This paper describes the application of a remote sensing Trophic State Index (TSI) to assess changes in eutrophication suffered by Ibitinga hydroelectric reservoir from 1985 to 2008. The TSI, from now on referred as TSIRTM4/TM3, was calibrated with images acquired in 2005. Its application to a Landsat/TM time series was based on the following assumptions: i) the conversion from digital numbers to water surface reflectance would normalize the data set reducing the need for further calibration; ii) images with Sun elevation larger then 40o would minimize surface effects on underwater signal ; ii) images acquired under similar hydraulic residence time would guarantee uniform limnological environment. A set of 15 images were selected, calibrated, atmospheric corrected and geo-referenced. A water mask was built using band TM5 to create a comparable reference among the dates and applied to all scenes. A series of filters were tested and applied to images in order to reduce stripe. After noise removal TM4/TM3 ratio was computed and the TSIRTM4/TM3 look up table developed for the 2005 image was applied to all dates. The area occupied for each trophic state class was then computed. The results showed that the Ibitinga reservoir since 1985 can be classified as super-hypereutrophic. These findings are corroborated by data available in the literature showing that the TSIRTM4/TM3 can be used as a semi-quantitative tool for assessing the degree of water eutrophication.Pages: 6556-656
