Abstract
Abstract. Daily total suspended solids concentrations (TSS, mg L-1), yields (Y, kg day-1 km-2) and runoff (q, L s-1 km-2) in world rivers are described by the median (C50), the upper percentile (C99), the discharge-weighted average concentrations (C*), and by their corresponding yields (Y50, Y99, Y*) and runoff (q*, q50, q99). These intra-station descriptors range over two to six orders of magnitude at a given station. Inter-station variability is considered through three sets of dimensionless metrics: (i) q*/q50, C*/C50 and Y*/Y50, defining the general temporal variability indicators, and q99/q50, C99/C50 and Y99/Y50, defining the extreme variability indicators; (ii) river flow duration (W2) and flux duration (M2) in 2% of time; and (iii) the truncated rating curve exponent (b50sup) of the C vs q relationship for the upper flows. The TSS and Y variability, measured on US, French and world rivers, are first explained by hydrological variability through the b50sup metric, the variability amplifier, then by basin size, erodibility, relief and lake occurrence. Yield variability is the product of runoff variability × TSS variability. All metrics are considerably modified after river damming. The control of river particulate matter (RPM) composition by TSS or yields depends on the targeted component. For major elements (Al, Fe, Mn, Ti, Si, Ca, Mg, Na, K), the average RPM chemistry is not dependent on C* and Y* in most world hydroregions, except in the tropical hydrobelt where it is controlled by basin relief. By contrast, the particulate organic carbon content (POC, as a percentage of RPM) is inversely correlated to TSS concentrations for (i) intra-station measurements in any hydroregion, and (ii) inter-station average POC and TSS figures in world rivers. TSS controls heavy metal content (ppm) in highly contaminated basins (e.g. Cd in the Seine vs the Rhone), and total metal concentration (ng/L) in all cases. Relations between RPM composition and TSS should be taken into account when assessing riverine fluxes, as ignoring them could lead to overestimation.
Highlights
Since the pioneering survey of the Mississippi by Humphrey & Abbot (1861), who measured daily concentrations of total suspended solids (TSS, mg L-1) and fluxes, river hydrologists have been familiar with the great temporal variability of total suspended sediments (TSS) and how it is affected by river flow (Q)
The temporality of riverine fluxes has been investigated mostly by river engineers and geographers through the integrative concept of sediment flux duration (ASCE, 1970; Walling, 1977; Dunne,1979; Meybeck et al, 2003; Moatar et al, 2013), little used by river hydrologists
The temporal variations of TSS in world rivers at the daily scale can be addressed by specific metrics, for river runoff, SPM concentrations and for daily fluxes or yields
Summary
Since the pioneering survey of the Mississippi by Humphrey & Abbot (1861), who measured daily concentrations of total suspended solids (TSS, mg L-1) and fluxes, river hydrologists have been familiar with the great temporal variability of TSS and how it is affected by river flow (Q). We can focus on the upper half of flow values (q > q50), which corresponds to the greatest part of riverine fluxes (Meybeck & Moatar, 2012) To this end, the (C, q) couples are first split into two groups described by two segmented exponents of the rating curve, generally the log-log linear correlation C = a qb, b50inf and b50sup (Fig. 4). The upper half of the rating curve corresponds to the great majority of riverine fluxes for all types of material, even when they are diluted during flows: 61 to 90% of total fluxes for TDS, 67 to 97.8% for nitrate and phosphate, and 85 to 99% for total phosphorus and total Kjeldahl nitrogen For suspended solids, this proportion ranges between 79 and 99.9% (median 97.2%) (Meybeck & Moatar, 2012). In mountainous river basins, where sediment yields exceed 200 t year-1 km-2
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