Free Evapotranspiration calculator with clear step-by-step results.
Implements the FAO-56 Penman-Monteith reference evapotranspiration for a short grass surface. The result splits into two parts worth reading separately: the radiation term, which dominates in still humid conditions, and the aerodynamic term, which takes over when it is dry and windy.
Saturation vapour pressure
es = 0.6108 x exp(17.27 T / (T + 237.3))
Reference ET
ET0 = (0.408 x slope x net radiation + gamma x (900 / (T + 273)) x wind x VPD) / (slope + gamma x (1 + 0.34 x wind))
No. ET0 is for a reference grass surface. Multiply by the crop coefficient for your crop and growth stage to get actual crop evapotranspiration.
Wind strips away the humid boundary layer above the surface, keeping the vapour pressure deficit at the leaf high. In dry conditions the aerodynamic term can exceed the radiation term.