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I I I I I I I I I I I I I I I I I I I CN = 1000 10+8 Where: Equations 5 and 6 CN = curve number, I. = initial abstraction in inches, and S = potential maximum retention after runoff begins in inches (SCS) fa = 0,28 Solving these two equations using the curve number gave an initial abstraction, or initial loss, of 0,8165 in, or 20,75 rom, For consistency, the SCS method was selected for the transform analysis with a lag time of9 hr, Chapter 2 of the SCS Engineering Field Manual also gave an equation to find the lag time: A",8[(1000)_9]"7 Where: T = CN T c = time of concentration in hours, c 1140 yO,' Equation A. = flow length in feet, CN = runoff curve number, and Y = average watershed slope in percent (SCS The flow length was found in the HEC-GeoHMS ArcView extension and was 43,017 feet, The curve number was as indicated above and the average watershed slope was determined to be 2.34% by dividing the total rise of the watershed by its length, Plugging these values into the above equation yielded the time of concentration, or lag time, to be 9 hours, No baseflow was used since any groundwater exfi1tration would be negligible for a 100-year precipitation event. The user must also supply the meteorological model and control specifications required to calculate the flow at the select point in the basin, The frequency storm method was selected for the meteorological model with a 1% exceedence probability, a 3 hour maximum intensity duration, a storm duration of24 hours and the peak center at 50%, The Colorado NOAA Atlas gave the precipitation for a 100-year 24-hour storm as 2,6 inches, Therefore, the precipitation depth was entered as 0.325,0,65, 1.3, and 2,6 in for the 3, 6, 12, and 24 hour durations, respectively, The control specifications were randomly selected as the week 20