LINKING GROUNDWATER RECHARGE, FLOW, AND STREAM TEMPERATURE MODELS TO SIMULATE THE EFFECTS OF LOCAL LAND-USE ON A STREAM

Groundwater inflow is critical for maintaining base flows and constant temperatures in coldwater trout streams. Urbanization often causes reductions in recharge, and the resulting changes in groundwater-flow regimes caused by increases in impermeable surface area threaten many rural watersheds with coldwater fisheries. Reductions in recharge can decrease the discharge of cold groundwater to streams, which in turn can negatively impact fish habitat and reproduction. These effects have been very difficult to quantify and predict because, until recently, models to estimate the spatial distribution of groundwater recharge were not readily available. Most groundwater models at the watershed scale simulate recharge using simplistic zoning that does not incorporate small-scale variability of land use or topography. Furthermore, until recently, stream-temperature models have not been linked to groundwater-flow models.

We used three models to simulate recharge, groundwater flow, and stream temperature in the Rowan Creek drainage basin in southern Columbia County, Wisconsin. Spatially variable recharge rates were calculated using a GIS-based water-routing model that incorporates readily available topographic, soil, meteorological, and land-cover data. Using this model, we created recharge arrays for different land-use scenarios that were linked to a calibrated groundwater-flow model (using the U.S. Geological Survey MODFLOW code) of the basin. The groundwater-flow model was used to simulate groundwater discharge to 12 reaches of Rowan Creek and tributaries where baseflow had been measured. The simulated discharge values were then used as input to a stream-temperature model that uses streamflow, vegetation patterns, channel width, and meteorological conditions to predict stream temperatures.

The results of the modeling show that recharge variation from different kinds of land use can have a significant impact on base flow and stream temperature. The size, type, and location of the land-use activities control the overall effects on the stream. Model linkages of this type can be a powerful tool to assist land-use planners and fishery managers in predicting the effects of land-use changes on stream flow and stream temperatures.