Systematic procedure for the computation of evaporation from lakes - A research report by National Institute of Hydrology

The report presents a systematic procedure for the computation of evaporation from lakes, which constitutes one of the major outputs from a lake.

The report presents a systematic procedure for the computation of evaporation from lakes. Water balance of a lake depends upon the relative importance of various inputs and outputs to and from the lake. In Indian conditions, evaporation loss constitutes one of the major outputs from a lake and as such, the water regime of a lake and yield are influenced by it. In order to conserve and use the water, there is an urgent need for a first hand reasonable accurate estimate of its water balance.

There are a plethora of formulae and methods to estimate evaporation from a free water surface and it is very difficult to adopt a particular method universally particularly under Indian conditions. The study on hydrological aspect of lakes started some time back in India and the development of a suitable methodology will need some time for experimentation.

As evaporation constitutes a major output component of Indian lakes, an attempt has been made here to critically review  the important formulae available and their suitability for appropriate adoption and adaptation. It is found that 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 three hydrometeorological inputs (air temperature, water temperature and wind velocity at 10 m height) could be used for operational purpose if reliable measurements of air and water temperatures are made.

The energy-budget method is suitable for research purposes where accuracy is of prime concern and 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.

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