Assessing the level of reservoir’s Eutrophication has been regarded as one of the main issues in the water quality management by managers and policy makers. Estimating the eutrophication level is not possible by a determined method and complete certainty with respect to the high complexity of this phenomenon. Fuzzy theory is an approach by which the lack of transparency and uncertainty can be determined between the levels of eutrophication. In this study, Entropy- based Fuzzy Eutrophication Index (EFEI) and Trophic State index (TSI) were used for the trophic level classification of the Ilam reservoir epilimnion and hypolimnion during a year. These indexes were weighted with two scenarios based on the chlorophyll a, total phosphorus and oxygen saturation parameters. In the first scenario, the relative weights of indexes were calculated by the combination of entropy- based fuzzy approach and analytical hierarchy process. In second scenario, equal weight was considered for all parameters. The results showed that based on both scenarios and in most months of the year the trophic level of the epilimnion was oligotrophic with suitable certainty and it was only mesotrophic in July and August. In the hypolimnion, the trophic level was similar to the epilimnion, but uncertainties have been increased.
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