## 3 Results and Analysis
### Correlation Analysis
Significant correlations are observed at specific soil moisture depths for EVI and GPP in arid and semi-humid regions, while such correlations are non-significant in the humid region. The correlation between EVI, GPP, and soil moisture across different land cover types from 2001 to 2020 is depicted in Figure 10. For croplands, the correlation coefficients between EVI and soil moisture layers SM1, SM2, SM3, and SM4 are \(-\)0.02, 0.01, \(-\)0.08, and -0.71, respectively, whereas for GPP, they are 0.04, 0.08, \(-\)0.01, and \(-\)0.61, respectively. Notably, SM4 exhibits a significant negative correlation with both EVI and GPP, indicating a decline in these values as SM4 increases (Figure 10a). For forests, the correlation coefficients for EVI with SM1, SM2, SM3, and SM4 are 0.13, 0.14, 0.10, and \(-\)0.30, respectively, and for GPP, they are 0.21, 0.23, 0.19, and \(-\)0.17, respectively, none of which are statistically significant (Figure 10b). For shrubs, the correlation coefficients for EVI with SM1, SM2, SM3, and SM4 are \(-\)0.01, 0.05, 0.04, and \(-\)0.27, respectively, and for GPP, they are 0.03, 0.08, 0.07, and \(-\)0.19, respectively, all of which are non-significant (Figure 10c). Grasslands show correlation coefficients for EVI with SM1, SM2, SM3, and SM4 of \(-\)0.11, 0.09, 0.14, and -0.36, respectively, none of which are significant. The corresponding GPP coefficients are -0.18, 0.02, 0.08, and \(-\)0.40, respectively, with SM4 showing a significant negative correlation, suggesting a decrease in GPP as SM4 increases (Figure 10d). Wetlands exhibit positive correlations for EVI with SM1, SM2, SM3, and SM4 of 0.23, 0.24, 0.22, and 0.16, respectively, though none are statistically significant. The GPP coefficients for SM1, SM2, SM3, and SM4 are 0.45, 0.48, 0.46, and 0.38, respectively, all of which are positively correlated and statistically significant, indicating an increase in GPP with increasing soil moisture (Figure 10e). SM4 significantly impacts GPP in cropland and grassland ecosystems, with GPP decreasing as soil moisture increases; conversely, GPP in wetlands significantly increases with soil moisture. Other vegetation types show no significant correlation with soil moisture. Figure 10: Correlation between different land cover types and soil moisture (**notes:** (**a**): correlation between EVI and GPP with soil moisture in croplands; (**b**): correlation between EVI and GPP with soil moisture in shrinks; (**d**): correlation between EVI and GPP with soil moisture in grasslands; (**e**): correlation between EVI and GPP with soil moisture in wetlands. “*” indicates significant correlation (\(p<0.05\)); “***” indicates extremely significant correlation (\(p<0.01\)). Figure 9: Correlation between EVI, GPP and soil moisture in different climate zones (**notes:** (**a**): the correlation of EVI and GPP with soil moisture in China; (**b**): correlation between EVI and GPP with soil moisture in arid region; (**c**): correlation between EVI and GPP with soil moisture in semi-arid region; (**d**): correlation between EVI and GPP with soil moisture in sub-humid region; (**e**): correlation between EVI and GPP with soil moisture in humid region. “***” indicates extremely significant correlation (\(p<0.01\))).