## 3 Results and Discussion
### Analysis of Factors Affecting the Model's Estimation of Soil Temperature
It should be noted that a contrasting pattern was observed in the IM region, where the model estimating accuracy decreased with the increasing soil water content. This phenomenon might be attributed to the relatively low soil water content in this region. For most stations in the IM region, their soil water contents were lower than 10%, which was distinct from the other six regions in the study. Therefore, soil water content might not play an important role in determining the pattern of model estimating accuracy in the IM region, in which other factors such as soil organic matter might be more influential. RH was highly significantly correlated with the accuracy of model-estimated \(T_{s}\) in the CC region, which might be due to the fact that the region is in a humid area with high perennial air humidity (Figure 6c). When the relative humidity is high, there is considerable moisture in the air. Given the fact that the specific heat capacity of water is larger than air, the heat transferred by the air temperature and solar radiation to soils is partially absorbed by the moisture in the air, thus reaching the amount of heat to the surface of the soil. [PERSON] [83] also suggested that RH had a strong influence on air temperature variation and indirectly affected the heat absorbed by the soil. Solar radiation is one of the main sources of soil heat. In our study, \(R_{s}\) showed significant correlation with model accuracy for estimating \(T_{s}\) in the NWC and CC regions, which might be caused by the longer insolation time in the NWC region and the higher insolation intensity in the low-latitude CC region (Figure 6d). Solar radiation can be directed to the surface, allowing the surface to absorb heat and thus increase the \(T_{s}\). High latitudes have a high tilt of solar irradiation, so that less solar radiation energy is absorbed on the soil per unit area. This results in lower \(T_{s}\). In contrast, the \(T_{s}\) is higher at low latitudes because the solar radiation absorbed per unit area of soil is higher due to direct sunlight to the ground [62]. [PERSON] et al. [84] also confirmed the important role of solar radiation in the conversion of \(T_{s}\). Wind speed affects the flow of air. As shown Figure 6e, \(U_{2}\) was highly significantly correlated with the accuracy of model-estimated \(T_{s}\) in the NEC region, which might be due to the fact that the NEC is in a cold-temperate region with frequent cold air and high wind speed all year round. When the \(U_{2}\) is low, heat exchange between soil and air is reduced, thus reducing the variation in \(T_{s}\). Conversely, the heat loss from air is generally fast when \(U_{2}\) is high, resulting in significant variation in \(T_{s}\). [PERSON] et al. [85] studied the effect of seasonal variation in \(U_{2}\) on \(T_{s}\) and suggested that \(U_{2}\) affects the heat uptake by soils to some extent, thus causing changes in \(T_{s}\). We also explored the extent of the contribution of these factors to the estimating error of the model and the relationship between them through principal component analysis (PCA) and correlation coefficients (Figure 8). PCA was used to explain the degree of influence of the factors on the model error. As shown in Figure 8a, the total model error explained by each factor was 97.9%. Among them, PC1 accounted for 96.2% and PC2 accounted for 1.7%. Soil water content, \(T_{mean}\), \(R_{s}\), and \(U_{2}\) were all located in the same quadrant. These factors, illustrated in Figure 8a, are shown to be the main factors contributing to the error in estimating \(T_{s}\). [PERSON] [86], when he studied the effect of snow cover on \(T_{s}\), also proposed that air temperature alone does not fully explain the variation in \(T_{s}\). The \(R_{s}\), soil water content, and RH can also combine to influence the variation of \(T_{s}\), whereas soil organic matter content and \(U_{2}\) were additionally proposed to explain the \(T_{s}\) variation in this study.