The spectral upwelling radiance distribution in optically shallow waters

The upwelling radiance distribution in optically shallow water is investigated with experiments and Hydrolight numerical simulations for two different benthic surfaces and a range of solar zenith angles (,608). Over a bright sand surface (water depth 5 m) the upwelling radiance distribution was brightest at nadir and decreased toward the horizon. The upwelling radiance distribution was nearly azimuthally symmetric but was strongly influenced by wave focusing. Q (Eu/Lu, where Eu is the upwelling irradiance and Lu is the upwelling radiance) for this case was significantly less than p. Q, at 440 nm and 670 nm, was almost independent of solar zenith angle. The Hydrolight model results agreed well with the experimental measurements in this case. Over a seagrass surface (water depth 8 m) at 440 nm the radiance distribution was more uniform, while at 670 nm the benthic surface has a negligible effect on the upwelling radiance distribution. In this case, Q was dependent on solar zenith angle, with Q(670 nm) close to the values expected in optically deep water. At 440 nm, the agreement between the radiance distribution obtained from Hydrolight and the data is better at larger solar zenith angles, but at small solar zenith angles the experiments indicated that there were significant non-Lambertian effects over the seagrass surface, with a much higher reflectance at small nadir view angles. In optically shallow water the light reflected from the benthic surface affects the remotely sensed water leaving radiance. Many workers have developed models that account for the effect of the benthic surface on the upwelling irradiance field (e.g., Joseph 1950; Plass and Kattawar 1972; Gordon and Brown 1974; Maritorena et al. 1994) and nadir upwelling radiance (Lee et al. 1998). However, most often the remotely sensed signal is not at nadir, but at some other viewing angle. In this case the variation of the upwelling radiance with view angle is required. The radiance distribution is the collection of information on the angular variation of the radiance in the ambient light field. In optically shallow water, reflectance from the benthic surface will modify the radiance distribution from that expected in optically deep water. Thus to understand remote sensing in shallow waters, we need to understand the effect of the benthic surface on the radiance distribution. The radiance distribution also provides a very detailed data set to 1 Corresponding author ([email protected]).