Calculation of river sediment fluxes from uncertain and infrequent measurements

15 16 This paper addresses feasibility issues in the calculation of fluxes of suspended particulate matter 17 (SPM) from degraded-quality data for flow discharge (Q) and sediment concentration (C) under the 18 additional constraints of infrequent and irregular sediment concentration samplings. A crucial setting 19 of the scope involves establishing the number of data required to counterbalance limitations in the 20 measurement accuracy and frequency of data collection. This study also compares the merits and 21 drawbacks of the classical rating curve (C=aQ b ) with those of an improved rating curve approach 22 (IRCA: C=aQ b +a1S) in which the correction term is an indicator of the variations in sediment storage, 23 thus relating it to flow dynamics. This alternative formulation remedies the known systematic 24 underestimations in the classical rating curve and correctly resists the degradation in data quality and 25 availability, as shown in a series of problematic though realistic cases. For example, monthly 26 concentration samplings (in average) with a random relative error in the [-30%, +30%] range 27 combined with daily discharge records with a systematic relative error in the [-5%, +5%] range still 28 yield SPM fluxes within factors of 0.60-1.65 of the real value, provided that 15 years of data are 29 available. A shorter 5-day time interval (on average) between samplings lowers the relative error in 30 the SPM fluxes to below 10%, a result directly related to the increased number of Q-C pairs available 31 for fitting. For regional-scale applications, this study may be used to define the data quality level 32 (uncertainty, frequency and/or number) compatible with reliable computation of river sediment 33 fluxes. Provided that at least 200 concentration samplings are available, the use of a sediment rating 34 curve model augmented to account for storage effects fulfils this purpose with satisfactory accuracy 35 under real-life conditions. 36 37 Author-produced version of the article published in Journal of Hydrology, 2014, N°508, p. 364-373 The original publication is available at http://www.sciencedirect.com Doi: 10.1016/j.jhydrol.2013.10.057