Isotopic hydrograph separation in the Hydrological Open Air Laboratory, Austria

Exploring the contributions of new and old water to runoff during precipitation events in agricultural catchments is essential for understanding runoff generation, solute transport, and soil erosion. The aim of this study was to compare two isotope hydrograph separation methods in the Hydrological Open Air Laboratory (HOAL) in Austria, a 66-ha large experimental catchment dominated by agricultural land use. The classical two-component (IHS) and the ensemble isotope hydrograph separation (EIHS) methods were applied to multiple events in May-October of 2013-2018 using δ18O and δ2H. The new water contributions obtained by the IHS during peak flow were compared with the average new water fraction from the EIHS. The results showed that EIHS provided average new water fractions during peak flows (0.46 for δ18O and 0.47 for δ2H) that were close to the averages obtained by IHS (0.48 for δ18O and 0.50 for δ2H). While the EIHS may be a more robust approach compared to IHS, as it relaxes some of the assumptions of IHS and it gives a reliable average of the new water contribution, the IHS can provide useful information on the new water contribution variability for individual events.