Effects of optically shallow bottoms on upwelling radiances: Bidirectional reflectance distribution function effects

Radiative transfer simulations were carried out for a variety of measured and modeled benthic bidirectional reflectance distribution functions (BRDFs), incident lighting conditions, bottom depths, and water inherent optical properties. These simulations quantify the errors that occur in predictions of above-surface remote-sensing reflectances and in-water upwelling radiances if non-Lambertian ocean bottoms are replaced by Lambertian bottoms having the same irradiance reflectance. We found that when computing water-leaving radiances, non-Lambertian BRDFs can be replaced by a Lambertian BRDF having the same irradiance reflectance, with errors seldom greater than 10% and often much less, for considerations of above-surface ocean-color remote sensing by Ocean Portable Hyperspectral Imager for Low-Light Spectroscopy (Ocean PHILLS) or similar systems using near-nadir viewing directions. The crucial measurement to make in characterizing the reflectance properties of a benthic surface for such applications is the spectral irradiance reflectance. Optically shallow bottoms affect the reflected, upwelling radiance in various ways. The magnitude and angular distribution of the bottom-reflected radiance are determined by the bidirectional reflectance distribution function (BRDF) of the bottom. If the bottom is inhomogeneous, or patchy, the upwelling radiance is a spatial function of horizontal location as well as depth; the same is true if the bottom is not level. This paper considers BRDF effects for level, homogeneous bottoms. A companion paper (Mobley and Sundman 2003) considers the effects of patchy and sloping bottoms. Consider a horizontal surface within a water body. This can be a physical surface such as the water–sand interface of a sandy bottom, or it can be simply a particular depth in the water column, e.g., 1 m above a seagrass bed or a given depth in optically deep water. In any case, we can think of light being incident onto the (real or conceptual) surface from all directions, with some of the incident light being reflected by the surface into all directions. To completely understand the optical properties of the surface, it is necessary to know how the surface reflects light going in any incident direction into any reflected direction. This information is given by the bidirectional reflectance distribution function (BRDF) of the surface. To define the BRDF, let the reflecting surface lie in the xy plane of a Cartesian coordinate system whose z axis is 1 Corresponding author ([email protected]).