EXAMINING CHANGES IN RAINFALL-RUNOFF RESPONSES DUE TO SUBSURFACE PERMEABILITY UPSCALING USING A FULLY-COUPLED 3D SURFACE/SUBSURFACE FLOW MODEL

The Integrated Hydrology Model (InHM) is a fully-coupled and physically-based distributed model which can simulate water flow and solute transport on the 2D land surface and in the 3D subsurface under variably-saturated conditions. Full coupling of the surface and subsurface components of the hydrologic cycle is accomplished by simultaneously solving one system of equations for overland flow rates and water depths, stream flow rates, subsurface pressure heads, saturations and velocities, as well as water fluxes between continua. Solving for the advection-dispersion transport equations in the surface and subsurface flow domains is achieved in a conceptually identical manner. The results of high-resolution numerical experiments performed in the Laurel Creek Watershed with InHM are presented which examine how rainfall-runoff responses are affected due to spatial discretization of the subsurface permeability field. The catchment, located in southern Ontario, Canada, is about 75 km^2 in area and includes both urban and rural land use. In addition to distinct land use and surface cover, the watershed has more than 100 m of topographic relief and is underlain by a very complex hydrostratigraphic suite of glaciofluvial deposits. While maintaining the mean log K of the system, the permeability field is up-scaled incrementally from the fully-heterogeneous case to the fully-homogeneous case, discrete precipitation events are input and the resulting hydrographs examined. Results show there is a critical threshold of subsurface permeability characterization which must be maintained if the rainfall-runoff responses calculated by physically-based distributed models are to be meaningful.