Analysis of Effect of the Climate Parameters Change on Runoff and Evapotranspiration of Mehrgerd Watershed

In the past century, the climate has been changing on both regional and global scales over the earth. It is also expected that such changes will continue in the near future. Climate change is due to increased greenhouse gas emissions in the atmosphere. The concentration of these gases is directly related to the temperature increase. Climate change affects the hydrological cycle through changes in time, amount, the shape of precipitation, evaporation rates and transfer, soil moisture, runoff, etc. Today, the use of hydrological models have been developed to have the factors affecting the hydrological cycle in the watershed. The Soil and Water Assessment Tool (SWAT) is an example of these models. The common method of assessing the effects of climate change on flow is using hydrological models along with general circulation models (GCMS) or regional weather models (RCMS). The purpose of this study is to investigate the effect of climate change on runoff and evapotranspiration (real and potential) of Mehrgerd Watershed using the SWAT hydrologic model and the CanESM2 climatic model.

For modeling the change rate of regional climate parameters in the future period (2017-2030) and the effect of these changes on hydrological parameters, the daily data of minimum and maximum temperature of the Borujen station and precipitation of the Tange Zardaloo station for the base period (1984-2005) were used as inputs of the CanESM2 model. Accordingly, using the model of SDSM5.2 under the scenario of RCP8.5 was performed the downscaling operation. To evaluate the efficiency of the SDSM model were used statistical criteria R2, RMSE, and NS. In the next step, the SWAT 2012 model was used to simulate the hydrologic conditions. After introducing the DEM map with a precision of 20 meters, the region was divided into 18 sub-basins. From the combination of land use maps, soil, and slope, 54 units of hydrological response (HRU) were obtained. Then, climatic data including precipitation, minimum and maximum temperature, relative humidity, wind speed, and solar radiation were introduced to the model. Due to the presence of the dam and the two water transfer lines in the area, physical data and discharge were calculated and introduced into the model. The calibration and validation of the model were done by Sufi-2 algorithm. The calibration process was conducted for the period 2004 to 2012 while the validation process was from 2013 to 2016. In order to evaluate the performance of the model, coefficients NS, R2, P-Factor and R-Factor were used. For this purpose, the model was restarted to obtain the appropriate range for each parameter. After calibrating the hydrological model was introduced the simulated climate to the SWAT model. Finally, the effect of climate change was investigated on runoff and evapotranspiration (real and potential) of Mehrgerd Watershed.

The results of the downscaling of the climatic model in this region indicate a decrease of 53.48% of precipitation and increase minimum and maximum temperatures for a future period (2017-2030), 0.84 and 3.99%, respectively. Based on the results of the sensitivity analysis of the SWAT model, 10 parameters were identified as the most sensitive parameters. In the hydrological section, the statistical criteria of R2, NS, P-Factor and R-Factor were obtained for the calibration period 0.73, 0.69, 0.52 and 0.24, respectively and for the validation period, 0.71, 0.58, 0.45 and 0.29, respectively. Comparing runoff simulation in the future period under the influence of climate change and comparison of its values with the base period showed a decrease of 23.82% in an annual average of runoff. Climate change will also reduce actual evapotranspiration by 26.03% and increase potential evapotranspiration by 10.20%.

Based on the results of the SDSM model, it was determined that the precipitation is strongly reduced in comparison with the observation period, and the minimum and maximum temperatures increase with a slight difference compared to the observation period. According to statistical criteria, the SDMS model has succeeded in simulating the parameters for the future period. Accordingly, the values of R2, RMSE, and NS for precipitation, were equal to 0.92, 5.81 and 0.39, respectively, and for the minimum and maximum temperatures were obtained 0.99, 0.16, 0.99 and 0.99, 0.21, 0.99, respectively. In the hydrological section, the statistical criteria were acceptable values for the calibration period and the validation. Finally, it was found that under the influence of climate change, runoff decreases. Real evapotranspiration is also declining due to a lack of available water, but potential evapotranspiration is increasing due to the close relationship with temperature.