CONTROL OF SOIL TEXTURE ON WATER FLUXES AT PERCHED BOREAL WETLANDS IN A SUB-HUMID CLIMATE

Wetlands with perched groundwater tables are common near a regional groundwater flow divide in Northern Alberta, Canada. These wetlands provide important habitat for boreal plant and animal species. Determination of hydrological processes sustaining these wetlands is needed to develop a framework to assess the susceptibility and permanence of these wetlands to anthropogenic impacts. The study site is characterized by wetlands and aspen uplands situated on a thick package of heterogeneous glacial deposits with subdued topography and sub-humid climate. At our study site, an extensive unsaturated zone separates numerous wetlands from the regional water table, resulting in isolated, local perched flow systems. Instrumentation transects from a shallow pond and a peatland, to their adjacent hillslopes, were used to quantify groundwater, surface water, and soil moisture regimes adjacent to and beneath the wetlands. Data including water levels, surface water flow, soil storage, soil tension, and atmospheric fluxes allowed the determination of a water budget and perched wetland sustainability.

Vertical water fluxes dominate the water budget due to the climate and isolation of the wetlands from regional groundwater flow. However, during infrequent periods of water surplus, lateral fluxes at wetland/upland transitions and inflow from ephemeral draws are important elements of perched wetland sustainability. Our field observations show that the presence of a low-permeability confining layer is the main control on antecedent soil moisture conditions thus controlling the magnitude of observed hydrological processes and water fluxes. 1-D and 2-D modeling using a fully coupled saturated/unsaturated groundwater and surface water flow model quantified the vertical and lateral water fluxes. Modeling results confirm field observations that permeability contrasts are the dominant control on water flux associated with perched wetlands and that groundwater/surface water interaction only occurs where a near-surface confining layer is present. This demonstrates that the spatial distribution of confining layers and atmospheric fluxes are key elements of a framework to interpret soil moisture regimes, runoff potential, wetland connectivity, vegetation patterns, and groundwater/surface water interaction in this environment.