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- W95493835 abstract "Lake Biwa is the largest freshwater lake in Japan, located in west central Japanese main land. We report the result of long-term trend analysis of time series of Chlorophyll-A in Lake Biwa. The concentration of Chlorophyll-A is a barometer of phytoplankton growth. The higher the concentration, the worse the water quality and the more probable we have the danger of red tides and blue-green algae. Data has been observed through the water quality investigations conducted by Shiga Prefecture and the Ministry of Land, Infrastructure, Transport and Tourism. Data are generally observed from April 1979 to March 2003, measured at the fixed points once in a month, though the data lengths and the observed items vary. The first part of this presentation works on the removal of seasonal pattern from a given time series. Because the periodic pattern due to yearly climate change or some general social activities is a nuisance for trend- analysis, it should be removed in advance. For this aim, we employ unobserved component time series models where the observation is decomposed into several unobserved components. Stochastic constrains on the smoothness of the variability of the components lead to constrained least squares solution, which can be represented in a Markov form, a state space representation. Based on the trend estimates obtained by the observation sites, we show a contour plot of the Chlorophyll-A concentration for a given time point via spatial smoothing and interpolation. Station-wise plots of the estimated trends of Chlorophyll concentration reveal that the clarification of north Lake Biwa made progress in 1980s while the south lake finally purified around 2000. This can be visually confirmed by the spatially smoothed contour plots. One of the background for the recent low concentration of Chlorophyll-A can be sought for the decrease of effluent impact, most of which could be accounted for the improvement of sewerage system of the towns around the lake. After this preliminary stage, we obtain the trend series of measured substance by every monitoring point. In the next step, we examine the significance of the estimated trend via formal statistical tests. We report the results of two different kinds of unit root tests, ADF test and KPSS test that are standard tools in the context of econometric time series analysis. Unit root tests are originally the tools to determine whether the process is non-stationary or trend stationary. In this paper, however, we are not interested in such a distinction." @default.
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- W95493835 date "2009-01-01" @default.
- W95493835 modified "2023-09-27" @default.
- W95493835 title "Long-term trend analysis of water quality in Lake Biwa" @default.
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