Estimation of underwater visibility in coastal and inland waters using remote sensing data.

An optical method is developed to estimate water transparency (or underwater visibility) in terms of Secchi depth ( Z ), which follows the remote sensing and contrast transmittance theory. The major factors governing the variation in Z , namely, turbidity and length attenuation coefficient (1/( c + K ), c = beam attenuation coefficient; K = diffuse attenuation coefficient at 531 nm), are obtained based on band rationing techniques. It was found that the band ratio of remote sensing reflectance (expressed as ( R (443) + R (490))/( R (555) + R (670)) contains essential information about the water column optical properties and thereby positively correlates to turbidity. The beam attenuation coefficient ( c) at 531 nm is obtained by a linear relationship with turbidity. To derive the vertical diffuse attenuation coefficient ( K ) at 531 nm, K (490) is estimated as a function of reflectance ratio ( R (670)/ R (490)), which provides the bio-optical link between chlorophyll concentration and K (531). The present algorithm was applied to MODIS-Aqua images, and the results were evaluated by matchup comparisons between the remotely estimated Z and in situ Z in coastal waters off Point Calimere and its adjoining regions on the southeast coast of India. The results showed the pattern of increasing Z from shallow turbid waters to deep clear waters. The statistical evaluation of the results showed that the percent mean relative error between the MODIS-Aqua-derived Z and in situ Z values was within ±25%. A close agreement achieved in spatial contours of MODIS-Aqua-derived Z and in situ Z for the month of January 2014 and August 2013 promises the model capability to yield accurate estimates of Z in coastal, estuarine, and inland waters. The spatial contours have been included to provide the best data visualization of the measured, modeled (in situ), and satellite-derived Z products. The modeled and satellite-derived Z values were compared with measurement data which yielded RMSE = 0.079, MRE = −0.016, and R = 0.95 for the modeled Z and RMSE = 0.075, MRE = 0.020, and R = 0.95 for the satellite-derived Z products. [ABSTRACT FROM AUTHOR]