Estimation of reference evapo-transpiration (ET0)

Gidelines for estimating Evapo-Transpiration ina perticuler crop is given by ,

ET^O = C [W.Rn + (1-w). f (U). (ea – ed)]  

where, 
Rn = net radiation in equivalent evaporation expressed as mm/day, W=temperature of altitude related factor, f(U) = wind related function, Ea–ed= vapor pressure deficit (mili bar),   C=the adjustment factor (ratio of U day to U night), Rn (0.75-Rns), Ea=Saturated vapor pressure (m.bar), Ed=mean actual vapor pressure of the air (mili bar). In this method, radiation, air temperature, relative humidity and wind speed are incorporated into the ETo estimate.