Closure Between Apparent and Inherent Optical Properties of the Ocean with Applications to the Determination of Spectral Bottom Reflectance

This study focuses on comparing six different marine optical models, field measurements, and laboratory measurements. Inherent Optical Properties (IOPs) of the water column depend only on the constituents within the water, not on the ambient light field. Apparent Optical Properties (AOPs) depend both on IOPs and the geometric underwater light field resulting from solar irradiance. Absorption (a) and scattering (b) are IOPs. Scattering can be partitioned into backscattering (b[subscript b]). Remote Sensing Reflectance (R[subscript rs]), the ratio of radiant light leaving the water to the light entering the water surface plane (E[subscript d]), is an AOP. R[subscript rs] is proportional to b[subscript b]/(a + b[subscript b]). Using this relationship, R[subscript rs] is inverted to determine both absorption and backscattering. The constituents contributing to both absorption and backscattering can then be further deconvolved using modeling techniques. The in situ instruments usually have a fixed path length while AOP measurement path length depends on the penetration and/or return of downwelling solar irradiance. As a consequence, AOP measurements use a longer path length than in situ instruments. If the path length of a direct IOP measurement instrument is too short, there may not be sufficient signal to determine a change in value. While the AOP inversions require more empirical assumptions to determine IOP values than in situ instruments, they provide a higher signal to noise ratio in clearer waters. This study defines closure as the statistical agreement between instruments and methods in order to determine the same optical property. No method is considered absolute truth. An R[subscript rs] inversion algorithm was best under most of the test stations for measuring IOP values. One exception was when bottom reflectance was significant, an inversion of diffuse attenuation (the change in the natural log of E[subscript d] over depth) was better for determining absorption and a field instrument was better for determining backscattering. The relationships between AOPs and IOPs provide estimates of unmeasured optical properties. A method was developed to determine the spectral reflectance of the bottom using IOP estimates and R[subscript rs]