Extracting remotely-sensed water quality parameters from shallow intertidal estuaries

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Sentinel-2 imagery is potentially ideal for providing a rapid assessment of the ecological condition of estuarine water due to its high temporal and spatial resolution and coverage. However, for optically shallow waters, the problem of isolating the effect of seabed reflectance from the influence of water properties makes it difficult to use the observed surface reflectance to monitor water quality.

In this study, we adopt a methodology based on Lyzenga’s model to estimate water quality properties such as the dominant wavelength and diffuse attenuation coefficient (K_d) of shallow estuarine waters. Lyzenga models the observed reflectance (R) using four parameters: total water depth (z), sea-bed reflectance (R_b), water reflectance (R_w) and K_d. If R_b is known a priori and multiple observations of R are available from different total water depths, we show that Lyzenga’s model can be used to estimate the values of the remaining two parameters, K_d and R_w. 

Tests in our case study estuary show that K_d and R_w can be estimated at water depths less than 6.4 m. The methodology provides a foundation for future work assessing rates of primary production in shallow estuaries on large scales.