Abstract
The Secchi disk depth (ZSD, 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 ZSD, 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 (Rrs, sr−1) plays a key role in determining the reliability of estimated ZSD, we first evaluated a few quasi-analytical algorithms (QAA) specifically tuned for turbid inland waters and determined the one (QAATI) 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 QAATI-derived absorption coefficients agree well with field measurements (r2 > 0.85, and mean absolute percentage difference (MAPD) smaller than ~39%). Furthermore, with QAATI-derived IOPs, the MAPD was less than 25% between the estimated and field-measured ZSD (r2 > 0.67, ZSD in a range of 0.1–1.7 m). Furthermore, using matchup data between Rrs from the Medium Resolution Imaging Spectrometer (MERIS) and in-situ ZSD, a similar performance in the estimation of ZSD from remote sensing was obtained (r2 = 0.73, MAPD = 37%, ZSD in a range of 0.1–0.9 m). Based on such performances, we are confident to apply the ZSD remote sensing scheme to MERIS measurements to characterize the spatial and temporal variations of ZSD in Lake Taihu during the period of 2003–2011.
Highlights
A white or black-and-white disc is commonly lowered into the water to measure water clarity of aquatic environments, and the depth at which it just disappears from a viewer at the surface is called the Secchi disk depth (ZSD, m) [1]
For the evaluated with the other independent dataset (Erie) dataset, the ZSD derived by all above algorithms were significantly underestimated with the mean absolute percentage difference (MAPD) ranging from 35% to 62% (Figure 3), which is consistent with the systematical overestimation of inherent optical properties (IOPs) (Figures 2a–2e)
Using in-situ measurements collected in Lake Taihu and Lake Erie, we found that the revised quasi-analytical algorithms (QAA) algorithm for turbid waters showed very good performance for the retrieval of IOPs and ZSD
Summary
A white or black-and-white disc (diameter 20 to 30 cm) is commonly lowered into the water to measure water clarity (or transparency) of aquatic environments, and the depth at which it just disappears from a viewer at the surface is called the Secchi disk depth (ZSD, m) [1]. Substantial efforts have been made to tune QAA for turbid eutrophic waters by shifting the reference wavelength to a longer wavelength or re-parameterize the semi-analytical relationships [23,25,26] Those studies are still limited to small areas [23] with a relatively small range of the total absorption coefficient, (e.g., a(443) from ~0.5 to 8.0 m−1 [25]) or chlorophyll concentrations, (e.g., from ~0.5 to 12.9 μg/L [26]). Before application of the new ZSD scheme to such turbid or eutrophic waters, it is imperative to revise the QAA in order to obtain more accurate estimates of IOPs. the empirical relationships employed in QAA_v6 are modified first with a dataset of great dynamic range (chlorophyll concentration in a range of 4.9 to 412.7 μg/L) measured in Lake Taihu, China. Using a 2003–2011 time series of Rrs from MERIS, we characterized the spatial and temporal variations of water clarity in Lake Taihu, and the potential factors associated with such variation are discussed
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