For the Savannah River monitors where dam release DO concentrations are the primary
concern, frequent calibration visits (at least once a week) during summer months reduce the
degree of drift resulting from biological activity. Calibration drift is assumed to be linear, which
allows corrections to be based on the degree of drift per hour for the period between
calibrations. Each reading is then corrected for drift by adding or subtracting this value to it,
with the drift at the time of the first calibration being equal to zero. The causative factors
leading to drift vary depending on the site, the parameters being measured, and the equipment
being used. (The instruments used for monitoring the Savannah River hydroprojects have a
resolution of 0.2 mg/L; therefore, drift must be >0.2 mg/L before corrections are made.) Drift
must be determined for each site and should be factored into the data set prior to its
incorporation with other project data (Whitfield and Wade 1993).
Data should be incorporated with other project data prior to final analysis. By combining the
available data into a comprehensive project data set, "windows of reflectiveness" can be better
identified and data interpretation will be more accurate. For example, the release monitor at
Hartwell Dam, a Corps project located on the Savannah River (Figure 4), is deployed in the
tailrace (Figure 5). It consists of a submersible pump and pipeline to pass water from the tailrace
to a water quality sonde in a nearby building. Because it samples water from the tailrace, the
monitor represents release water quality only during periods when Hartwell Dam is releasing
water. Data for periods of nonrelease reflect the tailwater conditions only in the area localized
around the sample intake line.
Figure 4. Savannah River basin
Figure 5. Hartwell Dam release monitor
7
Water Quality Technical Note AM-02 (January 1998)
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