## 3 Results
### Effects of Drought on WUE in China during 1982-2015  
The correlation between WUE and SPEI showed a large spatial heterogeneity in China from 1982 to 2015 (Figure 4). The WUE positively correlated to SPEI in approximately 65.4% of Chinese territory (significant in 13.7% of Chinese territory at the 0.05 level) whereas negatively correlated to SPEI in approximately 34.6% of Chinese territory (significant in 4.5% of Chinese territory, mainly in the northeast of Inner Mongolia and Northeast China and for DBF, DNF and SHR). The correlation coefficients were relatively higher in southwest parts of Inner Mongolia, central of Southeast China, Southwest China, Central China and South China and most regions of North China, Northwest China and Tibetan Plateau.  
In order to explore the drought lagging effects on WUE in China, in our study we also calculated Pearson correlation coefficient between WUE and previous-year SPEI. As shown in Figure 5, the spatial distribution of correlations mostly coincides with that shown Figure 4, which indicated that the WUE responded to current-year drought and previous-year drought in the same direction in most parts of China. However, the WUE was positively correlated with current-year SPEI but negatively correlated with previous-year SPEI in the central of Inner Mongolia and some areas of South China, Central China and Southeast China. WUE was negatively correlated with current-year SPEI but positively correlated with previous-year SPEI in the central of Southwest China, north and east parts of Northeast China. In the DBF, DNF and SHR vegetation types, the WUE showed positive correlations with previous-year drought but negative correlations with current-year drought, which led to even greater effects on carbon and water exchange processes during long-lasting droughts. In our study, we also used Akaike Information Criterion (AIC) determine whether the inclusion of previous-year SPEI improved performances of the regression model between WUE and SPEI, which could also reveal the lagging effects of previous-year drought on WUE. Figure 5 showed that the difference in AIC of the two equations was less than \(-2\) mainly in the east parts in Inner Mongolia and Northeast China, west and east parts in North China and central parts in Tibetan Plateau, which indicated that previous-year drought lagging effects on WUE in these regions were relatively larger than other regions.  
Figure 4: The spatial distribution of Pearson correlation coefficient between WUE and Standardized Precipitation-Evapotranspiration Index (SPEI) (**a**) in China during the period 1982–2015 (the white color indicates non-vegetated areas, and the black and red lines showed the boundaries of sub-regions and eco-climatic zones). The regional mean correlation coefficients for different vegetation types (CRO: croplands, EBF: evergreen broadleaf forests, DBF: deciduous broadleaf forests, ENF: evergreen needle-leaf forests, DNF: deciduous needle-leaf forests, MF: mixed forests, SHR: shrub lands and GRA: grasslands) are exhibited in (**b**). The error bars in (**b**) were standard deviation of Pearson correlation coefficient in each vegetation type.