Abstract: The thermal-moisture dynamics of the active layer directly affects the stability of permafrost and engineering projects in cold regions. Previous studies mainly focus on the thermal stability of permafrost based on the adherent layer theory. The migration process of liquid water and water vapor and its effects on the active layer are still lack of consideration. Considering the physical process and mechanism of liquid water-vapor transfer in unsaturated soils, a new heat and mass transfer model in saturated-unsaturated partially frozen soil was developed, in which the moisture migration in both vapor and liquid phases and heat transfer by means of conduction, convection and phase change were accommodated. The established water-vapor-heat transport model was used to analyze the water-vapor-heat transport in the shallow unsaturated zone of active layer under the in-situ meteorological conditions. The results show that the liquid water and water vapor are driven by the temperature gradient flow downward during daytime but upward at night, the liquid water and water vapor are driven by the temperature gradient flow upward in warm season but downward in cold season. Vapor water accounts for more than 15% of the water flux in the active layer and the water vapor driven by the pressure head can be neglected throughout the year. The moisture transport is mainly controlled by temperature gradient in sunny days. During and after rainfall events, rainfall infiltration is significantly enhanced and liquid water and water vapor mainly infiltrate downward. Rainfall can significantly decrease surface soil heat flux, heat conduction and soil temperature, which mitigate the permafrost degradation process.

Key words: unsaturated soil, active layer, water transport, heat transfer, vapor flow, evaporation