Modification of the Coefficients of some Equations for Estimation of Evapotranspiration of the Reference Plant

Iran is located in the arid and semi-arid belt of the world, which is characterized by low rainfall, thunderstorms, flood flows, and high evapotranspiration. Therefore, quantitative evaluation of evapotranspiration on a regional scale is necessary for water resources management, crop production, and environmental assessments in irrigated lands. In this study, to estimate ETo in four synoptic stations with arid, semi-arid, humid, and semi-humid climates, meteorological data such as temperature, net radiation, relative humidity, and wind speed were used for 1990-2010. Potational evapotranspiration was calculated using 5 different methods, including Hargreaves-Samani, Blaney-Criddle, Priestly-Tailor, Turc, and Makkink, besides, the standard FAO-56 was used (because there was no Lysimetric regionally data) to evaluate the applied formulas. In order to evaluate these methods, were used the mean absolute error (MAE), root mean squared error (RMSE), mean bias error (MBE), correlation coefficient (r), Nash -Satclif index (N.S). Based on these indices, Blaney-Criddle method, after the FAO-56 method, which was used as the basis, was selected as the superior method for the four climates in Iran.