Modified Hargreaves-Samani Model Based on Altitude in the High-Altitude Regions of Iran

The Food and Agriculture Organization (FAO) has proposed the FAO 56 Penman-Monteith model as a standard method for estimating reference evapotranspiration (ETo). However, using this model for the high-altitude regions mostly faces uncertainty due to the lack of meteorological stations, observation problems, and incomplete meteorological data. Therefore, in these regions models should be used that require fewer meteorological variables and can also be considered as a function of altitude. In the present study, the data from 28 synoptic stations in Iran with an altitude of more than 2000 meters above sea level in the period of 1989-2019 were used to modify the Hargreaves-Samani model based on the altitude and also correct the temperature factor coefficients. The comparison of the results of the FAO 56 Penman-Monteith model as a standard method, the original Hargreaves-Samani, and the modified Hargreaves-Samani models showed that the modified model offers better results than the original model and is able to estimate the ETo more accurately in the high-altitude regions of Iran. Based on the results of MBE, MAE, RMSE, PVC, LVC, d, r, and PI statistical indices, the highest agreement in estimating ETo with modified Hargreaves-Samani and FAO 56 Penman-Monteith models was observed at Firuzkuh, Baft, Baladeh, Sisakht, Avaj, Khansar, Abali, Daran, Fereidunshahr, Borujen, Samirom, Aligudarz, Farokhshahr, Damavand, Sepidan and Saman stations, which had an average error reduction of 14% compared to the values obtained from the original Hargreaves-Samani model.