Abstract: The purpose of this study is to reveal the spatial distribution of water quality in Caohai Lake watershed, and to provide a theoretical basis for pollution prevention and rational development in Caohai Lake watershed. Water samples were collected from different sections of the lake and were tested to identify major pollutants in line with Environmental Quality Standards for Surface Water (GB 3838-2002). Furthermore the water quality of Caohai Lake was evaluated. Results showed that the concentrations of DO, COD_Mn, COD_Cr and TP of Caohai Lake in wet season are higher than those in dry season, and other pollutants are also lower in dry season. COD_Mn, COD_Cr and TP have greatest impact on water quality, among which COD_Mn and COD_Cr are two major standard-exceeding indexes. In general, the water quality in the area between pier and lake center is the worst, and the water quality at the outlet of the lake is also inferior. The water quality of Caohai Lake is generally between Ⅳ-class and Ⅴ-class. Treating Ⅳ-class water, expanding Ⅲ-class waters, and reducing Ⅴ-class water area is of great significance for improving the water quality of Caohai watershed.

Key words: water quality, spatial and temporal distribution, water pollution, single factor index method, Caohai Lake watershed