Investigating changes in the process of evaporation from the free water surface in the context of future climate change using the MLR-E_pan model (Case Study: Shahid Abbaspur Dam Reservoir)

In this study, the future perspective of evaporation from the free water surface of Shahid Abbaspur dam reservoir was investigated using the multiple linear regression analysis of pan evaporation (MLR-Epan), the micro-scale atmospheric circulation model of LARS-WG6, and HadGEM2-ES model output, under RCPs emission scenarios for three time periods of 2021-2040, 2041-2060, and 2061-2080. The results of the Mann-Kendall test from 2021 to 2080 showed that under the RCP2.6 scenario, the trend of annual changes in the minimum temperature, maximum temperature, and evapotranspiration of the Karun catchment basin was insignificant. However, under RCP4.5 and RCP8.5 emission patterns, the annual trend of the mentioned parameters increased significantly (α = 0.01). The accuracy of MLR-Epan in the base period was confirmed by the results of the hypotheses tests, so that 87% of the changes in evaporation from the pan was explained by the three variables of minimum temperature, maximum temperature, and evapotranspiration. The findings showed that in the coming periods and based on all three radiative forcing patterns, the average annual surface evaporation from the Shahid Abbaspur dam reservoir will increase from 7.2 to 20.2 percent compared to the observation period. In the next sixty-year period (2021-2080) under the RCP2.6 scenario, the annual evaporation from the free water surface will be on the raise, but the increasing trend is not significant. Nevertheless, under RCP4.5 and RCP8.5 scenarios, the increasing trend is estimated to be significance (α = 0.01). Based on Sen's Slope estimator, the amount of evaporation from the reservoir surface is predicted to be 1.4, 19.1, and 31.9 mm, respectively, in the next 60-year period and under the investigated scenarios. Consequently, according to the future climatic conditions, it is necessary to reduce evaporation losses in dam reservoirs with proper planning and informed decisions.