INVESTIGATING THE ROLE OF CRITICAL ZONE HYDRODYNAMICS ON STREAM TEMPERATURE DISTRIBUTIONS

Rising air temperature and decreasing stream flow trends are predicted to result in corresponding increases in stream temperatures. As a result, the future of ectothermic stream fishes, which rely on seasonal and spatial temperature distributions for growth and survival, could be in jeopardy. Fortunately, contradicting stream temperature trends in forested headwater catchments suggest that non-climatic variables, such as baseflow indices and catchment geologic structure, may have an important confounding influence on the future of stream temperature. Most significantly, the intra-annual variability of groundwater temperature has long been recognized as an important contributor to the advective heat budget of streams. In this study we move beyond the hyporheic zone to investigate the drivers of shallow groundwater temperature variability in the recharge zone of a till-mantled/shallow bedrock headwater catchment. By applying the concept of saturated conductivity thresholds we demonstrate how the coupling of air and stream temperature can be mitigated by the hydrologic dynamics of the critical zone interface. We use distributed temperature, isotopic, and hydrometric analyses to evaluate how advective and conductive heat signals in the near surface behave with respect to soil saturation, thermal conductivity, and threshold discharge events. We examine how antecedent moisture conditions in the near-surface impact the thermal conduction of air temperature into shallow water tables, and how that translates to temperature distributions in baseflow on seasonal and inter-annual cycles.