Evapotranspiration after precipitation is a major component of the hydrological cycle that has widely been studied both theoretically and experimentally including hydrological balance, design and management of irrigation systems, agricultural productivity, irrigation, drainage planning, and estimation of plant water requirements and environment. The rate of evapotranspiration is directly related to air temperature, a key parameter in definition and classification of every region climate. Air temperature also is the main indicator of the global warming and consequent climate change. Therefore, evapotranspiration is expected to be related to climate change, too. Accordingly, a study was designed to investigate the impact of climate change on evapotranspiration in a western mountainous region of Iran. The study was carried out using the outputs of HadGEM2 and CanESM2 global models under RCP2.6, RCP4.5, and RCP8.5 scenarios for the future period of 2021–2050 considering the period of 1989–2018 representative of the region climate. Then two LARS-WG and SDSM statistical downscaling models were used to reduce the scale of outpots to the region of study. The evapotranspiration was calculated using two Hargreaves-Samani and Priestley- Taylor methods. The results showed that on average both minimum and mximum temperatures will increase by 0.6 to 1.6, and 0.8 to 1.9 °C, respectively. The amount of radiation will also increase on average by 0.2 MJ m−2 day−1. Consequently, it is expected that the evapotranspiration rate to be increased between 1 and 7.3%, too. The changes in colder area of the northern part of the studied region is estimated to be higher than that of the southern area. Comparison has also been made between the LARS-WG and SDSM models and Hargreaves-Samani and Priestley- Taylor methods on both estimation of changes in and the amount of evapotranspirtion. LARS-WG and Hargraves- Samani have made higher estimation than the SDSM and Priestley-Taylor model and method.
