Concentration-Discharge Relationships Revisited: Overused But Underutilised?

Over the past 50 years, concentration-discharge (cQ) relationships have been widely used to analyse water quality dynamics. Nowadays improved availability of concentration (c) and discharge (Q) data at different spatial and temporal scales have led to a high popularity of cQ applications. However, despite their widespread use, we see persistent challenges in the integration of cQ relationships across temporal scales, and in the identification of the encoded processes. In this commentary, we show that different catchment processes may lead to similar cQ responses resulting in a lack of clear causality. We emphasise that cQ relationships applied at different time scales integrate different parts of the catchment and may, therefore, convey different information. Finally, we advocate for the careful use of cQ relationship as one, but not the only, tool in addressing ecohydrological questions.