Using Active-DTS measurements in a lowland river to characterize the spatiotemporal variability of stream water infiltration
Résumé
Characterizing the spatiotemporal variability of groundwater fluxes at the stream/groundwater interface is extremely challenging, because of streambed heterogeneities that control hyporheic flow paths at different scales. By burying few hundred meters of heatable Fiber-Optic cables within streambed sediments, we demonstrate the great potential of Active-Distributed Temperature Sensing (DTS) measurements for mapping and measuring stream water infiltration in a lowland stream. Experiments were conducted in a large meander, where permanent stream-losing conditions are observed across the neck. In this case, the temperature elevation measured during heating periods is affected by the natural stream temperature variations. We thus introduce a new methodology to filter ambient temperature variations, extending the application of Active-DTS to losing streams. After data processing, results show that, along both lateral and longitudinal stream profiles, thermal conductivities and groundwater fluxes show relatively small variations, with groundwater fluxes values varying only over one order of magnitude. Another interesting result is the absence of correlation between groundwater fluxes and streambed topography variations, suggesting that the groundwater fluxes variability is mainly controlled by local streambed heterogeneities. It means that the spatiotemporal variability of groundwater fluxes might be used as a marker of the variability of streambed hydraulic conductivities. The relatively low spatial groundwater fluxes variability suggests a small variability of streambed properties, which is an interesting outcome for calibrating models assessing hyporheic processes. This is also an interesting insight for the understanding of flow paths in hyporheic zone, since it is the first time that the streambed heterogeneities is assessed at high spatial resolution for such spatial scale. Interestingly, almost no variations of thermal conductivities and groundwater fluxes has been detected from measurements made at different times for three years. This shows the excellent reproducibility of measurements and the remarkable stability of hyporheic flows through times.
