Abstract
The quantification of solute and sediment export from drainage basins is challenging. A large proportion of annual or decadal loads of most constituents is exported during relatively short periods of time, a “hot moment”, which vary between constituents and catchments. We developed a new framework based on concentration-discharge (C-Q) relationship to characterize the export regime of stream particulates and solutes during high water periods when the majority of annual and inter-annual load is transported. We evaluated the load flashiness index (percentage of cumulative load that occurs during the highest 2 % of daily load, M2), a function of flow flashiness (percentage of cumulative Q during the highest 2% of daily Q, W2) and export pattern (slope of the logC-logQ relationship for Q higher than the daily median Q, b50high). We established this relationship based on long-term water quality and discharge datasets of 580 streams sites of France and USA, corresponding to 2507 concentration time series of total suspended sediments (TSS), total dissolved solutes (TDS), total phosphorus (TP), nitrate (NO3) and dissolved organic carbon (DOC), generating 1.5 million data points in highly diverse geologic, climatic and anthropogenic contexts. Load flashiness (M2) increased with b50high and/or W2. Also, M2 varied as a function of the constituent transported. M2 had the highest values for total suspended sediments (TSS) and decreased for the other constituents in the following order: total phosphorus (TP), dissolved organic carbon (DOC), nitrate (NO3), total dissolved solutes (TDS). Based on these results, we constructed a load-flashiness diagram to determine optimal monitoring frequency of dissolved or particulate constituents as a function of b50high and W2. Based on M2, optimal temporal monitoring frequency of the studied constituents decreases in the following order: TSS, TP, DOC, NO3, and TDS. Finally, we analyzed relationships between these metrics and catchments characteristics. Depending on the constituent, we explained between 30 to 40% of their M2 variance with simple catchment characteristics, such as stream network density or percentage of intensive agriculture. Therefore, catchment characteristics can be used as a first approach to set up water quality monitoring design where no hydrological and/or water quality monitoring exist.
Highlights
Quantifying solute and sediment export from drainage basins is important to understanding stream biogeochemical processes (Jarvie et al, 2018), soil erosion (Vanmaercke et al, 2011), chemical weathering rates (Meybeck, 1987; Gaillardet et al, 1999), and water quality issues, such as eutrophication of inland and coastal waters (Le Moal et al, 2019) or coastal anoxia (Breitburg et al, 2018)
We were able to demonstrate that load flashiness (M2), i.e., percentage of cumulative load that occurs during the highest 2% of daily load values is a useful proxy of solute and sediment export regimes
We demonstrated that these two proxies are related to load flashiness (M2) with a simple equation
Summary
Quantifying solute and sediment export from drainage basins is important to understanding stream biogeochemical processes (Jarvie et al, 2018), soil erosion (Vanmaercke et al, 2011), chemical weathering rates (Meybeck, 1987; Gaillardet et al, 1999), and water quality issues, such as eutrophication of inland and coastal waters (Le Moal et al, 2019) or coastal anoxia (Breitburg et al, 2018). Solute and sediment export are related to climate, lithology, landscape structure, and land use (Burt and Pinay, 2005; Gall et al, 2013; Musolff et al, 2015; Van Meter et al, 2017), a large proportion of the annual or decadal load of sediment, particulate contaminants and even some solutes are exported during relatively short hydrological events (Syvitski and Morehead, 1999). Meybeck et al (2003) showed that large sediment loads often occur over short periods of time, e.g., hours to days, leading to skewed distributions of suspended sediment daily loads. As total phosphorus (TP) is largely transported in particulate form, its load distribution is often skewed compared to discharge (Johnes, 2007; Minaudo et al, 2017). A substantial portion of annual phosphate export is associated with large events while nitrate tends to present a more uniform flux regime (Frazar et al, 2019)
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