EXH0199018521

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Sea-Tac Airport
Third Runway
PCHBPollution Control Hearings Board of the State of Washington

Average simulated and observed streamflows for each 7-day low-flow period are listed in Table 2-3 (1993 through 1996) for the gage near the mouth and Table 2-4 (1991 through 1996) for the gage near the wetland. In general, with the exception of 1995, the observed 7-day low flows exceeded the predicted 7-day low flows at both gages. - -_" Table2-3. WalkerCreekatthe mouth,7-daylowflowsforwater-years1993through1996. Observed Calibrated Water-Year AverageFlow(cfs) AverageFlow (efs) Difference(cfs) 1993 1.502 0.923 0.579 1994 0.987 0.833 0.154 1995 0.915 1.077 -0.163 1996 1.719 1.287 0.432 AverageDifference 1.281 1.030 0.250 Table2-4. WalkerCreeknearwetland,7-daylowflowsforwater-years1991through1996. Observed Calibrated Water-Year AverageFlow (cfs) AverageFlow(efs) Difference(cfs) 1991 1.208 0.786 0.422 1992 1.098 0.682 0.416 1993 0.800 0.666 0.134 1994 0.670 0.614 0.056 •J 1995 0256 0.750 -0.494 1996 0.896 0.870 0.026 AverageDifference 0.656 0.725 -0.069 2.2.2.3 Des Moines Creek Low Streamflow Two streamflowgageslocatedinthe'DesMoinesCreekwatershedwereusedinthelow-streamflow calibrationreview(seeFigure2-I).One ofthesestreamflowgageswas locatednearthemouth of Des MoinesCreek,andtheothergage(IIc)was locatedfurtherupstream. Averagesimulated andobservedstreamflowsforeach7-daylow-flow periodarelistedinTable2-5 0992 through1996)forthegagenearthemouth andTable2-6(1991through1996)forgage1Ic. Ingeneral,theobserved7-daylow flowswereclosetothepredicted7-daylow flowsatthegage nearthemouth,whiletheobserved7

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