A semi-analytical scheme to estimate Secchi-disk depth from Landsat-8 measurements

The newly developed semi-analytical scheme (Lee et al., 2015a) for remote sensing of the Secchi disk depth (ZSD, m) was modified and applied to Landsat-8 data to obtain high-spatial-resolution map of water clarity. In order to implement the quasi-analytical algorithm (QAA) for the derivation of absorption and backscattering coefficients from Landsat-8 data, which are key optical properties for the estimation of ZSD, the representative wavelengths of Landsat-8 bands in the visible domain are verified; so are the absorption and backscattering coefficients of pure water for these bands. This semi-analytical scheme was then applied to a dataset having both in situ measurements of ZSD (~ 0.1-30 m) and remote-sensing reflectance and found that the estimated ZSD from remote sensing matches measured ZSD very well (R2 = 0.96, average absolute percent difference ~ 17%, N = 197). This scheme was further applied to a Landsat-8 image collected in an estuary to obtain high-spatial resolution ZSD map, and the obtained spatial distribution of ZSD is found quite consistent with in situ measurements and visual observations. These results indicate an important application of Landsat data - to provide reliable high-resolution water clarity product of bays, estuaries, and lakes with a unified mechanistic system.