FEEDBACK BETWEEN SUBSURFACE WATER FLOW AND HEAT TRANSFER IN A SUBARCTIC PERMAFROST PEATLAND

Peatlands cover a large portion of the northern North America. Much of Canadian peatlands occur in the discontinuous permafrost regions of the Subarctic and low Arctic, where climate warming is expected to have pronounced effects on the hydrology and ecology. Peatlands in the Subarctic region of the Mackenzie River Basin are characterized by a mosaic of forested uplands (peat plateau) underlain by relatively impermeable permafrost, and wetlands (bog and fen) serving as conduits of surface water and groundwater. Historic aerial photographs and satellite images indicate that peat plateaus are shrinking and wetlands are expanding in some areas, which may be a response of the system to climate warming. In order to understand the interaction between climate and hydrology in this region, a field study has been conducted since 1999 at the Scotty Creek watershed near Fort Simpson, Northwest Territories. The thickness of the active layer under peat plateaus is typically less than one meter, and the shallow groundwater in the active layer flows horizontally because the top of the frozen peat (i.e. frost table) is an impermeable boundary. As the frost table descends during thaw periods, the majority of flow in the active layer is transmitted through a relatively thin saturated layer on top of the frost table. The permeability of the peat is very high near the ground surface and decreases by orders of magnitude with depth. As a result, the depth to the frost table is a critical factor controlling the drainage of peat plateaus. The descending rate of the frost table (i.e. thawing rate) is essentially controlled by heat conduction from the surface, which is strongly dependent on water content because the peat has very high porosity (> 0.7). Therefore, a strong feedback mechanism exists between the subsurface water flow and heat conduction. Understanding this feedback is critical for predicting the effects of climate warming on peatland patterns and hydrological processes in the Subarctic region.