Automated Sequential Salt Injection to Estimate Subsurface Contributions to Streamflow in Headwater Catchments in Germany

Sequential salt dilution measurements of discharge along streams allow us to determine stream gains and losses on a reach-by-reach basis. This information is especially useful in the context of studying the spatial variability of subsurface flow contributions to overall streamflow. However, these sorts of measurements are time-intensive and laborious and therefore usually only carried out during snapshot campaigns. In our study we explore the potential of an automated sequential salt injection method, applying the same methodology in three headwater catchments in typical mountainous regions in Germany (Black Forest, Sauerland, Ore Mountains).

Several automated salt injection units were spaced approximately 200 m apart at each site, set to inject at a scheduled daily interval as well as on rainfall event-based triggers. Electrical conductivity is measured at 5-second intervals both upstream and downstream of each injection point to obtain discharge estimates. This approach opens the possibility of measuring local gains and losses at scales on the order of 200 m but at a much higher temporal frequency than is usually achieved with manual snapshot campaigns. This higher frequency has the advantage of sampling over a larger range of conditions and thus providing a much more detailed picture of runoff generation along the stream.

Given the required need for highly accurate discharge estimates in the here studied small streams the feasibility of using this automated method to quantify subsurface contributions to streamflow along the stream is assessed, with a key focus on evaluating uncertainties.