Assessing reference evapotranspiration changes during the 21st century in some semi-arid regions of Iran

Changes of ET0 and its atmospheric control were investigated in six stations located in West Iran during the period of 1966-2100. The outputs related to HadCM3 under B2 emission scenario were downscaled through SDSM. The results revealed that ET0, as averaged across all stations, would increase by 5.12, 7.33 and 11.01% respectively over the early, middle and late 21st century relative to the baseline period (1966-2010). The results obtained through Mann-Kendall test revealed that there was an insignificant positive trend in ET0 at the level of 95% over 1966-2010 in most of the surveyed sites. This increasing trend could be explained by an upward trend in temperature and solar radiation vs. a negative trend of relative humidity within the study area. The increasing trend in ET0 will likely be significant during 2011-2040 and 2071-2100 while insignificant during 2041-2070. The occurrence of ET0 upward trend in the future is most likely due to temperature rise.