Using high-frequency solute synchronies to determine simple two-end-member mixing in catchments during storm events

Brekenfeld, Nicolai; Cotel, Solenn; Faucheux, Mikaël; Floury, Paul; Fourtet, Colin; Gaillardet, Jérôme; Guillon, Sophie; Hamon, Yannick; Henine, Hocine; Petitjean, Patrice; Pierson-Wickmann, Anne-Catherine; Pierret, Marie-Claire; Fovet, Ophélie

Stream water chemistry at catchment outlets is commonly used to infer the flow paths of water through the catchment and to quantify the relative contributions of various flow paths and/or end-members, especially during storm events. For this purpose, the number and nature of these flow paths or end-members are commonly determined with principal component analysis based on all available conservative solute data. Here, for a given pair of measured solutes, we propose a methodology to determine the minimum number of required end-members, based on the ion’s synchronous variation during storm events. This allows identifying solute pairs, for which a simple two end-member mixing model is sufficient to explain their variation during storm events and solute pairs, which show a more complex pattern, requiring a higher-order end-member mixing model. We analysed the concentration-concentration relationships of several major ion pairs on the storm-event scale, using multi-year, high-frequency (< 60 minutes) monitoring data from the outlet of two small (0.8 to 5 km²) French catchments with contrasting land-use, climate and geology. A large number of storm-events (56 to 92 %) could be interpreted as the result of the mixture of only two end-members, depending on the catchment and the ion pairs used. Even though some of these results could have been expected (e.g. a two-end-member model for the Na +/Cl - pair in a catchment close to the Atlantic coast), others were more surprising and in contrast to previous studies. These findings might help to revise or improve the perceptual catchment understanding of flow path or end-member contributions and of biogeochemical processes. In addition, this methodology can identify, which solute pairs are governed by identical hydro-biogeochemical processes and which solutes are modified by more complex and diverse processes.

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Brekenfeld, Nicolai / Cotel, Solenn / Faucheux, Mikaël / et al: Using high-frequency solute synchronies to determine simple two-end-member mixing in catchments during storm events. 2023. Copernicus Publications.

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