## 5 Conclusions  
The coupling relationship between SM and EF in the climate transition zone is an important aspect of research on land-atmosphere coupling because it directly reflects the exchange of energy and moisture between the land and the atmosphere. In this study, we applied observation data on fluxes obtained from the SACOL station in the climate transition zone of China to analyze the coupling relationship between SM and EF during the season of vegetation growth. We also explored the relationships between the GPP and EF, LAI and EF, and NDVI and EF that reflect the coupling between vegetation and the atmosphere.  
The results indicated that the indices representing the characteristics of vegetation growth, namely GPP, LAI, and NDVI, did not exhibit strong coupling relationships with the EF in the transition zone of China in the summer monsoon. This contrasts significantly with findings on the southern Great Plains of North America. Furthermore, both the components of surface energy and SM exhibited significant interannual variations. The components of surface energy showed the most pronounced interannual differences in July and August. Except for a few extremely wet years, the SM exhibited the greatest interannual differences  
Figure 9: A flowchart for analysis of the coupling relationship between SM and EF over China’s transitional climate zone.  
in August of each year in the study period. An analysis of the coupling relationship between SM and EF revealed that both exhibited strong third\(-\)order polynomial correlations, with R-squared values ranging from 0.44 to 0.51 in both the shallower layer (0-40 cm) and the deeper layer of soil (40-80 cm). Further analysis showed that when the SMP reached 35%, there was a significant transition in the linear relationship between the SMP and EF that was consistent between the shallower (0-40 cm) and deeper layers (40-80 cm) of soil.  
The SM in the shallower layer (0-40 cm) was higher than in the deeper layer (40-80 cm). Moreover, soils in the climate transition zone of China were characterized by high rates of infiltration of water. The responses of the deep SM to dry and wet years were more significant than those of the SM of the shallow layer. The findings of this study are expected to provide a valuable reference for optimizing the relationships of land-atmosphere coupling in land surface models of the zones of climate transition.  
Conceptualization, Q.Z.; methodology, L.Z. and Q.Z.; software, S.S.; validation, J.W. and Y.H.; formal analysis, L.Z.; investigation, F.Z., S.W. and H.H.; resources, H.L.; writing--original draft preparation, L.Z.; writing--review and editing, S.S.; visualization, J.Z. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Natural Science Foundation of China (grant Nos. 42230611, 41875020, and 42005097), the Meteorological Capacity Enhancement Joint Research Special Key Projects (23 NLITSZ008), and the Natural Science Foundation of Gansu Province (grant No. 22 JR5 RA748).  
The NDVI data are available at [[https://doi.org/10.5067/MODIS/MOD09](https://doi.org/10.5067/MODIS/MOD09) GA.006]([https://doi.org/10.5067/MODIS/MOD09](https://doi.org/10.5067/MODIS/MOD09) GA.006), accessed on 6 October 2023; and the LAI data are available at [[https://lpdaac.usgs.gov/products/mod15a2](https://lpdaac.usgs.gov/products/mod15a2) hv061/]([https://lpdaac.usgs.gov/products/mod15a2](https://lpdaac.usgs.gov/products/mod15a2) hv061/), accessed on 6 October 2023.  
The authors thank SACOL and NASA for the data support.  
The authors declare no conflict of interest.