# Predicting Runoff from the Weigan River under Climate Change
## 3 Results
### Prediction and Analysis of Future Runoff Evolution Trends  
The \(r\)-MME temperature and precipitation data under four different radiative forcing scenarios were inputted into the well-constructed SWAT model to predict the future evolution of outflow runoff from 2020 to 2050 (Figure 6). From the statistics of the runoff data output by the SWAT model, it can be seen that the runoff out of the mountain in the upper Weigan River Basin under the four radiative forcing scenarios is maintained at 25.68-30.89 \(\times\) 10\({}^{8}\) m\({}^{3}\) in 2030 and 2050, which is an increase of 1.35-21.91% compared with the current runoff. Under the four different radiative forcing scenarios of SSP1-2.6, 2-4.5, 3-7.0 and 5-8.5, the average annual flow in the next 30 years shows an increasing trend, with an increase of 84.58 m\({}^{3}\).s\({}^{-1}\), 85.52 m\({}^{3}\).s\({}^{-1}\), 87.13 m\({}^{3}\).s\({}^{-1}\) and 90.01 m\({}^{3}\).s\({}^{-1}\), respectively. The average annual flow from 2036 to 2050 increases by 6.89%, 9.37%, 7.77% and 18.82%, respectively, compared to 2021 and 2035. Under the SSP2-4.5 radiative forcing scenario, the average annual outflow runoff of the Weigan River increases from 25.52 \(\times\) 10\({}^{8}\) m\({}^{3}\) in 2020 to 30.89 \(\times\) 10\({}^{8}\) m\({}^{3}\) in 2050, which is an increase of 5.37 \(\times\) 10\({}^{8}\) m\({}^{3}\) or 21.03% compared to 2020, with the fastest growth rate being about 1.79 \(\times\) 10\({}^{8}\) m\({}^{3}\)/10a. The SSP1-2.6 scenario has the next fastest growth rate of future runoff out of the mountain of about 1.61 \(\times\) 10\({}^{8}\) m\({}^{3}\)/10a. The SSP3-7.0 radiative forcing scenario has the slowest growth rate of future runoff, at only 1.26 \(\times\) 10\({}^{8}\) m\({}^{3}\)/10a. And although the growth rate of the SSP5-8.5 scenario is 1.59 \(\times\) 10\({}^{8}\) m\({}^{3}\)/10a, the fluctuation of its future runoff is the largest during the next 30 years, with the runoff out of the mountain as high as 34.41 \(\times\) 10\({}^{8}\) m\({}^{3}\) in 2049. It can be seen that the annual runoff of the upper Weigan River increases with an increase in radiative forcing intensity, and the different radiative forcing scenarios show different future runoff evolutions during the period 2020-2050.The future runoff predicted by the SSP3-7.0 radiative forcing scenario is relatively large, and the runoff predicted by the other three radiative forcing scenarios is similar during the same period. For the whole period, as the radiative forcing intensity increases, the upward trend of future runoff is more pronounced. As shown in Figure 6, the SSP5-8.5 radiative forcing scenario shows the most obvious increase in annual runoff in the study area, followed by the SSP3-7.0 radiative forcing scenario, and then the SSP2-4.5 and SSP1-2.6 scenarios.  
From the interdecadal changes in future runoff out of the mountain (Table 3), it can be seen that under different radiative forcing scenarios, the runoff from the Weigan River increases with increasing radiative forcing. Compared to the historical runoff, the average annual flow in all time periods shows an increasing trend. The rate of change of runoff in the first time period (2021-2023) is lower under the SSP2-4.5 and SSP5-8.5 radiative forcing  
Figure 6: Evolution of future runoff out of the mountain under different radiative forcing scenarios.  
scenarios than under the SSP1-2.6 and SSP3-7.0 radiative forcing scenarios. The third time period (2041-2050) shows the most significant increase under all scenarios, and the increase in runoff is more pronounced with increasing radiative forcing. It can be seen that under different radiative forcing scenarios, the future runoff from the Weigan River out of the mountain shows a fluctuating upward trend, and with an increase in time, the impact of radiative forcing on runoff is more pronounced.