Remote sensing reflectance and its relationship to optical properties of natural waters

Abstract

Monte Carlo simulations of photon propagation through natural water have been utilized to determine the sub-surface remote sensing reflectance, RRSW (the sub-surface value of the ratio of upwelling radiance from the nadir to the downwelling irradiance) as a function of water type (defined by the ratio of the backscattering coefficient to the absorption coefficient Bb/a), solar zenith angle, and incident radiation distribution (direct or diffuse). RRSW, as opposed to volume reflectance, RV (the sub-surface value of the ratio of upwelling to downwelling vector irradiance), is directly applicable to remotely sensed data collected over natural waters. It is shown that, for a nadir viewing direction, (a) RRSW is essentially independent of solar zenith angle and incident radiation distribution and (b) the dominant factor in determining RRSW is the optical nature of the water body itself (expressed as Bb/a). A relationship between the sub-surface remote sensing reflectance averaged over solar zenith angle between 15° and 89°, RRSW and water type is found to predict RRSW with an r.m.s. error of 9 per cent. Also addressed is the determination of the aquatic optical property, Bb/a, from the sub-surface remote sensing reflectance, RRSW This capability along with the specific absorption and scattering coefficients of aquatic constituents can, through bio-optical models, be used to estimate the concentrations of these aquatic constituents in non-Case I waters. The empirical relationship obtained to estimate Bb/a (with a r.m.s. error of 9·3 per cent) from the nadir value of the sub-surface remote sensing reflectance is Bb/a = 0·0027 + 987RRSW − 34·5(RRSW)2 + 1534(RRSW)3. © 1996 Taylor & Francis Group, LLC.