Importance of solar radiation, temperature, relative humidity, and wind speed for calculation of reference evapotranspiration

This study aims to compare two form of Valiantzas' evapotranspiration methods (one of the newest models) to detect the best method under different weather conditions. For this purpose, weather data were gathered from 181 synoptic stations in 31 provinces of Iran. The reference evapotranspiration was compared with the Food and Agriculture Organization Penman–Monteith method. The results showed that they are not suitable for provinces of Iran. Therefore, the methods were calibrated and preciseness of estimation was increased (the values of R2 were less than 0.99 for only five provinces in the modified methods). The new formulas estimated the reference evapotranspiration in the southeast of Iran and near the Persian Gulf (solar radiation >24.4 MJ m−2 day−1 and wind speed 2.50–3.50 m s−1) better than other provinces. The more precise methods were for SK (east of Iran) and for GO (north of Iran). Finally, a list of the best performances of each method was presented (using sensitive analysis) to use other regions and next studies according to temperature, relative humidity, solar radiation, and wind speed. The results are also useful for selecting the best model when we must apply the new formulas based on available data.