A large number of more or less empirical methods have been developed over the last 50 years by numerous scientists and specialists worldwide to estimate evapotranspiration from different climatic variables. To meet this need, guidelines were developed and published in the FAO Irrigation and Drainage Paper No. 24 ‘Crop water requirements’. In 1948, Penman combined the energy balance with the mass transfer method and derived an equation to compute the evaporation from an open water surface from standard climatological records of sunshine, temperature, humidity and wind speed.
\[ET_{O} = \frac{0.408 \Delta (R_n - G) + \gamma \frac{900}{T+ 273} U (e_s - e_a)}{\Delta + \gamma (1 + 0.34 U) }\]
ETo:reference evapotranspiration [mm day-1]
Rn:net radiation at the crop surface [MJ m-2 day-1]
G:soil heat flux density [MJ m-2 day-1]
T:mean daily air temperature at 2 m height [°C]
u:wind speed at 2 m height [m s-1]
\(e_s\):saturation vapour pressure [kPa]
\(e_a\): actual vapour pressure [kPa]
\(e_s- e_a\): saturation vapour pressure deficit [kPa]
\(\Delta\): slope vapour pressure curve [kPa °C-1]
\(\gamma\): psychrometric constant [kPa °C-1]