The characteristics of seasonal variation of water quality at South Lake is analyzed. A valuable defined by the ratio between time series data and the annual average is introduced to discuss frequency and random components on the time series variations of eutrophic indices at South Lake. From the analysis of this valuable, it is shown that the variations of water temperature, NO
-3-N and Inorg-N depend mainly on the annual periodic effect while the annual periodicities on NH
+4-N and PO
3-4-P are not detected. The time series variations of the other eutrophic indices, such as chlorophyll-a, T-COD, T-N and T-P are partially in-fluenced by the effect of periodicity. The differences on seasonal variation patterns of some sampling locations are significantly detected on the indices of water temperature and NO
-3-N. To separate rationally the data into four seasons, the one-way analysis of variance is applied to the valuable, so that the classification into the following months suth as Jan.-Mar., Apr.-Jun., Jul.-Sep. and Oct.-Dec. becomes most suitable for the understanding of the periodic variation on the eutrophic indices at the lake.
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