MODELING THE RADIOMETRIC COLOR OF INLAND WATERS - IMPLICATIONS TO A) REMOTE-SENSING AND B) LIMNOLOGICAL COLOR SCALES

Utilizing a bio-optical model previously developed for Lake Ontario, t he responsiveness of chromaticity coordinates (X, Y, Z), dominant wave length (lambda(dom)), and associated spectral purity (p) to the abunda nce of color-producing agents (CPA) residing within the Lake Ladoga wa ter column was determined. CPA considered were phytoplankton (chl), su spended minerals (sm), and dissolved organic carbon (doc). Waters that contain simultaneously low concentrations of chl, sm, and doc are sho wn to appear blue to turquoise in color (472-500 nm). Highly turbid wa ters (i.e., waters containing high concentrations of chl and/or sm) wi th low concentrations of doc are shown to display colours ranging from green to brown (> 500 nm). Waters with large concentrations of doc, i rrespective of turbidity, are shown to be invariable brownish (560-570 nm). With increasing CPA content, X, Y, and Z (and, consequently, lam bda(dom)) asymptotically approach constant limit values. An ''end-poin t'' dominant wavelength at about 572 nm appears to be intrinsically ch aracteristic of all natural waters. It is shown that when one or more CPA exceeds a critical concentration, the spectral purity p asymptotic ally approaches values in the range 0.35 to 0.45 for all waters (excep tive of those containing solely chl in the restricted concentration ra nge less than or equal to 0.5 mu g/L. Optical distinctiveness, particu larly with respect to indigenous doc, of natural waters, while impacti ng the spectral purity of the ''end-point'' radiometric color, does no t produce comparable impact on rite ''end-point'' color itself. This w ork reinforces the restrictive application of chromaticity analyses to the remote sensing of binary aquatic systems comprised of water plus one CPA. It also illustrates that neither panchromatic nor two-channel ratio images can provide unambiguous inference of water quality param eters. Correspondence between radiomimetic water color descriptors (X, Y, Z, lambda(dom') and p) and water color scales traditionally used i n limnology is established, illustrating that the platinum-cobalt scal e would be most appropriate for assessing waters that were radiometric ally yellow, provided thar the yellow hue were not invariably attribut ed to doc.