High-frequency chemical measurements in streamwater give detailed information on the different hydrochemical processes occurring in the catchment. They can provide clues about the end-members (water sources) whose mixing produces the ionic stream concentrations observed. To identify and quantify the end-members and their respective contribution to streamwater chemistry, we developed a new methodology, termed “IQEA” (Identification and Quantification of End-members and their Apportionment). It consists in the resolution by a minimization process of an objective function based on downstream high-frequency chemical data and the size of the shape of a pre-defined fixed number of three end-members. We applied the IQEA method on the high-frequency concentration dataset of nitrates and calcium from the ORACLE-Orgeval Observatory in France. The results were compared with field data obtained from various potential end-members. The IQEA methodology yielded robust and very satisfactory results, apart from an exceptional flood event that occurred during the 2 years of measurements used in this study. While the IQEA model treats concentration and discharge datasets independently, its results show a strong relationship between the dominant end-member computed and the hydrological features.

Concentration–discharge (C–Q) relationships are widely used to assess the link between hydrological and biogeochemical processes at catchment scale. They can be fitted to represent either concentrations or discharge-weighted concentrations (i.e., load). In this technical note, we test a multi-objective calibration using both concentration and load, and compare it to a mono-objective calibration applied on either concentrations or load: although widely used in hydrological modeling, multi-objective calibrations are rarely applied to fit C–Q relationships. The multi-objective calibration yielded parameters which were more representative of the concentration behavior during the rarest high-flow events, avoiding issues of under-representation of high-discharge, low-concentration periods. In this technical note, we use the multi-objective calibration on a high-frequency dataset acquired on the ORACLE-Orgeval Observatory (France).