Do End-member Definitions and Tracer Choice have a Significant Influence on the Results of Isotope-based Hydrograph Separation?

English Abstract:

Surface and subsurface runoff volumes are known to be made of new water – brought by the current precipitation event – as well as old water – that was already stored in a watershed before the onset of rainfall or snowmelt. To determine the relative contributions of each in a stormflow, stable water isotopes (δ¹⁸O and δ²H) are usually employed as tracers in a mass balance equation to separate the hydrograph. However, there is no consensus in the literature as to whether δ¹⁸O or δ²H should be preferentially used for hydrograph separation. Similarly, hydrograph separation often assumes for simplification purposes that end-members have constant isotopic signatures, but there is a lack of studies measuring the differences in results when constant versus temporally varying end-member signatures are used. In order to test the sensitivity of computed fractions of new and old water to tracer choice and end-member definition, hydrograph separation was performed in two nested Prairie watersheds within the larger 74.6 km² South Tobacco Creek Watershed (south-central Manitoba, Canada). Constant and temporally varying isotopic definitions of the new water end-member were used (i.e., yearly average versus weekly values in precipitation). Similarly, several definitions of the old water end-member (e.g., average baseflow isotopic composition versus weekly isotopic ratios in soil water and deeper groundwater) were tested using δ¹⁸O on one end, and δ²H on the other hand. Preliminary results show that the computed fractions of new and old water were not very sensitive to the chosen tracer but very sensitive to the definition of the old water end-member. Future work will involve carrying sensitivity analyses at the scale of specific hydrological events to assess the cumulative impact of antecedent moisture conditions and methodological choices on hydrograph separation results.