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Satellite Observation and Model Simulation of Water Turbidity in the Chesapeake Bay. Introduction
Excess sediment is one of the most important contributors to degraded water quality and has adverse effects on critical habitats and living resources in the Chesapeake Bay. Satellite measurements of the ocean color are very important resources to study the sediment concentration. In this study, we use the MODIS-Aqua ocean color products derived with the shortwave infrared (SWIR)-based algorithm to study the turbidity spatial and temporal variability in the Chesapeake Bay, and results of numerical simulations of sediment concentration using Regional Ocean Modeling System (ROMS) will also be presented and compared with the satellite measurements.
The 2002-2008 multi-year averaged Kd(490) data for January, April, July and October derived from MODIS-Aqua show a detail spatial distribution pattern of the turbidity in the Chesapeake Bay. Generally, the region in the upper Chesapeake Bay is the most turbid with Kd(490) ~2-3 m-1, and the water gradually becomes less turbid down to the middle and lower Bay regions. This spatial distribution patter is found in all the four seasons. The data also show a clear seasonal variability of the turbidity in the Chesapeake Bay: the water is the most turbid in January, the turbidity gradually decreases in April, and reaches its minimum in July, and then gradually increases in October.
Normalized Water-Leaving Reflectance Spectra
Chesapeake Bay Kd(490)
Water-leaving radiance spectra retrieved in the Chesapeake Bay using MODIS standard atmospheric correction algorithm are inaccurate because of its invalid black water assumption in the near-infrared (NIR) bands in highly turbid waters. Recently, a SWIR-based atmospheric correction algorithm (Wang and Shi, 2005, 2007; Wang, 2007) has been developed for deriving MODIS-Aqua ocean color products, and it has been demonstrated that the SWIR-based data processing produces better quality ocean color products over the turbid coastal waters such as Chesapeake Bay (Wang et al. 2009).
The upper Chesapeake Bay region is also featured with enhanced water-leaving reflectance in the green and red bands between 555 and 645 nm. Reflecting the seasonal Kd(490) variation, highest reflectance at 645 nm occurs in the winter with value over 6% and the lowest during the summer with value less than 3%.
2002-2008 Multi-Year Average
Location of (39.29N, 76.32W)
ROMS implementation in the Chesapeake Bay region by the ChesROMS community is used in this study, and the built-in sediment model (Waner et al. 2008) is used to simulate sediment processes which represents separate cohesive and non-cohesive categories. Sensitivities of the sediment concentrations to the tidal and wind forcing, sediment settling speed, wind induced waves, are tested to investigate the mechanisms of the seasonal and spatial variations of sediment concentrations in the Chesapeake Bay.
Comparisons of MODIS Ocean Color Products from NIR, SWIR, and NIR-SWIR Combined Methods
Chlorophyll-a0.01-10 (mg/m3)(Log scale)
Standard Data Processing
Wang, M. and W. Shi (2007), “The NIR-SWIR combined atmospheric correction approach for MODIS ocean color data processing,” Optics Express, 15,15722-15733.
Wang, M., S. Son, and W. Shi (2009), “Evaluation of MODIS SWIR and NIR-SWIR atmospheric correction algorithms using SeaBASS data,” Remote Sens. Environ., 113,635-644.
Tidal Current vs. Wind
Seasonal Forcing vs. Sediment Settling Speed
Wind and Tide
Winter, 0.01 mm/s
Summer, 0.01 mm/s
Summer, 0.1 mm/s
NIR-SWIR Data Processing
Satellite derived diffuse attenuation coefficient at the wavelength 490 nm (Kd(490)) can be used to relate water turbidity. Wang et al. (2009) developed a new algorithm for the Kd(490) derived from MODIS-Aqua for turbid water, and it has been applied and evaluated for the Chesapeake Bay.
Kd(PAR) Matchup Comparisons for the Chesapeake Bay
New Kd(490) for U.S. East Coastal Region
The satellite measurement of Kd(490) from MODIS-Aqua shows a significant seasonal turbidity variation in the Chesapeake Bay. Kd(490) has its maximum in winter, and gradually decreases from spring to summer. Spatially, the water is the most turbid in the upper bay, and the turbidity decreases to the middle bay. Sensitivity tests using ROMS with sediment model show that the tidal forcing dominates the sediment processes. Sediment concentrations are also very sensitive to the sediment settling speed, which could be a contributor to the seasonal variations. The wind forcing with the wind-wave effect enabled is being further investigated.
NASA Standard Data Processing
Wang, M., S. Son, and L. W. Harding Jr., “Retrieval of diffuse attenuation coefficient in the Chesapeake Bay and turbid ocean regions for satellite ocean color applications,” J. Geophys. Res. (2009).