SPECIFIC GRAVITY (RELATIVE DENSITY) EVALUATION OF HYPERSALINE LAKES BY IN SITU HYPERSPECTRAL MEASUREMENTS
An evaluation of the density of Lake Acıgöl (Turkey) at the end of rainy season was made using in situ above surface hyperspectral data. In situ spectral and specific gravity measurements were made simultaneously on sunny days. The hyperspectral signatures of lake water were obtained using an in situ high spectral resolution spectroradiometer with a spectral range of 327–1075 nm. The density of the lake water was determined by measuring the specific gravity with a DKK-TOA multiparameter sensor. While the average specific gravity across the lake was 1.021, this dropped to 1.015 close to the Akpınar springs that feed into the lake. At the NW part of lake, where no spring feeds the lake, the specific gravity is 1.025.
The spectral characteristics of the lake water and the effect of specific gravity upon the water spectral signatures were analyzed. The potential for using hyperspectral reflectance measurement for the monitoring of lake density was evaluated using univariate correlation and band ratio analysis (BRA). Univariate correlation analysis suggested that two spectral regions at 750-780 and 830-850 nm were sensitive to the specific gravity of the lake water. A decrease in specific gravity caused an increase in reflectance at the 750-770 nm wavelength region of the lake water reflectance spectrum. BRA involves regression of band ratio values against the measured specific gravity for all possible bands. For hypersaline lake water, we found that the ratios of reflectance at 763 and 580 nm were strongly related to the specific gravity (R2=-0.91). The band ratio of the region within 750-780 nm reflectance to the region within 550-585 nm reflectance (BR=[R(750-780nm)/R(550-585nm)]) produced highly correlated results with lake specific gravity in the hypersaline Lake Acıgöl.