Abstract

Water quality estimation in fresh and marine water systems with <i>in situ</i> above-water spectroscopy requires measurement of the volume reflectance (ρ<sub>v</sub>) of water bodies. However, the above-water radiometric measurements include surface reflection (<i>L</i><sub>r</sub>) as a significant component along with volume reflection. The <i>L</i><sub>r</sub> carries no information on water quality, and hence it is considered as a major source of error in <i>in situ</i> above-water spectroscopy. Currently, there are no methods to directly measure <i>L</i><sub>r</sub>. The common method to estimate <i>L</i><sub>r</sub> assumes a constant water surface reflectance (ρ<sub>s</sub>) of 2%, and then subtracts the <i>L</i><sub>r</sub> thus calculated from the above-water radiance measurements to obtain the volume reflection (<i>L</i><sub>v</sub>). The problem with this method is that the amount of ρ<sub>s</sub> varies with environmental conditions. Therefore, a methodology was developed in this study for direct measurement of water volume reflectance above water at nadir view geometry. Other objectives of this study were to analyze the contribution of L<sub>r</sub> to the total water reflectance under various environmental conditions in a controlled setup and to develop an artificial neural network (ANN) model to estimate ρ<sub>s</sub> from environmental conditions. The results showed that <i>L</i><sub>r</sub> contributed 20–54% of total upwelling radiance from water at nadir. The ρ<sub>s</sub> was highly variable with environmental conditions. Using sun altitude, wind speed, diffuse lighting, and wavelength as inputs, the ANN model was able to accurately simulate ρ<sub>s</sub>, with a low root mean square error of 0.003. A sensitivity analysis with the ANN model indicated that sun altitude and diffuse light had the highest influence on ρ<sub>s</sub>, contributing to over 82% of predictability of the ANN model. Therefore, the ANN modeling framework can be an accurate tool for estimating surface reflectance in applications that require volume reflectance of water.

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