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Hargreaves equation
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As an alternative when solar radiation data, relative humidity data and/or wind speed data are missing, reference evapotranspiration, ET0 (mm d-1), can be estimated using the Hargreaves equation (Hargreaves and Samani, 1985). The FAO-56 Hargreaves equation (Allen et al., 1998) for daily computation is given by:

whereTmax(°C) is the maximum daily air temperature, Tmin(°C) is the minimum daily air temperature, Ra (MJ m-2 d-1) is the extra-terrestrial solar radiation. The parameters a (mm d-1) and b are calibrated coefficients, determined on a monthly or yearly basis by regression analysis or visual fitting. An unadjusted version of Hargreaves equation (given by default) is given with a=0 and b=1.