## 3 Result
### Seasonal Variation Characteristics of WUE Rates in the Loess Plateau  
The spatial distribution of the interannual variation of the WUE rates in the Loess Plateau from 2001 to 2020 is shown in Figure 6. The WUE of the entire Loess Plateau is increasing at the rate of 0.02 g C mm\({}^{-1}\) H\({}_{2}\)O per year, and the areas with increasing trends account for 93.36% of the Loess Plateau (Figure 6a). While the increasing areas are mainly distributed in the southern and central parts of the Loess Plateau, the decreasing areas are mainly distributed in the northwestern part of the Loess Plateau. In terms of the seasonal distribution, the WUE changes are slowest in spring, increasing at 0.01g C mm\({}^{-1}\) H\({}_{2}\)O per year (Figure 6b), with 12.63% of the area showing a decreasing trend, especially in the western part of the Loess Plateau in Qinghai Province. The summer WUE increased at a rate of 0.03 g C mm\({}^{-1}\) H\({}_{2}\)O per year, which was the fastest growth season of the year, and 18.94% of the regions showed an annual increase of more than 0.06 g C mm\({}^{-1}\) H\({}_{2}\)O (Figure 6c) but showed a more obvious decreasing trend in the southern part of the Loess Plateau. Autumn showed a similar distribution pattern as summer, with 86.25% of the area showing an increasing trend (Figure 6d). Figure 3: Spatial and temporal variations of NDVI values in the Loess Plateau during 2001–2020: (**a**) spatial distribution of annual mean NDVI values; (**b**) distribution of spatial characteristics of NDVI variation trends; (**c**) annual mean NDVI values for different land cover types; (**d**) statistics of NDVI variation trends for different land cover types. Figure 4: Spatial distribution characteristics of WUE rates annual (**a**) spring (**b**) summer (**c**) autumn (**d**) in the Loess Plateau from 2001 to 2020.The variation pattern of the multi-year average WUE rates with the drought index rates for different vegetation types is shown in Figure 5. The data are averaged over the entire Loess Plateau region. From the figure, it can be seen that the WUE rates of the different vegetation types in descending order were forest > scrub > crop > grassland, with forest areas reaching 1.72 g C mm\({}^{-1}\) H\({}_{2}\)O (Figure 5a). This paper also found that the WUE rates of all vegetation types decreased with the increase in CWSI, while the response rates of the WUE for each vegetation type to the increase in CWSI were ranked as grassland > crop > forest > shrub (Figure 5a). The rate of decline for the grassland WUE with the increase in CWSI was the fastest, indicating that the grassland WUE was the most sensitive to the changes in drought patterns. From the change pattern of the WUE rates with the drought index rates in different seasons, it can be seen that summer had the same response pattern as the whole-year pattern, while crop was the most sensitive to drought change responses in spring (Figure 5b); forest was the most sensitive to drought change responses in autumn (Figure 5d); and in both spring and autumn, grassland showed lower sensitivity to drought index changes (Figure 5b,d). The variation curves of interannual WUE trends with the drought index rates for different vegetation types are shown in Figure 7. For the entire Loess Plateau, although the WUE rates of different vegetation types showed an increasing trend in the CWSI in the interval of 0.2-0.5, there are still large differences in the variation patterns of WUE growth rates with the drought index values for the different vegetation types. In order to consider the completeness of image information, the interval of CWSI in this paper starts from 0. However, when CWSI is less than 0.3, no artificial vegetation is subjected to water stress, so the analysis results are based on CWSI of 0.3 as the starting point.