Soil Hydrothermal Response to Seasonal Freeze–Thaw Processes in Low-Water-Content Sandy Gravel Deposits

Seasonal freeze–thaw processes affect soil hydrothermal conditions in high-altitude valleys, yet evaluations based only on air temperature or maximum freezing depth may overlook the distinction between surface-connected freezing and delayed thawing within the soil profile. Meteorological conditions, ground surface temperature (GST), ground-temperature profiles, freezing depth, and volumetric water content (VWC) were continuously monitored in an arid valley on the Qinghai–Tibet Plateau. Mean annual GST was 3.23 °C higher than mean annual air temperature, and the freezing and thawing n-factors were 0.72 and 1.54, respectively, indicating weakened cold accumulation and enhanced heat accumulation at the ground surface. The maximum surface-connected freezing depth reached 3.30 m, whereas ground temperatures at 3.5 m and below remained above 0 °C. During spring thawing, a residual frozen layer persisted for 49 days after the shallow layer had thawed, with a maximum thickness of 3.24 m. GST-based freezing degree days represented freezing depth better than air-temperature-based freezing degree days. Soil VWC remained low, and precipitation responses were mainly confined to 0.2 m depth. These findings reveal a thermally dominated freeze–thaw regime with weak deep moisture response and show that distinguishing surface-connected freezing from residual frozen layers improves hydrothermal-state identification in low-water-content sandy gravel deposits.