Extended Abstract



Introduction



Climate change refers to irreversible changes in the average weather conditions in a region. Climate change as a change in the weather pattern is mainly caused by the emission of greenhouse gases from natural systems and human activities (Fawzy et al., 2020), which in the last few decades has caused an increase in the overall temperature of the earth, followed by the phenomenon that climate change has occured. According to the latest report of the Intergovernmental Panel on Climate Change, since 1971, the temperature of the earth's surface has been increasing at a faster rate than other fifty-year periods, at least during the last 2100 years (IPCC, 2022). In this research, it is tried to investigate the effects of climate change on the hydrological regime of the watershed and the condition of its floods in the future.

CanESM2 climate model has been used under RCP2.6, RCP4.5 and RCP8.5 emission scenarios with SDSM statistical microscale model in the future time period of 2021-2050, and then geographic information system (GIS) and one of the decision methods. Multi-criteria decision making (MCDM) will be used for risk analysis and their ranking for this simulation. The resulting flood potential maps can be a suitable tool for strategic planning in the future. Therefore, the objectives of this research are as follows: 1) Simulating the runoff of the Nakarod watershed using the SWAT physical model, 2) Zoning the flood risk of the Nakarod watershed using the ANP model.



Methodology

The watershed of the Nekarud River is one of the watersheds of the Caspian Sea and is located in the geographical range of 53 degrees 17 minutes to 54 degrees 44 minutes east longitude and 36 degrees 28 minutes to 36 degrees 42 minutes north latitude. The basic data used in this research for the calibration and implementation of the micro-scale statistical model includes the minimum, maximum, precipitation and sunshine hours of the synoptic station of Baikla Nekarud in the period from 2001 to 2020. In this research, the three release scenarios of RCP2.6, RCP4.5 and RCP8.5 of the CanESM2 model in the statistical period of 2021 to 2050 were microscaled with the help of the observational data of the synoptic station of Baikla-Nakarod.

In this research, three maps of distance from the river, basin shape coefficient and average maximum runoff in the upcoming period of 2021-2050 have been used for the flood risk zoning map due to climate change. To predict future temperature and precipitation variables, CanESM2 model under RCP8.5 scenario has been scaled with SDSM model, and SWAT model has been used to predict daily runoff in the future period. To complete the maps, the outputs of the SWAT model, including the construction of waterway lines from the map, which include DEM, the division of sub-basins, and the runoff of each sub-basin, which include other information on soil maps, land use, and precipitation, have been used. Finally, the layers are combined with the addition of ANP and the final flood risk map due to climate change is obtained.



Results and Discussion

The results show that both the maximum temperature and the minimum temperature of this station have increased in the future period. The range of temperature changes of the maximum station is between 0.08 and 0.28 degrees Celsius and the range of temperature changes of the minimum station is between 0.12 and 0.21 degrees Celsius. In the maximum temperature changes of this period, the highest temperature increase is related to this station under the RCP8.5 emission scenario with an increase of 0.28 degrees Celsius and the lowest temperature increase is related to the RCP2.6 emission scenario with an increase of 0.08 degrees Celsius. . Also, in the minimum temperature changes of this period, the highest temperature increase is related to the RCP8.5 release scenario with an increase of 0.21°C and the lowest temperature increase is related to the RCP2.6 release scenario with an increase of 0.12°C.

The amount of runoff decreases in the hot months of the year, early autumn and late spring. Of course, the decrease in the hot season follows a uniform trend. There are exceptions in different scenarios. An increase is seen in late winter and early spring. The reason for this increase is directly related to the increase in temperature and the increase in rainfall in the coming period. The increase in temperature will increase the rate of snow melting in the region, and over time, the water sources in the high altitudes and cold areas will melt, which will lead to floods in the region. Most of the runoff occurred in late winter and early spring (months of January, February, March and April). So that the peak in RCP8.5 occurred in March, which can be justified due to the increase in temperature and the simultaneous snow melting in the basin of this peak. Also, the reason for the decrease in the scenario of RCP4.5 compared to both scenarios is more extreme rainfall occurred in this scenario in the summer and autumn months, but in RCP8.5, despite the decrease in runoff compared to RCP2.6, the runoff increased due to the effect of serious rains occurred in the months of March, April and May.



Conclusion

With the identification of the most critical climatic scenario in extreme events and maximum discharges, flood risk zoning of the region was carried out. For this purpose, from three average runoff maps of up to 30 years, basin shape factor and distance from the river were combined using ANP approach and classified into five classes. According to the zoning, 41.61 percent of the area of the basin is in very low and low risk (746.18 and 213.73 square kilometers), 20.78 percent in medium risk (479.46 square kilometers) and 35.71 percent in high and very high risk (368.65 and 455.55 square kilometers) in the range It is around the main river, which has many villages nearby



Keywords: Climate change, flood risk, modeling, GIS, SWAT, Necarod.