The EXPECT method: a multi-tracer approach for reliable high temporal resolution young water fraction estimates

The portion of recently introduced water molecules in a stream, known as the young water fraction, is crucial in catchment intercomparison studies. The unweighted or flow-weighted average young water fraction in a catchment, over the period of isotope sampling, can be assessed through the ratio of flow-weighted or unweighted seasonal isotope cycles amplitudes in streamwater (A(*)S) and precipitation (AP), respectively. The symbol ‘*’ here indicates a flow-weighted variable. However, the young water fraction resulted to be a no-stationary quantity within individual catchments. Indeed, past studies revealed that young water fractions increase with stream discharge (Q). Accordingly, the rate of increase in young water fraction with increasing Q has been defined as the discharge sensitivity of young water fraction (S*d). S*d has been quantified as the parameter of a non-linear equation that expresses how AS(Q) varies with Q. Such parameter is directly obtained by fitting a sine curve, with amplitude AS(Q), on streamwater isotope data. Accordingly, in catchments with sparse isotope data S*d could be highly uncertain. In this study, we introduce a novel approach designed to enhance the temporal resolution of young water fraction estimates, consequently refining the determination of S*d. Our proposed method, referred to as EXPECT, is grounded in three fundamental assumptions. We propose a mixing relationship that follows an exponential decay of EC with an increasing young water fraction. We posit that the two-component hydrograph separation technique, utilizing measured Electrical Conductivity (EC) as a proxy of water age and the aforementioned exponential mixing relationship, can effectively delineate the proportion of young and old water in a stream by using appropriate end-members. We assume that the EC value of the young water endmember (ECyw) is lower than that of the old water endmember (ECow). The two endmembers, ECyw and ECow, have been adjusted through a calibration process by aligning the unweighted and flow-weighted average young water fractions obtained through hydrograph separation with the corresponding values derived from seasonal isotope cycles (AS/AP  and A*S/AP, respectively). The method has been tested in three small catchments in the Alptal valley, Switzerland, returning promising results. Nevertheless, we emphasize the importance of considering the limitations of EC as a tracer and the peculiar characteristics of the catchments under investigation for the appropriate application of the EXPECT method. Keywords: Stable water isotopes, Electrical Conductivity, Young water fraction, Discharge Sensitivity Acknowledgements: This publication is part of the project NODES which has received funding from the MUR –M4C2 1.5 of PNRR with grant agreement no. ECS00000036. References Gentile, A., von Freyberg, J., Gisolo, D., Canone, D., and Ferraris, S.: Technical Note: two-component Electrical Conductivity-based hydrograph separaTion employing an EXPonential mixing model (EXPECT) provides reliable high temporal resolution young water fraction estimates in three small Swiss catchments, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2023-1797, 2023.