Carlson's Trophic State Index (CTSI) has long been used in the Taiwan region to assess reservoir eutrophication, however this approach can often lead to confusion because the assessment criteria for CTSI (such as Secchi disk Depth) can be degraded by factors such as turbidity, which does not actually reflect the increased eutrophication. Mountainous subtropical islands suffer from serious stormdriven erosion problems, leaving reservoir water quality subject to seasonal weather conditions, torrential rains, typhoons and turbidity. Thus the CTSI alone may be insufficient or inappropriate at certain times to evaluate water quality. In pratice, we found that this indicator could produce biased results due to high flooding, seasonality, and high turbidity. In this study we use fuzzy sets to assess water quality rating, to investigate the appropriateness of its use in evaluating water quality, and to improve the evaluation for reservoir eutrophication. Results indicate that using fuzzy sets as a method of analysis is appropriate for determining water quality levels at Taiwan's Feitsui Reservoir, and that it can be used to represent water quality caused by hydrological phenomena.

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