Spectral irradiance profiles of suspended marine clay for the estimation of suspended sediment concentration in tropical waters

Reflectance spectra of water containing varying concentrations of organic and inorganic sediments isolated from the coastal waters of Singapore were measured using a portable spectroradiometer under controlled experimental conditions and natural sunlight. The effect of different sizes of sediments on the spectral profile of water was also investigated. In the presence of organic sediments, maximum reflectance of water was observed over a broad band between 440-580 nm, with a peak reflectance at about 550 nm, followed by two smaller peaks at 600-645 nm and 665-690 nm. In contrast, inorganic sediments produced a distinct band peak between 595 and 690 nm. For both sediment types, characteristic features in the infra-red region include a reflectance trough at 754 nm and peak at 814 nm. The empirical colour ratios, OD550-OD754 and OD 595-OD754, were used to estimate the organic and inorganic sediment concentrations respectively. In the case of organic sediments, a power function was found to fit the data well (R²=0.89-0.98), whereas a linear fit was found for inorganic sediments (R²=0.53-0.86). In general, decreases in particle size resulted in overall increases in spectral reflectance.