Investigating and analyzing the effects of climate change on the amount of runoff and sediment using the 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 hydrological cycle and the temporal and spatial distribution of 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).Temperature changes as well as precipitation patterns can have a distinct effect on water resources and soil erosion (Field and Barros, 2014; Mullan at al., 2012). In some cases, climate change can lead to changes in the amount, intensity and distribution of rainfall, in addition to affecting water resources, it can increase the amount of erosion (Ghazavi at al., 2018). In the field of the effects of climate change, such as changing the characteristics and amount of precipitation, increasing temperature, and reducing water resources, various studies have been conducted worldwide (Fallah-Ghalhari et al., 2019; Sharafati et al., 2020). Evaluation of the mentioned changes on hydrological resources and ecological change can provide a scientific basis for the design, use and management of water resource systems. Therefore, the current research was carried out with the aim of investigating the impact of climate changes 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 using the sixth IPCC report, the minimum and maximum temperature and daily precipitation for the period 2021-2100 using the SSP2-4.5 scenario (moderate scenario) and the SSP5-8.5 scenario (Very pessimistic scenario) was predicted. The second step is the run of the SWAT model to simulate runoff and sediment in the past and future periods. For this purpose, in the 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 is 0.66 and 0.65, respectively. The values of Nash Sutcliffe criteria for the validation period for discharge and sediment were 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 cubic meters per second, respectively, which shows a 48.9% reduction in runoff. in SSP2-4.5 scenario and 42.8% reduction of runoff in 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 million tons, 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) was 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 per year, respectively, and the average temperature in the SSP2-4.5 and SSP5-8.5 scenarios is 18.4 and 19.3 degrees Celsius, respectively. has been 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, due to the relative decrease in the amount of rainfall and definite increase in temperature, can be related to the occurrence of extreme rainfall in the future years. Considering the increase in flood conditions in the basin and the increase in the amount of sediment load in the horizon of 2100, in order to deal with and adapt to climate change, it is better to take appropriate solutions in the Ferizi basin.