Comparison of Hargreaves and FAO56 equations for estimating monthly evapotranspiration for semi-arid and arid environments

The objectives of this study are to compare reference evapotranspiration (ET0) computed using different forms of the Hargreaves (HG) equation with the standard FAO56PM; and to compare ET0 computed using the FAO56PM with limited data with the standard FAO56PM complete data set method. For this purpose, climatic data from 12 weather stations covering an area of ∼90 km2 of predominantly arid and semi-arid conditions are used. Ratios of ET0 estimated using the updated HG equation were closer to 1 when compared with the HG equations and ET0-est. Also, RMSE, ME and R 2 ranged from 0.235 to 0.526 mm day−1, −0.526 to 0.383 mm day−1, and 0.945 to 0.989, respectively. Therefore, the updated HG equation can be used for ET0 estimates instead of using the FAO56PM method with acceptable accuracy. In addition, it was found that the standard FAO56PM can be used with estimated solar radiation and humidity from temperature data and estimates of mean wind speed. In conclusion, this study produced two accurate methods that can be used to calculate monthly ET0 in conditions characterized by limited data, arid and semi-arid environments.