Thawing and freezing processes of active layer in Wudaoliang region of Tibetan Plateau

The interaction between permafrost and atmosphere is accomplished through t ransfer of heat and moisture in the overlay active layer. Thus, the researc h on the thermal and hydrodynamics of active layer during the thawing and f reezing processes was considered a key to revealing the heat and moisture e xchanges between permafrost and atmosphere. The monitoring and research on active layer were conducted because permafrost occupies about two thirds of the total area of the Tibetan Plateau. Based on the analysis of the ground temperature data and soil moisture data of monitoring near the Wudaoliang region of the Tibetan Plateau, the thawing and freezing processes of active layer were divided into four stages, i.e. summer thawing stage (ST), autum n freezing stage (AF), winter cooling stage (WC) and spring warming stage ( SW). Coupled heat and water flow is much more complicated in ST and AF, and more amount of water is migrating in these two stages. Heat is transferred mainly via conductive heat flow in the other two stages, and less water mi grated. Four water migration and coupled heat flow processes were addressed for the thawing and freezing stages, which are water infiltration driven b y gravity, moisture advection and distillation driven by temperature and os motic gradients, water migration driven by capillarity and unfrozen water m igration driven by temperature gradient. The water content near the permafr ost table tends to increase after one thawing and freezing cycle, which is the main reason for the development of thick ground ice layer near permafro st table.