The determination of evaporation from water masses: a case Study of Nyumba Ya Mungu Reservoir, Tanzania

Determination of evaporation from water surface is important in water control and water regulation of reservoir; however it is very difficult to get the accurate measurements. This study focused on estimating evaporation of the reservoir using Pan and other meteorological variables from the land surface and compared with the reservoir evaporation measured direct from the water surface and correlating the two then recommend the approach to be used in estimating evaporation from water masses. Four methods i.e. Pan, Herbeck , Bulk aerodynamic and Hargreaves were used to estimate reservoir evaporation from the land surface using meteorological variables from NyM weather station and one method, SEBS was used to estimate the evaporation of the reservoir direct from the water surface. It was realized that the SEBS estimates were higher than the estimates of the other four methods and the differences were tested using Wilcoxon’s test, between SEBS against other estimates, which showed the differences to be statistically significant. Also regression analysis was carried out, SEBS method gave good correlation with Pan and Hargreaves estimates. Therefore, determination of evaporation estimates on the water masses directly should be encouraged whenever possible. If not possible, determining evaporation from the water masses using the meteorological variables gathered outside the reservoir on land surface should continue but using many stations and which are very close to the reservoir in order to represent the same weather condition as that of the reservoir.