## 3 Results
### Drivers of WUE Variability  
Figure 9 shows the negative and positive contributions of human activity and climate change to WUE in Inner Mongolia from 2001 to 2020. The result shows that human activities are the main driving factor of WUE changes in Inner Mongolia from 2001 to 2020, with human activities and climate contributing to 53.52% and 46.46%, respectively. Human activity exhibited positive effects on WUE in approximately 70.34% of the pixels, dispersed in central Xilin Gol, Hohhot, Bugutu, eastern Bayan Nur, eastern Ordos, and eastern and western Hulun Buir. Nevertheless, human activities showed minor negative effects in approximately 29.66% of the pixels, mainly focused on the Da Hinggan Ling Mountains, Horqin sandy land, Otindag sandy land, and western Mu Us sandy land. Among them, areas with a negative contribution of less than 75% (2.45%) were scattered in the south foot of the Da Hinggan Ling Mountains. Areas with positive and negative contributions of climate change to WUE accounted for 68.20% and 31.80% of the total pixels, respectively. Areas with positive contributions are widely spread in the eastern and western foot of the Da Hinggan Ling Mountains, the Horqin sandy land, the Otindag sandy land, and the western regions. Areas with negative contribution were located in central Xilin Gol, northern Chifeng, northern Tongliao, and the Da Hinggan Ling Mountains. In particular, areas with negative contributions exceeding 75% accounted for 2.34% of the entire study area, mainly concentrated in the Da Hinggan Ling Mountains. Figure 10 shows the spatial pattern of sensitivity coefficients between WUE and climate factors in Inner Mongolia from 2001 to 2020. The results show that WUE is more sensitive to precipitation than other climatic variables. The sensitivity coefficient of WUE to precipitation was 0.345 \(\pm\) 0.325, with high sensitivity in the western region and low sensitivity in the Da Hinggan Ling Mountains. With positive sensitivity coefficient in most areas except northeast regions, with a value of 0.447, accounted for 85.88% of the areas, indicating that WUE increases with increasing precipitation. 14.12% of regions  
Figure 8: Spatial pattern of relative contributions of (**a**) GPP and (**b**) ET to WUE in Inner Mongolia from 2001 to 2020; (**c**) Spatial distributions of dominant controlling factors (i.e., GPP or ET). showed a negatively sensitive (\(-0.276\), Figure 10a) between WUE and precipitation, and it is primarily observed in the Da Hinggan Ling Mountains. According to Figure 10b, when precipitation is less than 400 mm, the WUE of Inner Mongolia increases with an increase of precipitation. However, when precipitation exceeds 400 mm, the sensitivity of the WUE to precipitation decreases. The temperature was the second most important climate factor influencing WUE and it mainly exhibited a negative sensitivity, with a value of \(-0.025\)\(\pm\) 0.217. Spatially, WUE was negatively sensitive to temperature in approximately 52.76% of regions (\(-0.189\), Figure 10c), these negative sensitivity regions existed in the central and eastern parts of Inner Mongolia, indicating that the increase of temperature would decrease WUE. Furthermore, the high negative sensitivity coefficients areas located in the Otindag sandy land. A total of 47.24% of regions showed a positive sensitivity (0.159, Figure 10c) between WUE and temperature, which was mainly distributed in the northeast, southeast, and southwest part of the region. In addition, the sensitivity coefficient is predominantly positive when the temperature > 6 \({}^{\circ}\)C (Figure 10d), indicating that temperatures above 6 \({}^{\circ}\)C are advantageous for the exchange of water and carbon in vegetation, and it is predominantly negative when the temperature < 6 \({}^{\circ}\)C, indicating that temperatures below 6 \({}^{\circ}\)C are unfavorable for the exchange of water and carbon. The mostly positive sensitivity of WUE to solar radiation, with a value of 0.019 \(\pm\) 0.231.