Automated discharge measurements with salt dilution in Alpine creeks and uncertainty quantification

The extent to which climate change affects the frequency and magnitude of flood events in mountain catchments is still unclear due to strong regional differences and a limitation in streamflow observations in space and time. However, recent flood events in Western and Southern Alps from August 2021 highlight the need for new monitoring strategies of peak flood events to better compute return periods of flood events. In particular, measuring peak events in small Alpine catchments can be enhanced by using automated tracer measurement systems. We present results from a hydrometric program using an automated salt dilution system at three different sites in the Tyrolean and South Tyrolean Alps from 2020 and 2021. The AutoSalt system triggers salt slug injections in response to water level changes in the creek while two downstream electrical conductivity probes record the passage of the breakthrough curve with high temporal resolution (5 sec). We collected in total 288 discharge measurements ranging from 0.1 m³/s to 15 m³/s. Besides the requirement of complete mixing of the tracer, the main challenge in automated salt dilution is the monitoring and control of the system components to ensure a high reliability and quality of observation data. The internal quality check of the AutoSalt system allowed us to record mainly discharge measurements (81 %) with a very low measurement error < 7% while 19 % of the discharge measurements had an error range of 7 % to 15 %. Based on the collected discharge measurements and their uncertainties, we constructed robust rating curves with error bars for each site. In a next step, we will use the collected observation data to validate hydrological model results for these three different Alpine catchments to strengthen the robustness of the model for long-term climate change modeling.