We establish the average wavelength dependence for rough-surface reflectance Srs(λ) in Lake Superior and determine its magnitude at any individual site by extrapolating the total reflectance or raw remote sensing reflectance RrsT(λ) measured at the surface to the asymptotic limit of zero scattering where Srs(λ) ≅RrsT(λ). The results show that Srs(λ) differs from the flat-surface Fresnel reflectance of sky radiance used in standard determination of the remote sensing reflectance Rrs(λ) attributed to the scattering of light by particles and molecules in bulk water. In waters containing colored dissolved organic matter (CDOM), Rrs(λ) can be very low and radiometric measurements at ground level can often lead to negative estimates of Rrs(λ) if we assume simple mirror-like reflectance of the sky radiance as the basis for correcting radiometric data for surface reflectance. We examine the differences between Fresnel reflectance and Srs(λ) and estimate the ratio of the concentrations of CDOM versus suspended particles that could produce negative values of Rrs(λ) if assume Fresnel reflectance in calculation of Rrs(λ).
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Vol. 32 • No. 3