End-Member Selection in Two-Component Isotope-Based Hydrograph Separation

The science that underpins our knowledge and understanding of Isotope-Based Hydrograph separation (IHS) has gained grounds, over the last few decades, in the identification of streamflow sources. However, challenges still exist in identifying appropriate tracers and the right combination of end-members for the IHS process. In a two-component IHS analysis, the application of the dual isotopes tracers, δ18O and (or) δ2H, is regarded as the simplest method. We undertook an IHS study within a nested system of eight Prairie watersheds located in South central Manitoba, Canada. The work evaluated about 17,000 results emanating from the application of a combination of two potential tracers (δ18O and δ2H) and eight each of potential “old” and “new” water end-members in a two-component IHS process. The outcome showed occurrences of many mathematically possible but hydrologically unacceptable IHS results. The observation was particularly predominant within relatively larger perennial sub-catchments of the watershed. It is also shown that inter-site sub-catchment isotopic end-member transferability is possible within watersheds of similar physio-hydrographic characteristics. We suggest that a careful evaluation of the physio-hydrographic characteristics of catchments be considered in IHS studies in addition to the recommended guidelines in the selection of tracers and end-members.