INFLUENCE OF SEASONAL CLIMATE VARIATIONS AND LOCALIZED WATERSHED DISTURBANCE ON LAKE-GROUNDWATER INTERACTION IN A SUB-HUMID ENVIRONMENT

A fully-coupled, surface-water/groundwater model has been applied to study the hydrologic controls on lake-groundwater interaction in the sub-humid, Boreal Plains of northern Alberta. Lakes and ponds in the Boreal Plain exist on a coarse-textured, hummocky landscape that has poorly developed surface drainage. Field results have shown that lake connectivity with larger-scale groundwater flow systems and subsurface capture zones are primarily responsible for lake permanence in the sub-humid climate, where precipitation and evaporation are balanced. Most lakes behave as evaporation windows during the ice-free season. In upland areas, vertical moisture transfer is the dominant hydrologic process, and unsaturated zone thickness is a key hydrologic control, compared to topographic slope indices. We applied the numerical model to an area that contained a flow-through lake, which has no surface-water input and can only generate overland outflow through a small channel when lake levels are high. Even though the fully-coupled model was designed to simulate overland flow, we found that the robust, first-order coupling mechanism allowed lake-groundwater exchange fluxes to be computed as part of the simulation, and that the lake did not need to be explicitly defined as a hydraulic boundary condition. The model was used to simulate response of lake-groundwater exchange and transient water table configuration for short-term (<10 yrs) variations in precipitation and evaporation. Using a nested approach, a refined model was constructed to investigate enhanced snowmelt recharge at an area of localized disturbance in the watershed. Recharge and the transport simulations were supported by stable isotopic measurements. We also investigated the ability of the model to simulate near-surface moisture transfer in a climate where annual precipitation and evaporation fluxes are both less than 500 mm per year. Representing such processes in a numerical model is required to evaluate the impacts of subtle shifts in the timing of summer rainfall events and the onset of evaporation, which could be caused by anticipated future climate scenarios. We speculate that such a comprehensive surface-water/groundwater model could be applicable for simulating longer-term changes in climate.