SIMULATING LAKE-GROUNDWATER EXCHANGE USING A FULLY-COUPLED SURFACE-WATER/GROUNDWATER MODEL

Two different approaches of representing lake-groundwater exchange in numerical flow models are investigated for a generalized glacial outwash lake in Northern Alberta. The LAK3 package for the USGS finite-difference model (MODFLOW) is compared to the finite-element, Integrated Hydrology Model (InHM), which uses a recently devised, fully-coupled approach to simulate surface-water/groundwater interaction. The models are based on a study lake situated on 15 metres of glacially deposited sand and gravel overlying a thick sequence of clay till. The lake receives a significant portion of inflow from groundwater discharge and outflow is through groundwater recharge and an ephemeral channel when lake levels are high enough to overcome basin topography. Long-term, precipitation and evaporative fluxes are approximately equal, though subtle annual differences appear to impact local water budgets significantly.

The regional groundwater conditions are represented equally well using either numerical model, especially during steady-state analyses. However, field data confirm the findings of similar studies, in that lake-groundwater interaction is transient, with seasonal variation in fluxes and the presence of flow reversals. We compared the ability of each model to represent the observed field behaviour. Each model's input data requirements and numerical complexity are evaluated against the dominance of a lake body within a hydrologic system, and the physical mechanics of groundwater exchange. Although the fully-coupled modelling approach depends greatly on basin topography and requires many additional parameters, the differences in numerical solution scheme allow for a more rigorous treatment of lake-groundwater exchange processes. Thus, this approach to simulating lake-groundwater interaction may better represent near-shore lakebed geometry, overall lake bathymetry, and the relationship between rainfall and stream outflow for many geologic settings in a plains region.