Knowledge of the amount of unfrozen water and its migration in permafrost soils is important for understanding physico-chemical and biological processes. Here, we developed sub-routines in FREZCHEM and embedded them in the WATEREGO2 soil environmental model to: (a) estimate unfrozen water content under changing soil temperatures and water-ice phase changes; and (b) determine the effects of Van der Waals (VdW) and rheological forces driven by seasonal temperature variations on the transport of residual water and the long-term evolution of ground ice content over depths of 30 m. Together, the seasonal thermal regime and associated VdW and rheological forces on the transport of residual water lead to the evolution of distinct zones of ice-enrichment: near the surface of permafrost, at 3-5 m, 11-13 m and 17-19 m depth. The depths of ice enrichment are a function of soil thermal diffusivity, and the time needed to evolve the ground ice content is dependent on soil type, soil water chemistry and permafrost temperature. The model can explain observed variations with depth in ground ice content of icy permafrost soils and indicate that these conditions evolve over time. The findings can be used to assess the stability of permafrost to climate change under different temperature scenarios.

There is an obvious trend of climate warming and wetting on the Qinghai-Tibet Plateau during the past fifty years. Climate changes in air temperature or precipitation will inevitably influence the stability of permafrost. Previous studies mainly focus on the thermal influence of climate warming, but little is known about the induced rainfall infiltration and the hydrothermal response mechanism. Based on the meteorological data observed at Beiluhe observation station during 2013, the established water-vapor-heat transport model is used to predict the response under 1 degrees C and 2 degrees C increment of temperature, which considering the influences of rainfall. Climate change influences the thermal-moisture of permafrost mainly by changing the surface energy budget and soil hydrothermal transport components. The results show that climate warming greatly increased the surface net radiation, latent heat of evaporation and soil heat flux, decreased the sensible heat and rainfall infiltration. The rising air temperature reduces the soil moisture and soil hydraulic conductivity. Temperature gradient increases dramatically with temperature arising, further increases the moisture and energy components and reduces the components related to the water potential gradient. Climate warming increases the surface evaporation and thickness of active layer and accelerates the degradation of permafrost, which is contrary to the thermal effects of rainfall increasing.