Estimation of the scattering coefficient of natural waters using underwater irradiance measurements

Abstract

Certain relationships between irradiance reflectance (R), average cosine (μ) and the ratio of the scattering and absorption coefficients (b/a), previously derived by Monte Carlo simulation of underwater light, have been used as the basis for a new method of estimating the scattering and absorption coefficients of natural waters from irradiance measurements made within the water body concerned. Standard curves are presented from which, given R at a certain optical depth, the values of μ at that depth and b/a for the water may be read off. The product of μ and the vertical attenuation coefficient, KE for net downward irradiance, (Ed - Ea, provides an estimate of a; this multiplied by b/a then gives the value of b. When applied to published data for Lake Pend Oreille the method gives a value for b differing by only 5% from that which may be calculated by subtracting the absorption coefficient from the beam attenuation coefficient for that lake. Values for scattering coefficient have been calculated by the new method for five water bodies in the southern tablelands of New South Wales for various dates over a 3-year period. The values of b correlate very closely with nephelometric turbidity, an independent measure of light scattering. The new procedure gives higher values than those obtained by an earlier method, but is considered to be more accurate.