• • l rodi•• .I1el .llydro I, a",pAy•. (1977), 28 4, 507·614 556 · 535 (282 ' 253)0 Estimation of water balance of lower Sutlej catchment upto Bhakra dam site by Thornthwaite's method K. K. SIUVASTAVA, S. D. S. ABBI a lU l ll. H. LOE Jl eleorologirol Offiee , N etc l k/M (Reeeived J.J December 1975) AIlSTRACT • Water balance by Thnrnthwaitc's method of lo,n T Sut lej catchment between Rampur and Bhak re da m 6ite ha l! been dlecnseed. Maps showing the monthly water deficit, water surplus, eoil moisture utilisa- tio n an d Mil moisture recharge for the years loo.t-68 have been An estimation of the annual runoff in ri ver S.,IIej on the besle of the wat er bele nce paremcte re for the same period h&8 been attempted and presented in the papt"r. • • t. The concept of Book keeping procedure using meteorulogicnl parameters fur the ostimntion of climatic water balance developed by Thor n- rhwnite (194R). The climatic wat er budget dove- lope.! wns modified by Thornth wnitc ami Matl ier ( 1955) to make it more useful over a wide range of soil and vegeta tion conditions. For this, field capacity of the soil was.increased from IO em to 30 em and th er at e of soil mositure decrease was found to follow. an expon entia l decay law. Suba Rao and Subrahm anyam (1961) in their paper 00 estimation of yields from river basins by II modi- fic ation of water balance procedure of Thorn- thwaite have estimated monthly and yearly run - ofT from .1In 1&,,"adi, Kosi and Ashni rivers, Hac c1 al. (1970), have computed climati c wat er balance for about 350 stntions in India on the hnsiscf meth odology developed by Thorn th- waite and his associates, The above procedure has therefore been utilized broadly in the present 8tudy to fiud the moistur e stat us of the Sutl ej basin. The study of the water balance provides of potential evapo transpiration, wat er deficit a nd wat er surplus and esti matio n of ru noff in 8 basin. A knowledge of the moisture deficit facili- tat es the understanding of the economic fea8j- bility of irrigat ion, and t hat of the water surplus provide, informat ion on the recharge of gr ou nd wat er or runoff in streams or rivers. Similarly, the magnitude of eva pot ra n8p ira t ion gives the limit of ficld irr igat ion for o pt imum crop producti on. All these p arameters thu s provide useful inf er - metion for assessing irrigation requirements and in judging the hydrologicaland ngcieu lt u m l pot en- tinl of the hasin. 2. M'lb01lology III comput ing the water bnlnnce, th e precipi- tation (l') is compared with potential cvapotran- sp ira t ion (I'll ) on a mon thly basis. (1'- I'E) can be zero, positive or ncgativo. The positive value of (1'- 1' E ) indicates th e amount of excess wat er wh ich is available during certain periods in the year for soil moist ure recharge and run off. It is only when precipitat ion adds to the soil moi sture IIptOits maximum value of water holding capacity th at th e resultant wat er turns into run off. Fu rther, the value of act ua l evs potr ansp iration (..I E) is equal to pot ent ial evapotrunspi j-ation (1'8) when (1'- 1' g) is positive, as evapotrans- pirat ion can proceed unhindered because of the adequacy of available moi sture. When (1'- 1'E) is zero, the potential loss of moi sture du e to eva.. potranspiration is fu lly supplied by the preci- pit ati on. But thi s is the hypothetieal situation which may not always be realised in actual prae- tice. Lastly the negativevalue of (1'- I'E) ind i- cates the amount by which the precipitation falls short of the supply to the potential water needs of the vegetat ion covered area. The valu e of actual evapot ranspiration is t hen equal to precipitn tion p Itts th e amount of wat er drawn from the soil moisture storage. TIle amount of soil moisture thus utilized is th e change in storage. The accumu- lated value of ( 1'-1'8 ) gives an estimate of the moi stur e stored in the soil at th e end of the nega- live (1'- 1'&) peri od. . ti07