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 (Zsd), which follows the remote sensing and contrast transmittance theory. The major factors governing the variation in Zsd, namely, turbidity and length attenuation coefficient (1/(c + Kd), c = beam attenuation coefficient; Kd = 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 (Rrs(443) + Rrs(490))/(Rrs(555) + Rrs(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 (Kd) at 531 nm, Kd(490) is estimated as a function of reflectance ratio (Rrs(670)/Rrs(490)), which provides the bio-optical link between chlorophyll concentration and Kd(531). The present algorithm was applied to MODIS-Aqua images, and the results were evaluated by matchup comparisons between the remotely estimated Zsd and in situ Zsd in coastal waters off Point Calimere and its adjoining regions on the southeast coast of India. The results showed the pattern of increasing Zsd 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 Zsd and in situ Zsd values was within ±25%. A close agreement achieved in spatial contours of MODIS-Aqua-derived Zsd and in situ Zsd for the month of January 2014 and August 2013 promises the model capability to yield accurate estimates of Zsd 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 Zsd products. The modeled and satellite-derived Zsd values were compared with measurement data which yielded RMSE = 0.079, MRE = −0.016, and R2 = 0.95 for the modeled Zsd and RMSE = 0.075, MRE = 0.020, and R2 = 0.95 for the satellite-derived Zsd products.