CS(AR)-24/96-97 : Estimation of evaporation losses from water surface -a study of Tawa reservoir

Abstract

In order to assess the adaptability of different methods for estimated of evaporation from free water surfaces in semi-arid areas, this study for Tawa reservoir is third in the series of evaporation studies. The estimates of evaporation from free water surface obtained by four methods namely Penman, Kohler, Van Bavel and Morton are compared with observed pan values on monthly, seasonal and annual basis. The estimates of the mean value of evaporation in winter months are lower than those of corrected pan values, whereas in spring and summer season differences are comparatively less and estimates are closer to pan values. All the methods gave comparable estimates of evaporation for spring and summer months. The comparison of monthly values indicated that the estimates of evaporation by both Kohler and Morton methods gave better correlation with corrected pan values for all the months. Also on the basis of comparison of annual values, the results of both Kohler and Morton methods are in good agreement as compared to Penman and Van Bavel models.

Earlies studies conducted for the Malprabha and the Bargi reservoir sites had also come up with more or less similar results as that of the Tawa reservoir site. It may now be concluded that the both Kohler and Morton methods are reliable approaches for estimation of evaporation from reservoirs and lakes in semi arid areas. However, the coefficients used for adjusting Pan evaporation to lake are tentative and need confirmation by further studies for more places.