Monitoring zero water level in a drought-affected headwater stream network

Even largely perennial rivers can fall dry during drought events. A resulting partial or full drying-up of streambeds is difficult to monitor with conventional gauging stations, but important as it heavily impacts water availability, quality and aquatic ecosystems. With a predicted tendency towards more extreme droughts, event-based intermittency is likely to increase requiring a better longitudinal quantification of water level and streamflow conditions. The Dreisam River in the south-west of Germany is a stream with a highly dynamic hydrology. In the recent extreme drought years of 2015, 2018 and 2019 the main stream and tributaries partly fell dry; whereas the main gauging station still recorded flow. Furthermore, several tributaries fell dry in 2016, 2017 and 2019.To improve the understanding of the interaction between streamflow, groundwater and water usages in low flow and zero-flow situations, a flexible longitudinal water quality and quantity monitoring network was developed. Different techniques such as QR-code-reading camera systems and ultrasound devices to log water levels as well as water temperature and electrical conductivity sensors were used. The set-up was additionally equipped with conventional capacitive water level loggers. Here, we present a comparison of the different water level monitoring techniques in order to a) evaluate the advantages and limits of the novel techniques and b) investigate any added value of longitudinal, catchment wide zero level monitoring. The results show that the choice of the measurement sites' environment, including shading of QR-codes, light reflections of the water surface and streambed topography, is crucial for a successful application of the used techniques. The distributed gauges reveal a highly variable longitudinal drying pattern within the river network that appears to be event-specific and may not be explained without consideration of all natural and altered system fluxes.