The most Appropriate Method to Estimate Potential Evapotranspiration in Meteorological Data Scarce Condition in the Warm and Cold Months of the Year (Case Study of Isfahan)

Evapotranspiration causes water waste and moisture, and loss of vegetation cover and soil. The calculating of evapotranspiration by a proper approach is very important since the precipitation and water resources are very limited in Iran. There are several common methods to calculate the potential evapotranspiration. The present research was conducted in Isfahan; the accuracy of the six experimental methods for calculating ET° (namely radiation method - FAO 24, Blaney- Criddle FAO, Hargreaves-Samani, Priestley Taylor, Mecking 1957 and Tork) was compared to the reference method- FAO Penman-Monteith. The calculations were done by using Ref- ET software during the 47-year records (1964- 2010) for the warm and cold months of the year separately which have been divided by the average temperature etdel using the SAS software. The comparative results of this study which was performed by statistical indices showed that these six methods are applicable for the cold months of the year in most cases in which using the equations of Blaney- Criddle, Mecking, Radiation, Hargreaves, Tork, Taylor, respectively, and can be suitable an alternative for FAO Penman-Monteith method dueto a larger adjusted determination coefficient (close to one), less estimation error of RMSE (close to zero) and therefore ratio be would larger and for the warm months, due to low and higher error estimation used.