THERMAL SENSITIVITY OF SHALLOW AQUIFERS TO CLIMATE CHANGE AND LAND COVER DISTURBANCES: STREAM TEMPERATURE IMPLICATIONS

Climate change is expected to increase stream temperatures, and the projected warming may reduce the spatial extent of suitable habitat for coldwater fish and other aquatic taxa. Recent studies have proposed that measured stream thermal sensitivities to short term air temperature variations can be employed to infer future stream warming due to long term climate change. This approach utilizes air temperature as a proxy for surface heat fluxes and does not consider the potential for streambed heat fluxes to increase due to gradual warming of shallow groundwater. The temperature of shallow groundwater is particularly important for the thermal regimes of small streams and groundwater dominated river systems. Also, other recent stream temperature studies have investigated how land surface perturbations, such as wildfires or timber harvesting, can influence stream temperature dynamics by changing surface heat fluxes, but these studies have typically not considered how these land cover changes can also alter shallow groundwater temperatures and consequent streambed heat fluxes.

In this study, several analytical solutions to the one-dimensional unsteady advection-diffusion equation for subsurface heat transport are employed to investigate the timing and magnitude of groundwater warming due to seasonal and long term variability in land surface temperatures. These solutions illustrate that although groundwater temperatures may be relatively insensitive to seasonal changes in surface temperatures, shallow aquifers, and hence streambed heat fluxes, may be very sensitive to long term surface temperature changes. New aquifer thermal sensitivity mathematical formulae are proposed that accommodate different surface warming scenarios. The solutions demonstrate that shallow aquifers will warm in response to climate change and other surface perturbations, but the timing and magnitude of the groundwater warming depends on the surface warming, subsurface thermal properties, depth, and groundwater velocity. A novel approach is proposed for modifying stream temperature models to accommodate the timing and magnitude of subsurface warming in response to changes in land cover or climate.