## 4 Discussion  
### Dynamics and Drivers of WUE  
This study employed linear regression analysis and the EEMD technique to explore the variations in WUE in the YRB from 1982 to 2018. The WUE exhibited a significant \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**WUE** & **GPP and ET** & \multicolumn{2}{c}{**1982–2018**} & \multicolumn{2}{c}{**1982–2000**} & \multicolumn{2}{c}{**2000–2018**} \\ \cline{2-7}
**Change** & **Change** & **Area** & **Total** & **Area** & **Total** & **Area** & **Total** \\ \hline WUE & GPP \(\downarrow\) and ET \(\downarrow\) & 0.22 & & 0.02 & & 0.20 & \\  & GPP \(\downarrow\) and ET \(\uparrow\) & 1.42 & & 0.04 & & 0.50 & \\  & GPP \(\uparrow\) and ET \(\downarrow\) & / & / & & / & / & \\  & GPP \(\uparrow\) and ET \(\uparrow\) & 3.01 & & / & & 0.04 & \\ \hline WUE & GPP \(\downarrow\) and ET \(\downarrow\) & / & / & / & / & \\  & GPP \(\downarrow\) and ET \(\uparrow\) & / & 45.55 & / & 6.14 & / & \\  & GPP \(\uparrow\) and ET \(\downarrow\) & / & & / & 6.14 & / & \\  & GPP \(\uparrow\) and ET \(\uparrow\) & 45.55 & & 6.14 & & 18.89 & \\ \hline \hline \end{tabular} Note: “\(\uparrow\)” represents a significant increase in the trend, “\(\downarrow\)” represents a significant decrease in the trend. “\(//\)” indicates that the pixels has not passed the significance test.  
\end{table} 
Table 4: The percentage of area in response to the significant changes in WUE to GPP and ET at different time periods. Figure 10: WUE significant changes in response to GPP significant changes and ET significant changes across different time periods (significant test \(p\) < 0.05). upward trend (0.56 \(\times\) 10\({}^{-2}\) gC\(\cdot\)kg\({}^{-1}\cdot\)H\({}_{2}\)O\(\cdot\)a\({}^{-1}\), \(p\) \(<\) 0.05) from 1982 to 2018 in the YRB. This trend was consistent with the findings of other scholars [8; 9; 17; 30]. The area showing a significant increase in WUE (47.07%, Slope \(>\) 0, \(p\) \(<\) 0.05) was higher than the area with a significant decrease (14.64%, Slope \(<\) 0, \(p\) \(<\) 0.05). The spatial patterns of WUE changes in the YRB were significantly different between the periods of 1982-2000 and 2000-2018. It indicated that since the implementation of the policy of returning farmland to forest and grassland, the vegetation has increased to promote the increase of water use efficiency; the findings of this paper were consistent with those of other scholars [20; 21; 51; 52; 53; 54]. This study evaluated the results of WUE outcomes across various plant communities, determining that forest WUE (1.267 gC\(\cdot\)kg\({}^{-1}\cdot\)H\({}_{2}\)O) was greater than that of cropland WUE (0.972 gC\(\cdot\)kg\({}^{-1}\cdot\)H\({}_{2}\)O), which in turn surpassed that of grassland WUE (0.805 gC\(\cdot\)kg\({}^{-1}\cdot\)H\({}_{2}\)O). From the analysis of different watersheds, it was learned that the WUE in the upstream (1.04 gC\(\cdot\)kg\({}^{-1}\cdot\)H\({}_{2}\)O) was higher than that in the downstream (0.95 gC\(\cdot\)kg\({}^{-1}\cdot\)H\({}_{2}\)O) and the midstream (0.81 gC\(\cdot\)kg\({}^{-1}\cdot\)H\({}_{2}\)O), which may be caused by the difference in vegetation structure between the upstream and midstream/downstream regions [5; 23; 30]. In the YRB, a sequential distribution pattern of grasslands, farmlands, and forests has emerged from the upstream to the midstream and downstream regions. Grasslands in the southwestern part of the basin and forests in the southeastern part demonstrate relatively high WUE due to higher GPP [20; 21; 40; 55]. In contrast, the northern grassland ecosystems exhibit lower average annual precipitation and GPP. Consequently, more water is lost through soil evapotranspiration, resulting in comparatively lower WUE. This is primarily attributed to the \"Returning Farmland to Forests and grasslands\" project [26; 56; 57; 58], initiated in 2000, which expanded forest and grassland areas in the midstream region while decreasing farmland [6; 33; 54].