Holistic assessment of seasonally frozen ground changes on the Qinghai-Tibet Plateau

The seasonally frozen ground (SFG) on the Qinghai-Tibet Plateau (QTP) acts as a critical regulator of regional hydrology and ecosystem stability, yet its elevational dynamics remain inadequately quantified at large scales. In particular, the impacts of snowmelt on the freeze–thaw processes need further characterization. To address this gap, we developed a sinusoidal heat transfer model that links soil thermal diffusivity (as functions of soil moisture via the Gaussian fitting in different soil types) with surface temperature forcing, calibrated using in situ measurements from 58 meteorological stations. The model achieved an acceptable calibration performance for the maximum freezing depth (MFD) and indicated a mean first date of soil freeze (FDSF) delay of 0.37 day/yr, a mean MFD decline of 2.07 cm/yr, and shortening of both freezing duration (FDR) and freeze–thaw duration (FTDR) over 1980–2018. Compared with low-elevation areas, high-elevation regions generally exhibit an earlier FDSF, a deeper MFD, and longer FDR and FTDR, primarily driven by lower temperatures. The elevation-dependent effects of snowpack further modulate the freeze–thaw dynamics: at higher elevations, thinning snow reduces its insulating capacity, which partly counteracts the warming-induced delay in FDSF and dampens the sensitivity of freeze onset to temperature increases. These findings confirm that temperature and snowpack jointly control the SFG elevational dependence and underscore that our model can bridge observational gaps, offering improved assessments of freeze–thaw shifts and associated hydrological impacts in high-altitude regions.