Investigating the Performance of Regression Equations Methods in Determining Reference Evapotranspiration and Comparison with Hargreaves Method in Alborz Province

Determination of reference evapotranspiration is the basis of all calculations of water requirement and is considered as the main foundation in determining water requirement, therefore, proper estimation of reference evapotranspiration is of great importance. Globally, systems have been developed that provide users with this information in a location-based manner. In this study, using the database of the country's water requirement system, the evapotranspiration rates of the reference plant were evaluated using meteorological data at Karaj station as a combination regression function. In this study, using SPSS software, the values ​​of different curves based on the parameters of relative humidity, temperature and sunshine were calibrated. The results showed that there is a significant relationship between transpiration evaporation and parameters of relative humidity, temperature, and sunshine. Therefore, two combined and multivariate regression models were calibrated using 7300 data of Karaj station and for evaluation using 15432 data in Alborz province. The evaluation results showed that the combined regression model with normal error of 29% and root mean square error index of about 0.5 mm, agreement index of 0.98 and efficiency index of model 0.94 had better performance. Therefore, this method can be used to determine the reference evapotranspiration in the stations of IRAN.