Remote sensing of water quality in an Australian tropical freshwater impoundment using matrix inversion and MERIS images

The purpose of this study was to investigate how semi-analytical inversion techniques developed for the remote sensing of water quality parameters (chlorophyll a, tripton and coloured dissolved organic matter (CDOM)) in inland waters could be adapted or improved for application to Australian tropical and sub-tropical water bodies. The Matrix Inversion Method (MIM) with a semi-analytic model of the anisotropy of the in-water light field was applied to MERIS images of Burdekin Falls Dam, Australia, a tropical freshwater impoundment. Specific attention was required to improve the atmospheric correction of the MERIS data. The performance of the conventional three band exact solution of the MIM was compared to that of over-determined solutions that used constant and differential weighting for each sensor band.The results of the application of the MIM algorithm showed that the best weighting scheme had a mean chlorophyll a retrieval difference of 1.0 μgl− 1, the three band direct matrix inversion scheme had a mean difference of 4.2 μgl− 1 and the constant weight scheme had a mean difference of 5.5 μgl− 1. For tripton, the best performed weighting scheme had a mean difference of 1.2 mgl− 1, the three band scheme had a mean difference of 3.4 mgl− 1 and the constant weight scheme had a mean difference of 1.8 mgl− 1. For the CDOM retrieval, the mean difference was found to be 0.12 m− 1 for the best performed weighting scheme, 0.25 m− 1 for the three band scheme and 0.52 m− 1 for the constant weight scheme. It was found that significant improvements in the accuracy and precision of retrieved water quality parameter values can be obtained by using differentially weighted, over-determined systems of equations, rather than exact solutions. These more reliable estimates of water quality parameters will allow water resource managers to improve their monitoring regimes.

Research highlights
► We retrieve water quality parameters for a lake using a semi-analytical inversion.
► A site specific method was required to atmospherically correct the MERIS data.
► Optical closure can be used to identify the most appropriate SIOP set for a pixel.
► Using weighted over-determined systems of equations improved the MIM algorithm.