DERIVATIVE REFLECTANCE SPECTROSCOPY TO ESTIMATE SUSPENDED SEDIMENT CONCENTRATION

Remotely sensed data can be used to estimate successfully the concentration of sediment in water. Such estimation has relied on relationships between suspended sediment concentration (SSC) and radiation in one or two broad wavebands where it is assumed that the effects of environmental variability (irradiance, subpixel cloud, etc.) are either small, or can be considered as spectrally additive constants in all wavebands. Where these assumptions do not hold, an alternative and theoretically more robust relationship is proposed, that between SSC and derivative radiation (change in radiation per unit wavelength). Measurements of spectral reflectance (R(lambda)), derivative spectral reflectance (DR(lambda)) and SSC were measured in the laboratory, where the effects of environmental variability were small and at sea where the effects of environmental variability were large. There was a strong correlation between SSC and DR(lambda) both in the laboratory (max R2 = -0.98) and at sea (max R2 = -0.83) and DR(lambda) was used to estimate SSC in the laboratory to an error of less than 8% of the mean SSC. The correlation between R(lambda) and SSC was weaker at sea (max R2 = 0.46) than in the laboratory (max R2 = 0.96). This was due to the presence of large and spectrally variable environmental effects. We recommend the use of derivative spectra for the estimation of SSC when continuous spectra are available.