A Generic Relation Between Turbidity, Suspended Particulate Matter Concentration, and Sediment Characteristics

Optical turbidity sensors have become standard instrumentation to estimate suspended particulate matter concentration (SPMC). However, turbidity readings respond to factors other than SPMC, such as particle size and shape, organic fraction, sediment density and color. Therefore, local and site dependent calibrations are needed to transfer turbidity to SPMC. In this study, we propose a new relation between turbidity, suspended particulate matter (SPM) carbon content and particle size, which helps to move from local calibrations between turbidity and SPMC toward generic relations based on inherent SPM properties. Organic content is known to influence particle size, density and, as a result, turbidity, but an explicit formulation of turbidity accounting for organic content has not yet been established. We address this knowledge gap by collecting field data from four sites representing contrasting land use types, geological settings and watershed size. The sites were monitored with a turbidity meter, a LISST‐200X particle size analyzer and an automatic water sampler. SPM samples were collected and analyzed for particle size, carbon content, and color. Local calibrations between turbidity and SPMC derived at each site were compared with a generic calibration, where SPMC was obtained as a function of turbidity, carbon content, particle size and color. The mean relative error associated to predicted SPMCs was equal to 3% when using the generic calibration, which is only marginally lower than the mean relative error of 5% obtained using the local calibrations. Plain Language Summary Turbidity is the degree of scatter in the optical path of light caused by matter suspended in water. It is considered as a surrogate for suspended particulate matter concentration (SPMC), and is predominantly measured by optical sensors. These optical sensors are calibrated in the laboratory using manually collected suspended particulate matter (SPM) samples. A calibration is needed before a turbidity sensor can measure SPMC, because turbidity readings are highly influenced by SPM properties such as particle size, carbon content, color, shape and density. As a consequence, currently, a new empirical relation (hereinafter referred to as “local calibration”) has to be established at each new site. The process of developing a site‐specific local calibration requires a significant manual effort. This study proposes a site‐independent calibration approach (hereinafter referred to as “generi