Source-to-stream connectivity assessment through end-member mixing analysis

Streamflow sources across various hydrologic conditions were examined in a 5.1 ha temperate humid forested catchment (Laurentians, Canada). In that system, the relationship between rainfall and runoff is nonlinear, thus hinting towards complex processes involving critical, transient source areas and a changing catchment internal state of connectivity. Multiyear daily stream chemistry data were broken down into several hydrologic scenarios reflecting different conditions with respect to stream discharge and antecedent catchment wetness. End-member mixing analysis and mass balance calculations were performed to: (1) compare the dimensionality of the mixing spaces (i.e. the number of streamflow sources) obtained under 64 different hydrologic scenarios; (2) screen independently sampled end-members (i.e. the nature of sources) to assess catchment connectivity from a spatial perspective; and (3) estimate the relative contributions of end-members to streamflow to characterize hydrological connectivity from a volumetric standpoint. Mixing space dimensionality did not vary significantly among the tested hydrologic scenarios, as three end-members were generally required to account for most of the variance in stream geochemistry. Differences were significant in the ability of the tested end-members to fit in mixing spaces; for instance, throughfall and organic soil water end-members better fitted in mixing spaces associated with high rather than low discharges. The relative contributions of end-members to streamflow were highly variable in time. Scenarios involving low discharges and dry antecedent conditions were mostly associated with baseflow, while scenarios involving high discharges and wet antecedent conditions were associated with increased proportions of throughfall and organic soil water from downstream downslope and downstream upslope areas. These results suggest a cautious evaluation of the predictive power of one single mixing space with regards to the nature of streamflow sources across hydrologic conditions.