Investigation of Pan evaporation paradox and climatic parameters affecting it in half-west and center of Iran

Evaporation acts as an important component and a key control factor in land hydrological processes. In the context of global warming, pan evaporation demand is expected to increase, but the decrease in measured pan evaporation has challenged the hypothesis that climate change would increase evaporation. The main purposes of this study were (1) to investigate the existence of evaporation paradox, (2) to detect the temporal trends of pan evaporation and related climatic variables on annual time scale. In this study the data of 50 synoptic weather stations in half-west and center of Iran were used. Non-parametric tests Mann-Kendall and Sen's slope estimator were used to discover trend through time series. Results showed that pan evaporation in 10% and 28% of stations had significant (p<0.05) decreasing and increasing trend, respectively. But, trend analysis of mean temperature as source of energy required for evaporation, indicated that in 94% of the stations had increasing trend. Despite the increase in temperature in the all stations, evaporation paradox has occurred in 36% of stations and only in 10% of stations evaporation paradox was significant at 5% level (p<0.05). Data analysis in stations that had evaporation paradox ( with negative pan evaporation trend) indicated that decrease in wind speed and sunshine duration and more increasing in the minimum temperature compared to the maximum temperature, were the main climatic parameters that controlling the rate of pan evaporation. Decreasing in pan evaporation in arid and semiarid areas of Iran is important in climatic studies.