## 3 Results  
### Spatial Distribution and Spatiotemporal Trends in Annual WUE over China  
The spatial distribution of mean annual WUE showed large regional differences during the period 1982-2015 (Figure 2) with relatively higher values in southeastern China (sub-humid and humid zones) and lower values in northwestern China (sub-arid and arid zones). The multi-year average WUE was 1.55 g C kg\({}^{-1}\) H\({}_{2}\)O (varied from 0 to 5.21 g C kg\({}^{-1}\) H\({}_{2}\)O) in China with the highest regional WUE in Northeast China (2.53 g C kg\({}^{-1}\) H\({}_{2}\)O) and the second in North China (2.04 g C kg\({}^{-1}\) H\({}_{2}\)O). The average WUE in Central China, Inner Mongolia, Southeast China and Southwest China varied from 1.5 to 2.0 g C kg\({}^{-1}\) H\({}_{2}\)O and the lowest WUE (0.78 g C kg\({}^{-1}\) H\({}_{2}\)O) was in Tibetan Plateau among the nine sub-regions. For different vegetation types, the highest WUE was found for deciduous broadleaf forests (2.48 g C kg\({}^{-1}\) H\({}_{2}\)O), followed by deciduous needle-leaf forests (2.35 g C kg\({}^{-1}\) H\({}_{2}\)O) and evergreen broadleaf forests (2.16 g C kg\({}^{-1}\) H\({}_{2}\)O). The annual mean WUE were similar in croplands (1.85 g C kg\({}^{-1}\) H\({}_{2}\)O), shrub lands (1.81 g C kg\({}^{-1}\) H\({}_{2}\)O), evergreen needle-leaf forests (1.77 g C kg\({}^{-1}\) H\({}_{2}\)O) and mixed forests (1.75 g C kg\({}^{-1}\) H\({}_{2}\)O). The grasslands had the lowest WUE (0.97 g C kg\({}^{-1}\) H\({}_{2}\)O) among the eight vegetation categories.  
The annual WUE showed a significantly increasing trend (slope: 8.5 \(\times\) 10\({}^{-3}\) g C kg\({}^{-1}\) H\({}_{2}\)O per year) over China during the period of 1982-2015 (Figure 3a). Large inter-annual fluctuations were found in the WUE series with the highest value (1.75 g C kg\({}^{-1}\) H\({}_{2}\)O) in 2013 and the lowest value (1.40 g C kg\({}^{-1}\) H\({}_{2}\)O) in 1982. Spatially, WUE increased in approximately 77.1% of Chinese territory (significantly increased in 56.2% of Chinese territory) during the past 34 years, which was mostly distributed in the west parts in Inner Mongolia and Northeast China, the north parts in Southeast China, south parts in Southwest China and most regions in North China, South China and Tibetan Plateau. It should be noted that the increasing trends were relatively larger in the east of sub-humid zones. In contrast, the WUE in approximately 7.9% of Chinese territory exhibited negative trends, which distributed sporadically in the east of Inner Mongolia, Northeast China and Southeast China, south of North China and Northwest China, central region of Central China and north parts in Southwest China.  
Figure 3: Temporal (**a**) and spatial (**b**) trends in annual WUE over China from 1982 to 2015. The white color in (**b**) indicates non-vegetated areas, and the black and red lines show the boundaries of sub-regions and eco-climatic zones. The light gray and dark gray colors in (**b**) indicate that trends in annual WUE were not significantly increased and decreased at the 0.05 level.  
Figure 2: The spatial distribution of mean annual water use efficiency (WUE) (**a**) in China (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 annual WUEs for each sub-region and vegetation type (DBF: deciduous broadleaf forests, DNF: deciduous needle-leaf forests, EBF: evergreen broadleaf forests, CRO: croplands, SHR: shrub lands, ENF: evergreen needle-leaf forests, MF: mixed forests and GRA: grasslands) are exhibited in (**b**,**c**). The error bars in (**b**,**c**) were standard deviation of WUE in each sub-region and vegetation type.