## 3 Results
### Spatiotemporal Change Characteristics in the URB's Hydrological Process Factors  
Upon analyzing the changes in the URB's hydrological process factors during 2001-2019, it was found that precipitation, ET, surface runoff, and lateral flow showed a non-significant increasing trend at the rates of 1.24 mm/yr, 1.66 mm/yr, 0.47 mm/yr, and 0.056 mm/yr, respectively (Figure 3a-d). Groundwater recharge showed a significant increasing trend at a rate of 0.18 mm/yr (\(p<0.1\)), while snowmelt showed a non-significant decreasing trend at a rate of 0.01 mm/yr (Figure 3e). Precipitation, surface runoff, lateral flow, and groundwater recharge all peaked in 2012, while evaporation and snowmelt peaked in 2013.  
Figure 3: Characteristics of interannual variation in the URB’s hydrological process factors from 2001 to 2019: (**a**) precipitation, (**b**) ET, (**c**) surface runoff, (**d**) lateral flow, (**e**) groundwater recharge, and (**f**) snowmelt.  
Figure 2: Comparison of SWAT model simulation results and measured discharged (**a**), MODIS snow cover (**b**), and evapotranspiration (**c**) in the URB.  
Great spatial heterogeneity was observed in the spatial distribution and variation rates of the URB's different hydrological process factors (Figure 4). The annual total precipitation in the upstream region of the basin was 457.48 mm, and it gradually decreased from east to west; the overall precipitation variation rate was high, with the fastest variation reaching up to 2.033 mm/yr in the upstream, followed by that in the downstream (1.601 mm/yr; see Figure 4a). The ET in the southern part of URB reached 272.79 mm, while the ET in the northwestern part was lower at 261.74 mm, showing a decreasing trend from south to north. The rate of change was faster in the URB's upstream and southern parts and slower in its northern and downstream parts (Figure 4b). The runoff deep high value of the URB's runoff was mainly concentrated in the wetland area, decreasing from the wetland to the surrounding area. The rate of change varied similarly, with the downstream wetland area increasing at a rate of 8.23 mm/yr (Figure 4c). The lateral flow in the high-elevation area was higher than that in the low-elevation area; the lateral flow gradually increased from the URB's wetland to the surrounding area, and the change rate showed the same spatial distribution characteristics as the lateral flow did (Figure 4d). Groundwater recharge reached 3.12 mm in the URB's upper and western parts and was lower in the URB's southern and northern parts (1.5 mm); the rate of change was also faster in the higher groundwater recharge areas (Figure 4e). Snowmelt was observed more in the URB's upper and middle reaches, with the highest value being up to 16 mm. Snowmelt in the middle and south of the lower reaches was less than 6.27 mm. This shows a distribution pattern of gradual decrease from the URB's northeast to southwest, with the rate of decrease of snowmelt being higher in the middle (Figure 4f). Overall, the precipitation and snowmelt in the URB's upper and middle reaches were high and the runoff depth was small; for the downstream region, it was just the opposite, and the rate of change in the downstream was higher than that in the upstream.  
Figure 4: Spatial distribution and change rate of the URB’s hydrological process factors, 2001–2019: (**a**) precipitation, (**b**) ET, (**c**) runoff depth, (**d**) lateral flow, (**e**) groundwater recharge, and (**f**) snowmelt.