r a m ( B ) i s f o r c a se2 . T h e c o m p u t a t i o n a l p r o c e d u r e o f p r o g r a m m e A i s s h o w n i n t h e f lo w c h ar t( F i g . 1 ) . P r o g r a m m e B i n v o l v e s o n l y o n e i r r ig a t io n d e c i s i o n ( i n t h e 4 4 t hw e e k ) a n d f o r m s a s u b se t o f p r o g r a m m e A .RESULTS AND D ISCUSSIONS

Tota l cost o f tankT h e t o t a l c o s t p e r u n i t c a p a c i t y o f t h e t a n k fo r v a r i o u s s iz e s ( c a p a c i t y ) i s

d e p i c t e d i n F ig . 2 . H i g h c o s t s, u p t o R s 3 4 m - 3 w a s f o u n d f o r s m a l l ta n k sh a v i n g a c a p a c i t y b e l o w 1 0 0 0 m 3. T h i s c o s t r e d u c e s w h e n i ts s i z e i n c r e a s e s . I ti s b e c a u s e o f t h e d e c r e a se i n l in i n g c o s t d u e t o t h e r e d u c t i o n i n w e t t e d a r ear e l a ti v e t o t h e i n c r e a s e i n e x c a v a t i o n c o s t p e r u n i t v o l u m e d u e t o t h e i n c r e a s e ss u r f a c e ar e a o f l a n d . T h u s f o r t a n k s o f 1 0 0 0 , 1 0 0 0 0 a n d 1 0 0 0 0 0 m 3 c a p a c i t y ,t h e c o r r e s p o n d i n g c o s t i s a b o u t 1 3 . 2, 7 .8 a n d 5 . 9 R s m - 3 , r e s p e c t i v e ly . H e n c e ,

34"3 2 "3 0 ,2 82 62 / -

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T O T A L C A P A C f T Y O F T A N K ) m 3Fig . 2 . Tota l in i t ia l cos t o f tank cons truct ion .


8/13

2 0 2 H . N . V E R M A A N D P .B .S . S A R M A

100- cs9.5-~ 0 -85 .8 0 .

o7 5-70 -6 5 .60 ,

o-~ 55.~ 50-a:,,~ 45.

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20' i51050. r 0 10,000 20,000 3000 0 i*0 ,00 0 50,000 60,000 70 ,00 0 80,000 90,000 100000 110,000 120,000

TOTAL CAPACITY OF TANK~ m tF i g. 3 . C a t c h m e n t a r ea a n d c a p a c i t y o f t a n k a t d i f f e re n t p r o b a b i l i t y l e v el s o f s e a s on a l r u n o f f o na s a n d y l o a m s o i l .

i n o r d e r t o a c h i e v e e c o n o m y o n t a n k c o n s t r u c t i o n , t h e t a n k s h o u l d b e o f t h el a r g e s t p o s s i b l e s i z e .Probab i li ty o f assured runof f o r tank design an d cos t o f avai lable w ater forirrigation

I n g e n e r a l , t h e c a p a c i t y o f t h e t a n k s i n c r e a s e s a s th e p r o b a b i l it y l e v e l o fa s s u r e d r u n o f f d e c r e a s e s . T h i s i s t r u e fo r a ll s i z e s o f t h e c a t c h m e n t a r e a ( F i g .3 ) . F u r t h e r , t h e v o l u m e o f a v a i l a b l e w a t e r p e r u n i t o f ta n k c a p a c i t y i n c r e a s e sa s t h i s p r o b a b i l i t y l e v e l i n c r e a s e s f o r v a r i o u s s i z e s o f t a n k ( F i g . 4 ) .

I n o r d e r t o s e l e c t a n a p p r o p r i a t e p r o b a b i l i ty l e v e l o f a s s u r e d r u n o f f f o r t a n kd e s i g n t h e a n n u a l c o s t o f u n i t v o l u m e o f a v a il a b l e w a t e r a t t h e t i m e o f ir rig a-t i o n w a s c o m p u t e d . T h e a n n u a l c o s t p e r u n i t o f a va i la b l e w a t e r d e c r e as e sg r a d u a ll y u p t o a m i n i m u m a s t h e p r o b a b i l it y l e v e l o f a ss u r e d r u n o f f i n -c r e a s e s . F u r t h e r o n , it in c r e a s e s g r a d u a l l y ( F i g . 5 ) . T h i s i s d u e t o t h e f a c t t h a t


