Abstract
The Ocean and Land Color Imager (OLCI) on the Sentinel-3A satellite, which was launched by the European Space Agency in 2016, is a new-generation water color sensor with a spatial resolution of 300 m and 21 bands in the range of 400–1020 nm. The OLCI is important to the expansion of remote sensing monitoring of inland waters using water color satellite data. In this study, we developed a dual band ratio algorithm for the downwelling diffuse attenuation coefficient at 490 nm (Kd(490)) for the waters of Lake Taihu, a large shallow lake in China, based on data measured during seven surveys conducted between 2008 and 2017 in combination with Sentinel-3A-OLCI data. The results show that: (1) Compared to the available Kd(490) estimation algorithms, the dual band ratio (681 nm/560 nm and 754 nm/560 nm) algorithm developed in this study had a higher estimation accuracy (N = 26, coefficient of determination (R2) = 0.81, root-mean-square error (RMSE) = 0.99 m−1 and mean absolute percentage error (MAPE) = 19.55%) and validation accuracy (N = 14, R2 = 0.83, RMSE = 1.06 m−1 and MAPE = 27.30%), making it more suitable for turbid inland waters; (2) A comparison of the OLCI Kd(490) product and a similar Moderate Resolution Imaging Spectroradiometer (MODIS) product reveals a high consistency between the OLCI and MODIS products in terms of the spatial distribution of Kd(490). However, the OLCI product has a smoother spatial distribution and finer textural characteristics than the MODIS product and contains notably higher-quality data; (3) The Kd(490) values for Lake Taihu exhibit notable spatial and temporal variations. Kd(490) is higher in seasons with relatively high wind speeds and in open waters that are prone to wind- and wave-induced sediment resuspension. Finally, the Sentinel-3A-OLCI has a higher spatial resolution and is equipped with a relatively wide dynamic range of spectral bands suitable for inland waters. The Sentinel-3B satellite will be launched soon and, together with the Sentinel-3A satellite, will form a two-satellite network with the ability to make observations twice every three days. This satellite network will have a wider range of application and play an important role in the monitoring of inland waters with complex optical properties.
Highlights
After entering water, sunlight is absorbed and scattered by constituents in the water, such as suspended particulate matter (SPM), phytoplankton, and colored dissolved organic matter (CDOM), and its downwelling irradiance attenuates nearly exponentially with increasing depth
This study examined the temporal and spatial distributions of Kd(490) in Lake Taihu, which is a large shallow lake in China, based on data measured during seven surveys conducted between 2008 and 2017 in combination with the Sentinel-3A-Ocean and Land Color Imager (OLCI) dataset from 2016–2017 to determine the characteristics of the photoecological environment in the waters of Lake Taihu
Lake Taihu suffers from frequent algal blooms in the spring and summer, which severely impacts the normal life of the several million nearby residents [34,35]
Summary
Sunlight is absorbed and scattered by constituents in the water, such as suspended particulate matter (SPM), phytoplankton, and colored dissolved organic matter (CDOM), and its downwelling irradiance attenuates nearly exponentially with increasing depth. Kd is an apparent optical property that is determined collectively by factors such as absorption and backscattering by each constituent in the water, the incident light field, and the water depth [1,2]. The effects of light fields are generally insignificant, and Kd is mainly determined by the inherent optical properties (e.g., absorption and scattering) of the water. Kd is a basic parameter that characterizes underwater light fields [5] and plays a vital role in studying water turbidity [6], sediment transport and resuspension [7,8], heat transfer within the upper water [9,10], the photosynthesis of phytoplankton [11,12], and the net primary productivity of natural waters [13,14]
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