Evaluation and Development of Hargreaves-Samani Evapotranspiration Equation Based on Humidity Conditions

The Hargreaves-Samani method of estimating reference evapotranspiration (ETo) has been recommended by irrigation and drainage paper No.56 of the Food and Agricultural Organization of the United Nations (FAO)  in the situations that weather data are missing. since the method requires just measured temperature, the it has been used in several studies over the past decades. FAO paper No. 56 and other investigations has mentioned that Hargreaves-Samani method is an overestimating method in humid cliamtes. Hence, the method has been calibrated for different site and situations. This issue made the purpose of the current study. In this study, the history of the development of Hargreaves-Samani method has been reviewed. Thus, an equation for adjusting humidity condition is discussed. To compare the goodness of models,  ETo was calculated based on 3 different methods 1) Hargreaves-Samani method 2) Allen modified method of Hargreaves-Samani, and 3) the modified method of Hargreaves-Samani based on relative humidity. Also ETo was calculated according to FAO Penman-Montith method as a control estimation. The values were calculated for monthly weather reports of 4,169 stations from CLIMWAT v2.0 all around the world. The results showed that the Hargreaves-Samani’s method has a weakness of estimation under extreme conditions. Interestingly, the corrected method improved precision of the estimations especially in arid and semi-arid locations.