A WATERSHED-SCALE APPROACH TO GROUNDWATER-SURFACE WATER FLUXES IN PRAIRIE WETLANDS

The fluxes between groundwater and surface water are typically viewed as local processes occurring at the interface. However, understanding of such fluxes can benefit from viewing them also at a larger scale, including adjacent upland areas, and considering both subsurface and surface fluxes of water and dissolved mass. Such a larger-scale approach to water and mass fluxes, which may encompass the whole watershed, provides context for more detailed small-scale approaches. Here we illustrate such an approach for the wetlands (potholes) of the northern glaciated interior plains. Most of the wetlands or wetland complexes lie within small (typically 1-100 ha) topographically closed watersheds which lend themselves well to whole-watershed studies.

The hydrological regime of prairie wetlands is dominated by the cold dry climate and by the underlying clay-rich glacial till which has negligible permeability were it not for fractures. There are active exchanges of water and mass between the open water and the riparian zone through the shallow highly fractured till, often involving transient reversals in the direction of the fluxes. There are also slow exchanges with deeper groundwater through the much less fractured underlying till. The latter have a long-term cumulative effect on the overall water chemistry but little or no discernible impact on water balance. Contrasting behaviors of different wetlands with regard to water balance, salt concentrations, tracers and DOC can be understood through this conceptual framework. Soil development and salt distributions beneath and adjacent to the wetlands are also consistent with this model. These groundwater processes are accompanied by surface-water processes, in particular snowmelt runoff that transfers a large quantity of water and mass from the surrounding uplands to the wetlands. Therefore, both surface water fluxes and groundwater fluxes must be considered at the watershed scale to obtain a coherent picture of the wetlands.