The existence of significant trend and change point in the aridity index -AI-, precipitation, and reference evapotranspi- ration -ET0- was evaluated using the Mann-Kendall and Pettitt tests, respectively, over 22 semi-arid, humid, and sub- humid sites of Iran during 1966–2012. Detrending method was also employed to quantify each factor’s contribution to the AI and ET0 trends. The ET0 and AI sensitivity to their input perturbations was analyzed using a derivative sensitivity coefficient. The results indicated a downward trend in precipitation and AI and an upward trend in ET0 on both seasonal and annual scales for majority of sites. Compared with AI and precipitation, more significant change points were detected, mainly during the 1990s, in the ET0 series. Except for wintertime, solar radiation -SR- was found as the most sensitive factor on the AI and ET0 dynamics in the semi-arid regions. However, with an exception for summertime, AI and ET0 exhibited greater sensitivity to the RH changes followed by the SR changes in the humid/sub-humid areas. For more than 70% of semi-arid stations and Gorgan -a sub-humid location-, wind speed -U- was the most important variable contributing to seasonal and annual ET0 trends. However, the ET0 trend was primarily caused by the mean temperature -Tmean- changes for the humid environments. Further, the precipitation changes made the largest contri- bution to seasonal and annual AI trends for most cases followed by the U changes. An integrated water resource management is required to be implemented to reduce negative impacts of decreased AI and ET0 increment over Iran.

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