The synthetic data record is obtained by combining the long- and short-term equa¬ 
tions. A sample two-month period, August through September 1987 (indicated by 
the two, close-together vertical reference lines in Figure VI-23), is illustrated in 
Figure VI-24. This synthetic record can then be analyzed relative to the applicable 
criteria elements. In the example shown, the 9-meter depth at station CB4.2C is near 
or below the pycnocline and is, therefore, subject to criteria for the deep-water desig¬ 
nated use. Summer dissolved oxygen criteria for the deep-water designated use is a 
3 mg liter 1 30-day mean, 2.3 mg liter 1 1-day mean and 1.7 mg liter 1 instantaneous 
minimum. For demonstration purposes, let a 7-day mean of 2.5 mg liter' 1 also apply. 
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Dot=observed monthly dissolved oxygen; dashed line=long-term forecast; solid line=combined forecast 
Figure VI-24. Expanded view from Figure VI-23 of the two-month period August- 
September 1987 synthetic data record obtained by combining the long- and short-term 
spectral equations. 
Based on monitoring data alone (two observations each month), the August and 
September mean monthly values are 3.4 mg liter 1 and 4.2 mg liter 1 , respectively. 
Basing assessment on the synthetic data record, attainment can be measured either 
in sequential or rolling time windows, as described below. In some cases the results 
vary depending on which option is used (Table VI-5). For the 30-day duration, the 
sequential option results in two 30-day periods within the 61 days, between August 
1 and September 30, 1987; the rolling time window option yields 31 periods. If there 
was a 7-day criterion for deep-water designated use, there would be 8 sequential 
versus 55 rolling-window periods in those 61 days. For the 1-day minimum duration, 
the question of sequential and rolling-window is moot. 
Verifying the Spectral Analysis Approach. The number and distribution of high 
frequency semicontinuous dissolved oxygen data sets is small compared to the 
variety of habitats, times of year and layers of the water column that need to be char¬ 
acterized. There are gaps in critical seasons, geographic coverage and designated 
chapter vi 
Recommended Implementation Procedures 
