# Spatio-Temporal Changes in Water Use Efficiency and Its Driving Factors in Central Asia (2001-2021)
## 4 Discussion
### Driving Mechanisms of WUE Change in Central Asia  
Based on the geographic detector, we can not only assess the importance of the drivers quickly and effectively, but can also determine the importance of the interactions between drivers. In contrast to linear regression, the geographic detector works with continuous data and is not limited by multicollinearity [54]. In addition, through partial correlation analysis, we can determine the direction of correlation between drivers and GPP, ET, and WUE. We found that the atmospheric factors LST, DEF, VPD, RN, and TEM have a significant effect on WUE in Central Asia. Among these, LST has the strongest driving role and can explain 54.8% of the variation in WUE. The above five atmospheric factors correlate negatively with GPP and ET in most parts of Central Asia and positively with  
Figure 7: Spatial and temporal variation of monthly average WUE in Central Asia, 2001–2021. WUE (Figure 8). In contrast, the hydrological factors PRE and VSW1 correlate positively with GPP and ET and negatively with WUE in most of Central Asia (Figure 8). Because increases in LST and TEM cause drought, and the correlation between LST and VPD is high [34], both drought and high VPD promote the stomatal closure of plant leaves, leading to a decrease in evapotranspiration before a decrease in photosynthesis rate, causing an increase in WUE [55; 56]. Furthermore, with strong evapotranspiration in Central Asia, water in the soil evaporates rapidly after precipitation occurs, which, in turn, limits the ability of vegetation to absorb CO\({}_{2}\). For example, [PERSON] et al. found that WUE in sparsely vegetated and drier areas increased with decreasing precipitation [24]. We found that the seven drivers mentioned above explain WUE in Central Asia to a high degree, and all drive WUE by affecting vegetation photosynthesis or transpiration. We found that the interactions between the 20 selected drivers all strongly enhance the driving force on WUE, suggesting responses between drivers can better explain changes in WUE. A total of 22 interactions between atmospheric and vegetation factors explained more  
Figure 8: Deviation correlations of each driver with GPP, ET, and WUE; (**a–t**) for TEM, RN, LST, VPD, DEF, VAP, AOD, TSVEG, PRE, VSW1, GWS, RTZSM, SFSM, PDSI, RO, SW, CRSI, LAI, EVI, and FVC. than 60% of the variation in WUE. The vegetation factors CRSI, LAI, EVI, and FVC were all positively correlated with GPP and ET and negatively correlated with WUE (Figure 8). It has been shown that Central Asia is turning green and that the recovery of vegetation leads to a significant increase in GPP and ET [23; 38]. However, under drought stress, vegetation regulates its own water consumption mechanisms by reducing leaf area [36], reflecting the physiological adaptation of vegetation to water stress [57; 58]. It has also been suggested that increasing LAI may lead to stronger evapotranspiration than transpiration, resulting in a decrease in WUE [37]. This study likewise found that transpiration under climate stress could explain WUE changes well; for example, the combination of VAP and TSVEG drove a 12% increase in capacity. In addition, the interaction between AOD and GWS can substantially enhance the driving force, especially the interaction with vegetation factors, which has the most significant enhancement effect. It has been shown that aerosols can form foliar dust that affects the photosynthesis of vegetation, and that appropriate groundwater levels are beneficial to the growth of vegetation in arid zones [38; 59]. We found that vegetation factors are the most critical aspects of WUE change in Central Asia, with FVC being the strongest driving vegetation factor. Integrating the effects of aerosol and groundwater distribution on vegetation can provide a better understanding of WUE change in Central Asia. In summary, atmospheric and hydrological factors drive changes in WUE through their effects on vegetation photosynthesis and transpiration.