POTENTIAL CLIMATE CHANGE IMPACTS TO STREAM TEMPERATURE IN THE MARENGO RIVER HEADWATERS
ABSTRACT Cold-water trout habitat in Wisconsin is projected to significantly decrease as a result of climate change. The extent of habitat loss depends, in part, on groundwater discharge to the stream, which can provide cool water refuges for trout. Field measurements and models of groundwater flow and stream temperature were used to evaluate how climatic changes are likely to impact groundwater discharge and stream temperatures during summer low flow conditions in a small headwater stream in the Chequamegon-Nicolet National Forest in northern Wisconsin. Variations in groundwater discharge and stream temperature in the watershed correlate with the highly varied geology. Two cool tributaries receive groundwater from distinct areas: shallow groundwater flow in glacial sand deposits discharges to a wetland tributary with temperatures near 10-16°C; deeper, longer flow paths through fractured and faulted bedrock discharge to a tributary (<12°C) and the main stem, increasing flow by about 6 cfs over half a mile. Trout are commonly observed downstream of this focused discharge area. In contrast, warm stream segments in flat wetland areas with shallow crystalline bedrock reach temperatures upwards of 30°C, well above the lethal threshold for trout. A groundwater flow model was developed to simulate possible changes to the groundwater system. Climate change will impact long-term average baseflow through changes in groundwater recharge. A previously published soil-water balance model run using a suite of global circulation models suggest recharge in this watershed could change from -12% to +21% by the end of the century. Based on these changes in recharge, the main stem baseflow is simulated to change up to 30% from current long-term average conditions; flow in the groundwater-dominated tributaries remains fairly constant relative to current conditions. Thermal impacts from climate change were simulated by modifying flow, air temperature, and groundwater temperature in a mechanistic stream temperature model. Air temperature from 2018 was scaled to low, medium, and high possible future time series using three global circulation models that represent a range of increases in temperature; groundwater temperature thermal sensitivity was varied from 0 to 1. Results were compared to trout thermal tolerance limits over several time periods ranging from 1 to 42 days to evaluate habitat impacts. Stream temperature downstream of the focused groundwater discharge area was simulated to increase between 0.8 and 4.6°C in 30 to70 years depending on the climate scenario. This area remains below the trout thermal tolerance threshold under the low and medium air temperature scenarios, but exceeds the threshold for the high. An increase in the frequency and magnitude of extreme rain events will also increase stream temperatures in groundwater-dominated areas. Trout will most likely be impacted by a sustained increase in stream temperatures over weeks or months, rather than by increases in daily mean temperature, for both flooding and low flow conditions. This improved understanding of system dynamics will help the U.S. Forest Service manage the watershed for trout.