FAO Penman–Monteith method

Potential evapotranspiration (ET_p) is defined as the evapotranspiration rate from the reference surface, which is a hypothetical grass with height of 0.12 m, albedo of 0.23 and surface resistance of 70 s/m. The FAO Penman–Monteith equation has been recommended as the sole standard method for determining ET_p since 1990 by the Food and Agricultural Organization of the United Nations (FAO)¹⁵. Air temperature, relative humidity, solar radiation and wind speed data are required when using this combined method which allows estimation of ET_p even in the case of missing climatic variables and different climatic conditions. Daily ET_p values were calculated according to the FAO Penman–Monteith equation¹⁵.

Rn is the net radiation at the crop surface (MJ m⁻² day⁻¹), G is the soil heat flux density (MJ m⁻² day⁻¹), T is the mean daily air temperature at 2 m height (°C), U₂ is the wind speed at 2 m height (ms⁻¹), e_s is the pressure (kPa), e_a is the actual vapor pressure (kPa), Δ is the slope of the vapor pressure curve (k Pa °C⁻¹), and γ is the psychrometric constant (k Pa °C⁻¹)¹⁵.

Net radiation (Rn) was calculated as the difference between incoming net shortwave radiation and outgoing net longwave radiation and followed the procedure of Allen et al. based on global solar radiation, albedo (0.23), clear-sky solar radiation, T_min, T_max and e_a¹⁵. Based on the same guidelines, the magnitude of daily soil heat flux (G) beneath the reference grass surface is relatively small and therefore, may be neglected for 24-h time steps^28,30,31.