The aim of this study is to estimate water quality characteristics in up-stream area of Nagaragawa estuary barrage based on statistical analyses of “Nagaragawa Estuary Barrage Monitoring Data”. First, spectra of chlorophyll-a and DO data are evaluated by means of FFT (First Fourier Transform). Chlorophyll-a data have fluctuation component of about 10 day period affected by flood inflow in addition to fluctuation components of seasonal period and day period. Secondly, governing factors of chlorophyll-a and DO and their characteristics are investigated by means of Quantification Theory I. It is found that water temperature and growth of thermal stratification are the most important factors for the absolute value of chlorophyll-a and DO, though the governing factors seasonally change.
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