## 3 Results
### Spatiotemporal Change Characteristics of the URB's EQ  
The URB's EPI showed a significant increasing trend, at a rate of 0.43/yr from 2001 to 2019 (\(p<0.01\)). EPI's highest and lowest values were 46.60 and 31.33, which appeared in 2012 and 2007, respectively (Figure 5a). The multi-year mean value was 41.01, indicating that the URB's overall EPI was low. The higher EPI areas were mainly distributed across the arable land dominant upstream and the downstream southern woodland coverage areas. Most areas in the middle and downstream areas had a lower EPI. The area of Level I accounted for 74.7% of the total area; Level II accounted for 11.51%; and Levels III, IV, and V accounted for 7.42%, 5.31%, and 1.08%, respectively (Figure 5b). The URB's ESI showed a non-significant increasing trend at a rate of 0.12/yr during the study period. The ESI was the highest in 2007, with a value of 73.43, and lowest in 2011, with a value of 63.79 (Figure 5a). The multi-year average ESI value reached 67.85,  
Figure 5: Interannual variation of the URB’s EPI, ESI, and EBCI from 2001 to 2019 (**a**); the spatial distribution of EPI level (**b**); the spatial distribution of ESI level (**c**); and the spatial distribution of EBCI level (**d**). which is at a high level overall. As seen in Figure 5c, the URB's ESI from 2001 to 2019 had only two levels, Levels IV and V, with area ratios of 14.95% and 85.05%, respectively. Level V was mainly distributed in the downstream area, where grassland cover chiefly prevailed, whereas Level IV was mainly distributed in the upstream area, where arable land chiefly prevailed. The URB's EBCI showed a non-significant increasing trend at a rate of 0.74/yr. Over the last 19 years, the highest value of EBCI was 88.09, which occurred in 2012, and the lowest value was 24.24 in 2007 (Figure 5a). The multi-year average value was 57.45, which is at a higher level. The higher EBCI areas were mainly spread over the upstream and downstream southern areas. On the other hand, the EBCI value in the downstream northwestern area was lower, which is similar to the spatial distribution characteristics of the EPI (Figure 5d). As shown in Figure 6a, the EQ of the URB from 2001 to 2019 showed an overall distribution pattern of high in the northeast and southwest and low in the northwest. The best EQ levels were concentrated in the upstream and downstream southern areas of the URB, whereas the worst EQ areas lay in and around the wetlands of the downstream area. The highest area percentage at the medium level (III) was 51.97%, mainly in the central part of the URB downstream dominated by grass cover, which was followed by the higher level (IV) area percentage of 32.54, mainly in the upstream and midstream areas of the basin and the southern downstream parts. Statistics about area percentages at different EQ levels, across different reaches of the basin, revealed that levels III and IV have been dominant over the last 19 years. However, the difference in EQ among the reaches was relatively significant (Figure 6b). The areas of levels III and IV decreased gradually from the upstream to the downstream of the URB, whereas the areas of levels I and II increased subsequently. Moreover, the level IV area in the URB upstream had the highest proportion of 89.96%, whereas the level III area in the downstream region occupied the highest proportion of 54.80%. Thus, the upstream and midstream areas are at a high level, whereas the downstream area is at an intermediate level. The URB's EQ showed a significant increasing trend at a rate of 0.39/yr from 2001 to 2019 (\(p<0.01\); see Figure 7a). EQ was the highest in the year 2012 at 64.91, fluctuating widely before and after. It was the lowest in 2007, at 40.99. The overall EQ was seen to be at a higher level between 2012 and 2017. Its multi-year average was 53.66, indicating that the URB's overall EQ is at a medium level. The URB's overall EQ showed an increasing trend (slope \(>\) 0) over the last 19 years, accounting for 95.14% of the total study area.