The ƒ'/Q Factor in the Relationship Between Inherent and Apparent Optical Properties in a Typical Inland Water (Lake Taihu, China)

Abstract

Water reflectance anisotropy has serious implications for the remote sensing of water surfaces. The ${\bm{f}^{\prime}}/{\bm{Q}}$ factor serves as a crucial bridge connecting the inherent and apparent optical properties of turbid water bodies; ${\bm{f}^{\prime}}/{\bm{Q}}$ varies not only with angular geometry and inherent optical properties (IOPs), but also with wavelength. However, there was limited research on ${\bm{f}^{\prime}}/{\bm{Q}}$ in turbid water bodies. This research conducted preliminary exploration on ${\bm{f}^{\prime}}/{\bm{Q}}$ in inland turbid water bodies based on measurements of in-situ multi-angle bidirectionality reflectance, including 17 angles and 251 bands when the solar zenith angle (${{\bm{\theta }}}_{\bm{s}}$) was between 40° and 50°. This study indicated that ${\bm{f}^{\prime}}/{\bm{Q}}$ exhibited highly sensitivity in the wavelength range of 690–750 nm with a peak near 727 nm, as well as angular geometry played an important role in the BRDF. The ${\bm{f}^{\prime}}/{\bm{Q}}$ varied over the range of 0.13–0.18 sr−1, variation of 0.06 sr−1 across different bands, when viewing zenith angle (${{\bm{\theta }}}_{\bm{v}}$) and particle backscattering ratio (${{\tilde{\bm{b}}}}_{{\bm{bp}}}$) were 0 and 0.0183, respectively; higher than the range observed for the ocean (0.08–0.15 sr−1). The variability in ${\bm{f}^{\prime}}/{\bm{Q}}$ as a function of wavelength must be accounted for in turbid waters. The ${\bm{f}^{\prime}}/{\bm{Q}}$ was correlated with the IOPs in the range of 690–750 nm, with coefficient of determination (R2) values higher than 0.94 and root mean square error (RMSE) values lower than 0.004. Our findings are of great significance for understanding the relationship between inherent and apparent optical properties in inland water bodies, for BRDF studies, and for improving the accuracy of satellite product retrieval.

Keywords

• Bidirectional reflectance distribution function (BRDF)
• inland waters
• multi-angle reflectance
• radiative transfer
• remote sensing