9/13

D E S I G N O F S T O R A G E T A N K S 203

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Fig. 4. Total capacity of tank and expected available water for presowing irrigation of wheat fortanks designed on different probabili ty levels of seasonal runoff on a sandy loam soil.a s t h e p r o b a b i l i ty l e v e l i n c r e a s e s t h e t a n k s s iz e d e c re a s e s a n d t h e v o l u m e o fa v a i l a b le w a t e r p e r u n i t o f t a n k c a p a c i t y in c r e a s e s ( F i g . 4 ) . T h u s a t a h ig h e rp r o b a b i l i t y l e v e l, t h e c o s t o f t a n k p e r u n i t v o l u m e w i l l b e h i g h a n d v i c e v er s aa t l o w e r p r o b a b i l i ty l e v e ls . T h e a p p r o p r i a t e p r o b a b i l i t y l e v e ls o f a t a n k d e s ig ng i v i n g t h e l o w e s t a n n u a l c o s t p e r u n i t v o l u m e o f a v a i l a b le w a t e r f o r i r r ig a t i o nf o r d i f f e r e n t c a se s a r e p r e s e n t e d i n T a b l e 2 . T h i s p r o b a b i l i t y l e v e l v a r i e s f r o m4 0 t o 8 0 % f o r c a t c h m e n t s i z e s v a r y i n g f r o m 1 t o 1 0 0 h a . I n a l l c a s e s, t h e p r o b -a b i l it y l e v el fo r t a n k d e s i g n i n c r e a s e d w i t h i n c r e a s i n g c a t c h m e n t s iz e.Economic feasibility o f tank based irrigation

T h e b e n e f i t - c o s t a n a l y s is o f t h e t a n k s d e s i g n e d o n b a s i s o f p e r i o d i c a l r u n o f fw a s d o n e a n d t h e a p p r o p r i a t e p r o b a b i l i t y l e v el w a s i d e n t i f i e d ( T a b l e 3 ). F u r -t h e r , t h e f o l l o w i n g f a c t s w e r e o b s e r v e d :( i ) T h e a m o u n t o f w a t e r a v a il a b l e in t h e 3 5 t h w e e k f o r i r ri g a t i o n o f m a i z ei s a l w a y s m o r e t h a n t h a t o f w a t e r s a v e d u p t o t h e 4 4 t h w e e k fo r p r e s o w i n gi r ri g a t i o n o f w h e a t . T h i s i s b e c a u s e s e e p a g e a n d e v a p o r a t i o n l os se s d u r i n g t h e


10/13

204 H.N. VERMA AN D P.B.S. SARMA

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l o i o ; o i o ~ o 6 o ; o ; o ~ o 1 ; oPROBABILITY OF LOW EST ASSUREDRUNOFFs, /oFig . 5 . Cost o f w a t e r a v a i l a b l e i n t a n k s d e s i g n e d a t d i f f e r e n t p r o b a b i l i t y l e v e l s o f p e r i o d i c a l( w e e ks 2 4 - 3 5 ) r u n o f f f o r m a i z e i r r ig a t i o n i n s a n d y l o a m s o il ( c a s e 1 ) .

TA BLE 2Appropriate probability levels of tank design for different s iz e of catchm entsCase Appropriate prob ability level in percent(catchm ent area in ha)( 1 ) T35 for 50 60 70 80( i ) M a i z e ir ri ga tio n ( 1 - 2 ) ( 3 - 1 0 ) ( 2 0 - 5 0 ) ( 6 0 - 1 0 0 )(i i) Presowing irrigation to 40 50 60 70 80w h e at ( 1 ) ( 2 - 5 ) ( 6 - 2 0 ) ( 3 0 - 9 0 ) ( 1 0 0 )(2 ) T39 for presowing 50 60 70 80ir ri ga tio n t o w h ea t ( 1 - 2 ) ( 3 - 7 ) ( 8 - 3 0 ) ( 4 0 - 1 0 0 )


11/13

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12/13

20 6 H.N. VERMAAND P.B.S. SARMAT A B L E 4C o s t a n d b e n e f i t o f t a n k s d e s i g n e d o n a s e a s o n a l r u n o f f b a s is a n d p r o b a b i l i t y l e v e l a g i v i n g t h e l o w e s tc o s t p e r u n i t v o l u m e o f e x p e c t e d w a t e r in t h e 4 4 t h w e e k f o r w h e a t i r r i g a ti o n o n s a n d y l o a m s o i ls ( c a s e2)C a t c h - T o t a l t a n k T o t a l A v a i l a b l e A r e a A n n u a l N e t B e n e f i t - c o s tm e n t c a p a c i t y a n n u a l w a t e r i r r i g a t e d i r r i g a ti o n a n n u a l r a t i o ,a r e a ( m 3 ) c o s t o f ( m 3) ( h a ) c o s t , C 2 r e t u r n o f B/(CI+C2)( h a ) t a n k , C I ( R s ) i r r i g a t i o n ,

