This study was undertaken to investigate the refer- ence evapotranspiration (ET0) changes in semi-arid and hu- mid regions of Iran during the past (1966–2010) and future (2011–2099). For detecting possible trend in ET0 over 1966– 2010, the Mann–Kendall trend test was employed. The out- puts of Hadley Centre coupled model version 3 (HadCM3) and the third generation couple global climate model (CGCM3) under A2, B2, and A1B emission scenarios were also used to simulate the future ET0 changes by statistical downscaling model (SDSM). The results indicated upward trends in annual ET0 during 1966–2010 in the most sites. Furthermore, the significant increasing ET0 trends were iden- tified for 54.5, 18.2, 27.3, 22.7, and 36.3% of studied locations during winter, spring, summer, autumn, and entire year, re- spectively. Positive trends in ET0 were mostly found in north- east, west, and northwest Iran, and insignificant downward ET0 trends were primarily detected in southwestern and south- ern stations in 1966–2010. The ET0 changes were attributed to wind speed changes in semi-arid regions and mean tempera- ture changes in humid areas in the past period. An increase in ET0 was projected under all scenarios due mainly to temper- ature rise and declined relative humidity in the investigated regions from 2011 to 2100. Averaged over all stations, the lowest and highest ET0 increment were, respectively, modeled for autumn and summer using CGCM3 outputs and winter and autumn using HadCM3 outputs. Given significant ET0 increase over the twenty-first century, appropriate adaptive measures are required to reduce negative impacts of climate change on water resources and agricultural productions.

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