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

Abstract

Abstract. Studying the response of streamwater chemistry to changes in discharge can provide valuable insights into how catchments store and release water and solutes. Previous studies have determined concentration–discharge (cQ) relationships from long-term, low-frequency data of a wide range of solutes. These analyses, however, provide little insight into the coupling of solute concentrations and flow during individual hydrologic events. Event-scale cQ relationships have rarely been investigated across a wide range of solutes and over extended periods of time, and thus little is known about differences and similarities between event-scale and long-term cQ relationships. Differences between event-scale and long-term cQ behavior may provide useful information about the processes regulating their transport through the landscape. Here we analyze cQ relationships of 14 different solutes, ranging from major ions to trace metals, as well as electrical conductivity, in the Swiss Erlenbach catchment. From a 2-year time series of sub-hourly solute concentration data, we determined 2-year cQ relationships for each solute and compared them to cQ relationships of 30 individual events. The 2-year cQ behavior of groundwater-sourced solutes was representative of their cQ behavior during hydrologic events. Other solutes, however, exhibited very different cQ patterns at the event scale and across 2 consecutive years. This was particularly true for trace metals and atmospheric and/or biologically active solutes, many of which exhibited highly variable cQ behavior from one event to the next. Most of this inter-event variability in cQ behavior could be explained by factors such as catchment wetness, season, event size, input concentrations, and event-water contributions. We present an overview of the processes regulating different groups of solutes, depending on their origin in and pathways through the catchment. Our analysis thus provides insight into controls on solute variations at the hydrologic event scale.

Do stream water solute concentrations reflect when connectivity occurs in a small, pre-Alpine headwater catchment?

“…We found that the event water fractions from two-component hydrograph separation (isotopes) and EMMA (multi-tracer) were comparable (Table 4). Similar to our results, Ladouche et al (2001) found for the 0.8 km 2 Strengbach catchment in France that the hydrograph separation results based on δ 18 O (f pe -10 %) were relatively similar to the results of their mixing analyses (including DOC, Si, Ba and U), and that a multi-tracer approach allowed them to distinguish between pre-event water contributions from the upper and lower part of the catchment. We found that concentrations of metals, such as iron or copper, were much higher than expected from mixing of rainfall and baseflow, whereas weathering-derived solutes, such as sodium or calcium, were lower than expected from mixing of rainfall and baseflow.…”

“…Knowledge of which areas are connected and contribute to streamflow is important because it helps us to shape our conceptual understanding of how catchments function. For example, Ladouche et al (2001) showed for the 0.8 km 2 Strengbach catchment in France that the upper layers of saturated areas contributed up to 30 % of the discharge during the initial stages of a rainfall event, even though these areas occupied only 2 % of the catchment area. However, during the final stage of the event, upslope and downslope areas contributed equally to flow.…”

Differential C-Q Analysis: A New Approach to Inferring Lateral Transport and Hydrologic Transients Within Multiple Reaches of a Mountainous Headwater Catchment

“…While chemodynamic behavior of redox-sensitive and biogeochemically active solutes is commonly observed, chemodynamic behavior of weatheringbased ions has also been reported in some carbonate-dominated catchments (e.g., Sullivan et al, 2018). This is usually attributed to spatial heterogeneity, activation of different flow paths and/or mineral control (Koger et al, 2018;Rose et al, 2018;Molins et al, 2019;Knapp et al, 2020). At PH, sulfuric acid weathering is estimated to account for 35-75% carbonate dissolution (Winnick et al, 2017), thus resulting in source-limitation of weathering solutes at this location.…”

“…The log-log plot here indicates that the hypothetical solute shows dilution with increasing discharge at both stations A and B, albeit to different degrees. Although recent availability of highfrequency data sets are expanding the scope and application of these techniques (e.g., Rose et al, 2018;Knapp et al, 2020), log-log plots are typically applied to long time-frame datasets and are used to compare solutes across different catchments, while standard C-Q relationships are typically used to study solutes intensively at one station with temporal differentiation. More importantly, both the standard and logarithmic C-Q relationships provide solute behavior characteristics at a single location within a stream.…”

Fingerprinting hydrological and biogeochemical drivers of freshwater quality

“…Seasonal c-q patterns were found to differ from event-based ones for phosphorus and nitrate in 219 French catchments covering a wide range of soil, climate and land use characteristics (Minaudo et al, 2019). These differences between long-term and event-based c-q responses likely indicate different controls on solute transport and storage processes on different temporal scales (Knapp et al, 2020). More recently, the notion that biogeochemical cycling is switched off or severely dampened, depending on the magnitude of the storm event has gained prominence, with higher magnitude storm events thought to have a greater damping effect (Bernhardt et al, 2018;Raymond et al, 2016) compared to low-magnitude storm events that can preserve diel cycles (Burns et al, 2016).…”