Investigating and analyzing the effect of climate change on the runoff and sediment using SWAT model (Case study: Ferizi Watershed)

Introduction
Climate change has become a concern among the scientific community, governments, and the public. Climate change will inevitably affect the temporal and spatial distribution of the hydrological cycle and water resources. These changes, in turn, may lead to changes in soil erosion in watersheds, among other environmental problems. Evaluating changes in runoff and sediment performance in the context of climate change and showing the relationships between climate change and hydrological resources and ecological change can provide a scientific basis for the design, use, and management of water resource systems. Temperature changes and precipitation patterns can be predicted by regional climate models (RCM) and global climate models (GCM), which can have a distinct effect on water resources and soil erosion. In some cases, climate change can lead to changes in the amount, intensity, and distribution of rainfall. These changes, such as altering precipitation characteristics, increasing temperature, and reducing water resources have been extensively studied worldwide. Evaluating these impacts on hydrological and ecological systems can provide a scientific basis for the design, use, and management of water resource systems. Therefore, the current research was carried out to investigate the impact of climate change on the runoff and sediment of the Ferizi Watershed located in Razavi Khorasan Province.
 
Materials and Methods
This research has been done in two main steps. In the first step, the climate of the region was studied. First, the meteorological data of the region, including the minimum and maximum temperature and daily rainfall, were obtained from the Meteorological Organization. To investigate the climate change of the region using atmospheric general circulation models and the sixth IPCC report, the minimum and maximum temperature and daily precipitation for the period 2021-2100 were predicted using the SSP2-4.5 scenario (moderate) and the SSP5-8.5 scenario (very pessimistic). The second step is the application of the SWAT model for simulating past and future runoff and sediment patterns. For this purpose, first, the SWAT model was run using the data of the study area, such as the digital elevation model map, soil characteristics map, land use map, and meteorological data, and then using the observed runoff and sediment from the area and SUFI-2 algorithm in the SWAT-CUP software was calibrated. Finally, the SWAT calibrated model was run with forecasted precipitation and temperature data until the year 2100 and the impact of climate change on runoff and sediment was investigated.
 
Results and Discussion
The results of SWAT model calibration showed that the Nash-Sutcliffe criterion for discharge and monthly sediment in the calibration period was 0.66 and 0.65, respectively, while for the validation period, it was 0.57 and 0.56, respectively. The results of runoff and sediment from the simulation of the SWAT model under future climate conditions with two scenarios SSP2-4.5 and SSP5-8.5 showed that the average runoff in these two scenarios were estimated to be 0.25 and 0.28 m³ s^-1, respectively, which shows a 48.9 % reduction in runoff. In the SSP2-4.5 scenario and a 42.8 % reduction of runoff in the SSP5-8.5 scenario compared to the previous period. Also, the total sediment for the SSP2-4.5 and SSP5-8.5 scenarios is predicted to be 3.57 and 4.94 × 10⁶ ton, respectively, which shows a 7.2% increase in sediment in the SSP2-4.5 scenario and a 48.3 % increase in the sediment in the SSP5-8.5 scenario compared to the previous period. Also the average runoff and total sediment in the pessimistic scenario (SSP5-8.5) were predicted higher than the average scenario (SSP2-4.5). The total average precipitation in the SSP2-4.5 and SSP5-8.5 scenarios is 266.1 and 281.4 mm y^-1, respectively, and the average temperature in the SSP2-4.5 and SSP5-8.5 scenarios is 18.4 and 19.3 ^°C, respectively. Therefore, the reason for the increase in runoff and sedimentation in the pessimistic scenario compared to the average scenario is the higher amount of precipitation in the pessimistic scenario.
 
Conclusion
The investigations carried out in this research show that in general, runoff and sediment will decrease between 2021 and 2100 compared to the previous period, and the amount of reduction in the very pessimistic scenario (SSP5-8.5) will be higher than the average scenario (SSP2-4.5). This result, attributed to a relative reduction in rainfall and an increase in temperature, can be linked to extreme rainfall events projected for future years. Considering the increase in flood conditions in the basin and the increase in the amount of sediment load on the horizon of 2100, to deal with and adapt to climate change, it is better to take appropriate solutions in the Ferizi Watershed.