expression of the diverse aspects of the trophic state found in multi-parameter indices yet also has the simplicity of a single parameter index, for water quality assessment of impounded water bodies. Carlson's trophic state index can be computed from any of three interrelated water quality parameters: secchi disk depth (SDD), chlorophyll-a concentration (Chla), and total phosphorous measurement (TP). The index has since then been widely accepted owing to its calculation simplicity and ability to communicate between researchers, government agencies, and local community residents.

Determination of trophic state using field-collected water quality data is time and cost consuming, and whether the limited number of field data can adequately represent the overall quality of a vast water body is often disputed. To circumvent the disadvantages of traditional data-collection method, utilization of remote sensing data for water quality assessment has been investigated. Previous work (Lillesand, et al., 1983; Ekstrand, 1992; Tassan, 1993; Lavery et al., 1993; Han, 1997; Rundquist, 1997; Avard et al., 2000: Thiemann and Kaufmann, 2000) has shown that remote sensing data can not only be used to estimate certain individual water quality parameters, these data can also be related to trophic state indices developed from single or multiple water quality parameters.

Although empirical models have been developed for water quality monitoring using remote sensing data, most of these studies were conducted in coastal or estuarine waters, or used field-spectrometer measurements in inland lakes. Also chlorophyll and sediment concentrations in several Taiwan's reservoirs are found much higher than that reported in above studies. Therefore, the objective of this study is to use TM data to determine water quality of the Te-Chi reservoir, located in Central Taiwan, and give a probability-based decision on the overall trophic state of the reservoir.

2. Carlson Trophic State Index
Carlson's TSI model converts raw data of SDD, Chla, and TP to a standard numerical scale ranging from 0 to 100 based on empirical relationships between water quality parameters. Individual indices are calculated by the following equations:



The units are meters for SDD and mg/l for Chla and TP. A commonly applied criterion for trophic classification of lakes is: Oligotrophic - TSI £ 40; mesotrophic - 40 < TSI £ 50; eutrophic - 50 < TSI.