## 3 Results
### The Spatiotemporal Dynamics of WUE and Drought in Central Asia  
Figure 2 clearly indicates that arid and slightly arid areas are mainly distributed throughout Central Asia, and these areas cover approximately 80% of the total area. The areas of severe drought and drought showed increasing trends, while the slight drought, normal, and humid areas showed decreasing trends from 2000 to 2014. The area of drought showed a continuously increasing trend from 2000 to 2008, while the areas were relatively small from 2009 to 2011; then, the drought area increased after 2012; the trend of the change in slight drought area was opposite the change in drought area, which suggests that Central Asia experienced a continuous drought from 2000 to 2008. This drought pattern was found in other studies that examined the occurrence of serious drought due to an increase in ET after 2000 [40]. The WUE changes of different drought classifications over time are shown in Figure 3. Overall, the WUE declined starting in 2000, and the decreasing trend stopped in 2010, except for the severely arid region, where the WUE declined starting in 2001, and the decreasing trend stopped in 2011. The longer duration of the decrease in the WUE in the severely arid region with the subsequent increase in the level of drought results in a lower multiyear mean WUE, which means that the drier conditions in the later years result in a decrease in WUE to the average annual WUE. For example, the WUE was less than the multiyear mean WUE in the humid region (2000), normal region (2000), slight drought region (2002), drought region (2004), and severe drought region (2005). Figure 1: (**a**) Average water use efficiency (WUE) of Central Asia for the period of 2000–2014. White areas represent barren, snow, and water body pixels. Differences in the mean annual WUE among (**b**) vegetation types, (**c**) drought classifications, and (**d**) countries. C-Shrublands represent closed shrublands and O-Shrublands represent open shrublands. Different letters represent significant differences at the 0.05 level. Figure 2: The area ratios of different drought levels from 2000 to 2014. We applied the Mann-Kendall test to determine the trend of WUE and drought over the period of 2000-2014. The spatial distributions of the TVDI trends are shown in Figure 4a. Over the past 15 years, only 1.04% of the vegetated areas in Central Asia exhibited significant wetting. However, 20.01% of the vegetated areas exhibited significant drying trends (\(p<0.05\)), and were mainly located in the northwestern part of Kazakhstan and the Tianshan Mountains in Xinjiang; therefore, the significantly arid areas were larger than the significantly wetting areas. Approximately 19.22% of the vegetated areas in Central Asia presented significant increasing trends in WUE, and these areas were mainly located in Kyrgyzstan, the northern region of Xinjiang, and north of Kazakhstan. Only 1.24% of the vegetated areas showed significant decreases in WUE in the south of Xinjiang and southwestern Turkmenistan. The areas where an increase in the WUE was accompanied by drying were observed in Kyrgyzstan and Kazakhstan, except for the northern region of Balkhash Lake. The areas where the drying trends combined with decreasing WUE trends were observed in Turkmenistan and Uzbekistan (Figure 4b). Overall, Central Asia exhibited a drying trend and the WUE increased from 2000 to 2014. Our observations are consistent with those of a previous study, in which Central Asia experienced a serious drought in 2008. The ecosystem WUE decreased linearly, and the TVDI increased rapidly from 2000-2010 (Figure 3). The WUE increased slightly and the TVDI decreased slightly from 2011 to 2014. Therefore, the correlations between the annual WUE and TVDI were divided into two parts in this study, which represent the drought stress period from 2000 to 2010, and the post-drought stress period from 2011 to 2014.