# Responses of Water Use Efficiency to Drought in Southwest China
## 3 Results
### Responses of WUE to Drought in Southwest China  
The spatially averaged mean annual WUE values under different wet or dry conditions are shown in Figure 6. The mean annual WUE averaged over all vegetated grid cells increased as the wet or dry conditions changed from moderate humid to moderate drought. The mean annual WUE of forests and shrublands generally increased with the wet or dry conditions changing from humid to drought, and reached their peak values (2.43 and 2.13 gC kg\({}^{-1}\)H\({}_{2}\)O, respectively) in moderate drought. The grasslands were more sensitive to drought with WUE increased to the peak value (1.2 gC kg\({}^{-1}\)H\({}_{2}\)O) under slight drought. The WUE of croplands was the highest for the slight humid category and then decreased as the wet or dry conditions changed from slight humid to moderate drought. The percentage of drought area (scPDSI \(<-1\)) was used to identify drought years (Figure 2a). We selected two drought years, 2006 and 2010, for further analysis. The average data of the study  
Figure 5: The mean annual WUE and the trend in annual WUE over Southwest China from 2000 to 2017: (**a**) mean annual WUE; (**b**) trend of annual WUE. Figure 6: Mean annual WUE spatially averaged for all vegetated grid cells and each of the four broad vegetation types under different wet or dry conditions. period 2000-2017, excluding the two identified drought years (2006 and 2010), were used as the normal year. The \(\Delta\)WUE was used to study the impacts of drought on ecosystem WUE. Although the spatial distributions of WUE in 2006, 2010, and the normal year were generally similar (Figure 7), the WUE anomalies for 2006 (\(\Delta\)WUE\({}_{2006}\)) and 2010 (\(\Delta\)WUE\({}_{2010}\)) showed positive and negative values (Figure 8). \(\Delta\)WUE\({}_{2006}\) and \(\Delta\)WUE\({}_{2010}\) generally had opposite values in the Sichuan Basin, southwestern Yunnan, and Guizhou Province (Figure 8). The effects of drought on WUE were also different for different vegetation types (Table 3). In 2006, WUE increased for 55.3%, 70.2%, 79.1%, and 80.9% of the grid cells covered by forests, shrublands, grasslands, and croplands, respectively, and increased for more than 70% of all the vegetated grid cells for the whole region. In 2010, WUE increased for more grid cells for forests and grasslands and decreased for more grid cells for croplands. The areas with severe drought in 2006 and 2010 (Figure 8) were selected to further analyze the impacts of drought on ecosystem WUE. In 2006, southwestern Yunnan was selected as a typical region of drought occurrence. In this area, the scPDSI was mostly negatively correlated with WUE because the moderate and severe drought (\(-4<\) scPDSI \(<-2\)) enhanced WUE (Figure 9a,c). In 2010, the WUE in the area highlighted in Figure 8b showed a quadratic correlation with scPDSI and a peak value was observed because the extreme drought (\(-5<\) scPDSI \(<-4\)) reduced WUE (Figure 9b). We further explored the responses of ecosystem WUE to drought at the monthly timescale for different vegetation types (Figure 10). The monthly WUE of both forests and shrublands increased first and then decreased with the aggravation of drought, and the inflection points of WUE were \(-3.4\) and \(-3.2\) for scPDSI, respectively. However, no inflection points in monthly WUE were found for grasslands and croplands. The monthly WUE of grasslands and croplands linearly and nonlinearly decreased with increasing drought severity, respectively. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Vegetation Type** & \(\Delta\)**WUE\({}_{2006}\) \textless{}0** & \(\Delta\)**WUE\({}_{2006}\) \textgreater{}0** & \(\Delta\)**WUE\({}_{2010}\) \textless{}0** & \(\Delta\)**WUE\({}_{2010}\) \textgreater{}0** \\ \hline Forests & 44.7\% & 55.3\% & 46.4\% & 53.6\% \\ Shrublands & 29.8\% & 70.2\% & 50.6\% & 49.4\% \\ Grasslands & 20.9\% & 79.1\% & 46.0\% & 54.0\% \\ Croplands & 19.1\% & 80.9\% & 78.8\% & 21.2\% \\ All & 27.7\% & 72.3\% & 56.4\% & 43.6\% \\ \hline \hline \end{tabular}
\end{table}