Monte Carlo study of the nature of the underwater light field in, and the relationships between optical properties of, turbid yellow waters

Abstract

A Monte Carlo calculation procedure has been developed for computer modelling of the penetration of light into turbid, coloured waters of the type commonly found on the Australian continent. For monochromatic light the structure of the underwater light field is described in terms of the variation of the average cosine (total, downward, upward) and irradiance reflectance, as a function of optical depth at a given ratio of scattering to absorption (bla), and as a function of bia at a given optical depth. The vertical attenuation coefficient for downward irradiance at the mid-point of the euphotic zone [K_d(z_a)] has been studied as a function of the absorption (a) and scattering (b) coefficients and, with the particular normalized volume scattering function used in these simulations. has been found to conform closely to the equation

K_d(z_m) = (a² + 0.256ab)^½

The data generated by the model have been used in conjunction with radiative transfer theory to determine the contribution of the different components of the attenuation mechanism to the vertical diminution of irradiance at different ratios of b to a.

Simulation of the penetration of photosynthetically available radiation into four water bodies in the southern tablelands of New South Wales has been carried out using measured absorption coefficients and indirectly estimated values for scattering coefficients. The calculated values for vertical attenuation coefficients are in good agreement with those obtained by measurement within the water bodies. The possible use of this modelling procedure as a predictive tool is discussed.