Loss of reservoir water would mainly take place due to evaporation and a number of methods have been suggest for controlling such loss. The Bureau of Indian Standard code IS: 14654 - 1999 “Minimizing evaporation losses from reservoirs- guidelines” describes the cause of evaporation reduction methods in detail, some important aspects of which are described in the subsequent paragraphs. As such, percolation or seepage loss is small for most of the reservoirs and progressively gets lowered with the passage of time since the sediment getting deposited at the reservoir bottom helps to reduce percolation losses.  Of course, in some hills and valleys forming the reservoirs, there may be continuous seams of porous rock strata or limestone caverns which cause huge amount of water to get drained out of the reservoirs. 

The reservoir of the Kopili Hydroelectric Project in Assam-Meghalaya border had faced similar problems due to the presence of large caverns which had to be sealed later at quite large cost at a later stage. 

A number of factors affect the evaporation from open water surface, of which, the major factors are water spread area and frequent change of speed and direction of wind over the water body. Other meteorological factors like.  

1. Vapour pressure difference_ between water surface and the layer of air above

2. Temperature of water and air

3. Atmospheric pressure

4. Radiation

5. Heat storage in water body

6. Quality of water

have direct influence on the rate of evaporation. 

Since the meteorological factors affecting evaporation cannot be controlled under normal conditions, efforts are made for inhibition of evaporation by control of flow of wind over water surface or by protection of the water surface area by physical or chemical methods.

The methods generally used are as follows: 

1. Wind breakers

2. Covering the water surface

3. Reduction of exposed water surface

4. Integrated operation of reservoirs

5. Treatment with chemical water evaporetardants (WERs).