Concentration-discharge relationships vary among hydrological events, reflecting differences in event characteristics

Studying the response of streamwater chemistry to changes in discharge can improve our understanding of how catchments store and release water and solutes. Previous studies have determined concentration-discharge (cQ) relationships from long-term, low-frequency data for many different solutes. These analyses, however, provide little insight into the coupling of solute concentrations and flow during individual hydrologic events. Although intra-event cQ relationships have been determined for selected solutes and storm events, they have rarely been investigated across a wide range of solutes and over extended periods of time. Thus, little is known about how intra-event and longer-term cQ relationships may differ, potentially providing different perspectives on processes regulating transport through the landscape.

We present cQ relationships of 14 different solutes, ranging from major ions to trace metals, as well as electrical conductivity, in the Swiss Erlenbach catchment (0.7 km²). From a 2-year time series of sub-hourly solute concentration measurements, we determined long-term cQ relationships for each solute. We compared these to cQ relationships spanning the hydrograph recessions of 30 individual events. Solutes sharing the same dominant water sources exhibited similar behavior. Groundwater-sourced solutes exhibited dilution patterns, and their long-term cQ behavior was representative of their cQ behavior during hydrologic events. Other solutes, however, exhibited highly variable cQ behavior from one event to the next, and very different cQ patterns at intra-event and longer-term time scales. This was particularly true for trace metals as well as atmospherically derived and/or biologically active solutes. Most of the observed event-to-event variability in cQ behavior could be explained by factors such as catchment wetness, season, event size, input concentrations, and event-water contributions. These relationships help to clarify how the release of solutes depends on their catchment sources and pathways. Our analysis thus provides insight into controls on solute variations at the hydrologic event scale.