where ET0 is the reference evapotranspiration (mmd−1), Rn is the net radiation at the crop surface (MJ m−2 d−1), G is the soil heat flux density (MJ m−2 d−1), T is the mean daily air temperature (°C), u2 is the daily average wind speed at 2 m above ground level (ms−1), es is the saturation vapor pressure (kPa), ea is the actual vapor pressure (kPa), es − ea is the saturation vapor pressure deficit (kPa), Δ is the slope of the saturated vapor pressure in relation to air temperature (kPa °C −1) and γ is the psychrometric constant (kPa °C −1).