Calculation of the Reference Evapotranspiration based on the Statistical Analysis of Air Temperature (case study: Tabriz area)

Irrigation is necessary for the crop growth and determining crops water requirement is essential for supplying it. In arid and semi-arid regions, air temperature plays a fundamental role in evapotranspiration process. In this study, the temperature occurrence probabilities at the levels of 50%, 65% and 80% were applied for calculation of evapotranspiration. The Penman-Monteith FAO and Hargreaves equations were used to calculate the evapotranspiration in Tabriz area. The results showed that the Wakeby distribution function had the best adaptation with the temperature data set in the study area. Using different levels of probability for temperature in determining reference evapotranspiration in the Penman-Monteith FAO method, negligible effects were seen in the obtained results so the averaged temperature data was enough for calculations at the cases that there were no needs for precise values. It was concluded that the Penman-Monteith FAO equation was not sensitive to small changes in temperature values, on the contrary in Hargreaves model, using different levels of temperature probability had significant effects on the calculated reference evapotranspiration values. Finally, results of the both methods were compared with the exact evaporation data obtained from pan and the pan coefficient was obtained for each method and suitable pan coefficient values of 0.63 and 1.22 were determined for methods of Penman-Monteith FAO and Hargreaves models, respectively.