IDENTIFICATION OF HEAT-TRANSFER PROCESSES DURING SOIL COOLING, FREEZING, AND THAW IN CENTRAL ALASKA

Hourly time series of temperature and a surrogate measure of the soil water ionic (solute) concentration were obtained in the upper 50 cm of soil from two adjacent sites in central Alaska from August 1991 to May 1992. One site is located in a ground water seepage zone and is characterized by seasonal freezing of the upper soil. The other site is located over a body of permafrost. The time series, used simultaneously, display a unique signature diagnostic of the operative conductive and non-conductive heat-transfer process. During soil cooling in autumn, internal distillation (concurrent evaporation, vertical transport, and condensation of water vapour in soil column) moderates heat transfer in the active layer. A 'zero curtain' develops in winter and persists for three months; maintenance of this isothermal layer is associated with water advection and internal distillation within the unfrozen zone. Conductive heat transfer is not possible in the isothermal layer during this period but dominates during late winter following soil freezing and closure of the zero curtain. Warming of the active layer in spring occurs nearly instantaneously and results from infiltration of snow cover meltwater and downward vapour migration. In contrast, water saturation of pore space at the seepage site inhibits vertical migration of vapour and water.