The Pearl River system is mainly located in the Guangdong Province in southern China, with the length of 2214 km and total area of 453,690 km2. The Pearl River estuary is the largest estuary in the South China Sea (SCS), with a mean annual discharge of 326 billion m3, of which are about 30 million tons of dissolved matters annually discharged into the estuary. The high concentration of suspended sediments and dissolved matters makes the optical properties of the coastal waters very complex.
The spectral absorption coefficient of yellow substance [Ay(λ)] is one of the inherent optical properties that influence the reflectance (or water-leaving radiance) of the water body. It is essential to measure Ay(λ) and to quantify its contributions to the total absorption of the water body. In this study, the Gelbstoff Optical Analyse Laboratory System (GOALS), with spectral range from 200 to 850 nm and with spectral resolution of 0.37 nm per pixel, was used to measure Ay(λ) in the Pearl River estuary and in the adjacent coastal waters in July 2002. Ay(400) was around 1.5 m-1 near the river mouth (zero salinity). It decreases with increasing salinity following an apparent non-linear mixing line. There is no apparent relationship between Ay(400) and dissolved organic carbon (DOC) concentration, indicating that the estuary is a complex, non-point source environment. This presents a great challenge to remote sensing study in this area.