## 4 Discussion  
### Seasonal Variation Characteristics of eWUE  
The findings of this research indicate that there are discernible distinctions to be found in the patterns of the geographical distribution of eWUE between seasons. It is possible that this is due to the fact that various types of flora make use of water in different ways at different times of the year. For instance, the distribution ranges of DNF and GRA both exhibited increased WUE during the spring, but CRO and SA both showed lower WUE over the same time period. In the summer, the distribution ranges of CRO and SA showed lower WUE, whereas the distribution ranges of GRA and DBF showed greater WUE. This indicates that in the summer, when there is considerable evaporation, grassland with poor carbon fixation is more likely to be found. Instead, it may adjust to its surroundings by modifying its tactics for water usage and become more efficient in its water use. Conversely, MF with higher carbon fixation was less efficient in water use. In autumn, the distribution range of DNF showed higher WUE, while GRA showed lower WUE. The vegetation eWUE of DNF in different seasons in the study area was higher than that of DBF, which was inconsistent with the conclusions drawn by other researchers [33]. This may have been caused by different vegetation site conditions caused by differences in soil, climate, hydrology and other environmental factors [34]. Compared with coniferous forests, DBF has a stronger canopy interception capacity, which makes its evapotranspiration higher, so the eWUE of DBF is lower [35]. In addition, the eWUE of DBF is also slightly lower than that of GRA, which is mainly due to the low stomatal conductance of forest vegetation, and thus, the low photosynthetic rate [36]. At the same time, forests in Inner Mongolia are mainly distributed in high-altitude areas where the annual average temperature is lower than zero, and a low-temperature environment is not conducive to the photosynthesis of vegetation. In addition, typical shrubs (sea buckthorn, caragana, etc.) have small leaves and low chlorophyll content, so the remote sensing signal cannot truly reflect the vegetation coverage of low shrubs, and GPP remote sensing products have a certain degree of underestimation [37; 38]. This is also one of the reasons for the lower eWUE of shrubs. The differences in eWUE caused by natural vegetation types in different seasons may be related to the heterogeneity of the vegetation community structure in the ecosystem, the water use strategies of different vegetation types and the limitations of geobotanical factors [39; 40; 41].  
In terms of trend, eWUE in different seasons in Inner Mongolia generally showed a downward trend, but eWUE in some areas showed an increasing trend, and the downward trend in summer was significantly higher than that in spring and autumn, indicating that there were differences in vegetation water use strategies and capacities in different growth stages. The eWUE showed a reverse trend in the eastern and western regions, indicating that the western arid region will greatly promote the increase in surface evapotranspiration under the condition of more precipitation in summer, which will lead to the decline in eWUE, while the eastern region will be affected by increased precipitation. The favorable climatic conditions of increasing temperature increased the photosynthesis of vegetation, and the increase in GPP was greater than that in ET, thus, effectively promoting the significant increase in eWUE.