Validation of MERIS water quality products in Murchison Bay, Lake Victoria – Preliminary results

Abstract

As a step forward to validating the retrieval algorithm associated with the MERIS (Medium Resolution Imaging Spectrometer) sensor onboard the European Space Agency's ENVISAT satellite, we compare scaling factors for (442) (pigment absorption coefficient at 442 nm) versus chl-a concentration, and for (scattering coefficient for particles at 440 nm) versus TSM (total suspended matter) concentration, as well as in situ measurements of chl-a and TSM concentrations in Murchison Bay, Lake Victoria, with MERIS level 2 products. Our comparisons show that MERIS chl-a data products underestimate the in situ chl-a concentration by a factor of about 0.6, and that MERIS level 2 products overestimate the in situ TSM concentration and the yellow substance absorption coefficient by a factor of about 2. Application of a local inherent optical property (IOP) scaling factor improves the retrieval provided by MERIS level 2 products. The MERIS level 2 products also underestimate the water-leaving reflectance. These discrepancies between retrievals from MERIS data and in situ measurements are due to the current imperfect atmospheric correction algorithm for inland waters. Therefore, a need arises to develop site-specific bio-optical models and to improve existing MERIS retrieval algorithms.

Acknowledgements

This work was supported by the Norwegian State Educational Loan Fund and the NUFU Project No. 33/02. We would like to thank Barbro Silde for performing the spectrophotometric analyses and Merete Grung for the HPLC measurements at NIVA. The work was also supported by the ESA and Norwegian Space Agency project VAMP (Validation of MERIS data products), ESA PRODEX contract no. 14849/00/NL/Sfe(IC). The MERIS data used are from this project. We would like to thank three anonymous reviewers for useful comments that improved the article.