Hydrologic Functioning of Glacial Moraine Landscapes Within Alberta's Boreal Plains

Within the Boreal Plains of north-central Alberta, catchments situated within low permeability glacial terrain are composed of a mosaic of landscape units including ponds, peatlands, and upland aspen forest ecosystems within a sub-humid climatic zone where water deficit conditions are frequent. These ecosystems host ecologically and commercially significant habitat and natural resources; however, they are threatened by expanding anthropogenic development and climate change. Within this framework, characterization of the processes governing water movement within and between landscape units is paramount for proper management of existing ecosystems and restoration of disturbed landscapes. Hydrologic data were collected over eleven years to evaluate hydrologic interactions occurring between landscape units. Two-dimensional numerical models were developed using the fully-integrated groundwater-surface water model HydroGeoSphere to evaluate key landscape features and processes that allow these ecosystems to persist within the sub-humid climate. Results show that dynamic interactions between the pond and peatlands are driven by precipitation and evapotranspiration, with pond and peatland water levels reflecting recent climatic trends. Limited hillslope contributions to the peatlands occur, indicating they are not required within this climatic setting for long-term maintenance. Instead, the peatlands conserve water within the landscape and supply it to adjacent landscape units. By contrast, the pond and the aspen forested hillslopes are dominated by high rates of evapotranspiration, and represent net water sinks within the landscape. A two-dimensional numerical model was also developed using MODFLOW-SURFACT to quantify the effects of seasonal peatland freezing on water distribution and water table position through investigation of changes due to variations in peatland hydraulic conductivity and storage properties. Results indicate that seasonal freezing is expected to maintain higher water table conditions by restricting infiltration of snowmelt and spring precipitation, thereby supporting higher rates of spring evapotranspiration, with discharge at the peat surface as surface ponding and overland flow. Subsurface hydrologic connectivity between the peatland and pond is also restricted due to the lower hydraulic conductivity of frozen peat. The degree of influence of the frozen peat is dependent on the relative timing of snowmelt and peatland ice recession. Where sufficient ice remains to prevent infiltration of spring meltwater and rains, less water may be available to the peatlands. This decrease in available water may have negative implications for growing season productivity and fire susceptibility, as well as hydrologic interactions with neighboring ecosystems. A two-dimensional numerical model was also developed using HydroGeoSphere to assess the hydrologic impact of aspen harvesting. Study results indicate that aspen harvesting has limited impact on groundwater levels and stream flows. This outcome is because of the sub-humid climate, with low-frequency of large storms, large soil-moisture storage capacity of heterogeneous glacial materials, and high evapotranspiration rates of regenerating aspen. Despite an estimated increase in hillslope groundwater levels of up to 3 m, pond and peatland water levels increased by less than 0.3 m and were accompanied by increased stream flows of less than 10 mm/yr. However, groundwater level and stream flow predictions were sensitive to regenerating aspen evapotranspiration rates, which can be enhanced by appropriate harvesting techniques but may be reduced by climate change. These results are consistent with previous results for the Boreal Plains, but they differ from aspen harvesting studies conducted in other settings where appreciable increases in stream flows have been reported. This disparity highlights the need to consider the integrated response of the hydrologic system when evaluating impacts from disturbance and making comparisons between settings. Two-dimensional numerical simulations were also conducted using HydroGeoSphere to predict potential climate change impacts for a range of projected scenarios. Results indicate peatland water levels may decline by up to 1 m; however, sensitivity simulations indicate that the decline in water levels may be moderated by several feedback mechanisms that restrict evaporative losses and moderate water level changes. In contrast, higher evapotranspiration losses from the aspen hillslopes are predicted to result in near-surface soils becoming increasingly drier. Thus, the aspen may frequently be water-stressed and increasingly susceptible to secondary maladies such as pests and disease. Reduced pond water levels are also predicted with the development of frequent ephemeral conditions in warmer and drier scenarios. Concurrent decreases in stream flow may further impact downstream ecosystems. Further research into the regional health and sustainability of Boreal Plains ecosystems is warranted.

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