# Spatiotemporal Analysis of Evapotranspiration and Effects of Water and Heat on Water Use Efficiency
## 3 Results and Discussion
### Spatiotemporal Characteristic of WUEs Defined by GPP/E, GPP/Et and GPP/P  
There are obvious spatial differences between different WUEs in which the spatial distribution patterns of eWUE and pWUE are generally consistent. Spatiotemporal WUE (2000-2014 average) consists of GPP/E (eWUE), GPP/Et (tWUE), and GPP/P (pWUE) (Figure 6). During the period of 2000-2014, the value of WUE showed slight changes, and the spatial distribution of WUEs was uneven (the white area means no value). The spatial distribution of eWUE presents a high-low interval distribution. The eWUE around the Taklimakan Desert was relatively high, indicating that the WUE of the desert ecosystem is generally high. However, the tWUE around the Taklimakan Desert is higher than the eWUE. Secondly, the spatial distribution of pWUE and eWUE is basically the same (Figure 6a,c), but they are different from the trend of tWUE. It shows that precipitation and evapotranspiration have consistent changes in semi-arid areas. In arid areas, 90% of the surface precipitation returns to the atmosphere in the form of vapor [45; 46]; therefore, the precipitation and evaporation show the same pattern. The range of WUE was 0-5 g C-mm\({}^{-1}\).m\({}^{-2}\), and [PERSON] et al. analyzed the range of nearly 15 ecosystems in Central Asia and Xinjiang, confirming the accuracy of the calculation results [41]. We normalized data from 0-5, mapped them to values between 0-1, and divided WUE into five levels for statistical analysis (Table 2): When 0 \(<\) WUE \(<\) 0.2, the proportion of eWUE and pWUE is similar (65.25% and 67.11%), but the proportion of tWUE (23.66%) is less than eWUE and pWUE. When 0.2 \(<\) WUE \(<\) 0.4, overall, the values of eWUE and pWUE range mainly from 0-0.4, which is more than 85% of the total area. However, the tWUE at 0-0.2, 0.2-0.4, and \(>\)0.8 levels accounted for 23.66%, 26.78%, and 25.66%, respectively. In summary, the mean annual value of tWUE is larger than that of eWUE and pWUE,  
Figure 5: Spatial distribution of Hurst index in Xinjiang from 2000 to 2014. (**a**) \(E\); (**b**) \(Ep\). and the spatial difference is obvious. According to previous studies [47; 48], there has been an increase in temperature and precipitation since the mid-1980s, with the increase in precipitation in Xinjiang being particularly significant compared with other regions. However, according to the results of this study, WUEs has a negative effect on precipitation. We think the increase in precipitation is bad news for the vegetation that has adapted to the arid environment. It takes a while for the vegetation to adapt. Such a research phenomenon may be the lag effect. Firstly, we found that there was a high degree of spatial similarity between pWUE and NDVI in the growing season [20]. Secondly, pWUE values were also higher in the regions with higher NDVI values in the growing season, except for sparse vegetation around the desert. Such a phenomenon occurs around the desert. We believe that the reason why pWUE is high around the desert is that mainly coniferous vegetation around the desert leads to the reduction of transpiration, which leads to a high WUE. Finally, we believe that there is a difference between NDVI in the growing season and pWUE, which may be caused by the difference in time scale. We divided the change trends (Slope) into five categories (Table 1). The regions with increasing eWUE and pWUE accounted for 68.18% and 81.47%, respectively (Table 2), while the regions with decreasing trends were mainly distributed in the north, central, and a few southern regions of Xinjiang (Figure 7a,c). Secondly, the area with a slight increasing trend of TWUE accounted for 41.32%. The spatial distribution is scattered and irregular, which is mainly because the vegetation cover distribution and precipitation distribution are jointly determined. Surprisingly, the regions with high and relatively high increasing trends in southern Xinjiang accounted for 5.74% of the total.