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" . , .,' , .' TABLE 4 RESERVOIR INFORMATION Si2:e at spillway Normal Pool capacity Reservoir River (Acres) (cfs) ~ S Clear Cr S Clear Cr 36.6 31,300 Hydropower Clear Lake S Clear Cr 26.9 190 Water Supply Green Lake S Clear Cr 10.3 1,200 Water Supply G'Town Res S Clear Cr 2.3 770 Water Supply Clear Cr Res Clear Cr 51. 5 12,500 Water Supply Model Calibration The SWMM was calibrated to the rainfall discharge frequency relationship at the Laison gage. A frequency analysis of streamflow records for ~clusively rainfall events at the Lawson gage was conducted using the flood flow frequency analysis computer program, HECWRC (USACE, 1982). Annual rainfall peak flow records for Lawson were taken from the united States Geological Survey (USGS) report, "Annual Snowmelt and Rainfall Peak-Flow Data on Selected Foothills Region Streams, South Platte River, Arkansas River, and Colorado River Basins, ~Colorado", Open-File Report 82-426 (USDOI, 1982). Records were available from 1946 through 1977. Appendix A contains the / annual peak discharge values obtained from that report. A gen- eralized skew coefficient 0.22, for rainfall discharge frequency data in the South Platte River basin, was obtained from "Flood Hydrology of Foothill and Mountain Streams in Colorado "(Jarrett, 1987). The pervious surface storage, infiltration rates, impervious area, channel roughness coefficient and overland roughness coefficient were varied in order to calibrate the model. Results Results of the calibration indicated that a value of 4 percent should be used for impervious area. The Manning "n" coefficient used for the channel and overbank conditions was 0.50. The resistance factors used for overland flow were 0.013 for impervious areas and 0.8 for pervious areas. Surface storage values used for impervious and pervious areas were 0.06 and 0.18 inches, respectively. A uniform infiltration rate of 0.95 inches per hour was used. 5