# Responses of Water Use Efficiency to Drought in Southwest China
## 3 Results
### Spatial and Temporal Variations of WUE in Southwest China  
The annual WUE of the terrestrial ecosystems in Southwest China fluctuated greatly from 2000 to 2017 with a multi-year mean value of 1.70 gC kg\({}^{-1}\)H\({}_{2}\)O (Figure 4a). Over the entire study period 2000-2017, the annual WUE generally showed a decreasing trend that was statistically insignificant except for croplands (Figure 4a-e). With the start (2000) and end (2017) years with particularly low WUE values excluded, the trends of WUE were statistically significant for all vegetated grid cells (Figure 4a) and croplands (Figure 4e). Moreover, WUE also exhibited a declining trend for each of the  
Figure 3: The spatial distribution, frequency, and trend of drought as measured by scPDSI in Southwest China over the period 2000–2017: (**a**) the spatial distribution of the long-term mean scPDSI; (**b**) the frequency map of drought occurrence; (**c**) the scPDSI trend.  
Figure 2: The interannual variations of the drought area (scPDSI \(<-1\)) and precipitation in Southwest China over the period 2000–2017: (**a**) the drought area and annual precipitation for all vegetated grid cells; (**b**) the drought area for different vegetation types.  
four main vegetation types (Figure 4b-e). In addition, the mean annual WUE differed among vegetation types: Forests had the largest WUE (3.25 gC kg\({}^{-1}\)H\({}_{2}\)O), followed by shrublands (2.00 gC kg\({}^{-1}\)H\({}_{2}\)O), croplands (1.76 gC kg\({}^{-1}\)H\({}_{2}\)O), and grasslands (1.04 gC kg\({}^{-1}\)H\({}_{2}\)O) (Figure 4b-e).  
The annual WUE was relatively high in Yunnan, southern Sichuan, and Southwestern Sichuan Basin and relatively low in the Western Sichuan Plateau and Guizhou (Figure 5a). The WUE trends based on the Mann-Kendall method were divided into three categories: Significant increase (slope \(>\) 0, \(p\) \(<\) 0.05), no significant trend (\(p\) \(<\) 0.5), and significant decrease (slope \(<\) 0, \(p\) \(<\) 0.05) [49]. The WUE trends from the Mann-Kendall method are shown in Figure 5b. From 2000 to 2017, most vegetated grid cells in the study region had no significant trends; The annual WUE in the Sichuan Basin and northern Sichuan showed significant decreases while the annual WUE in the Northwest, Northeast, and eastern Yunnan significantly increased. From Figures 3b and 5b, it can be found that the areas with significant increases in scPDSI (drought relief or wetting) were generally consistent with those areas with significant decreases in WUE; the areas with significant decreases in scPDSI (drying or drought aggravation) were generally consistent with those areas with significant increases in WUE. Therefore, drought had an enhancement effect on WUE over Southwest China.  
Figure 4: The variations and trends of spatially averaged annual WUE in Southwest China over the period 2000–2017: (**a**) all vegetated grid cells; (**b**) forests; (**c**) shrublands; (**d**) grasslands; and (**e**) croplands. The dashed, red lines are the trend lines over the period 2001–2016, and the start (2000) and end (2017) years of the study period (2000–2017) are excluded to remove the effects of the start and the end years. The trend lines over the period 2000–2017 are not statistically significant except for croplands and are not plotted.