## 4 Discussion
### Sensitivity Analysis of WUE in the YRB  
Global warming and other climatic factors have driven changes in WUE; WUE could diminish if ambient temperatures surpass the ideal range for botanical development. Moreover, elevated heat levels can accelerate the rate of water loss through transpiration, consequently amplifying the hydration requirements of vegetation [3; 61; 62; 63]. Research has demonstrated that climate change modifies vegetation structure, thereby impacting the interplay between the carbon and hydrological cycles of ecosystems and consequently affecting WUE [61; 64; 65; 66]. [PERSON] et al. [4] demonstrated a decreasing trend in WUE in Xinjiang from 2000 to 2014 (Slope = \(-\)0.001), and determined that thermal conditions exerted a substantial influence on WUE, accounting for 87.7% of the variation. [PERSON] et al. [25] investigated WUE and its influencing factors in the Tekeze River region of northern Ethiopia from 1982 to 2014, identifying temperature and precipitation as key climatic factors affecting changes in Net Primary Productivity (NPP), ET, and WUE.  
Traditional correlation analysis methods may not sufficiently account for the interrelationships among multiple variables [20; 44; 51]. In contrast, the partial correlation analysis method allows for the examination of the effects of multiple independent variables' effects on a dependent variable while considering their interactions and excluding the influence of other independent variables. This method enables a more accurate assessment of each variable's individual contribution to the dependent variable. The PCA method was utilized to assess the effects of temperature, precipitation, vapor pressure deficit, and soil moisture on WUE. The findings revealed that the regions showing a positive and high correlation were 58.27%, 79.08%, 47.70%, and 66.47%, respectively, with the correlation between vapor pressure deficit and WUE being relatively lower. Precipitation and soil moisture are the primary climatic factors influencing the WUE of ecosystems in the YRB. Precipitation directly affects the availability of soil moisture, which in turn influences vegetation growth, water absorption, photosynthesis, and evapotranspiration processes [9, 62, 67, 9]. Consequently, precipitation is closely linked to the WUE of vegetation [2, 34, 49]. The soil moisture affects the balance of photosynthesis and transpiration in plants [50, 66, 67], and when soil moisture becomes too low (e.g., drought conditions), the plant root system has difficulty in absorbing sufficient water [1, 12, 66, 67, 68]. This deficiency leads to reductions in photosynthesis and transpiration, stomatal closure, and ultimately a decline in WUE.  
Meanwhile, the changes in ecosystem WUE were influenced by GPP and ET. Approximately 70.39% of the WUE in the YRB exhibited an increasing trend, primarily driven by increases in GPP and ET. WUE plays a regulatory role between GPP and ET. Under conditions of sufficient water supply, plants can increase GPP and WUE by increasing transpiration. However, under conditions of insufficient water supply, plants may reduce transpiration to maintain water, which may lead to a decrease in GPP, but WUE may increase due to a decrease in transpiration. The inconsistent rate of changes in GPP and ET could affect the change in WUE. [PERSON] et al. [64] discovered that the increase in WUE was primarily driven by the rise in GPP, while ET exhibited no significant change under projected future warming scenarios. Amidst the context of global warming, the sequestration of carbon and the management of water resources have exhibited notable patterns. The disproportionate changes between GPP and ET highlight the important role of vegetation in the dynamics of the WUE ecosystem in the YRB. This research offers empirical evidence to facilitate the optimization of land management and water resource optimization.