THE ROLE OF INDIVIDUAL TERRAIN UNITS IN THE WATER-BALANCE OF WETLAND TUNDRA

The water balance of a High Subarctic wetland was measured in a newly established research basin near Churchill, Manitoba, Canada. Measureme nts spanned the growing season from early June through to the end of A ugust 1991. The watershed was instrumented such that the water budget could be monitored over each of the dominant terrain units. The study basin has 5 major terrain types consisting of sedge-dominated wetland, upland lichen-heath, tundra lakes and ponds, willow-birch wetland, an d open spruce forest, in decreasing order of coverage. These terrain u nits, and the ecosystem as a whole, are representative of coastal wetl ands underlain by permafrost in the Hudson Bay Lowland. Runoff and thu s streamflow were closely tied to the moisture status of the peatlands and to the depth of the active layer. During 2 unusually dry periods, the peatlands remained wet, which indicates a lack of water mobility when the water table drops below the surface of the wetland. Streamflo w response to rainfall during these periods was small in comparison to early summer when the frost table was near the surface, or to wet per iods when the water table rose above the surface. Energy balance measu rements showed the lakes and ponds to have about 15% higher net radiat ion than wetlands and 29% greater than upland lichen-heath. From 9 to 18% of net radiation was used in thawing and heating the ground wherea s the latent heat flux for the various terrain types utilized from 49 to 83% of net radiation. Even with unusually large rainfalls that occu rred in July, evaporation exceeded precipitation over the course of th e summer. Despite this, runoff was high and required a large negative change in groundwater storage to account for it. The Lichen-heath and ponds exhibited the greatest change in water storage. The drainage of water from the uplands to the lowlands maintained the soil moisture of the peatlands and contributed to streamflow. Each terrain type had a characteristic water balance due to differences in one or more of the hydrologic variables (evapotranspiration, runoff and storage). This ha s important implications concerning the impact on the water balance of a change in surface cover brought about by a change in climate.