# Water Use Efficiency Spatiotemporal Change and Its Driving Analysis on the Mongolian Plateau
## 3 Results
### Potential Factors Affecting WUE Changes  
2.1 Correlations Between WUE and Meteorological Factors (PRE, TEM, SW, and DSI) on the Mongolian Plateau  
We estimated the Hurst index of the annual WUE data for the Mongolian Plateau (Figure 6a). Areas with a Hurst index less than 0.5, greater than 0.5, and equal to 0.5 accounted for 92.78%, 6.22%, and 0%, respectively. This suggests that most areas on the Mongolian Plateau may experience a reversal in their WUE trends in the future. Before the change point, 42.5% of pixels showed a decline and 57.4% showed an increase. After the change point, 34.95% showed a decline and 65.05% showed an increase, indicating that the WUE reversed from a declining trend to an increasing one in 7.65% of the pixels, mainly in broadleaved forests in Northeastern and Central Inner Mongolia (Figure 6b). As shown in Figure 7a, 88.47% of the pixels of PRE and WUE showed a positive correlation, 36.22% of which were significantly correlated (\(p<0.05\)). These pixels were mainly distributed in Eastern and Northeastern Inner Mongolia. This indicated that an increase in precipitation contributed to an increase in WUE. In total, 77.22% of the pixels of PRE and WUE showed a negative correlation, and only 9.58% of these correlations were significant (\(p<0.05\)). These pixels were mainly distributed in the broadleaf forest areas of Eastern and Northeastern Inner Mongolia. TEM and WUE were generally negatively correlated; areas with this negative correlation were mainly distributed in the middle and  
Figure 5: (**a**) Change points of WUE in the Mongolian Plateau; (**b**) trends before and (**c**) trends after the change points, respectively. east of Mongolia and the northeast and middle of Inner Mongolia, but their significance was not high (Figure 7b). Pixels were distributed in the meadow steppe belt in Eastern Mongolia and in the central and southern areas of Inner Mongolia; however, only 4.62% of these pixels were significant (\(p<0.05\)). A total of 85.47% of the pixels of SW and WUE showed a positive correlation, of which 35.28% were significant (\(p<0.05\)). These pixels were mainly distributed in the meadow steppe belt in Eastern Mongolia and Eastern and Southeastern Inner Mongolia (Figure 7c). This part of the region overlaps with areas with a significant positive correlation between PRE and WUE and with areas with a negative correlation  
Figure 6: Hurst index: (**a**) WUE and (**b**) future WUE trends in the Mongolian Plateau from 1982 to 2018. Figure 7: Partial correlation analysis between WUE and meteorological factors, where (**a–d**) show the partial correlation coefficients and significance analysis of PRE, TEM, SW, and DSI with WUE, respectively. Note: Ns indicates a significant negative correlation, Nn indicates a nonsignificant negative correlation, Pn indicates a nonsignificant positive correlation, and Ps indicates a significant positive correlation. between TEM and WUE. This indicates that water is an important factor driving the change in WUE in this area, and that increases in temperature and water consumption will lead to a decrease in WUE. Therefore, in the arid and semiarid areas of the Mongolian Plateau, water is very important for changes in WUE. Water loss led to a decrease in NPP, and a change in temperature led to an increase in ET, which in turn led to a decrease in WUE. On the Mongolian Plateau, 93.57% of the pixels of DSI and WUE showed a negative correlation; 53.03% of these correlations were significant (\(p<0.05\)) and they were mainly distributed in the central and eastern parts of Mongolia and most areas of Inner Mongolia, except for the northeast (Figure 7d). This shows that drought caused by water loss is the main factor in reducing the WUE and that moisture is helpful in increasing the WUE. However, there was a negative correlation between DSI and WUE in the forested areas of Northeastern Inner Mongolia, which may have been caused by significant growth in ET in this area. Phenological vegetative parameters are important indicators of vegetative growth.