Evaluation and Calibration of Priestley-Taylor equation for Estimating Monthly Reference Evapotranspiration in windy areas (case study of Sistan region)

Document Type : Research Paper


1 Water Engineering Department, Faculty of water and soil, University of Zabol, Zabol, Iran.

2 Water Engineering Department, Faculty of water and soil, University of Zabol, Zabol, Iran



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 methods require a few meteorological data and their 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 of Prestley-Taylor compared to the PMF-56 method in the windy region of Sistan 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 of this equation which is used in the evaluation of the equation. The results showed that the value of the evaporation coefficient in the main equation (equal to 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 transpiration evaporation obtained using the correction factor based on wind speed (α_(PT-U2)) has the best results.


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