Penman’s evapo-transpiration equation is based on

• Evapotranspiration Equation:
There are large number of evapotranspiration equations available, they are purely empirical and not theoretical. The most used equation is Penman's Equation.

• Penman's Equation:
Penman's equation is based on sound theoretical reasoning and is obtained by combination of energy and mass transfer approach.
γγPET=AHn+Eaγ/Aγ
PET = Daily potential evapotranspiration (in mm per day)
A = Slope of the saturation vapour pressure vs. temperature curve at the mean air temperature (in mm of Hg per °C)
Hn = Net radiation
Ea = Parameter including wind velocity and saturation deficit.
γ = Psychromrtic constant = 0.49 mm of Hg/°C