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 R rs (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 R rs (λ): R rs (λ), derived from radiometric measurements; and R rs (λ), derived from simultaneous IOP measurements. Five subsequent steps have been taken to achieve this goal, including (1) estimation of the R rs (λ) from radiometric measurements; (2) scattering correction for the non-water spectral absorption coefficient a pd (λ); (3) estimation of the below-water spectral remote-sensing reflectance r rs (λ) from IOPs measurements; (4) the estimation of the R rs (λ) from the r rs (λ) values; and (5) the comparison between the R rs (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 R rs (λ) may be rather significant, especially in the long-wavelength spectrum range. Nevertheless, spectral features were similar between these R rs (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.

Fusion of Landsat-8/OLI and GOCI Data for Hourly Mapping of Suspended Particulate Matter at High Spatial Resolution: A Case Study in the Yangtze (Changjiang) Estuary

“…Additional details about the R rs calculation can be found in Sokoletsky and Shen [42]. The [SPM] were determined gravimetrically by filtering the surface water samples on 0.7-µm Whatman GF/F glass fiber filters.…”

Validation and Calibration of QAA Algorithm for CDOM Absorption Retrieval in the Changjiang (Yangtze) Estuarine and Coastal Waters

“…The estimations of ρsky(λ), Ls,sky(λ) and Rrs(λ) were detailed in Sokoletsky and Shen [47]. Because a(λ) is affected by temperature, salinity, pure water absorption and total scattering, it must be corrected [47][48][49]. For the most important and simultaneously the most difficult scattering Typical spectra of remote-sensing reflectance collected in the Changjiang estuarine and its adjacent coastal waters are shown in Figure 2a Typical spectra of remote-sensing reflectance collected in the Changjiang estuarine and its adjacent coastal waters are shown in Figure 2a …”

“…Through in situ measurements of downwelling spectral irradiance, Ed(λ), incident spectral radiance, Ls(λ), total upwelling spectral radiance, Ltot(λ), Rrs(λ) is estimated by Sokoletsky and Shen [47]:…”

“…The estimations of ρsky(λ), Ls,sky(λ) and Rrs(λ) were detailed in Sokoletsky and Shen [47]. Because a(λ) is affected by temperature, salinity, pure water absorption and total scattering, it must be corrected [47][48][49].…”

Automatic Sun Glint Removal of Multispectral High-Resolution Worldview-2 Imagery for Retrieving Coastal Shallow Water Parameters

“…where ρ TOC pλq is the surface reflectance; R rs pλq is the remote sensing reflectance; R d rs pλq is the deglinted remote sensing reflectance; min pR rs pN IRqq is the minima NIR reflectance of the areas; r rs is the subsurface remote sensing reflectance; α pθ s q and β pθ s q are coefficients, based on Hydrolight simulations [10], which depend on the zenithal angle [11], α pθ s q = (0˝= 0.5236, 30˝= 0.5169, 60˝= 0.4933), β pθ s q = (0˝= 2.1941, 30˝= 2.3001, 60˝= 2.6796).…”