The diffuse attenuation coefficient in water indicates how strongly light intensity at a specified wavelength is attenuated within the water column. This parameter has wide applicability in ocean optics, as it is directly related to the presence of scattering particles in the water column, either organic or inorganic, and thus is an indication of water clarity.
The diffuse attenuation coefficient at 490 nm (Kd490) indicates the turbidity of the water column - how visible light in the blue to green region of the spectrum penetrates within the water column. The value of Kd490 represents the rate which light at 490 nm is attenuated with depth. For example a Kd490 of 0.1/meter means that light intensity will be reduced one natural log within 10 meters of water. Thus, for a Kd490 of 0.1, one attenuation length is 10 meters. Higher Kd490 value means smaller attenuation depth, and lower clarity of ocean water.
CoastWatch provides high-resolution Kd490 products in near-real time from the Moderate Resolution Imaging Spectroradiometer (MODIS) on-board the NASA Earth Observing System (EOS) Aqua satellite. This high-resolution Kd490 is well-suited to characterize turbidity in coastal and high-turbid water (Tomlinson et al, 2019). It utilizes the 250 m bands on MODIS to achieve the high spatial resolution rather than the 1 km spatial resolution of other MODIS Kd490 products.
Surface reflectance (rho_s) from MODIS bands 1 (red), 2 (near infrared) and 3 (blue) are used to derive Kd490 using the following difference ratio technique developed by NOAA's National Ocean Service: ( Red - NIR ) / ( Blue - NIR ). A red/blue ratio is often used to estimate Kd490 in coastal turbid waters. A correction factor is also applied to adjust the resulting difference ratio to coastal water.