University of Tehran, College of Aburaihan Water and Irrigation Management 2251-6298 10 3 2020 12 21 Evaluation of the accuracy of different combined methods for estimating the potential evapotranspiration in different climates (case study: Iran) Evaluation of the accuracy of different combined methods for estimating the potential evapotranspiration in different climates (case study: Iran) 443 452 79466 10.22059/jwim.2021.308145.813 FA Saeid Nasseri Ph.D. Candidate, Department of Water Engineering, Campus of Agriculture and Natural Resources, Razi University, Kermanshah, Iran Houshang Ghamarnia Professor, Department of Water Engineering, Campus of Agriculture and Natural Resources, Razi University, Kermanshah, Iran Rezvan Khosravi Ph.D. Candidate, Department of Water Engineering, Campus of Agriculture and Natural Resources, Razi University, Kermanshah, Iran Yazdan Mohamadi Ph.D. Candidate, Department of Water Engineering, Campus of Agriculture and Natural Resources, Razi University, Kermanshah, Iran Farhang Sargordi Assistant Professor, Department of Water Engineering, Campus of Agriculture and Natural Resources, Razi University, Kermanshah, Iran Journal Article 2020 08 26 Estimating the exact value of reference evapotranspiration is very important in irrigation planning and determining the irrigation frequency. There are different methods for estimating the reference crop evapotranspiration. The aim of this study was to find the best combined method for estimating the reference evapotranspiration in arid, semi-arid, semi-humid and humid climates of Iran. In this study, based on different necessary data from first order weather stations in the period of 1995 to 2014, the reference evapotranspiration was calculated by some combined methods and compared with FAO 56 Penman Monteith method. The results showed that all methods except Businger and Van bavel method have a good correlation with FAO 56 Penman Monteith (R^2 > 0.96). The accuracy of Businger and Van bavel methods in all regions were low with RMSE and MAE values greater than 0.9 and 0.55 mm per day, respectively. In arid, humid and semi-humid regions of Iran, Penman 1948 and Penman Wright 1972 methods with RMSE and MAE values less than 0.39 mm per day and in semi- arid regions of Iran Penman Wright 1972 methods and Penman 1948 with RMSE and MAE values less than 0.35 mm per day had the highest accuracy. Estimating the exact value of reference evapotranspiration is very important in irrigation planning and determining the irrigation frequency. There are different methods for estimating the reference crop evapotranspiration. The aim of this study was to find the best combined method for estimating the reference evapotranspiration in arid, semi-arid, semi-humid and humid climates of Iran. In this study, based on different necessary data from first order weather stations in the period of 1995 to 2014, the reference evapotranspiration was calculated by some combined methods and compared with FAO 56 Penman Monteith method. The results showed that all methods except Businger and Van bavel method have a good correlation with FAO 56 Penman Monteith (R^2 > 0.96). The accuracy of Businger and Van bavel methods in all regions were low with RMSE and MAE values greater than 0.9 and 0.55 mm per day, respectively. In arid, humid and semi-humid regions of Iran, Penman 1948 and Penman Wright 1972 methods with RMSE and MAE values less than 0.39 mm per day and in semi- arid regions of Iran Penman Wright 1972 methods and Penman 1948 with RMSE and MAE values less than 0.35 mm per day had the highest accuracy. https://jwim.ut.ac.ir/article_79466_70155249e336143a1e05c717baa2c669.pdf