Deriving inherent optical properties from classical water color measurements: Forel-Ule index and Secchi disk depth

Secchi disk depth (Z(SD)) and Forel-Ule index (FUI) are the two oldest and easiest measurements of water optical properties based on visual determination. With an overarching objective to obtain water inherent optical properties (IOPs) using these historical measurements, this study presents a model for associating remote-sensing reflectance (R-rs) with FUI and Z(SD). Based upon this, a scheme (FZ2ab) for converting FUI and Z(SD) to absorption (a) and backscattering coefficients (b(b)) is developed and evaluated. For a data set from HydroLight simulations, the difference is <11% between FZ2ab-derived a and known a, and < 28% between FZ2ab-derived b(b) and known b(b). Further, for a data set from field measurements, the difference is < 30% between FZ2ab-derived a and measured a. These results indicate that FZ2ab can bridge the gap between historical measurements and the focus of IOP measurements in modern marine optics, and potentially extend our knowledge on the bio-optical properties of global seas to the past century through the historical measurements of FUI and Z(SD). (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement