# Water Use Efficiency Spatiotemporal Change and Its Driving Analysis on the Mongolian Plateau
## 4 Discussion  
In this study, we analyzed the linear trends in WUE and the trends and alterations in the WUE\({}_{\text{Season}}\). We found that the trend in WUE over the whole year was significantly reduced; however, the trend in the WUE\({}_{\text{Season}}\) was significantly enhanced because NPP weakly contributed to WUE in the non-growing season. Simultaneously, the ET during the non-growing season decreased the overall WUE for the entire year. Spatial differentiation is important in analyzing the effects of ET and NPP on WUE. For example, if an area with significant ET growth also exhibits significant NPP growth, the change in WUE depends on the proportional growth rates of NPP and ET. However, sometimes, the NPP decreases significantly in areas with significant growth in ET, which leads to a significant reduction in WUE; this has already occurred in the meadow grasslands in Northeastern Mongolia (Figure 4b,d,f). The results of the breakpoint analysis showed that the spatial distribution of the sudden increasing trend in WUE\({}_{\text{Season}}\) was not the same over the past 37 years. The  
Figure 10: (**a**) shows the 37-year average value of WUE\({}_{\text{Residual}}\) in the Mongolian Plateau; (**b**) shows the spatial distribution of the WUE\({}_{\text{Residual}}\) trend from 1982 to 2018.  
two alterations that occurred between 1997 and 2009 were mainly distributed in Eastern Mongolia and Central Inner Mongolia. During this period, the WUE Season increased significantly, and these regions overlapped with the WUE, PRE, and especially the DSI regions, which were significantly and positively related. This shows that the steep increase in WUE Season before 2009 may have been related to changes in water content. An extremely significant increase from 2010 to 2018 was observed in the central and southern regions of Inner Mongolia, where more artificial vegetation was distributed; these regions overlapped with regions where WUE Resideal significantly increased (Figure 10b). Human activities inevitably interfere with the growth of vegetation [64; 77] and even indirectly interfere with ET values [66] owing to changes in vegetation transpiration, which has been confirmed in previous studies. The results show that human activity may lead to an increase in WUE in forests and a decrease in WUE in grasslands. Moreover, as shown by the results of the annual WUE trend analysis, the most significant changes were observed in agricultural land (with more artificial vegetation) and desert vegetation in Central and Southern Inner Mongolia [30]. This shows that human activities have greatly contributed to the sharp increase in WUE Season after 2010. The results of the trend change point detection showed that, except from 1989 to 1993, the trend in the WUE of the Mongolian Plateau changed in most years. In addition, the broadleaf forest areas in Northeastern and Eastern Inner Mongolia reversed from an original decreasing trend to an increasing trend. The analysis of WUE trends at different spatiotemporal scales will lead to different information. Among the meteorological factors, changes in water significantly affected WUE, whereas the effect of the change in temperature was not significant. This is consistent with the results of previous studies [30]. An increase in environmental humidity also significantly improves WUE, mainly in grassland ecosystems. The DSI had the most significant positive contribution to WUE, indicating that drought greatly affects WUE. In addition, an increase in the DSI will have an insignificant effect on wetter forest areas. In contrast to the Pearson correlation analysis results for the interannual WUE and meteorological factors, WUE had strong resistance to abnormal water disturbances but weak resistance to anomalous temperature disturbances. In addition, the positive and negative contributions of TEM to WUE were strongly influenced by the latitude. Forest areas at high latitudes were usually positively correlated, whereas grasslands at low latitudes were negatively correlated.