## 3 Results and Discussion
### Response of WUE to Climatic Factors and Drought  
Extensive studies have reported that climatic conditions can result in spatial and temporal changes in WUE. For instance, slight changes in precipitation reduce runoff [46] and can improve the plant growth environment [47], therefore increasing the NPP, which has a positive effect on WUE. Thus, the relationship between WUE and climate factors and drought were investigated at a large regional scale in the TMS. As shown in Figure 5a-c, temperature and wind speed had a positive influence on WUE in the central and eastern TMS. Precipitation can also play a positive role in enhancing WUE, especially on the northern slope of the TMS, which is consistent with a previous study that was carried out on the Chinese Loess Plateau [48]. The authors also demonstrated that the positive correlation between WUE and the mean annual precipitation range in the Chinese Loess Plateau was larger for forest vegetation than for steppe vegetation [48]. The two climatic factors (wind speed and precipitation) both had a negative influence on WUE in the IIi region in the western part of the TMS (Figure 5a-c). It is easy to understand that higher wind speeds decrease NPP and further decrease WUE. The IIi region is located in a river valley and is mainly covered by grassland, and a previous study revealed that precipitation is the main factor controlling WUE variation in grassland [5]. Our results are in line with this finding, and the decrease in precipitation was found to reduce ET, which may lead to enhancing WUE in the IIi region. This phenomenon occurs because the relationship between WUE and precipitation is primarily controlled by the carbon process rather than the hydrological process [5]. As presented in Figure 5d,e, the correlation coefficients of annual mean WUE with sunshine duration and solar radiation in the TMS showed strong spatial heterogeneity, and the spatial distribution of these correlation coefficients had good consistency. Additionally, the areas having a positive correlation were scattered in the central and western TMS, and the areas having a negative correlation were distributed in the eastern regions.  
At present, the analysis of the effect of drought on water cycles' terrestrial carbon is a popular and meaningful research topic. It was reported that drought can cause a significant increase in WUE in natural ecosystems [49; 50], but other researchers demonstrated that severe drought may obviously reduce ecosystem WUE [51; 52]. In the present study, although there was strong spatial heterogeneity of the correlation coefficient (0.68, \(p<0.05\)) between WUE and the TVDI (Figure 5f), the slopes of WUE and the TVDI presented good consistency in spatial distribution (Figure 4f,h). Thus, we concluded that drought can enhance the WUE of natural ecosystems, which agrees with results obtained by [PERSON] et al. [49]. However, considering that the adopted drought index, spatiotemporal scale, and statistical method can vary, the conclusion regarding the relationship between WUE and drought may differ. In future, it will be necessary to analyze the mechanism by which drought influences WUE, in addition to making field observations. This will improve the comprehension of the WUE of ecosystems and enhance the scientific basis for water resource management in the TMS.