水库水质之时间数列分析研究

本研究将以德基水库中总磷资料计算 TSI（Trophic Status Index）优养指数，并利用相乘性分解方法（multiplicative decomposition method）进行德基水库水质优养化时间数列分析研究，用以了解德基水库水质之优养化变化情形。研究显示，德基水库季节变化（S）之水质优养化变动情形每年有一高峰期，从 4～10月，总长度达 7个月之久。此外，德基水库优养指数长期趋势（T）乃显示随时间（t）呈缓慢成长状态，其间的关系可表示为 T =51.4231+ 0.0605t，其中 t=1，2，3……，161。至于在循环变动（C）的研究中则发现，循环变动周期自早期的 2～3年，逐渐缩减至 1年左右。由此可知，德基水库水质优养化的情形有日益严重的趋势。

Time Series Analysis Method on Water Quality of Reservoir

The objective of this study reported herein is to maturate a practical procedure for the time series analysis of an eutrophication condition in reservoir.Application is made to the mostimportantreservoirin the central partof Taiwan,Te-Chi.A multiplicative decomposition method as well as TSI (Trophic Status Index) has been adapted to determine the trophic varia-tions including seasonal(S),circular(C),longterm(T) and irregular(I) changes.The re-sults obtained indicate that(1) there is along high peak for seven months from April to Octo-ber yearly;(2) the long-term trend(T) increases with time(t) as following relationship:T = 51.4231 +0.0605t,and(3) circular change period decreases from two or three years to aboutone year.The methodology is feasible to presentthe eutrophication changes numerically.