## 3 Results
### Response Mechanism Analysis
#### 3.4.3 Results of Contribution Rate Analysis  
In this study, we quantified the contribution of NPP and ET to WUE during the growing season of alpine grassland from 2001 to 2018 using the contribution rate calculation method, and the spatial distribution of the contribution rate of NPP to the change in WUE is shown in Figure 15a. The results showed that the contribution of NPP to the change in WUE during the growing season was mostly positive, with 56.76% of the regions having a contribution rate of more than 50%. The regions with higher NPP contribution rates were mainly located in the northeast of the Tanggula Mountain Range and near the Lancangjiang River source area, with contribution rates of up to 95% or more, while those with lower NPP contribution rates were mainly located in the east of the Yangtze River source, the southeast of the Qinghai Lake, and the northwest of the Qilian Mountains, The contribution of ET to WUE in the growing season was spatially heterogeneous, with 23.71% of the area having a positive ET contribution, mainly located in the northeastern region of the Yangtze River source area, and 19.73% of the area with negative ET contribution, mainly located in the southern side of the Qinghai Lake, as shown in Figure 15b. Figure 14: Structural equation modeling of WUE: (**a**) \(<\)3000 m, (**b**) 3000–3700 m, and (**c**) \(>\)3700 m elevation zones; path coefficients that pass the significance test of \(p<0.05\) are shown. In this study, areas with an NPP or ET greater than 50% (less than \(-\)50%) were identified as these being the dominant factors influencing WUE changes based on the magnitude of the contribution of NPP and ET to WUE, and the results are shown in Figure 15c. In terms of spatial distribution, the areas negatively dominated by ET were mainly concentrated in the central and southern parts of the study area, indicating that the increase in ET in this region had a significant negative effect on WUE, which might be related to enhanced evapotranspiration and increased water consumption. And the ET positive dominant region was mainly distributed in the eastern and northern local areas, indicating that the reduction in ET contributed to an increase in WUE. In addition, the areas positively dominated by NPP were in the northeastern Tanggula Mountain region, the Lancang River source area, and the northwestern Qinghai Lake region, indicating that the increase in NPP contributed significantly to WUE. In contrast, there were fewer areas negatively dominated by NPP, indicating that the negative impact of NPP reduction on WUE was relatively weak. Overall, changes in WUE in the region are more evenly distributed across the regions affected by ET and NPP, with increases in ET in the south and center of the region having the most prominent suppressive effect on WUE, while the positive effect of NPP was mainly seen in the southeast. In this study, the overall contribution of NPP and ET to the change in WUE in alpine grassland during the growing season from 2001 to 2018 was counted in three elevation zones, <3000 m, 3000-3700 m, and >3700 m, and the results are shown in Figure 16. Below 3000 m elevation, the contribution of WUE change driven by NPP and ET was 17.60% and \(-\)82.5%, respectively, indicating that ET negatively dominated the contribution to WUE change and NPP showed a positive contribution. At the elevation of 3000-3700 m, the contribution of the change in WUE driven by NPP and ET was 42.10% and 57.90%, respectively, indicating that the contribution of ET to the change in WUE is positively  
Figure 15: Spatial distribution of NPP and ET contribution to WUE change: (**a**) NPP contribution to WUE, (**b**) ET contribution to WUE, (**c**) spatial distribution of dominant factors. dominant in this elevation sub-region.