Application of different cases of Hargreaves-Samani and Jensen-Haise methods for estimating Alfalfa Reference crop evapotranspiration in Isfahan

Estimating rate of actual evapotranspiration is very important for designing of the irrigation systems, storage construction and water conveyance structures. Many methods have been developed for estimating reference crop evapotranspiration. Amount of these methods are need different climatic parameters but providing all of the data those are necessary for ET0 is not always available and if have been provided, these data may be is not completely correct. Therefore some methods have developed to estimating ET0 with minimum data such as Hargreaves-Samani and Jensen-Haise methods. In this research, eight cases of these methods have been compared. These comparisons were made on the basis alfalfa lysimeter data from Kabotarabud, Isfahan. The results show that through of these cases, which were used Samani-Pessarakly equation for calculating Extraterrestrial Radiation, were estimating ET0 better than the others with 15 Percent Average Maximum Absolute Relative Error (MARE). In addition, these above mention cases were had higher correlation coefficient as compare to other cases, so with improved the main equations with linear regression, MARE were decreased until 9 Percent. Finally, as compared the two methods of calculating the HargreavesœSamani coefficient, Kt, the results showed that there was no considerable different between Allen's equation and Samani-Pessarakly's equation.