TR(BR)-15/97-98 : Systematic procedure for the components of water computation of the lakes Part I: Evaporation

Abstract

Water balance of a lake depends upon the relative importance of various inputs and outputs to and from the lake. Evaporation loss constitutes one of the major output from a lake under Indian condition and as such the water regime of a lake and yield from it are influenced by the evaporation under Indian conditions. There are a plethora of formulae and methods to estimate evaporation from a free water surface as in a lake and it is very difficult to adopt a particular method universally and more so under Indian condition. Study on hydrological aspect of lake in India has started some time back and the development of most suitable methodology for lakes in a region will need lot of experimentation and will take time to identify. In order to conserve and use our valuable water in our lakes for the community, there is an urgent need to have a first hand reasonably accurate estimate of water balance of lakes. As evaporation constitute the major output component of Indian lakes, an attempt has been made herein to review critically the major and important formulae available and their suitability for appropriate adoption and adaptation under Indian condition are examined with respect to objective of the study and desired level of accuracy.

Meyer Equation and Class A pan data multiplied by 0.7 are likely to give acceptable estimates of evaporation for design purpose. The equation which requires simple three hydro meteorological inputs (air temperature, water temperature and Wind Velocity at 10 meter height) even could be used for operational purpose if reliable measurements of air and water temperatures are made.

The energy-budget method is suitable for research purpose where very accurate estimate of evaporation is warranted. It can be resorted to for small areas only as the requirements for detailed meteorological data are great. The practical utility of the method for large lakes is limited.