Assessment of Different Methods to Estimate Reference Evapotranspiration in Qazvin Plain

Determining reference evapotranspiration is considered as a key factor for estimating crop water requirement and managing irrigation plans. Different methods have been presented to estimate this parameter. The accuracy of those methods varies based on meteorological data and climate conditions. So, it is necessary to evaluate different methods and determine appropriate method for estimating reference evapotranspiration in each region. In this study, reference evapotranspiration values were measured using lysimeter data collected from Ismail Abad research station located in Qazvin province, Iran, during a 4-years period. In order to compare different methods, lysimeter values was considered as reference method. So as to find the best evaporation method, Hargreaves-Samani, Blany-Criddle, Turc, FAO-Penman-Monteith, Makkink, Priestly- Taylor, FAO-Penman and Evaporation Pan were used. Evaporation Pan was the most appropriate method to estimate reference evapotranspiration based on the highest correlation coefficient (0.973) and the lowest standard error (3.81). In addition, Priestly- Taylor had the lowest accuracy based on with the lowest correlation coefficient (0.896) and the highest standard error (23.16). FAO-Penman-Monteith and Hargreaves-Samani ranked 2nd and 3rd among other methods because of high correlation coefficients and low standard error.