Freeze–thaw phase-dependent hydrothermal regimes across soil depths and contrasting landscape positions in an alpine basin

The annual freeze–thaw cycle regulates water availability and energy exchange in permafrost-affected alpine basins; however, its internal phase structure remains poorly constrained by field observations. We developed an operational four-phase framework using soil temperature records at depths of 40, 70, and 100 cm and characterized the resulting phases using synchronized volumetric water content (VWC) and soil heat flux observations from three contrasting sites in the Shaliu River Basin on the northeastern Qinghai-Tibet Plateau. Four fixed analysis years (October 2021–September 2025) covered three complete event-defined freeze–thaw cycles. The fully frozen phase occupied the longest portion of the cycle, with a median duration of 182 d, whereas the freezing advancement phase had a median duration of only 20 d. The mean freezing-associated zero-curtain duration was 14.9 d, more than twice the thawing-associated duration of 6.4 d, indicating a pronounced asymmetry in near-zero thermal persistence. VWC and soil heat flux also exhibited distinct phase-dependent responses. Deep-layer thermal peaks lagged behind shallow-layer peaks by more than 10 d in some years, while thermal amplitude attenuation from 40 to 100 cm reached approximately 40–55% at S1. The timing of freeze–thaw transitions and hydrothermal behavior varied substantially across the three sites. However, because the sites differed jointly in elevation, slope aspect, vegetation, soil properties, and topography, these contrasts are interpreted as site-level differences rather than as independent environmental effects. Overall, the framework provides an operational, field-based approach for identifying profile-scale freeze–thaw transitions and characterizing their associated hydrothermal responses across cold-region soils.