## 4 Discussion  
The relationship between the SM and EF is important for research on land-atmosphere coupling. It reflects strong land-atmosphere coupling in the climate transition zones, representing the distributions of water and energy when land surfaces interact with the atmosphere. The results of this study show that synergistic variations in the SM and EF had significant impacts on the regional weather and climate, and this has been confirmed by numerous past studies [7; 33; 34; 35]. We investigated the relationships of land-atmosphere coupling between the SM and EF, GPP and EF, LAI and EF, and NDVI and EF in the climate transition zone of China. The results showed that the latter three relations (GPP\(-\)EF, LAI\(-\)EF, and NDVI\(-\)EF) did not exhibit strong coupling compared with their values in other regions of the world. This can be attributed to the relatively low annual precipitation in the Chinese climate transition zone and the dominance of desert grasslands as the primary type of vegetation that led to less vigorous transpiration. Moreover, the weak vegetation transpiration led to the lack of vigorous vegetation growth and the low values of GPP, LAI, and NDVI. Furthermore (Figure 9), the dry soil and strong incident radiation resulted in the surface precipitation being returned to the atmosphere through the evaporation process in the climate transition zones. Only a small amount of precipitation was absorbed by vegetation, which was not enough to meet the needs of sufficient vegetation growth [9]. Overall, the relationship between the EF and SM, as recorded at the SACOL station, exhibited strong coupling that was characterized by non\(-\)linear and non\(-\)monotonic variations. This result is evident not only from the third\(-\)order equations of polynomial fitting of the two variables but also from the segmented relationship of fitting between the EF and the SMP. The turning point in the relationship between the EF and SMP occurred within the SM range of 0.08-0.11 m\({}^{3}\) m\({}^{-3}\) and was relatively close to the wilting coefficient of soil. The SM-EF relationship recorded at the SACOL station exhibited a stronger coupling-based correlation than in regions with a humid climate and did not show decoupling when the SM approached or reached saturation, as reported by [PERSON] et al. in 2015 [9]. This was because the soil moisture values in the transitional climate zone are relatively small and cannot reach a sufficiently moist state (>0.3 m\({}^{3}\) m\({}^{-3}\)). Additionally, this characteristic of soil moisture can influence the land-atmosphere coupling relationship [32]. This indirectly indicates the characteristics of a low SM, high ecological vulnerability, and weak resilience against droughts in the area of the SACOL station, which is dominated by desert grasslands. Figure 8: Fitted normal probability distribution function of the SM in dry and wet years at (**a**) 0–40 cm and (**b**) 40–80 cm in the growing season from 2007 to 2012 (excluding 2008) over the transitional climate zone in China. (The dashed line represents the mean values for the different study periods). The coupling relationship between the SM and EF is a crucial parameterization in land surface models [7]. In the transitional climate zone, this strong coupling relationship can offer more options for simulating land-atmosphere exchanges processes. Moreover, this constraint relationship between the soil moisture and evaporative fraction, as reflected by SM\(-\)EF coupling, is utilized to analyze and assess the drought characteristics of a region [35]. The coupling relationship between the SM and EF is a crucial parameterization in land surface models [7]. This strong coupling relationship can provide more options for simulating land-atmosphere exchanges processes in the transitional climate zone. Furthermore, the constraint relationship between soil water moisture and surface fluxes reflected by SM\(-\)EF coupling was also used to analyze and assess the drought characteristics in a region [35].