High resolution temperature observations and modeling to quantify groundwater-surface water interactions

Hyporheic exchange may influence the water quality of a stream or river significantly. However, it is often difficult to quantify this flux, since there is no net loss or gain of water. One way to estimate this flux is to use tracers. However, the spatial resolution of tracer experiments is often low. In this study, we use high resolution temperature observation to quantify hyporheic exchange at the scale of one meter. Since November 2007, we have been applying a DTS (Distributed Temperature Sensing) fiber optic cable to measure temperature in a first order stream in Luxembourg. The system used has a resolution of 2 m and 3 minutes, resulting in an accuracy of ~0.1°C. The studied stream is approximately 600m long and discharge is measured at the upstream and downstream end. Four distinct lateral inflow points are identified. If the temperature of an inflow is known, it can be quantified using a mass balance. Because heat is not a conservative tracer, we developed a coupled hydraulic-energy balance model. By confronting the simulated temperatures with the observed ones, we estimated diffuse lateral inflows, as well as losses of water, with the constraint that the water balance should be closed. This is done in an iterative approach. However, the gains and losses of water, combined with the energy balance, could not explain the whole observed temperature signal. We hypothesize that hyporheic flow is responsible for the discrepancy between the observed and simulated temperature. If the stream is warmer than the subsoil, hyporheic flow cools down the stream water and vice versa. Consequently, hyporheic exchange can be quantified by matching the simulated temperature with the observed.