Investigating the relationship between particle size of suspended sediments and optical sensor turbidity readings

Optical sensors are widely used for turbidity measurements in suspended sediment concentration studies. When conducting continuous measurements of turbidity in the field with optical sensors, results are determined according to the calibration curve (relationship between suspended sediment concentration and turbidity readings from the optical sensors). The calibration curve is developed based on the samples of the material present in and/or around the investigated stream. The concentration data are useful in water-quality-related investigations as well as for evaluating the amounts of transported (flushed, eroded) material from the catchments as suspended load. The amounts and particle sizes of transported material depend on the hydrological conditions. Usually, the particles’ size is not directly considered when developing the calibration curve. However, different particle sizes of the material from the same study site can result in different turbidity readings. Taking into account one general calibration curve for suspended sediment concentration determination can lead to misestimation of the transported material amounts. Here, the results of turbidity sensor calibration for different particle size classes are presented. Additionally, the uncertainty of the suspended material concentrations due to this effect is estimated. Further, we show how different calibration curves affect the assessment of the amount of the transported suspended load from the selected experimental catchment.