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The complex relationships between indicators and water conditions cause fuzzy and gray uncertainties in evaluation of water quality.Compared to conventional single-factor evaluation methods,the combination evaluation method can consider these two uncertainties to produce more objective and reasonable evaluation results.In this paper,we propose a combination evaluation method with two main parts:(1) the use of fuzzy comprehensive evaluation and gray correlation analysis as submodels with which to consider the fuzzy and gray uncertainties and (2) the establishment of a combination model based on minimum bias squares.In addition,using this method,we evaluate the water quality of a ditch in a typical rice-wheat system of Yixing city in the Taihu Lake Basin during three rainfall events.The results show that the ditch water quality is not good and we found the chemical oxygen demand to be the key indicator that affects water quality most significantly.The proposed combination evaluation method is more accurate and practical than single-factor evaluation methods in that it considers the uncertainties of fuzziness and grayness.