## 3 Results
### Synergy Relationships between the URB's Hydrological Process Factors and EQ  
First, we analyzed the synergy between the hydrological process factors and EQ, using the gray correlation method. It was found that a high synergy existed between the URB's hydrological process factors and EQ during the study period, with correlations ranging between 0.7 and 0.95, along with large differences between different river sections (Figure 8a). The correlations of ET, precipitation, and lateral flow with EQ in the last 19 years were 0.95, 0.91, and 0.90, respectively. Taking 2012, the year with the largest EQ fluctuations, as a reference point, the synergy between EQ and the hydrological process factors was found to be high until 2012, with a mean value of 0.89, but decreased later, with a mean value of 0.83. The synergy between EQ and precipitation was found to be the highest in 2007 and the lowest in 2017 for ET. Moreover, surface runoff, lateral flow, and groundwater recharge mainly occurred during the early part of the study period; snowmelt was mainly concentrated toward the latter part (Figure 8b). The synergy between the hydrological process factors and EQ at the subbasin scale is in Figure 8c, with synergy coefficients higher than 0.8. The areas with lower synergy were mainly located west and south of the downstream regions. The synergy between the hydrological process factors and EQ decreased from the upstream to the downstream regions, according to varied river sections (Figure 8d). Second, we analyzed the synergistic effects of the hydrological process factors and EQ through Pearson correlation analysis. Between 2001 and 2019, except for individual subbasins where ET was negatively correlated with EQ (e.g., Subbasins 11 and 13), all ET-EQ correlations were positive. Moreover, a large spatial heterogeneity was observed in the synergy (Figure 9). The synergistic relationships between precipitation, evapotranspiration, surface runoff, lateral flow, and groundwater recharge and EQ are stronger in the northwestern part of the downstream of the basin and in the southern part of the upstream--both around 0.5. However, the synergistic effect of the two is weaker in the southern part of the downstream. The synergistic coefficient between lateral flow and EQ in the lower reaches of the basin reached around 0.7. The synergy between snowmelt and EQ in the northwest region of the downstream basin was 0.37, while that in the upstream was only 0.1. In general, the synergy between the hydrological process factors and EQ was found to be stronger in the downstream basin and weaker in the upstream basin during 2001-2019. Figure 7: Interannual trends (**a**) and spatial distribution (**b**) of ecological quality in the URB from 2001 to 2019. Figure 8: Synergy of hydrological process factors and EQ in URB (**a**), the most synergistic hydrological process factors (**b**), spatial distribution of average synergy between hydrological process factors and EQ in subbasins (**c**), and synergy between hydrological process factors and EQ system in different river sections (**d**). Figure 9: Synergy relationships between the URB’s hydrological process factors and EQ: (**a**) precipitation, (**b**) ET, (**c**) runoff depth, (**d**) lateral flow, (**e**) groundwater recharge, and (**f**) snowmelt. To gain deeper insights into the key hydrological process factors that account for EQ improvements in the URB, we further compared the synergistic coefficients of the hydrological process factors with EQ in each subbasin. As seen in Figure 10, EQ had the highest synergy with groundwater, lateral flow, ET, and snowmelt in the basin. The synergy between EQ and lateral flow is high in most parts of the basin, with the highest value of 0.626. It passed the significance test, accounting for 53.37% of the total basin area, mainly concentrated downstream. This was followed by groundwater, which accounts for 22.5% of the total basin area, and is mainly concentrated in the Southeastern URB. ET was most synergistic in the central part of the downstream area.