Abstract
Bio-optical measurements were carried out on eutrophic Lake Zug and oligotrophic Lake Lucerne, to provide data and models for thein situ calibration of multispectral imagery collected in 1991, during an AVIRIS flight over Central Switzerland. The results indicate that Secchi depth, chlorophyll and total suspended solid concentrations, vertical extinction coefficient, absorption coefficient and irradiance reflectance in the PAR region can be used to discriminate between the two lakes. Dissolved organics concentration, scattering and total attenuation coefficients estimates, as well as backscattering probability, are less acceptable, because of instrumental limitations. Relationships between optical and water quality parameters were investigated and found to behave according to accepted theoretical frameworks. Both lakes exhibited vertical and horizontal heterogeneities in chlorophyll and turbidity, and had contrasting mean bio-optical characteristics: although suspended solid concentrations were similar, transparency was lower in Lake Zug, but reflectance was substantially higher in Lake Lucerne. Water colour determined by chromaticity analysis was blue-green in Lake Lucerne and green in Lake Zug. Reflectance spectra simulated through a three-component optochemical model did not completely match those derived fromin situ measurements, because of lack of site-specific optical cross-sections for suspended minerals and dissolved organics. The monitoring of water quality in subalpine lakes by remote sensing, both with present and future technology, is discussed and considered as possible, provided that their optical behaviour is known, and preferably expressed by a valid optochemical model.
Highlights
Introduction and scopeThe colour of water is one of its most obvious organoleptic descriptors
In Lake Zug, for instance, a definite turbidity maximum occurs at an approximate depth of 5 m, together with a smaller surface peak gaining in importance from North to South (Stn. 3B)
These patterns are confirmed by the results of chlorophyll measurements (Table 1), and can be ascribed to the stronger anthropic impact on the Northern part of the watershed
Summary
The colour of water is one of its most obvious organoleptic descriptors. The pioneering work of Bukata and co-workers is significant (Bukata et al, 1981 a, b) They developed a method relating irradiance reflectance to backscatter and absorption coefficients, themselves functions of water, chlorophyll, suspended minerals and dissolved organic carbon optical crosssections. This modelling approach provided a link between the water optical signature (or colour) and water quality, necessary for the use of air- or satelliteborne sensors (Mittenzwey et al, 1988)
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