( R s ) B(Rs)1 4 9 4 .3 8 5 7 .5 2 4 9 .8 0 .5 0 0 1 9 5 1 6 9 0 1 .6 02 1 1 0 1 .3 1 5 1 3 .7 6 0 5 .9 1 .2 1 2 4 7 3 4 0 9 6 2 .0 63 1 4 0 1 .9 1 8 0 4 .5 8 5 0 .6 1 .7 0 1 6 6 3 5 7 4 9 2 .3 34 1 9 3 3 .1 2 2 8 5 .9 1 1 9 9 .9 2 .4 0 0 9 3 6 8 1 1 1 2 .5 25 2 4 7 6 .8 2 7 6 0 .0 1 5 6 3 .9 3 .1 2 8 1 2 2 0 1 0 5 7 1 2 .6 66 3 0 1 7 .3 3 1 9 4 .7 1 9 2 1 .3 3 .8 4 3 1 4 9 9 1 2 9 8 7 2 .7 77 3 5 6 8 . 4 3 6 3 4 . 9 2 2 9 2 . 4 4 . 5 8 5 1 5 1 8 1 5 4 9 5 3 . 0 08 3 2 4 8 . 8 3 3 9 3 . 3 2 2 6 4 . 4 4 . 4 5 3 1 4 7 4 1 5 0 4 9 3 . 0 99 3 6 8 7 .4 3 7 3 4 .0 2 5 4 0 .6 5 .0 8 1 1 6 8 2 1 7 1 7 1 3 .1 7

1 0 4 1 2 5 . 2 4 0 6 7 . 1 2 8 5 3 . 7 5 . 7 0 7 1 8 8 9 1 9 2 8 7 3 . 2 42 0 8 6 4 3 .2 7 3 2 8 .1 6 1 5 8 .8 1 2 .3 1 8 3 6 7 1 4 1 6 2 9 3 .7 83 0 1 3 2 1 0 .2 1 0 3 9 7 .1 9 5 1 4 .9 1 9 .0 3 0 5 6 7 1 6 4 3 1 2 4 .0 04 0 1 3 3 6 8 . 9 1 0 4 9 9 . 0 1 0 2 1 3 . 4 2 0 . 4 2 7 6 0 8 7 6 9 0 3 3 4 . 1 65 0 1 6 8 2 8 .4 1 2 7 5 6 .5 1 2 9 0 7 .7 2 5 .8 1 5 7 6 9 3 8 7 2 4 2 4 .2 76 0 2 0 3 6 6 .1 1 4 9 9 3 .6 1 5 7 0 7 .2 3 1 .4 1 4 9 3 6 1 1 0 6 1 6 4 4 .3 67 0 2 3 8 3 3 . l 1 7 1 9 3 .5 1 8 4 1 0 .1 3 6 .8 2 0 1 0 9 7 2 1 2 4 4 3 3 4 .4 28 0 2 7 3 4 0 .9 1 9 3 7 7 .7 2 1 1 6 7 .9 4 2 .3 3 6 1 2 6 1 6 1 4 3 0 7 5 4 .4 79 0 3 0 9 0 6 . 8 2 1 5 4 2 . 9 2 4 0 0 3 . 9 4 8 . 0 0 8 1 4 3 0 6 1 6 2 2 4 3 4 . 5 2

1 0 0 3 4 3 8 2 . 8 2 3 6 9 8 . 3 2 6 7 1 7 . 9 5 3 . 4 3 6 1 5 9 2 4 1 8 0 5 8 7 4 . 5 6a L o w e s t c o s t p e r u n i t o f e x p e c t e d w a t e r v o l u m e i s a t 5 0 , 6 0, 7 0 a n d 8 0 % p r o b a b i l i t y le v e l s f o r 1 - 2 , 3 -7, 8 - 3 0 a n d 4 0 - 1 0 0 h a w a t e r s h e d a r e a s.

s to r a ge p e r i o d f r o m w e e k N o s . 3 6 t o 4 4 a re h i g h e r t h a n t h e r u n o f f r e c e i v e dd u r i n g t h e s a m e p e r i o d .

( i i ) T h e b e n e f i t - c o s t r a ti o in t h e c a se o f p r e s o w i n g i r r ig a ti o n o f w h e a t i sa l w a y s m o r e t h a n t h a t o f ir r ig a ti o n o f m a i z e . T h i s i s b e c a u s e o f t h e lo w r e-s p o n s e a n d t h e l o w p r ic e o f m a i z e a n d c h a n c e s o f pa r ti al b e n e f i ts f r o m ir ri-g a t io n o f m a i z e d u e t o t h e p o s s i b i l i t y o f t h e o c c u r r e n c e o f r a in j u s t a f te ri r r i g a t i o n .

( i i i ) T h e b e n e f i t - c o s t r a ti o o f th e t a n k b a s e d i r r ig a ti on o f m a i z e i s l es s t h a no n e f or ta n k s h a v i n g a c a t c h m e n t a r ea u p t o 1 0 h a . T h i s r a ti o in c r e a s e d w i t hi n c r e a s i n g c a t c h m e n t a r e a a n d v a r i e d f r o m 1 .1 t o 1 .3 fo r c a t c h m e n t a r e a s o f20 to 100 h a .

