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Daily variability of river concentrations and fluxes: indicators based on the segmentation of the rating curve

Abstract : The variability of water chemistry on a daily scale is rarely addressed due to the lack of records. Appropriate tools, such as typologies and dimensionless indicators, which permit comparisons between stations and between river materials, are missing. Such tools are developed here for daily concentrations (C), specific fluxes or yields (Y) and specific river flow (q). The data set includes 128 long-term daily records, for suspended particulate matter (SPM), total dissolved solids (TDS), dissolved and total nutrients, totalling 1236 years of records. These 86 river basins (103-106 km2) cover a wide range of environmental conditions in semi-arid and temperate regions. The segmentation--truncation of C-q rating curves into two parts at median flows (q50) generates two exponents (b50inf and b50sup) that are different for 66% of the analysed rating curves. After segmentation, the analysis of records results in the definition of nine major C-q types combining concentrating, diluting or stable patterns, showing inflexions, chevron and U shapes. SPM and TDS are preferentially distributed among a few types, while dissolved and total nutrients are more widely distributed. Four dimensionless indicators of daily variability combine median (C50, Y50), extreme (C99, Y99) and flow-weighted (C*, Y*) concentrations and yields (e.g. C99/C50, Y*/Y50). They vary over two to four orders of magnitude in the analysed records, discriminating stations and river material. A second set of four indicators of relative variability [e.g. (Y*/Y50)/(q*/q50)], takes into account the daily flow variability, as expressed by q*/q50 and q99/q50, which also vary over multiple orders of magnitude. The truncated exponent b50sup is used to describe fluxes at higher flows accounting for 75% (TDS) to 97% (SPM) of interannual fluxes. It ranges from − 0*61 to + 1*86 in the database. It can be regarded as the key amplificator (positive b50sup) or reductor (negative b50sup) of concentrations or yields variability. C50, Y50, b50sup can also be estimated in discrete surveys, which provides a new perspective for quantifying and mapping water quality variability at daily scale
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Michel Meybeck, Florentina Moatar. Daily variability of river concentrations and fluxes: indicators based on the segmentation of the rating curve. Hydrological Processes, Wiley, 2012, 26 (8), pp.1188-1207. ⟨10.1002/hyp.8211⟩. ⟨insu-00616551⟩

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