Both analysis methods of autocorrelation and mutual correlation are applied to the time series data of water quality indices at South Lake. The characteristics of temporal water quality variations are considered. From the analysis of autocorrelation, the magnitudes of frequency and random component in the time series data are discussed. The results show that the variations of water temperature, pH, chlorophyll-a, P-COD, S-COD and NO
-3-N are rather influenced by the frequency component of one year periodicity, while the variations of SS, NH
+4-N and PO
3-4-P are mainly influenced by the random component. In the latter indices, the coefficient of autocorrelation decreases rapidly with the increase of the lag-time, and becomes less than 0.5 at the lag-time of more than one month. The mutual correlations of water quality indices with chlorophyll-a are calculated and the relationships between the algal activity and water quality indices are discussed. From the analysis, it is shown that the concentrations of SS, P-COD and Org-N tend to change with the variation of chlorophyll-a concetration at the same time, but that those of pH, S-COD and NO
-3-N follow to the change of chlorophyll-a concentration with some days delay.
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