Can conductivity and stable isotope tracers determine water sources during flooding? An example from the Elbe River in 2002

Despite drastic runoff variations, stable isotope data of water in the upstream part of the Elbe River showed remarkable similarity during, and two months after the flood of 2002. This homogeneity indicates that water sources remained the same and most likely represents dominant groundwater input regardless of the flood. While the latter remains the most plausible explanation, it was difficult to prove with water stable isotopes for the upstream part of the river. Overlapping isotope compositions of long-term average precipitation data (as a proxy for the groundwater end member) and of the weighted average from precipitation events in August 2002 did not allow quantification of the groundwater component during flooding. This shows that better spatial and temporal sample resolution is necessary for enhanced understanding of water sources and mass balances during floods. Such mass balances were only possible in the estuary where conductivity and stable isotope tracers both revealed much stronger freshwater fluxes to the estuary during the flooding event of 2002 when compared to those of two months after the flood. Conductivity was also a good indicator for additions of more saline waters from the Saale River when the system was not affected by flooding. However, during the flood in August 2002, large volumes of low-saline waters, most likely from shallow groundwater, masked these usually high conductivity values. This indicates that conductivity better reveals input of dissolved constituents, while the stable isotopes can better indicate sources of water input. Both tracers become most powerful when applied in combination for better understanding of water sources and river basin management.