Correlation Structure of the Water Quality-Time Series

Statistical studies on eutrophication of Lake Biwa (Southern Basin) (I)

Abstract

The quality of water taken from the southern basin of Lake Biwa has been observed at Keage Water Station, in Kyoto City on a monthly basis, for about 30 years. This paper describes the results of this statistical analysis regarding these time series from an eutrophicative viewpoint. The results are as follows.
Generally, the correlation structure of time series varies in accordance with the time scale. In the correlation analysis of the monthly data taken from that viewpoint, it is necessary to analyze them on an annual time scale, for example, by previous transformation into the annual mean series.
The synthetic estimate standard for the trophic state was derived by applying the Principal Compornent Analysis to the annual mean series. As a result of using this standard, the eutrophication of the southern basin was estimated to be progressing continuously in a large scope (Fig. 5).
Additionally, it was found that annual periodicitiy existed in the alrnost all the variations of water quality and the seasonal pattern was able to be classified into several types (Figs. 6 and 7).
In conclusion, the causalities among the indices at seasonal and annual time scale were able to be collected (Fig. 8).