## 3 Results and Discussion
### Spatial Distribution Characteristics of NPP, ET, WUE, and TVDI  
The spatial patterns of the annual mean and slope of NPP, ET, WUE, and TVDI during 2000-2020 are depicted in Figure 4. The spatial patterns in the average value of ET showed a decreasing trend with decreasing latitude (southward) and the highest value (620 mm) appeared in the Turpan region (Figure 4a). From west to east, the slope of ET experienced an \"increase-decrease-increase-decrease\" mode in the TMS, and most of the rates of increase were in the range of 0.73-7.36 (Figure 4b). The spatial pattern of the average annual NPP showed that highest NPP generally occurred in the central region of the study area (Figure 4c). Furthermore, the regions having decreasing trends that were statistically significant at the 0.01 level for NPP were primarily distributed in the central and eastern TMS (Figure 4d). In addition, a majority of regions (approximately 86%) exhibited a generally increasing trend for NPP, with range of 0.54-8.25 gC\(\cdot\)m\({}^{-2}\cdot\)a\({}^{-1}\), which is consistent with a study conducted in the Hexi Corridor in north-western China [45].  
Figure 3: Temporal variation in WUE (**a**) and TVDI (**b**) under different land-use types during 2000–2020.  
Comparing the map for average annual NPP with that for average annual WUE, we noticed that the spatial distribution for average annual NPP was consistent with that of annual WUE (Figure 4c,e). For example, the high values of NPP and WUE appeared predominantly in the central and northern regions of the TMS. Overall, the number of regions in which WUE decreased was greater than that in which WUE increased, indicating that WUE had a decreasing variation in the TMS during 2000-2020. Compared with the multiyear mean spatial distribution of WUE, the slope indicated a different pattern that presented no significant changes for all of the areas, and only some areas in the central TMS showed a remarkably decreasing trend, at a rate of 0.03-0.66 gC\(\cdot\)kg\({}^{-1}\cdot\)H\({}_{2}\)O-a\({}^{-1}\) (Figure 4f). In addition, most areas (75%) experiencing an increasing trend in WUE were distributed in  
Figure 4: The spatial patterns of the average and change trend in NPP (**a,b**), ET (**c,d**), WUE (**e,f**), and TVDI (**g,h**) in the TMS from 2000 to 2020.  
the eastern region of the TMS. As shown in Figure 4g, notably, the majority of pixels having high TVDI values were distributed in low-altitude areas on the southern slope of the TMS, which indicates that these areas, especially those at the edge of oases, experienced drought and severe drought, which may be related to the increasing temperature (Figure 1) and lower vegetation coverage in these areas [31]. Overall, the percentage of pixels showing an increase in the TVDI was slightly larger than that showing a decrease in the TVDI in the TMS (Figure 4h). Interestingly, the region experiencing a significant increase (at the 0.05 significance level) in the TVDI corresponded to the area showing a significant decrease (at the 0.05 significance level) in WUE, indicating that the TVDI may reduce WUE and increase ET. This issue is discussed in the following section.