## 4 Discussion
### Response of WUE to Seasonal Drought  
Interestingly, spring drought increased WUE and summer drought decreased WUE, although all significant sensitivity of WUE to SPEI in two seasons was found in the northwest study region dominated by grassland. When drought occurs, the sensitivity of WUE to the drought index is low in spring and high in summer. The divergence of sensitivity of WUE to SPEI in spring and summer may be related to the drought tolerance of vegetation and the magnitude of drought [9; 18; 19; 22; 49; 50]. First, there is generally less precipitation in spring than in summer, and vegetation has adapted to the drier climate of spring so that when the drought is prolonged, the drought tolerance of grass in the spring is greater than that in the summer when drought continues [13; 18; 22; 51; 52]; therefore, spring drought led to increased WUE. Secondly, higher sensitivity of WUE to SPEI was found in grasslands because the grass is more sensitive to environmental and climate change than deep-rooted vegetation [18; 21]. In other words, shallow-rooted herbs rely on precipitation to supplement soil water, whereas deep-rooted trees or shrubs primarily use deep soil water or groundwater [53; 54; 55; 56]. Lastly, we found that mild drought caused lower sensitivity of WUE to SPEI, and moderate drought caused higher sensitivity of WUE to SPEI, implying that drought tolerance of vegetation may have a threshold effect, which is consistent with previous studies [32; 33]. The divergent influence of seasonal drought on WUE may be explained by the divergent change rate of GPP and ET [13; 24; 57]. Some studies investigated whether the ecosystem WUE response to drought was positive or negative depending on the divergent ecosystem GPP and ET sensitivity to drought [22]. Drought can reduce soil water content, as well as GPP and ET [24; 57], which can explain why summer drought causes a decrease in WUE, whereas spring drought can cause an increase in WUE to some extent. The response of ecosystem WUE to drought is an extremely complex process that is affected by a variety of biotic and environmental factors. WUE has strong inherent variability and plasticity [44; 58]. The results of our study show that the response of WUE to drought is varies in different climatic regions of the Loess Plateau, and the sensitivity of WUE to drought magnitude in the arid area was generally higher than that in the semi-arid, semi-humid, and humid regions. In most cases, the impact of drought on WUE is that it affects ecosystem productivity and ET to varying degrees. However, under different hydrothermal conditions, different biological types of productivity and ET exhibit different sensitivities to drought [18]. Persistent drought has little effect on vegetation growth in arid areas because vegetation in such regions have stronger tolerance to drought and adapt well to drought [18; 51; 52]. On the other hand, low vegetation coverage leads to more soil evaporation in arid regions and higher sensitivity of ET to changed hydroclimatic conditions [59]. In contrast, in semi-arid and semi-humid regions, ecosystem functions and activities depend largely on water availability [21]. Consequently, the change in WUE in arid areas is generally controlled by evaporation (ET) (which is a physical process), whereas that in semi-arid and semi-humid areas is primarily regulated by assimilation (GPP) (which is a biological process). Divergent changes in GPP and ET lead to different WUE sensitivities to drought in different climate regions [18]. [PERSON] et al. [13] pointed out that persistent water deficit (i.e., the drought time-scale) in arid and humid areas affects the sensitivity of vegetation WUE to drought. Vegetation in arid areas has a mechanism that allows it quickly adapt to the changing water resources, with vegetation quickly adapting when water resources are less than a threshold value in a short time [13].