Spatial and Temporal Variability in Concentration-Discharge Relationships at the Event Scale

The analysis of concentration-discharge (C-Q) relationships from low-frequency observations is commonly used to assess solute sources, mobilization, and reactive transport processes at the catchment scale. High-frequency concentration measurements are increasingly available and offer additional insights into event-scale export dynamics. However, only few studies have integrated inter-annual and event-scale C-Q relationships. Here, we analyze high-frequency measurements of specific conductance (EC), nitrate (NO3-N) concentrations and spectral absorbance at 254 nm (SAC(254), as a proxy for dissolved organic carbon) over a two year period for four neighboring catchments in Germany ranging from more pristine forested to agriculturally managed settings. We apply an integrated method that adds a hysteresis term to the established power law C-Q model so that concentration intercept, C-Q slope and hysteresis can be characterized simultaneously. We found that inter-event variability in C-Q hysteresis and slope were most pronounced for SAC(254) in all catchments and for NO3-N in forested catchments. SAC(254) and NO3-N event responses in the smallest forested catchment were closely coupled and explainable by antecedent conditions that hint to a common near-stream source. In contrast, the event-scale C-Q patterns of EC in all catchments and of NO3-N in the agricultural catchment without buffer zones around streams were less variable and similar to the inter-annual C-Q relationship indicating a homogeneity of mobilization processes over time. Event-scale C-Q analysis thus added key insights into catchment functioning whenever the inter-annual C-Q relationship contrasted with event-scale responses. Analyzing long-term and event-scale behavior in one coherent framework helps to disentangle these scattered C-Q patterns.

Automated summary

The study analyzed high-frequency measurements over a two year period in four neighboring catchments in Germany. It found that SAC(254) showed the most pronounced variability in C-Q hysteresis and slope in all catchments, while NO3-N variability was significant in forested catchments. Event-scale C-Q analysis provided key insights into catchment functioning, helping to disentangle scattered C-Q patterns.