Now You See Them, Now You Don’t: Temporal Change in the Mode and Extent of Connected and Disconnected Boreal Wetlands and Implications for Streamflow Estimation

Abstract:

In northern Boreal catchments the presence of a multitude of connected and disconnected wetland complexes is commonly believed to play a controlling role on the source and timing of streamflow, effective simulation of which is critical to flow forecasting in changing climates. A key factor in this control is the mode of connectivity between wetlands and downstream rivers, and temporal distribution thereof.

The local Lower Nelson River basin, Manitoba, Canada, has an area of approximately 90,000 km², of which 25% is estimated to be covered by wetlands. Assessment of a decade of aerial imagery indicates variation in the spatial extent of wetlands of up to 50% of the surface area of individual headwater basins on both an inter- and intra-annual basis. Aerial and ground reconnaissance of selected areas indicates that using generalised aerial-based reflectance imagery for land cover classification is hampered by the presence of a number of types of wetlands (treed, shrubby, grassed, open) and shallow groundwater in this flat landscape. The large, remote, catchment area renders detailed ground-truthing impractical. As an alternative, five headwater basins and the main stem of the river are gauged and monitored for stable isotopes of water. In this study linear regression is used to assess linkages between isotopic and wetland extent variation and dominant environmental variables. Mass balance modelling is used to assess the relative merits of a detailed re-analysis of wetland delineation using refined reflectance analysis, soil and isotopic data, and simply assigning wetland extent as a calibration variable. Results indicate that aerial imagery provides a useful tool to assess surface connectivity, but that explicit identification and representation of temporal variation in surface and subsurface connectivity is necessary to adequately estimate timing of streamflow in this flat, wetland-dominated catchment.