Evaluation and Calibration of Priestley-Taylor Equation for Estimating Monthly Reference Evapotranspiration in Windy Areas of Sistan

The FAO Penman-Monteith method as a standard method requires a lot of meteorological data. The accurate preparation of these data is not possible in all regions; as a result, alternative methods that require less data are investigated. Prestley-Taylor method require a few meteorological data and its application can be useful in areas where meteorological data is not available. The Sistan region in the southeast of Iran is one of the regions that is unique in Iran due to the 120-day winds and high day-night temperature changes. The purpose of this research is to evaluate Prestley-Taylor method compared to the PMF-56 method and to modify this equation according to the wind conditions for the region of Sistan. For this purpose, 30 years of meteorological data in Sistan region were used. The coefficient of Priestley-Taylor equation (α_PT) is one of the most important parameters which is used for evaluation of the equation. The results showed that the value of the evaporation coefficient in the main equation (1.26) for the Sistan region is too low and should be corrected. Its correction value varied between 1.02 and 6.11. The average value of α_PT was equal to 2.16, which is 71% different from the default value (1.26). Also, a regression relationship between wind speed and α_PT was presented. The results show that the amount of evapotranspiration obtained using the correction factor based on the wind speed (α_(PT-U2)) has the best results.