Abstract
Concentration–discharge (C-Q) relationships are a convenient and increasingly popular tool for interpreting the episodic hydrochemical response to the varying discharge in small basins, providing insights into solute transport and streamflow generation. While most studies are focused on total runoff, this study quantified C-Q relationships in four runoff components during precipitation events at the Hydrohill experimental catchment in Chuzhou, China. This unique artificial catchment is carefully engineered, allowing observations of the interacting runoff components that collectively determine total flow issuing from the catchment. The four runoff components, or flow paths, include surface runoff (SR), shallow interflow at 0–30 cm depth (SSR30), deeper interflow at 30–60 cm depth (SSR60), and groundwater flow at 60–100 cm depth (SSR100). Water samples were collected during three consecutive precipitation events to study how the concentrations of primary solutes vary with flow. Analysis of C-Q relationships reveals that concentrations of Na+, Ca2+, Mg2+, SO42−, and HCO3− in the four runoff components had a negative relationship with discharge, while the concentration of K+ and Cl− were negatively correlated with discharge in SR and SSR30 but positively correlated in SSR60 and SSR100. Further insights were gained from principal component analysis. Three eigenvectors explained 92% of the variability in hydrochemistry in surface runoff, while two eigenvectors explained most of the variability in the hydrochemistry of subsurface flows observed at various depths in the soil profile (73% for SSR30, 79% for SSR60, and 76% for SSR100). PC1 (the first Principal Component) can be interpreted as a salinity factor, deriving from carbonate minerals such as dolomites and limestone minerals. Results indicated that leaching and dilution processes, water–soil interaction, and macropore flows in soils are the primary factors controlling the C-Q relationships. Our work sheds light on the coupled processes and streamflow generation mechanisms that control water quality at the catchment scale.
Highlights
Concentration–discharge (C-Q) relationships are an increasingly popular tool for interpreting episodic hydrochemical responses to the varying discharge in small catchments [1,2,3]
Three principal component eigenvectors explained most of the variability in hydrochemistry in surface runoff, while two eigenvectors explained most of the variability in the hydrochemistry of subsurface flows observed at various depths in the soil profile
C-Q relationships reveal that the concentrations of Na+, Ca2+, Mg2+, SO4 2−, and HCO3 − in surface runoff (SR), shallow subsurface interflows (SSR30 and SSR60), and in groundwater flows (SSR100) have a negative relationship with discharge
Summary
Concentration–discharge (C-Q) relationships are an increasingly popular tool for interpreting episodic hydrochemical responses to the varying discharge in small catchments [1,2,3]. Solutes show different responses to the changing discharge with the origin, reactivity, and physical phase of materials influencing the C-Q relationship [1,4,5]. Water 2020, 12, 3033 many catchments have shown that the behavior within the watershed is influenced by flushing or concentrating effects of lithologic solutes [6,7,8]. Storm events provide opportunities to mobilize and transport these solutes. Describing and quantifying hydrological and geochemical responses to storm events, in turn, improves the understanding of the processes underlying solute and particulate export to catchment outlets [11]. The concentrations of most dissolved species in runoff decline during periods of high discharge, while a few increases in concentration or are unchanged
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