A national‐scale study of spatial variability in the relationship between turbidity and suspended sediment concentration and sediment properties

Abstract Turbidity, an index of light side‐scattering, depends on the mass concentration of suspended sediment (SS) within water. Turbidity of river water is regulated by the presence of suspended particulate matter and is used to identify visual changes in response to SS. We used data from the New Zealand National River Water Quality Network, to calculate “specific turbidity” (K; turbidity normalised to mass concentration of suspended particulates). Specific turbidity is shown here to be an effective metric to assess the effect of suspended material composition and particle size distribution of suspended particulate matter over different landscape characteristics, and the effect on SS‐turbidity relationships. Of the catchment characteristics considered in our study, specific turbidity was most responsive to lithological factors, and relatively insensitive to land use and soil parameters. Decreasing particle size has a positive linear response to K, attributed to the higher proportion of ultra‐fine particulate material that is generated by certain lithologies, underscoring the lithological influence on K. SS‐turbidity relationships, therefore, vary spatially across New Zealand's national record of water clarity, and K is considered a useful index for inter‐catchment comparison of SS than turbidity alone.