TOWARDS OPERATIONAL AIRBORNE REMOTE SENSING OF WATER QUALITY IN THE NETHERLANDS

Optical imaging techniques can be used to produce maps of concentrations of chlorophyll-a, total suspended matter and coloured dissolved organic carbon. The Rijkswaterstaat sets up a production chain for such maps using a hyperspectral imaging scanner installed in the Dutch coast guard aircraft. Multiple use of aircraft and scanner makes data acquisition more cost effective. Water quality parameters are retrieved from remote sensing images using successively; 1) A module that calculates subsurface reflectance spectra, it corrects for atmospheric and air-water interface effects using MODTRAN embedded in TOOLKIT. 2) A module that calculates concentrations of water constituents from the subsurface irradiance reflectance spectra and specific inherent optical properties using matrix inversion. Specific inherent optical properties of the water constituents are not necessarily simultaneous measured to the airborne measurements. Effects of instrumental noise, errors in the atmospheric correction and errors in the specific inherent optical properties on the derived concentrations of water constituents were estimated. An image was taken from Lake Veluwe in The Netherlands with complementary in situ measurements, including reflectance spectra and specific absorption and back scattering spectra of water constituents and concentrations of water constituents. The data were used to validate the system by 1) forward modelling of the reflectance spectra based on the Gordon model using a set of measured specific inherent optical properties, concentrations of the constituents and measured reflectance spectra, 2) comparison of the atmospheric corrected airborne measurements and reflectance spectra measured from a ship, and 3) comparison of the measured concentrations with the calculated concentrations.