Abstract
Evapotrao.otion forms a foundation for planning and d:e~ of most of the irrigation and drainage projli!~. It is usually the starting point in determining s~,f~and sub-surface water storage requirements, ~city of the deliverY' system and general operation ·pta€tices. The evapotranspiration depends upon the climate, time of year, cropping pattern and cropping intensity of the area. The effects of climate and time at the year are included for estimating the reference crop evapotranspiration. Prior to actual evapotranspiration, it is important to discuss the reference crop evapotranspiration (ETo) and crop coefficient (Kcl as these will be used to estimate the actual evaoot. anspiration. D08rer,bv"; and Pruitt (1977) defined reference crop evapotranspiration (ETa) as "the rate of evapotranspiration from an extensive surface of 8-15 cm tal' green grass cover of uoiforrr, height, actively growing, completely shading the ground surface and not short of water". In developing a guideline for determining the crop water requrrernenrs Doorenbos and Pruitt estimated the reference crop evapotranspiration (ETo), using Blanney-criddle, Radiation, Penman combination methods and pan evaporation method
