The depth of a Secchi disk (Z_sd) has been used for ~160 years as a measure of water clarity. In the classic theory regarding Z_sd, the contrast between the Secchi disk and water is attenuated following light beam attenuation; in other words, a Secchi disk is treated as a point target. However, a recent theory challenges this classical theory and suggests that the contrast between the Secchi disk and water attenuates following the law of diffuse light. In this effort, we assess the two attenuation schemes through both field and numerical experiments. In the field experiment, a ring and a disk of the same diameter were used to measure Z_sd simultaneously in varying coastal waters; and similarly, Monte-Carlo simulations of Z_sd for a ring and a disk of the same diameter, respectively, were caried out for different water properties. If the classical theory of Z_sd is sound, we should expect “same” Z_sd values from the ring and the disk, but both field and numerical experiments show consistently lower (~35%) Z_sd from the ring. These results further support the new theory regarding the Secchi disk observation.

Secchi disk,Ring,Contrast Attenuation,Monte-Carlo simulation