Water balance study of Krishnai River Basin based on Thornthwaite’s concept of Potential Evapotranspiration: A research report by National Institute of Hydrology

The study aims to determine the climatological water balance of Krishnai river basin in Goalpura, Assam using the popular Thornthwaite’s concept of Potential Evapotranspiration (PET) to calculate the availability of water for various uses. The method offers a firm basis for appraising the problems related to water development projects in the planning stage and provides a sound footing for operating and managing the system. 

A time series of mean monthly data on certain climatological parameters like temperature and rainfall were used for the nine year period of 1986-1994. These were analyzed to estimate the various elements of water balance like actual evapotranspiration, water surplus and water deficit. Thornthwaite’s method has been used to calculate the potential evapotranspiration. Mean monthly rainfall data for different months were computed using Isohyetal method. Field capacity for the soil was assumed as 100 mm for the Krishnai basin.

Various statistical methods were employed to analyze the vast amount of accumulated climatic data. On the basis of observed mean monthly PET and rainfall data, the normal values of PET and rainfall were plotted for various months. It reveals that on a normal annual basis, the basin has a water need of 1476.56 mm, whereas the rainfall is 1844.14 mm. It is observed that the highest mean monthly rainfall is 356.18 mm in the month of July and the water surplus in the month is 148.96 mm, which is the highest water surplus in any month.

PET exceeds rainfall for five months and the total deficit during the period is 70.43 mm. For the rest of the period i.e., April to November the value of rainfall exceeds PET (541 mm). Out of this amount, 388 mm appears as water surplus and remaining 153 mm adds to soil moisture as recharge. Out of twelve months, six are water surplus months. In four months there is water deficit and the surplus month of April recharges the moisture deficit of the soil mass. The study indicates that the basin can be considered as drought-free as the water surplus exceeds the water deficit. 

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