MOVING BEYOND COUPLED HYDROLOGY: USING GSFLOW OUTPUTS FOR CLIMATE CHANGE SIMULATIONS OF STREAM TEMPERATURE

The use of coupled surface-water and groundwater models to evaluate climate change effects has become more common in recent years. However, many societally relevant decisions require more than just accurate flow simulations. In two Wisconsin studies of cold-water fisheries, hydrogeologic flows have both a direct (ecological flow) and indirect (temperature conditioning) effect on fish habitat. Other drivers, such as air temperature and riparian shading, also affect stream temperature but are not well represented in current hydrologic models. To more comprehensively simulate the effect of climate change on cold-water fisheries, two watersheds were simulated using: 1) a GSFLOW coupled groundwater/surface-water model; and 2) a linked SNTEMP stream temperature model. SNTEMP is a steady-state, one-dimensional heat transport model that predicts daily mean and maximum temperatures as a function of stream distance and environmental heat flux. Multiple General Circulation Models (GCMs) and emissions scenarios were evaluated. Simulations showed that climate change influenced hydrologic flows, but larger effects were observed on simulated stream temperatures. Estimated increases in annual average stream temperature ranged from approximately 1 to 2 degrees Celsius by 2100 in a stream characterized by a high groundwater inflow rate, to 2 to 3 degrees Celsius in a stream with lower groundwater inflow rates. The climate drivers used for the climate-change scenarios had appreciable variation. Thus, as with all forecasts of this type, the results are still considered to approximate the range of potential outcomes of climate change despite our more sophisticated handling of important processes.