Topography scale effects on groundwater-surface water exchange fluxes in a Canadian Shield setting

Exchange fluxes occur when hydrological processes transfer water across the topographic surface between the groundwater and surface water systems. Many hydrological models are capable of estimating exchange fluxes at point-scales, however as models, including integrated hydrological models, are upscaled to encompass larger areas using non-point scale discretizations, the accuracy of hydrological process estimation declines such that the results can become empirical. Through analyses of topographic indices (e.g., slope, roughness, wetness index), hydraulic heads, and surface water/groundwater exchange fluxes at multiple horizontal spatial scales, this study indicates that it is possible to identify the scale dependencies and inherent empiricism in shallow groundwater hydrological models which contribute to variations in exchange flux estimates, and hence groundwater recharge/discharge rates, at different spatial scales. By identifying the scale dependencies and empiricism related to horizontal spatial scale choice, it should be possible to improve conceptual design choices when attempting to simulate locations with shallow water tables. This is important as a better understanding of the spatial variability of exchange fluxes will allow for better predictions of some catchment behaviours. The characterization of the models in this study are a reasonable approximation of a Canadian Shield site based upon field collected parameters. An analysis of the impact of model discretization scale on exchange fluxes, and specifically recharge, should be a component of all model studies.