Bio-optical modelling combined with remote sensing to assess water quality
A simple bio-optical model, with parameter values derived from measurements of the inherent optical properties (IOPs) and optically active substances that are known to influence the IOPs, has been developed. A large historical dataset of measurements of the concentration of chlorophyll a and phaeophytine a (Chl), suspended particulate inorganic material (SPIM) and the absorption coefficient of Coloured Dissolved Organic Matter (CDOM), spanning more than 25 years, has been used together with the model to develop algorithms for the retrieval of these water quality parameters, for a site in Lake Malaren, Sweden. The model takes as input the optically active substances and outputs a reflectance spectrum just above the water surface. From the modelled reflectance, algorithms were derived for Chl, SPIM and CDOM absorption at 420 nm. The algorithms were applied to atmospherically corrected remote sensing data, which were collected by the Compact Airborne Spectrographic Imager, CASI. The radiative transfer code 6S was used for the atmospheric correction of the data. Distribution maps for the three retrieved parameters were constructed and Chl and SPIM were validated by continuous field measurements of fluorescence and beam attenuation. The continuous data were calibrated with water analysis results from nine water samples. The time lag between the image acquisition and the ground data measurements was never more than 3 hours. Even though the model parameter values were collected at different times from that of the CASI over-flight, and from a larger geographic region of Lake Malaren than that used for the CASI measurements, the independently developed algorithms predicted the concentrations of the optically active substances within a reasonable level of accuracy, allowing spatial variations in the substances to be predicted.