Changes in near-surface freeze-thaw state and their impact on stream flow in the source region of the Yellow River

Based on soil temperature data at depths of 0-40 cm, we investigate the characteristics of soil temperature and freeze-thaw state dynamics in the Yellow River source region and their impacts on streamflow from 1982 to 2020. Using 1-km resolution gridded soil temperature data (0-40 cm depths) interpolated from station observations using a thin plate spline method, we analyzed long-term trends in soil temperature and associated changes in soil freezing states. In addition, we integrated meteorological variables (e.g., precipitation, air temperature, and evapotranspiration) and normalized difference vegetation index (NDVI) datasets to quantify the impact of soil freezing state variation on streamflow using random forest and Shapley Additive Explanations (SHAP). The results show a marked shift in the streamflow trend in the Yellow River source region: A sharp decline occurred during the earlier period (1982-2002), with an average annual reduction of 6.11×108 m3, followed by a notable increase during the later period (2002-2020), at an average rate of 6.02×108 m3 per year. Soil warming at 0-40 cm depths was accompanied by a substantial decrease in the frozen soil area across all layers and notable freezing depth shallowing in winter. Random forest modeling combined with SHAP interpretation revealed a nonlinear relationship between stratified soil freezing states and streamflow. Among the variables representing soil freezing states, the streamflow during 1982-2002 was mainly influenced by the frozen areas at 0 cm and 10 cm depths, with relative contributions of 8.3% and 7.2%, respectively. Meanwhile, during 2002-2020, the primary influencing factors shifted to the frozen areas at 0 and 40 cm, with relative contributions of 10.6% and 8.4%, respectively. Further, the interannual variations in streamflow were more sensitive to fluctuations in the surface soil freezing state (0-10 cm) than in the deeper freezing state (15-40 cm). This study elucidates the changes in near-surface soil temperature and associated freeze-thaw dynamics under climate warming and their effects on streamflow, providing key insights for hydrological process studies in cold regions.