Measurement differences between turbidity instruments, and their implications for suspended sediment concentration and load calculations: A sensor inter-comparison study

Predicting suspended sediment concentration from nephelometric turbidity in organic‐rich waters

“…The steeper regression slope coefficients between the riverine particulates and NTU are likely a consequence of the disproportionally lower turbidity values, for the same particulate concentrations, compared with the FNU results, supporting the observation that the ISO 7027 method is less influenced by the absorption of light by CDOM. These results are consistent with previous field‐based studies where different relationships between the SSC and turbidity in a variety of units have been observed (e.g., Barter & Deas, ; Hongve & Akesson, ; Lewis et al, ; Rymszewicz et al, ), although hydrological controls and variable sources are also key considerations (Gipple, ; Hughes et al, ; Walling, ). Instrument variability is also a key source of potential uncertainty in monitoring programmes that rely on turbidity, because turbidity values are not transferable between instruments of different manufacture, as evident from the interinstrument study of Rymszewicz et al ().…”

“…These results are consistent with previous field‐based studies where different relationships between the SSC and turbidity in a variety of units have been observed (e.g., Barter & Deas, ; Hongve & Akesson, ; Lewis et al, ; Rymszewicz et al, ), although hydrological controls and variable sources are also key considerations (Gipple, ; Hughes et al, ; Walling, ). Instrument variability is also a key source of potential uncertainty in monitoring programmes that rely on turbidity, because turbidity values are not transferable between instruments of different manufacture, as evident from the interinstrument study of Rymszewicz et al (). From a monitoring perspective, however, such influences are a commonly overlooked limitation on the use of turbidity as a predictor variable for suspended sediment and must be considered prior to examining patterns of hysteresis or other environmental processes.…”

“…As a result, there may be variability in turbidity response observed in natural water samples as a result of changing sample composition and a long‐recognized limitation of some turbidimeters (e.g., McCarthy, Pyle, & Griffin, ; McCluney, ; Sadar, ; USGS, ); however, such limitations and suitability of application are generally overlooked by users (Gipple, ). A comparison of 12 different nephelometric turbidimeters under laboratory conditions, for example, showed turbidity measurements varied five‐fold between instruments (Rymszewicz et al, ). Such limitations of turbidimeter suitability and vulnerability to dissolved optical properties are not an issue if turbidity is being used to assess suitability for drinking water; but this may be an issue when monitoring for environmental standards in natural river systems.…”

“…The objective of this study is to examine the effects of two turbidimeters on the ability to predict SSCs from natural river samples with regards to differences in POM that occur in differently vegetated catchments. Previous studies have examined the variability between turbidimeters (e.g., Barter & Deas, ; Hongve & Akesson, ; Lewis et al, ; Rymszewicz et al, ); however, the effects of such interferences on predictive relationships between SSC and turbidity have not been considered with regard to the influence of organic‐rich riverine suspended loads. We compare the response of two different turbidimeters and the predicted relationships between turbidity and suspended sediment, for natural river samples that include rivers under base flow and event flow conditions, as well as variable POM concentrations as a way to examine the effect of organic particulates on turbidity.…”

Understanding the effect of catchment characteristics on suspended sediment dynamics during flood events

“…Differences in the relationships between the sites may reflect intrinsic differences between the turbidity sensors, variability in sediment characteristics including size mixture, colour, mineral, and organic composition (Dogliotti, Ruddick, Nechad, Doxaran, & Knaeps, ; Down & Lehr, ; Fink, ; Fondriest Environmental, ; Murphy, Morse, & Cleveland, ). In regard to sensor variability, Rymszewicz et al () tested turbidity measurements of 12 different sensors to known pre‐prepared identical sediment concentrations. They found that, despite calibration to a Formazin standard, sensor responses to identical sediment concentrations varied considerably.…”