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                                        BOD 5   (mg/l)  10

                                         5

                                         0
                                           77  78  79  80  81  82  83  84  85  86 87  88  89  90  91  92
                                                              Year

                       FIGURE 3.4 Time series plot of BOD 5  concentration in the Fox River, Wisconsin.


                                        BOD 5   (mg/l)  10 5



                                         0
                                            J      F     M     A     M     J     J     A      S     O     N     D
                                                               Month

                       FIGURE 3.5 Seasonal subseries plot of BOD 5  concentration in the Fox River, Wisconsin.

                                                   8
                                                       90th percentile
                                                   7
                                                   6   75th
                                                 BOD 5   (mg/l)  5 4  50th





                                                   3   25th
                                                       10th
                                                   2

                                                   1
                                                       75          80           85          90
                                                      Starting Year of 5-year Interval

                       FIGURE 3.6 Percentile plot of the Fox River BOD 5  data.

                        Figure 3.4 is a time series plot of a 16-year record of monthly average BOD 5  concentrations measured at
                       one of many monitoring stations in the Fox River, Wisconsin. This is part of the data record that was analyzed
                       to assess improvements in the river due to a massive investment in pollution control facilities along this
                       heavily industrialized river. The fishermen in the area knew that water quality had improved, but improvement
                       was not apparent in these BOD data or in time series plots of other water quality data.
                        Figure 3.5 shows another way of looking at the same data. This is a seasonal subseries plot (Cleveland,
                       1994). The original times series is divided into a time series for each month. (These have unequal numbers
                       of data values because the monitoring was not complete in all years.) The annual time sequence is preserved
                       within each subseries. It does appear that BOD 5  in the summer months may be decreasing after about the
                       mid-1980s.
                        Figure 3.6 is a percentile plot of Fox River BOD 5  data. The values plotted at 1977 are percentiles of
                       monthly averages of BOD 5  concentrations for the 5-year period of 1975–1979. The reason for aggregating
                       data over 5-year periods is that a reliable estimate of the 90th percentile cannot be made from just the
                       12 monthly averages from 1975. This plot shows that the median (50th percentile) BOD 5  concentration
                       has not changed over the period of record, but there has been improvement at the extremes. The highest
                       © 2002 By CRC Press LLC
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