Secchi Disk Measurements in Turbid Water

Abstract In the classical theory of the Secchi disk depth, diffuse sunlight falling on the disk is reflected back to the observer's eye along the most direct route, as a beam. The disappearance depth, Z SD , of the disk is then expected to vary inversely with the sum of the beam and diffuse attenuation coefficients: c + K D . Observations presented here show that, in the most turbid waters sampled, the Secchi disk is visible at greater depths (by a factor of up to 4) than predicted by this theory. In these conditions, the disk appears blurry, and it seems likely that some of the light reflected by the disk returns to the eye as diffuse light, photons being scattered one or more times on their journey from the disk surface to the observer. We have modified the theory of the Secchi disk in turbid water to allow for a mixture of beamed and diffuse light contributing to disk visibility. The modified theory corrects the under‐estimate of Secchi depths in turbid waters and gives good agreement with observations over a wide range of turbidity. The insight gained allows a more informed interpretation of Secchi disk measurements in turbid water.