Investigation of Valiantzas' evapotranspiration equation in Iran

Several methods are available to estimate the reference evapotranspiration including mass transfer-based, radiation-based, temperature-based, and pan evaporation-based methods. The most important weather parameters are solar radiation, temperature, relative humidity, and wind speed for evapotranspiration models. This study aims to compare five forms of Valiantzas' evapotranspiration methods (one of the newest models) as well as Priestley-Taylor and Turc models to detect the best one 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 FAO Penman-Monteith method. The results show that they are suitable for provinces of Iran (coefficient of determination (R-2) was more than 0.9900). The Valiantzas 1 (T, R-s, RH, u) is more suitable for centre and south of Iran (9 provinces), and the Valiantzas 2 (T, Rs, RH, u) is suitable for west, east, and north of Iran (22 provinces). The most precise method was the Valiantzas 1 (T, Rs, RH, u) for ES. In addition, among limited data methods, the Valiantzas 2 (T, Rs, RH) is the best method (18 provinces). Finally, a list of the best performances of each method was presented to use other regions according to values of temperature, relative humidity, solar radiation, and wind speed. The best weather conditions for use in Valiantzas' methods are >24.2 MJ m(-2) day(-1), 16-18 degrees C, 40-50 %, and 1.50-2.50 m s(-1) for solar radiation, temperature, relative humidity, and wind speed, respectively. Results are also useful for selecting the best model when researchers must apply these models on the basis of the available data.