Stable isotope patterns of German rivers with aspects on scales, continuity and network status

In Germany, river monitoring for tritium started in the early 1970s. Today this monitoring network consists of 50 stations and includes stable isotopes. The stable isotope time series to the end of 2021 are at least four years and for some stations up to 30 years long. Daily river water samples were collected during an extraordinary dry season from October 2018 until end of January 2019 from six selected stations of the Rhine and five stations of the Elbe basin. The most dominating stable isotope effects in river water are the seasonal and altitude effects, but also a continental effect is visible. The isotopes indicate snow and ice melt contributions in the Rhine and Danube during the summer months and a consecutive dilution of these signals by mixing with tributary rivers. Close to the coasts in northern Germany, stable isotope patterns reflect influence of seawater and tides. Daily patterns during the dry season 2018/2019 surprisingly do not exhibit extreme changes but rather trends of enhanced groundwater contribution. Long-term continual data across scales are important for comparing and identifying hydrological processes in German river basins of different size and mean catchment altitudes, and highlight the benefits of a co-organized national network.

Automated summary

River monitoring for tritium in Germany began in the early 1970s and now includes 50 stations and stable isotopes. The stable isotope time series show contributions of snow and ice melt in the Rhine and Danube rivers during summer months. Close to the northern coasts, stable isotope patterns reflect the influence of seawater and tides. Surprisingly, there were no extreme changes in stable isotope patterns during the dry season of 2018/2019, but rather an indication of enhanced groundwater contribution. Continual long-term data across different river basins of varying sizes and altitudes highlight the importance of a co-organized national network.