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One of the bases of remote-sensing on water quality is to analyze reflectance just beneath water surface (0- deepness).
Because reflectance just beneath water surface can not be obtained directly, and remote sensing reflectance which is ratio
between water-leaving radiance and total radiance on water surface from sky can be obtained by spectroradiometer,
remote sensing reflectance is used commonly for building remote sensing model instead of reflectance just beneath water
surface. However, the water-leaving radiance obtained from the water surface includes some mirror reflections of the
water surface which reflect little information of the waterbodies. What's more, since the mirror reflections of the same
water surface are fluctuating when repeating the measurements in the same area of the water surface, the water-leaving
radiances obtained from this water surface are not identical in these measurements. Obviously, there is significant
difference between remote sensing reflectance and reflectance just beneath water surface. The remote-sensing model of
the waterbodies based on remote sensing reflectance exists some errors in retrieving the water quality. Therefore, it is
necessary to extracting reflectance just beneath water surface from measured remote sensing reflectance when building
the remote-sensing model of the waterbodies and retrieving the parameters of the water quality. This paper builds a novel
remote-sensing model and proposes an approach to extracting reflectance just beneath water surface from measured
remote sensing reflectance based on the model. In the proposed model, there are two assumptions: the radiance from the
underwater is steady and identical in all directions after the radiance entering into the under water is scattered entirely by
the water molecule and the particle of other material in the waterbodies, and the measurements of the radiance are
completed in a short period of time. Based on the above assumption, the differences among several remote sensing
reflectance which were measured repeatedly in same area of the waterbodies in a short time are related to fluctuation of
the water surface. In this model, the mirror reflectance model of water surface about wavelength is firstly obtained from
a group of remote sensing reflectance spectra measured repeatedly over same area of waterbodies, and then reflectance
just beneath water surface is extracted from the remote sensing reflectance using the least squared method and the
Levenberg -Marquardt algorithm. After the proposed model being built, three groups of experiments for remote sensing
reflectance measured above clean waterbodies, waterbodies with suspended sand and alga in Lake Taihu are conducted
and reflectance just beneath water surface of these three waterbodies are extracted successfully. Both the theoretical
deduction and the experimental result demonstrate that the proposed model is effective and efficient.
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