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Abstract
The Penman-Monteith is the most desirable method for estimating reference crop potential evapotranspiration (ETo). However, this method needs full weather data, but only a few stations with complete weather data exist in the south of Iran. On the other hand, the Hargreaves equation is a more simple equation for estimating ETo, and some new types of this equation have been proposed recently. In this study, using the weather data of 14 stations in the south of Iran, different types of the Hargreaves equation for monthly ETo estimates were compared with the Penman-Monteith method. By using the obtained results, monthly ETo can be estimated with appropriate type of Hargreaves equation for different places by considering the average ratios between monthly estimated ETo with Penman-Monteith to the appropriate type of Hargreaves equation. The results showed that the original type of Hargreaves equation was only better than the other types of this equation in three stations with humid climate and low value of windspeed during the year. On the other hand, in seven stations with arid and semi-arid climate, the new type of Hargreaves equation which contains monthly rainfall data was the best condition for ETo estimates, which was in agreement with the previous results. Furthermore, a new equation was derived for selected stations in the south of Iran, which can be used for monthly ETo estimates by using mean, maximum and minimum air temperature and monthly rainfall data.