The Analysis of Seasonally Varying Flowin a Crystalline Rock Watershed Using anIntegrated Surface Water andGroundwater Model

Abstract

Minimal scientific knowledge is available to describe the interaction of surface water and groundwater flow systems within watersheds in the crystalline rock setting of the Canadian Shield. The surface water and groundwater flow in a small watershed are investigated using the fully coupled groundwater / surface water model HydroGeoSphere. The watershed has an area of 6.92 square kilometers and contains multiple lakes, including Bass Lake (the largest lake) with a surface area of 0.94 square kilometers and a maximum depth of 9.0 meters. The surface water drainage system includes wetlands, small streams, and beaver dams while the topographic relief has a range of 50 meters. A bathymetric survey was completed and lake circulation patterns were determined for Bass Lake within the watershed using RMA2. The overburden is thin with granite bedrock outcrops occurring at numerous locations throughout the basin. The conceptual model for the subsurface system includes the surface sediments while three categories are used to characterize the bedrock: moderately fractured rock (MFR), sparsely fractured rock (SFR), and fracture zones (FZ). These categories are consistent with those used in the characterization of the crystalline rock at the Atomic Energy of Canada Limited (AECL) Underground Research Laboratory near Lac du Bonnet, Manitoba. ArcView GIS was used to facilitate data synthesis, analysis, and visualization. Data layers include orthophotos and digital Ontario Base Maps. Hydrologic data for the basin includes 2 rain gauges and a continuous water level recorder for Bass Lake. Additional climate data were obtained from nearby meteorological stations. The HydroGeoSphere model is based on the FRAC3DVS three-dimensional groundwater model and the MODHMS surface water simulator. HydroGeoSphere was used to investigate and calibrate the parameters for groundwater and surface water flow in the basin for a spring to fall time period. Model output is compared to the observed continuous hydrologic record. The simulation results are compared to expected trends and observed field data. The groundwater heads and flow vector fields show groundwater movement in expected directions with reasonable flow velocities, while the subsurface saturation levels confirm that the evapotranspiration component is withdrawing groundwater during plant transpiration. Surface water depths and locations of water accumulation are consistent with known and collected field data, while surface water flows and velocities are in the expected ranges and directions. Simulated Bass Lake surface elevations compared very well to observed data-logger water elevations. Low overland friction values produced the most accurate Bass Lake elevations, with high overland friction values slightly overestimating the Bass Lake water level throughout the simulation period. The integrated surface water-groundwater model HydroGeoSphere faithfully reproduced the observed conditions and physical processes of the Bass Lake model domain.