An optical method to assess water clarity in coastal waters



Accurate estimation of water clarity in coastal regions is highly desired by various activities such as search and recovery operations, dredging and water quality monitoring. This study intends to develop a practical method for estimating water clarity based on a larger in situ dataset, which includes Secchi depth (Z sd ), turbidity, chlorophyll and optical properties from several field campaigns in turbid coastal waters. The Secchi depth parameter is found to closely vary with the concentration of suspended sediments, vertical diffuse attenuation coefficient K d (m−1) and beam attenuation coefficient c (m−1). The optical relationships obtained for the selected wavelengths (i.e. 520, 530 and 540 nm) exhibit an inverse relationship between Secchi depth and the length attenuation coefficient (1/(c + K d )). The variation in Secchi depth is expressed in terms of undetermined coupling coefficient which is composed of light penetration factor (expressed by z(1 %)K d (λ)) and a correction factor (ξ) (essentially governed by turbidity of the water column). This method of estimating water clarity was validated using independent in situ data from turbid coastal waters, and its results were compared with those obtained from the existing methods. The statistical analysis of the measured and the estimated Z sd showed that the present method yields lower error when compared to the existing methods. The spatial structures of the measured and predicted Z sd are also highly consistent with in situ data, which indicates the potential of the present method for estimating the water clarity in turbid coastal and associated lagoon waters.


Water clarity Coastal and lagoon waters Secchi depth Attenuation coefficient Water quality 



Absorption and attenuation sensors


Apparent optical properties


Backscattering sensors


Coloured dissolved organic matter




Coefficient of variation


Turbidity and fluorescence chlorophyll sensors


Inherent optical properties


Mean root error


Nephelometric turbidity unit


Root mean square error


Submerged aquatic vegetation


Total suspended sediment



Absorption coefficient

at − aw

Particulate absorption coefficient


Backscattering coefficient

ct − cw

Particulate attenuation coefficient


Secchi depth


Vertical diffuse attenuation coefficient


Diffuse attenuation coefficient for photosynthetically available radiation


Beam attenuation coefficient

1/(c + Kd)

Length attenuation coefficient


Correction factor


Coupling coefficient


Inherent contrast of the disk (dimensionless)


Threshold contrast of the disk (dimensionless)


Correlation coefficient



The present work was supported by the NRB and was carried out by the extensive support of IIT Madras, Chennai-600036. We would like to extend our earnest thanks to D. Rajshekhar, The Head, Vessel Management Cell (VMC) and the Director of National Institute of Ocean Technology (NIOT), for providing the coastal research vessels (SagarPaschimi and SagarPurvi) to IIT Madras for carrying out various underwater light field measurements during the cruises and develop the bio-optical models. We also thank scientists N. Ravi and K. Sashikumar for their timely arrangement of the vessel and the VMC members for their valuable on-board help during the in situ measurements. We sincerely thank two anonymous reviewers for their insightful comments on our manuscript.


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Copyright information

© Springer International Publishing Switzerland 2015

Authors and Affiliations

  1. 1.Ocean Optics and Imaging Laboratory, Department of Ocean EngineeringIndian Institute of Technology MadrasChennaiIndia

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