Before revision: Equation ETo = p ·( 0.46 ·Tmean + 8 ) Where: ETo is the reference evapotranspiration [mm day−1] (monthly) Tmean is the mean daily temperature [°C] given as Tmean = (Tmax + Tmin )/ 2 p is the mean daily percentage of annual daytime hours.
After revision: Equation ETo = p ·( 0.457 ·Tmean + 8.128 ) Where: ETo is the reference evapotranspiration [mm day−1] (monthly) Tmean is the mean daily temperature [°C] given as Tmean = (Tmax + Tmin )/ 2 p is the mean daily percentage of annual daytime hours.