Optical closure for remote-sensing reflectance based on accurate radiative transfer approximations: the case of the Changjiang (Yangtze) River Estuary and its adjacent coastal area, China

Abstract

Optical closure exercises are pivotal for evaluating the accuracy of water quality remote-sensing techniques. The agreement between radiometrically derived and inherent optical property (IOP)-derived above-water spectral remote-sensing reflectance Rrs(λ) is necessary for resolving IOPs, the diffuse attenuation coefficient, and biogeochemical parameters from space. We combined spectral radiometric and IOP measurements to perform an optical closure exercise for two optically contrasting Chinese waters – the Changjiang (Yangtze) River Estuary and its adjacent coastal area in the East China Sea. The final aim of our investigation was to compare two derivations of Rrs(λ): Rrs(λ), derived from radiometric measurements; and Rrs(λ), derived from simultaneous IOP measurements. Five subsequent steps have been taken to achieve this goal, including (1) estimation of the Rrs(λ) from radiometric measurements; (2) scattering correction for the non-water spectral absorption coefficient apd(λ); (3) estimation of the below-water spectral remote-sensing reflectance rrs(λ) from IOPs measurements; (4) the estimation of the Rrs(λ) from the rrs(λ) values; and (5) the comparison between the Rrs(λ) derived from radiometric and IOP measurements. All steps were realized by using both direct measurements and different models based on radiative transfer theory. Results demonstrated that the impact of the errors caused by the scattering correction procedure and conversion of radiometric quantities into Rrs(λ) may be rather significant, especially in the long-wavelength spectrum range. Nevertheless, spectral features were similar between these Rrs(λ) sets for all waters – from relatively clear to very turbid. Exploiting this fact allows use of the spectral reflectance ratios for remote sensing of the estuarine and coastal Chinese waters.