A modification of chromaticity analysis to separate the effects of water quality variables

Abstract

The goals of the study were a) to investigate whether the addition of brightness to standard X and Y chromaticity coordinates would reduce confusion between the effects of suspended minerals (SM) and those of chlorophyll (C) and yellow substance (YS) in water quality analyses and b) to explain the V-shaped curve of the XY chromaticity locus for SM in Chilko Lake, British Columbia. To address these issues, a theoretical optical model was used to simulate Landsat MSS data, upon which chromaticity analyses were performed. The model was calibrated for Chilko Lake materials, and was tested using water samples taken at the same time as two MSS images of the lake. The study showed that the use of brightness and chromaticity X separated the effect of SM from that of C and YS, but that C and YS could not be separated because their absorption spectra were similar. Thus it may be possible to measure SM and a combined C and YS variable (CYS) simultaneously, but it is not possible to measure all three variables simultaneously using MSS imagery. The study also showed that the V-shaped locus, which is unlike those previously reported, is the natural consequence of a SM gradient as it approaches zero when atmospheric radiance is included in the image signal. The shape of the locus is not unique to Chilko Lake, but probably has not been reported before because most chromaticity studies have not encountered such low SM concentrations. Classified maps for SM and CYS were produced for Chilko Lake. As expected, highest SM concentrations were found at the mouths of creeks carrying glacial meltwater. Highest CYS concentrations were associated with known sites of high YS or with the western shore and south end of the lake. One interpretation for the latter is that hypolimnial water caused an increase of nutrients and consequently C in the area, though the suggestion cannot be confirmed.