GROUNDWATER-SURFACE WATER INTERACTIONS AT A WATERSHED SCALE

Watersheds located within a mountain to coast physiographic setting have been described as having one of the most highly inter-connected surface water and groundwater environments. The steep topographical setting, coupled with a large accumulation of valley fill sediments, creates conditions whereby the two water systems are dynamically coupled. This study uses MIKE SHE, a coupled surface water-groundwater model, to explore the seasonally and spatially dynamic nature of these interactions in the Cowichan Watershed on Vancouver Island, British Columbia, Canada. The 980 km² watershed extends from mountains to the coast, and ranges in elevation from 1500 to 0 masl. The hydrostratigraphy of the watershed was constructed using geostatistically modeled surfaces based on existing surficial and bedrock geology maps, the results of electrical resistivity surveys, and lithology records from the BC WELLS database. The spatial distribution and thickness of unconsolidated aquifers and confining units overlying bedrock was further constrained using a relative thickness mapping approach. The ArcGIS solar radiation analyst tool was utilized to temporally and spatially define insolation within the mountainous terrain, which was coupled with other climatic variables (temperature, windspeed, and relative humidity) to estimate potential evapotranspiration (PET). Leaf area index (LAI) was estimated using satellite reflection data, referenced to published measured relationships of LAI. Accumulation of water within Cowichan Lake and subsequent flow within the Cowichan River was simulated using MIKE 11, and dynamically coupled to MIKE SHE. Simulated snow water equivalent, lake levels, river discharge, and groundwater levels reflect the magnitude and seasonal variations in the measured data. The model topography (lake bathymetry, river bottom elevation, land surface) has a pronounced effect on model results. Actual evapotranspiration (AET) ranges from 0.5 to 10 mm/day, with the highest values in the highly vegetated north slopes at low elevation. Recharge occurs primarily from October through to April, largely within zones of permeable surficial material. The proportion of gaining and losing sections of the Cowichan River are close to equal, and the headwaters have the largest control on flows throughout the year.