Comparison of different empirical methods for estimating ddaily reference evapotranspiration in the humid cold climate (case study: Borujen, Shahrekord, Koohrang and Lordegan)

The proposed method for calculation of potential evapotranspiration is Penman-Monteith FAO method, but there are other methods that require less meteorological data but estimates close to the FAO Penman-Monteith method in different climatic conditions.  Performance evaluation of these methods on the same basis is prerequisite for selecting an alternative approach in accordance with available data. In this study, daily values of potential evapotranspiration in the period of 21 years (1994-2015) estimated using empirical methods ;Copais, Turc, Hargreaves-Samani, Hargreaves, Ritchie, Irmak  and three forms of  Valiantzas equation  for Borujen,  Shahrekord, Koohrang and Lordegan, located in humid cold climate region of Chaharmahal and Bakhtiary province and values were compared with Penman-Monteith FAO method. To compare the results, the root mean square error, error absolute value and coefficient of determination was used. The results show that the equations of Irmak and Valiantzas (RS, T,RH,U ) performs better than the other empirical methods in four stations and the Hargreaves method showed very poor estimation of evapotranspiration than FAO Penman-Monteith method with 92.25 percent relative error. Also, comparison of total potential  evapotranspiration estimated with empirical methods determined that equation of  Irmak and  Valiantzas (Rs, T, RH, U) for all the stations were the most efficient way.