# Spatiotemporal Analysis of Evapotranspiration and Effects of Water and Heat on Water Use Efficiency
## 3 Results and Discussion
### Stability Analysis of Evaporranspiration  
In order to further understand the spatial stability pattern of \(E\) and _Ep_ in Xinjiang, we used the coefficient of variation method for analysis. The CV of \(E\) and _Ep_ is divided into five levels, with CV less than 0.05 defined as low fluctuation, 0.05 to 0.10 defined as low to moderate fluctuation, 0.10 to 0.15 defined as moderate fluctuation, 0.15 to 0.20 defined as moderate to strong fluctuation, and greater than 0.20 defined as very strong fluctuation.  
According to Figure 4a, the CV value of \(E\) near Yili River Valley, Tacheng, and Kunlun Mountain is significantly larger than 0.2, which indicates that the interannual \(E\) in this region has large fluctuations.  
Figure 3: Annual and monthly variations of \(E\) and _Ep_ in Xinjiang during 2000–2014. (**a**) Annual mean changes; (**b**) Monthly mean changes; (**c**) Trends of PDSI monthly scale variation; (**d**) Trends of SPEI monthly scale variation).  
The \(E\) is the most unstable, while the CV of the Junggar Basin and the vicinity of the Tianshan Mountains is the lowest (less than 0.1), indicating that the interannual \(E\) fluctuation in this region is small. The time-series fluctuation is the most stable and the time-series fluctuation of \(E\) in northern Xinjiang (to the north of the Tianshan Mountains) is more durable than that in southern Xinjiang. This may be due to the continuous \"warm and wet\" conditions in the northwest [45]. From Figure 4b, we can see that the CV values of \(Ep\) in Xinjiang are all low (less than 0.1), and the minimum CV values (\(\leq\)0.05) are mainly distributed in the vicinity of the Junggar Basin. The larger CV (0.05-0.1) is primarily distributed near the Tianshan Mountains; it shows that the Ep's space sequence in Xinjiang is relatively stable, and the stability of the Junggar Basin is most significant. To sum up, the regions with large fluctuations in evapotranspiration in Xinjiang are mainly located around the Yili River Valley and the Tarim Basin. The possible reason is that the vegetation in these areas has poor water conservation capacity, causing the interannual evapotranspiration to vary greatly.  
LULC also affected the spatial pattern of \(E\) and \(Ep\). As can be seen from Figure 11, the regions with strong fluctuations of \(E\) are mainly distributed in cultivated land, grassland and forest land in mountainous areas such as Tianshan mountains, Altai mountains, Qilian mountains and Yili river valley. The regions with low \(ET\) fluctuation are mainly in the margin of Tarim Basin in southern Xinjiang and Junggar Basin in northern Xinjiang, Meanwhile, the \(ET\) of agricultural land in oasis is higher than that of unused land at oasis edge. According to the changes of different land cover types (Figure 11C), cultivated land increased by 1.08%, grassland decreased by 0.66%, and woodland decreased by 0.08%. From the perspective of spatial differences, the difference in dynamic and thermal properties of different land covers leads to the redistribution of energy in the land-air interaction [46]. Secondly, due to artificial planting and irrigation, vegetation coverage and soil moisture in cultivated areas are higher than those in other areas, which makes evapotranspiration show significant spatial heterogeneity.