Evaluation and Calibration of Thornthwaite equation for Estimating Reference Evapotranspiration in windy areas (case study of Sistan region)

There are many methods for calculating evapotranspiration that require a lot of data, but a few require only air temperature. One of these methods is Trontwait. The Sistan region in the southeast of Iran is one of the regions that is unique in Iran due to the 120 days of winds. The purpose of this research is: 1) to evaluate 6 different Trontwait methods available in the sources compared to the Fau-Penman-Monteith method and 2) to modify the equation for the windy region of Sistan. The results showed that the original Trontwhite equation underestimates the amount of evaporation-transpiration. Among the existing methods, the use of coefficient k=0.72 had the best results. In order to recalibrate the Trunthwaite equation, the effective temperature coefficient of the equation (k) must be modified. The results showed that the optimal value of k varies between 0.755 and 1.04. The annual average value of the root mean square error (RMSE), according to the variable k values, was equal to 0.14 mm per day. Also, by minimizing the square of the error, we considered the k value to be 0.802 as a constant, and the RMSE value was equal to 1.19 mm per day. It can be concluded that after correcting the Trontwhite equation, it can be used in the Sistan region.