The aim of this research is the study of the climate change impacts on the seasonal and annual reference evapotranspiration time scales in some selected stations located in the West of Iran. To this purpose, four stations including Sanandaj, Saghez, Khorramabad and Kermanshah synoptic stations with enough long-term data were selected and the climate change impact on the reference evapotranspiration of these stations under two RCP2.6 and RCP8.5 scenarios in three future time periods including 2011-2040, 2041-2070 and 2071-2100 in comparison with the 1970-1999 base period was studied. The FAO-Penman-Montieth method was applied to calculating reference evapotranspiration and the CanESM2 general circulation model and SDSM downscaling method were used to simulating future climate conditions under the climatic scenarios. The results showed that the mean reference evapotranspiration in the annual and autumn and winter time scales in comparison to the base period will significantly increase for all of the studied stations under all of the scenarios and periods at the 0.01 confidence level. For spring season, the only significant change of the future period mean reference evapotranspiration compared to the base period in the all of the studied area will be a significant increase at the 0.01 confidence level in the 2071-2100 period under the RCP8.5 scenario and for the summer season, this significant increasing rate will occur in the 2041-2070 and 2071-2100 periods under the RCP8.5 scenario. The overall results of this research showed that the highest increasing rate of the future periods in comparison with the base period for all of the seasonal and annual time periods and for all of the studied area will under RCP8.5 scenario and in the 2071-2100 time periods. by comparing the reference evapotranspiration change rates between the different seasonal and annual scales, the results showed that the increasing rate of the mean reference evapotranspiration at the West of Iran will be very remarkably in the autumn and winter seasons compared to the other time scales.

Reference evapotranspiration (ET₀) is a climatic parameter and can be computed from weather data. It is one of the most important hydrological parameters for calculating crop water demand, scheduling irrigation systems, preparing input data to hydrological water-balance models, regional water resources assessment, and planning and management for a region and/or a basin. The climatic data from synoptic stations with more than 20 years continues record in West Azarbaijan province was used. The well-known FAO-PM56 method was used to calculate the ET₀. Then Multiple linear Regression (MLR) was used to estimate the ET₀. The RMSE, MEA, NSH, and R² were used to evaluate the performance of the MLR model. Then, the correlation coefficient (r) between ET₀ and each of the meteorological parameters was obtained. And finally, with using Path analysis method, the influence of direct (P) and indirect effects of the meteorological parameters on ET₀ was calculated. In the studied synoptic stations, NSH between 0.91 and 0.99,   R² between 0.91 and 0.99, RMSE between 0.05 and 0.15, and MEA between 0.04 and 0.12 were obtained. Also, it was found that the wind speed at all of stations had a significant correlation (at the 0.01% level) with ET₀. The path analysis results showed that the maximum value of P (direct effect of meteorological parameters on ET₀) in all of the stations belongs to wind speed. The P value of wind speed in Urmia equal to 0.85, Piranshahr equal to 0.99, Takab equal to 0.97, Khoy equal to 0.90, Sardasht equal to 1.06, and Mahabad equal to 0.78 are obtained.