Remote Sensing of Secchi Depth in Highly Turbid Lake Waters and Its Application with MERIS Data

The Secchi disk depth (Z(SD), m) has been used globally for many decades to represent water clarity and an index of water quality and eutrophication. In recent studies, a new theory and model were developed for Z(SD), which enabled its semi-analytical remote sensing from the measurement of water color. Although excellent performance was reported for measurements in both oceanic and coastal waters, its reliability for highly turbid inland waters is still unknown. In this study, we extend this model and its evaluation to such environments. In particular, because the accuracy of the inherent optical properties (IOPs) derived from remote sensing reflectance (R-rs, sr(-1)) plays a key role in determining the reliability of estimated Z(SD), we first evaluated a few quasi-analytical algorithms (QAA) specifically tuned for turbid inland waters and determined the one (QAA(TI)) that performed the best in such environments. For the absorption coefficient at 443 nm (a(443), m(-1)) ranging from 0.2 to 12.5 m(-1), it is found that the QAA(TI)-derived absorption coefficients agree well with field measurements (r(2) > 0.85, and mean absolute percentage difference (MAPD) smaller than 39%). Furthermore, with QAA(TI)-derived IOPs, the MAPD was less than 25% between the estimated and field-measured Z(SD) (r(2) > 0.67, Z(SD) in a range of 0.1-1.7 m). Furthermore, using matchup data between R-rs from the Medium Resolution Imaging Spectrometer (MERIS) and in-situ Z(SD), a similar performance in the estimation of Z(SD) from remote sensing was obtained (r(2) = 0.73, MAPD = 37%, Z(SD) in a range of 0.1-0.9 m). Based on such performances, we are confident to apply the Z(SD) remote sensing scheme to MERIS measurements to characterize the spatial and temporal variations of Z(SD) in Lake Taihu during the period of 2003-2011.