USING MULTI-YEAR AIR AND STREAM TEMPERATURE SIGNALS TO INFER GROUNDWATER DISCHARGE DYNAMICS AT WATERSHED SCALES

There has been rigorous application of natural temperature signals collected in saturated bed sediments to quantify vertical groundwater discharge. However, there has been comparatively little exploration of the potential transfer of temperature signals from the shallow aquifer to channel water through groundwater discharge zones, and there is room for innovation. We have developed paired air and stream water annual temperature signal analysis techniques to elucidate the relative groundwater contribution to stream water and the effective groundwater flowpath depth. Groundwater discharge to streams attenuates surface water temperature signals, and this attenuation can be diagnostic of groundwater gaining systems. Additionally, discharge from shallow groundwater flowpaths can transfer lagged annual temperature signals from aquifer to stream water. Here we explore this concept using multi-year temperature records from 120 stream sites located across 16 mountain watersheds of Shenandoah National Park, VA, USA and a coastal watershed in Massachusetts, USA. Observed annual temperature signals indicate a dominance of shallow groundwater discharge to streams in the National Park, in contrast to the coastal watershed that has strong, apparently deeper, groundwater influence. The average phase lag from air to stream signals in Shenandoah National Park is 11 days; however, extended lags of approximately 1 month were observed in a subset of streams. In contrast, the coastal stream has pronounced attenuation of annual temperature signals without notable phase lag. To better understand these observed differences in signal characteristics, analytical and numerical models are used to quantify mixing of the annual temperature signals of surface and groundwater. The measurement of multi-seasonal paired air and water temperatures offers great promise toward understanding catchment processes and informing current cold-water habitat management at ecologically-relevant scales.