Dynamic streambed fluxes during rainfall-runoff events

Observed 3-D temperature distributions within a streambed were used to analyze the effects on exchange fluxes between groundwater and the stream during rainfall-runoff events. By combining a dense vertical and lateral monitoring network of streambed temperatures with coupled surface/subsurface 3-D flow and heat transport modeling, we demonstrate how temperature can be used directly as a calibration target. Three model setups with different hydraulic conductivity distributions were evaluated in an optimization approach using temperature and hydraulic head data. The hydraulic conductivity distributions were based on slug test surveys within the streambed and aquifer. A detailed characterization of the hydraulic conductivity of the streambed and aquifer is needed to accurately simulate observed temperatures. Hence, the most sensitive parameters, the vertical hydraulic conductivity and the thermal conductivity, were calibrated within different conductivity zones of the heterogeneous model. Simulated exchange fluxes across the streambed showed variations up to a factor of four within just a meter. Such differences may not have been correctly predicted using 1-D heat transport models due to lateral conduction amongst the different flow paths. During the rainfall-runoff event, fluxes decreased substantially (-50%) due to a decrease in the hydraulic gradient with increased stream stage. Although no flow reversals were observed during the studied conditions, it is possible that these can occur during larger rainfall-runoff events. We show that with the current sampling and modeling techniques, 3-D temperature data can be used to estimate dynamic exchange in heterogeneous flow fields encountered in the field. Key Points Three-dimensional streambed temperature patterns indicate heterogeneous flow fields Exchange fluxes vary in response to stream stage changes Hydraulic and thermal characterizations of streambed and aquifer are needed