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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/hess-2017-528
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 4.0 License.
Technical note
04 Oct 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Hydrology and Earth System Sciences (HESS).
Technical Note: False low turbidity readings during high suspendedsediment concentrations
Nicholas Voichick, David J. Topping, and Ronald E. Griffiths U.S. Geological Survey, Grand Canyon Monitoring and Research Center, 2255 N. Gemini Dr., Flagstaff, Arizona, 86001, USA
Abstract. Turbidity, a measure of water clarity, is monitored for a variety of purposes including: 1) to help determine whether water is safe to drink; 2) to establish background conditions of lakes and rivers and detect pollution caused by construction projects and storm water discharge; and 3) to establish connections with aquatic biological properties, such as primary production and predator-prey interactions. Turbidity is normally measured with an optical probe that detects light scattered from particles in the water. Probes have defined upper limits of the range of turbidity that they can measure. The general assumption is that when turbidity exceeds this upper limit, the values of turbidity will be constant, i.e., the probe is pegged; however, this assumption is not necessarily valid. In cases where turbidity greatly exceeds the upper measurement limit, turbidity probes can falsely report incorrectly low values of turbidity that appear to be within the limits of the probe. In rivers with limited variation in the physical properties of the suspended sediment, an increase in suspended-sediment concentration will initially cause a linear increase in turbidity. When the suspended-sediment concentration in these rivers causes turbidity levels that exceed the upper measurement limit of a probe, turbidity probes do not necessarily peg at a constant value. Data from the Colorado River in Grand Canyon, Arizona, USA and a laboratory experiment both demonstrate that when turbidity exceeds instrument-pegged conditions, increasing suspended-sediment concentration (and thus increasing turbidity) may cause optical probes to record decreasing false turbidity values that appear to be within the valid measurement range of the probe. Therefore, under high turbidity conditions, other surrogate measurements of turbidity (e.g., acoustic attenuation measurements or suspended-sediment samples) are necessary to correct these low turbidity measurements and accurately measure turbidity.

Citation: Voichick, N., Topping, D. J., and Griffiths, R. E.: Technical Note: False low turbidity readings during high suspendedsediment concentrations, Hydrol. Earth Syst. Sci. Discuss., https://doi.org/10.5194/hess-2017-528, in review, 2017.
Nicholas Voichick et al.
Nicholas Voichick et al.
Nicholas Voichick et al.

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