## 4 Discussion
### Response Mechanism of WUE to Human Activities and Climate Change  
This study found that human activities are the primary drivers of WUE changes in Inner Mongolia, and their positive effects are stronger than their negative effects, which is consistent with previous findings [5; 15]. Human activities induce an increase of WUE by increasing vegetation greenness through altered land cover [5]. The Chinese government has been implementing several large-scale ecological restoration projects since 2000, including the Grain for Green Project (since 1999), the Three Norths Shelter Forest Program (since 1978), and Grassland Ecological Protection Subsidy and Reward Policies (since 2010),all with the goal of restoring degraded ecosystem services by increasing vegetation greenness [32; 65; 66]. Driven by ecological restoration policies, positive changes have occurred in the forms of land cover. Figure 11 shows the Sankey diagram of land use transformations in Inner Mongolia from 2000 to 2020, the most dramatic transfer of land use mainly occurred in high-coverage grassland, medium-coverage grassland, and cropland, followed by forest, low-coverage grassland, and unused land. In particular, high-coverage grassland was largely converted into forest, medium-coverage grassland into high-coverage grassland, and cropland into high-coverage grassland. Furthermore, the transitions from forest to high-coverage grassland, low-coverage grassland to high-coverage grassland, and unused land to medium-coverage and high-coverage grassland were the most extensive. In general, high-coverage grassland and forest areas have increased in Inner Mongolia, and sandy land has decreased. Although the area of low-coverage grassland has slightly expanded, it is still much smaller than the increase of the area of high-coverage grassland and forest (Table S1). Therefore, the positive effects of human activities exceed the negative effects. Nevertheless, human activities have also negatively affected WUE changes in the Da Hinggan Ling Mountains in the northeast, the Horqin sandy land, and the western part of the Mu Us sandy land. The degree of land use dynamics for individual land use types confirmed that urban development and built-up land expansion are relatively serious in Hulun Buir, Xingan, Tongliao, Chifeng, and Xilin Gol (Figures 12 and S2), which has greatly reduced the green area of the land surface, leading to decreases in WUE [67]. Therefore, environmental protection and rational land use planning should be strengthened in these areas.  
The influence of climate factors on WUE is caused by changes in GPP and ET [15; 68]. Precipitation was found to be the dominant climate factor regulating WUE variability in Inner Mongolia. The sensitivity between WUE and precipitation was mostly positive, indicating that the increase of precipitation promotes the increase of vegetation WUE, which agrees with the previous findings [21; 69]. Located in arid and semi-arid regions, vegetation growth in Inner Mongolia is primarily limited by insufficient water supply, and precipitation is an important source of water for vegetation growth [21]. An increase of precipitation would increase GPP beyond the increase of ET, thus resulting in an increase of  
Figure 11: Sankey maps of land use transformations in Inner Mongolia from 2000 to 2020.  
WUE [16]. Good consistency was found in the sensitivity of WUE with solar radiation and temperature (Figure 10b,c). Negative sensitivity coefficients were observed in the Xilin Gol and the Da Hinggan Ling Mountains. A decline of WUE in these regions may be attributed to higher temperatures and increased solar radiation, leading to an increase of ET greater than of GPP [59; 70]. Furthermore, the decline of WUE in Xilin Gol could be attributed to the negative effects of climate change, while the decline of WUE in the Da Hinggan Ling Mountains could be attributed to the common inhibition effects of human activity and climate change.