J.E. Bruton

Use of chromaticity in remote measurements of water quality

Abstract Optical cross sections (absorption and scattering per unit concentration of chlorophyll a , suspended mineral, and dissolved organic carbon) are utilized to generate subsurface irradiance reflectance spectra and upwelling irradiance spectra for a variety of water masses. Standard CIE chromaticity analyses are applied to the upwelling irradiance spectra. The results of these analyses serve as a basis for a discussion of severe restrictions in the application of chromaticity to water quality prediction.

Optical water quality model of Lake Ontario. 1: Determination of the optical cross sections of organic and inorganic particulates in Lake Ontario.

A five-component optical model of Lake Ontario is discussed in terms of unique organics (as represented by chlorophyll a), unique inorganics (as represented by total suspended minerals), unique nonliving organics, and unique dissolved organics. Direct measurements of the irradiance attenuation coefficient K(0), the diffuse reflectance R(0), and the total atttenuation coefficient c are used in conjunction with simulated solutions of the radiative transfer equations to determine the inherent optical properties of the water masses. Multiple regressions between these inherent optical properties and directly measured water quality data are then performed to determine the absorption, scattering, and backscattering cross sections of the organic and inorganic components.

Optical water quality model of Lake Ontario. 2: Determination of chlorophyll a and suspended mineral concentrations of natural waters from submersible and low altitude optical sensors

Spectrooptical and water quality data collected from a 1979 coordinated in situ and airborne study of western Lake Ontario are used to devise a five-component model from which subsurface chlorophyll a and suspended solids concentrations may be determined from submersible optical sensors capable of measuring spectral irradiance reflectance just beneath the free-surface layer. A water-air interface model, which incorporates the effects of surface reflection, is also presented in an attempt to extend such water quality estimations to low altitude remote sensors. Special emphasis is given to the spectral wavelength bands of the Coastal Zone Color Scanner aboard Nimbus-7.

Particulate concentrations in Lake St. Clair as recorded by a shipborne multispectral optical monitoring system

Abstract Methodology is developed to convert the upwelling multispectral radiance observed by a ship-mounted dual radiometer reflectance system into the subsurface volume reflectance normalized to a standard solar zenith angle and a standard downwelling light field. This normalized multispectral subsurface volume reflectance is then utilized to evaluate the capacity of effectively conducting, from a moving research vessel, a continuous monitor of the total seston and suspended sediment concentrations in the generally turbid, shallow waters of Lake St. Clair. Good agreement is obtained between remotely predicted and directly sampled concentrations of both seston and suspended sediments.

Determination of inherent optical properties of Lake Ontario coastal waters.

Two optical models (one based upon Monte Carlo simulations of the solutions of the radiative transfer equations and one based upon exponential/quasi-single scattering simulations) relating the apparent and inherent optical properties of natural water masses are utilized in conjunction with directly measured values of the irradiance attenuation coefficient K(0), the diffuse reflectance R(0), and the total attenuation coefficient c to determine the inherent optical properties of Lake Ontario coastal waters. Tables are presented displaying the calculated values of scattering albedo omega(0), forwardscattering probability F, backscattering probability B, absorption coefficient a, and scattering coefficient b as a function of wavelength. From the tables of calculated values, it is shown that both F and b display a spectral invariance, while omega(0) displays distinct spectral variations, the spectral variations apparent in the measured values of c may be attributable to spectral variations in a, and B displays a spectral change that varies inversely with the spectral change in a and c. The volume scattering phase function beta(theta) appears to be altered by the absorption characteristics of the water mass, contrary to the generally accepted premise that absorption and particulate backscattering are independent processes.

Spectral Attenuation and Irradiance in the Laurentian Great Lakes

Abstract Optical data collected between 1973 and 1979 are utilized to study spectral attenuation and irradiance in the midlake waters of four Laurentian Great Lakes. Particular attention is given to the photosynthetic available radiation (PAR), its incident spectral distribution, its spectral attenuation, and its qualitative change with depth. Curves are shown illustrating these spectral properties as well as the relationships between PAR and the photosynthetic usable radiation (PUR) for each of the lakes. These curves, along with the included mathematical relationships, enable quantitative estimates to be obtained of incident PAR, subsurface PAR, and subsurface PUR, and qualitative information to be obtained on subsurface irradiance levels for the Laurentian Great Lakes.

Relationships Among Secchi Disk Depth, Beam Attenuation Coefficient, and Irradiance Attenuation Coefficient for Great Lakes Waters☆

Abstract Optical data collected between 1973 and 1979 are utilized to discuss the relationships among the directly observed Secchi disk depths and the directly measured total attenuation coefficients and irradiance attenuation coefficients in Lakes Erie, Ontario, Superior, and Huron, as well as Georgian Bay. Tables and curves are presented depicting these mathematical relationships obtained by statistical regressions. These relationships are used to effect an intercomparison of the four Laurentian Great Lakes. In addition, subsurface, vertically downward sighting ranges are estimated and compared to the Secchi disk depths as determined from the mathematical regressions. Since there exists a vast amount of historical Secchi disk depth measurements, the need for such relationships certainly exists. It is intended that the cautious use of these regressions will assist the interpretation of such historical data bases wherein Secchi depths comprise the only available optical information.

Relationship Among Optical Transmission, Volume Reflectance, Suspended Mineral Concentration, and Chlorophyll a Concentration in Lake Superior

Abstract A semi-empirical model is described wherein both the suspended inorganic and organic (represented by suspended mineral and chlorophyll a) concentrations of a water mass may be determined by simultaneous measurements of the optical transmission and near-surface volume reflectance of the water column. Predictions of the chlorophyll concentrations resulting from the optical model are compared with chlorophyll measurements taken from Great Lakes cruises. Excellent agreement is observed for the waters of Lake Superior while very poor agreement is seen for the more optically complex water masses of Lakes Ontario and Erie. Comparisons are also made between predicted and measured values of suspended mineral concentrations.

Determination of Available Subsurface Light for Photochemical and Photobiological Activity

Appropriate determinations of available subsurface light for photochemical and photobiological processes require accurate estimates of the subsurface scalar irradiation. Quite often only values of downwelling irradiation are available. Therefore, two sets of multiplicative factors are derived. The first set (Fθ) converts a daily irradiation just above the surface into a daily scalar irradiation just below the surface. The second set (Fzθ) converts an “effective” (i.e., depth-averaged) daily irradiation for a specific photic zone into an “effective” daily scalar irradiation for that photic zone. These multiplicative factors are presented as functions of the subsurface volume reflectance of the water for the volume reflectance range 0.0 to 0.14. For a latitude range appropriate to the Great Lakes, these multiplicative factors vary between 1.1 and 2.1.

Influence of scattering phenomena on the solar zenith angle dependence of in-water irradiance levels.

A Monte Carlo analysis is utilized to determine the influence of the inherent optical properties of a water mass on the solar zenith angle dependence of subsurface irradiance levels. It is shown that the proportion of scattering interactions (as represented by the scattering albedo omega) has a greater influence on this dependence than does the backscattering probability B. Several representations of direct and/or diffuse incident radiation are considered, and their effects on the solar zenith angle dependence are evaluated. Irradiance level data collected in Lake Erie are compared to the predictions of the Monte Carlo analysis.