ESTIMATION OF OPTICAL PROPERTIES OF WATER FROM SECCHI DISK DEPTHS¹

Abstract

Optical information on a water body is often required when only Secchi disk depths are available. Many limnologists and water managers have attempted to estimate diffuse light attenuation in water from Secchi depth data assuming a simple inverse relationship. However, we show theoretically that the product of Secchi depth and the diffuse attenuation coefficient is markedly dependent on the reflectance coefficient (“brightness”) of water. Data from 28 New Zealand lakes of diverse optical character demonstrated this dependence over a wide range of reflectance (1.1 to 35.9 percent). Uncritical estimation of attenuation coefficients for diffuse light from Secchi depths, using the currently available simple inverse expressions, is discouraged because of the possibility of bias.

The effect of properties of sediment type on the relationship between suspended sediment concentration and radiance

“…The radiometric response was recorded in four spectral bands for water. The output corresponding to the input radiation was observed digitally in watts per square centimetre per steradian per micrometre (W cnr 2 Sr 1 ^nr 1 )-A secchi disc of 0.2 m diameter was painted white and black on alternate quadrants and was used to measure the extinction depth (Davies-Colley & Vant, 1988). The average of two readings was used for the depth at which the disc disappeared during descending and reappeared during raising.…”

Dual-polarization airborne lidar for freshwater fisheries management and research

“…Often, the only optical properties available for a given lake are Secchi disk depths, which are rough estimates of the water optical properties. 71,72 Measurement of local freshwater lake trophic and optical properties may reduce this discrepancy as assuming parameters from other lakes is problematic; Kirk notes that estimates for the absorption coefficient of CDOM vary widely, even for lakes in the same general area. 73 Figure 9 shows an example single lidar shot with the presence of fish from Yellowstone Lake, and Fig.…”

Optical properties of Lake Coleridge: The impacts of turbid inflows

“…Criteria for visual clarity of waters for aesthetics (human vision) are not well established and such criteria are absent for vision of aquatic fauna. Visual range of low contrast objects in Lake Coleridge would be expected to average 5-6 m based on correlation of Secchi depths with black disk ranges (Davies-Colley 1988). This is still much greater than in "good" lake fisheries elsewhere.…”

“…Kirk (1976) has suggested that the absorption coefficient of filtered water samples at 440 nm (g 440 ) is a useful index of yellow substance concentration. Absorbances at 440 nm were too low for direct measurement in Lake Coleridge but since the spectrum of yellow substance absorption is nearly constant in shape (quasi exponential: Davies-Colley & Vant 1987) we can estimate g 440 from A270F measurements. This gives g 44 o~ 0.04 rrr 1 , a low value typical of coastal sea waters (Kirk 1983 : table 3.2, p. 57).…”

“…The underlying theory is discussed by Davies-Colley & Vant (1988). The absorption coefficient was taken as a constant equal to the average value of 0.073 nr 1 because it showed little dependence on constituents of the waters in Lake Coleridge.…”