T h e t a n k s d e s i g n e d o n t h e b a s is o f ca s e 2 ( s e a s o n a l r u n o f f f o r p r e s o w i n gi r r ig a t io n o f w h e a t ) g i v e m o r e a v a i l a b l e w a t e r t h a n t h a t o f c a s e 1 . T h e b e n e -f i t - c o s t r a t io i n c a s e 2 ( T a b l e 4 ) i s g r e a te r f o r a ll t h e c a t c h m e n t s i z e s a n d


13/13

DESIGN OF STORAGE TANKS 207v a r i e s f r o m 1 .6 t o 4 .5 6 f o r c a t c h m e n t a r e a s o f I t o 1 0 0 h a . T h e a b o v e r e s u l t sr e v e a l t h a t t a n k b a s e d i r r i g a ti o n o f w h e a t at p r e s o w i n g is m u c h m o r e b e n e f i -c i al t h a n t h a t o f i r r ig a t i o n o f m a i z e . H e n c e , t a n k s i n s a n d y l o a m s oi ls s h o u l db e d e s i g n e d o n a s e a s o n a l r u n o f f b as i s f o r a p r e s o w i n g i r r ig a t i o n o f w h e a t .CONCLUSIONS

( 1 ) A c o m p u t e r b a s e d p r o c e d u r e h a s b e e n d e v e l o p e d f o r d e si g n in g st or ag et a n k s an d c o m p u t i n g c o st s a n d t h e v o l u m e o f av a il a b le w a t er a t t h e ti m e o fi r r ig a t io n f o r p l a n n i n g w a t e r h a r v e s t i n g t a n k s y s t e m s i n t h e Kand i d r y f a r m -i n g b e lt o f n o r t h e r n P u n j a b .

( 2 ) S m a l l t a n k s a re c o s t li e r a n d , fo r e c o n o m y m e a s u r e s , t h e t a n k s h o u l d b eo f t h e l a rg e s t s i z e w i t h i n t h e l i m i t s s e t b y o t h e r f a c to r s .

( 3 ) T h e a p p r o p r i a te p r o b a b i l i ty l e v e l o f l o w e s t a s s u re d r u n o f f f o r ta n k d e -s ig n i n c r e a se s w i t h i n c r e a s in g c a t c h m e n t a r e a a n d v a r i es f r o m 4 0 t o 8 0% .( 4 ) F o r m a x i m u m b e n e f i t t h e t a n k s s h o u l d b e d e s i g n e d o n t h e b a s i s o f

l o w e s t a s s u r e d r u n o f f f o r p r e s o w i n g i r r ig a t io n t o w h e a t . T h e b e n e f i t - c o s t r a-t i o o f s u c h ta n k s v a r i e d f r o m 1 .6 0 t o 4 . 5 6 f o r c a t c h m e n t a r e a s 1 t o 1 0 0 h a .

REFERENCESAnonymous, 1975-1983. Records of tank construction. AICRPDA, Hoshiarpur Centre, PAU,

Ludhiana.Helweg, O.J. and Sharma, P.N., 1983. Optimum design of small reservoirs (tanks). Water Re-sour. Res., 19(4): 881-885.Palmer, W.L., Barfield, B.J. and Haan, C.T., 1982. Sizing farm reservoirs for supplemental ir-rigation of corn. Trans. Am. Soc. Agric. Eng., 25 (2): 372-387.Singh, R.P., 1983. Farm pond. Project Bulletin No. 6. All India Coord. Res. Project on DrylandAgriculture, Hyderabad.Singh, R., Prihar, S.S., Verma, H.N., Singh, N., Sandhu, K.S., Singh, Y., Singh, M., Singh, N.and Saggar, S., 1983. Dry/and cropping in the sub-montane Punjab. Bull. Punjab Agric. Univ.,Ludhiana.Verma, H.N., 1981. Water harvesting for life-saving irrigation of rainfed crops in the sub-mon-tane region of Punjab. J. Agric. Eng., 18(3-4): 64-72.Verma, H.N., 1987. Studies on efficient use of rainwater for rainfed crops. Ph.D. Thesis, I.A.R.I.,New Delhi.Verma, H.N. and Sarma, P.B.S., 1988. Analysis of short duration rainfall for planning rainfedcrops. J. Agric. Eng., XXV(3): 14-19.Verma, H.N. and Sarma, P.B.S., 1989. Critical dry spells and supplemental irrigation to rainfedcrops. J. Ind. Water Resour. Soc., 9(4): 12-16.

Desing of Storage Tanks for Water Harvesting in Rainfed Areas 24.10.11

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