## 3 Results and Analysis
### Correlation Analysis  
The spatial distribution of correlation coefficients between EVI, GPP, and soil moisture across China from 2001 to 2020 is illustrated in Figure 8. The correlation patterns between EVI and GPP with soil moisture are spatially similar, showing predominantly negative correlations in the lower reaches of the Haihe, Huaihe, and Yellow River basins, while positive correlations are mainly observed in the Pearl River basin, the Tibetan Plateau, and eastern Inner Mongolia. The areas with positive correlations between EVI and soil moisture at depths SM1, SM2, SM3, and SM4 account for 67.84%, 69.97%, 66.66%, and 54.47%, respectively, whereas the areas with negative correlations are 32.16%, 30.03%, 33.34%, and 45.53% (Figure 8a-d). For GPP, the areas with positive correlations with soil moisture at the same depths are 67.09%, 66.79%, 65.14%, and 52.70%, with negative correlation areas being 32.91%, 33.21%, 34.86%, and 47.30% (Figure 8e-h). Overall, the correlations between EVI and GPP with soil moisture exhibit significant regional and depth variations, with positive correlation areas generally exceeding those of negative correlation. Figure 8: Spatial distribution of correlation coefficients between EVI, GPP and soil moisture (**notes:**(**a–d**): spatial distribution of correlation between EVI and SM1, SM2, SM3 and SM4; (**e–h**): spatial distribution of correlation between GPP and SM1, SM2, SM3 and SM4). The correlation between the Enhanced Vegetation Index (EVI), Gross Primary Productivity (GPP), and soil moisture across different climatic zones in China from 2001 to 2020 is illustrated in Figure 9. At the national scale, the correlation coefficients between EVI and soil moisture at depths SM1, SM2, SM3, and SM4 are \(-\)0.09, 0.12, \(-\)0.02, and \(-\)0.46, respectively. Notably, a significant negative correlation is observed between SM4 and EVI, indicating that EVI values decrease as SM4 increases. The correlation coefficients between GPP and soil moisture at depths SM1, SM2, SM3, and SM4 are \(-\)0.38, 0.03, 0.15, and \(-\)0.07, respectively, none of which are statistically significant (Figure 9a). In arid regions, the correlation coefficients between EVI and soil moisture at depths SM1, SM2, SM3, and SM4 are 0.05, 0.46, \(-\)0.07, and 0.33, respectively. A significant positive correlation exists between SM2 and EVI, indicating that EVI values increase with increasing soil moisture at this depth. For GPP, the correlation coefficients with soil moisture at depths SM1, SM2, SM3, and SM4 are 0.22, 0.48, \(-\)0.09, and 0.46, respectively, with significant positive correlations for SM2 and SM4, suggesting that GPP values increase as soil moisture at these depths increases (Figure 9b). In semi-arid regions, the correlation coefficients of EVI with soil moisture at depths SM1, SM2, SM3, and SM4 are 0.09, 0.22, 0.30, and \(-\)0.00, respectively, and for GPP, they are 0.08, 0.22, 0.30, and \(-\)0.05, respectively. None of these correlations are statistically significant (Figure 9c). In semi-humid regions, the correlation coefficients of EVI with soil moisture at depths SM1, SM2, SM3, and SM4 are \(-\)0.25, \(-\)0.20, \(-\)0.23, and \(-\)0.80, respectively, all of which are negative. A significant negative correlation is observed for SM4, indicating that EVI values decrease as soil moisture at this depth increases. For GPP, the correlation coefficients with soil moisture at depths SM1, SM2, SM3, and SM4 are \(-\)0.21, \(-\)0.16, \(-\)0.22, and \(-\)0.80, respectively, with a significant negative correlation for SM4, showing that GPP values decrease as soil moisture at this depth increases (Figure 9d). In humid regions, the correlation coefficients of EVI with soil moisture at depths SM1, SM2, SM3, and SM4 are 0.28, 0.28, 0.26, and 0.08, respectively, and for GPP, they are 0.37, 0.37, 0.23, and 0.23, respectively. None of these correlations are statistically significant (Figure 9e). Overall, the correlations between EVI and GPP with soil moisture across different climatic regions exhibit distinct differences.