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Coastal Water Quality
Remote Sensing of Water BasicsAlgorithms | Depth Penetration | Atomospheric Correction Algorithms for Calculating Chlorophyll from Ocean ColorSince waters with higher chlorophyll concentrations tend to be green and those with lower concentrations tend to be blue, many chlorophyll algorithms are based on the ratio of blue to green radiances, or reflectances. For example the SeaWIFS Ocean Chlorophyll 2 (OC2) algorithm is based on the ratio of blue to green radiance.
These algorithms are empirical, and the mathematical relationship can be different for different water types. For example, a general algorithm for open-ocean water is C(chl) = N[L(λ1)/L(λ2)]exp(M) (Bukata, 2005) C(chl) = chlorophyll concentration Open-ocean water is often referred to as Case 1 water, in which the principal colorant is from chlorophyll pigments. Coastal and inland waters next to or enclosed by landforms are more optically complex than deep-ocean water. These water bodies are often referred to as Case 2 water and contain color-producing agents such as suspended sediments and dissolved organic matter, in addition to chlorophyll. While most inland and coastal waters fall under the Case 2 water category, some non-coastal water bodies with high turbidity can also be referred to as Case 2 water. Suspended sediments and dissolved organic matter also contribute to the overall reflectance of a water body, making it difficult to determine chlorophyll concentration in remotely sensed data. To determine chlorophyll concentration in coastal water bodies, the absorption and scattering characteristics of suspended sediments and dissolved organic matter must also be accounted for. The two images below show turbid waters associated with coastal plumes. The left image shows the Louisiana coastal plume as observed by SeaWIFS. Suspended sediments from the Mississippi river are primarily responsible for this turbidity. The right image shows the boundary between turbid freshwater and seawater at Winyah Bay, South Carolina. The fresher riverine water entering the coastal area is lower in clarity as it carries land-based sediments and organic material to the sea.
Depth PenetrationThe goal of water-quality remote sensing is to obtain information about the characteristics (such as turbidity, chlorophyll, etc.) of the water column. Remote sensing imagery is generally capable of providing information about the top few meters of the water column. In deep water, this means that imagery only provides information about the surface, or a small fraction, of the water column. Shallow turbid waters may also experience this limitation, while clear, shallow waters may experience the problem of ocean-bottom reflectance. When incident light penetrates the water column, reaches the ocean bottom, and exits the water, it will contribute to the total radiance making it difficult to disentangle the water column radiance from the bottom radiance. The penetration of incident light is limited primarily by water clarity, and absorption is highly dependent on wavelength — red wavelengths penetrate the water only a few centimeters, while blue wavelengths penetrate the furthest. Sometimes coastal managers need to monitor bathymetry or bottom characteristics. In this case, only the bottom radiance is of interest, and subsurface volume radiance must be removed. The optimal spectral wavelength for obtaining information about water depth is approximately 0.5 µM. For more information about using remote sensing information to measure bottom characteristics, please visit the benthic habitat mapping Web site. Atmospheric CorrectionA portion of the light that reaches a sensor flying overhead actually comes from the Earth's atmosphere, not the surface of the Earth. This signal from the atmosphere, which can obscure the signal emanating from the land or water of interest, is caused by molecular scattering in the atmosphere (or Rayleigh scattering). Particles such as dust, ash, and smoke also scatter light reflected from the Earth's surface. These particles are more commonly encountered near land and therefore are more problematic in the remote sensing of coastal waters than for waters farther offshore. Both the atmospheric conditions and the flying height of the sensor, which affects how much atmosphere the signal must travel through, can affect the strength and spectrum of the signal. Atmospheric correction algorithms can be applied to remove the effects of the atmosphere and provide a more accurate signal from the area of interest. |
