THREE-DIMENSIONAL ANALYSIS OF SURFACE-SUBSURFACE WATER AND SOLUTE EXCHANGE FLUXES NEAR STREAMS: AN INTEGRATED MODELLING APPROACH

The Integrated Hydrology Model (InHM) is a fully-coupled 3D control-volume finite element model which can simulate water flow and advective-dispersive solute transport on the 2D land surface and in the 3D subsurface under variably-saturated conditions. Full coupling of the surface and subsurface flow regimes is accomplished by simultaneously solving one system of non-linear discrete equations describing flow and mass transport in both flow regimes, as well as the water and solute fluxes between continua. The results of high-resolution 3D numerical experiments performed with InHM are presented which examine the impact of a subsurface contaminant plume discharging along a reach of a small stream within the Laurel Creek Watershed located in Southern Ontario, Canada. The sub-catchment under study is about 17 km2 in area, has about 60 m of topographic relief as defined by a 25m-scale DEM, and is highly heterogeneous in terms of its land use, near-surface soil types and Quaternary geology. Simulations are compared for cases where annual, monthly and daily precipitation averages are applied as rainfall inputs to the model in order to assess the effect of different temporal averaging scales on predicted downstream surface-water and stream-bottom sediment quality. Results show that predicted water and solute exchange fluxes across the streambed can vary rapidly in space and time due to individual rainfall events and that short duration, high intensity peaks are not captured if monthly or annual average rainfall is used as input. In addition, we also compare the predicted spatial and temporal patterns of surface runoff, infiltration and exfiltration over the entire land surface using the different temporal resolutions of rainfall, and for changes in land use as represented by changes in the near-surface hydraulic properties of the sub-catchment.