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.
Highlights
The movement of water and solutes through the landscape is inherently coupled
Our study aims to explore questions such as (1) how 2-year cQ behavior differs from cQ behavior observed on the event scale; (2) how variable cQ relationships between individual events are; and (3) if inter-event variability in cQ relationships can be explained by specific environmental controls
We quantified 15 different parameters for each event from the following five categories: (1) temperature and proximity to midyear as seasonality indicators; (2) relative input concentrations, which quantify the ratio between the volume-weighted average precipitation concentration during the event to the streamwater solute concentrations during pre-event baseflow; (3) groundwater levels, baseflow discharges, and antecedent precipitation as indicators of antecedent wetness conditions; (4) several measures of event magnitude and intensity; and (5) event and pre-event-water contributions determined from isotope hydrograph separation
Summary
The movement of water and solutes through the landscape is inherently coupled. Streamwater chemistry at a catchment outlet differs depending on the flow paths of water through the catchment and can be considered a “fingerprint” of catchment transport, mixing, and reaction processes. Changes in solute concentrations as functions of discharge, i.e., concentration– discharge (or cQ) relationships, have commonly been assessed using multi-year time series of low-frequency (weekly to monthly) streamwater chemistry measurements (Hall, 1970; Godsey et al, 2009, 2019; Musolff et al, 2015) At this temporal scale, cQ relationships can serve as indicators of hydrologic and biogeochemical processes. The cQ relationships obtained from long-term, low-frequency data are useful for characterizing the average behavior of a catchment (Clow and Drever, 1996; Godsey et al, 2009; Godsey and Kirchner, 2014) These long-term cQ relationships provide limited insight into the coupling of streamwater chemistry and discharge on shorter timescales, such as during hydrologic events. Our study aims to explore questions such as (1) how 2-year cQ behavior differs from cQ behavior observed on the event scale; (2) how variable cQ relationships between individual events are; and (3) if inter-event variability in cQ relationships can be explained by specific environmental controls
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