Secchi Disc Transparency and Turbidity

Abstract

The optical principles that govern the two common measures of clarity, turbidity (Tn,NTU) and Secchi disc transparency (SD, m) are reviewed. It is demonstrated that these measurements differ fundamentally in their sensitivity to light attenuating processes, and that they cannot be uniquely specified by each other. A generalized inverse relationship between SD and Tn is developed, of the form SD=N″/Tn (where N″ is a variable coefficient), that may be valuable in some cases in estimating one parameter from the other. The coefficient N″ can range from 4.0–10.1, depending on the relative contributions of absorption and scattering to light attenuation, the relationship between Tn and the scattering coefficient (b,m-1), and the conditions that prevail during measurement of SD. Optical measurements made on five test systems, encompassing a wide range of optical conditions, indicate Tn is a reliable estimator of b, and support the developed relationship between SD and Tn and related optical theory.