Evaluation of Different Evapotranspiration Methods, Optimizing the Best Model Based on Multivariate Linear Regression in Cold Moderate and Semi-Humid Climates

ET0 plays an essential role in water resources management. In this study, nine temperature-based, ten radiation-based and six mass transfer-based methods at three-time scales (monthly, seasonal, and annual) were evaluated versus the FAO56-PM model, and the best and worst methods were selected from each group for the cold moderate and semi-humid climate of Iran. At annual and seasonal time scales (spring, summer and autumn seasons) Hargreaves-M4 and Trajkovic methods (temperature-based methods), Priestley-Taylor and IRMAK1 equations (radiation-based methods) in winter season and for monthly time scale: Meyer and Rohwer, Hargreaves-M4, Trajkovic, IRMAK1, and Priestley-Taylor methods were shown to have the best performance in comparison to FAO56 PM. These equations are the best ET0 estimators in cold moderate, and semi-humid climate. In this study, two equations with the best performance, namely Hargreaves-M4 and Trajkovic were optimized as linear and power functions. The results showed that the power equation had better performance than the linear equation. Finally, two equations were presented based on the linear multivariate regression to simplify ET0 estimation. The results revealed that those kinds of equations, which used Tmax, Tmin as input parameters have high accuracy in comparison with Hargreaves-M4 and Trajkovic equations.