Abstract
AbstractIn 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, ZSD, of the disk is then expected to vary inversely with the sum of the beam and diffuse attenuation coefficients: c + KD. 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.
Highlights
Despite caveats about what it measures, the Secchi disk continues to be widely used to give a quick and simple estimate of water clarity
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
Light reflected from the disk travels back to the eye along the shortest route as a straight line, and its passage depends on the beam attenuation coefficient c
Summary
Despite caveats about what it measures, the Secchi disk continues to be widely used to give a quick and simple estimate of water clarity. The idea that the light from the disk travels to the eye in a straight line as a beam does not sit comfortably with what we see when making Secchi depth measurements in a very turbid estuary. Some of the light reflected by the disk, and traveling initially in a direction away from the observer, may be scattered toward the eye (Figure 1, track B). Because this scattered light appears to come from a different part of the disk to that from which it originates, it does not faithfully transmit the image of the disk. The practical benefit of this work is that it will allow for a better informed interpretation of the many measurements of Secchi depth that have been made, and continue to be made, in turbid fresh, estuarine, and coastal water
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