REMOTE-SENSING REFLECTANCE AND ITS RELATIONSHIP TO OPTICAL-PROPERTIESOF NATURAL-WATERS

Monte Carlo simulations of photon propagation through natural water ha ve been utilized to determine the sub-surface remote sensing reflectan ce, R(RSW) (the sub-surface value of the ratio of upwelling radiance f rom the nadir to the downwelling irradiance) as a function of water ty pe (defined by the ratio of the backscattering coefficient to the abso rption coefficient Bb/a), solar zenith angle, and incident radiation d istribution (direct or diffuse). R(RSW) as opposed to volume reflectan ce, R(V) (the sub-surface value of the ratio of upwelling to downwelli ng vector irradiance), is directly applicable to remotely sensed data collected over natural waters. It is shown that, for a nadir viewing d irection, (a) R(RSW) is essentially independent of solar zenith angle and incident radiation distribution and (b) the dominant factor in det ermining R(RSW) is the optical nature of the water body itself (expres sed as Bb/a). A relationship between the sub-surface remote sensing re flectance averaged over solar zenith angle between 15 degrees and 89 d egrees, (R) over bar(RSW), and water type is found to predict (R) over bar RSW with an r.m.s. error of 9 per cent. Also addressed is the det ermination of the aquatic optical property, Bb/a, from the sub-surface remote sensing reflectance, R(RSW). This capability along with the sp ecific absorption and scattering coefficients of aquatic constituents can, through bio-optical models, be used to estimate the concentration s of these aquatic constituents in non-Case I waters. The empirical re lationship obtained to estimate Bb/a (with a r.m.s. error of 9.3 per c ent) from the nadir value of the sub-surface remote sensing reflectanc e is Bb/a = 0.0027 + 9.87R(RSW) - 34.5(R(RSW))(2) + 1534(R(RSW))(3).