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<br />4.4 Soil Conservation Ser'!ice ~:E:thod (SCS)_ <br /> <br />The method presented in I1Procedures for Deter'mining Peak Flo""~s in <br />Colorado--1977 Edition" (D. S. Dept.. of Agriculture, Soil Conservation <br />Service, 1977) can be used to estimate peak flows and runoff volunes <br />in the mountainous watersheds of Larim',r County. Thls technique appears <br />to be applicable to watersheds whose average elevation is greater than <br />6,000 feet and whose area is greater than 200 acr.'s 'but less than 10 <br />square miles. Although the procedure allows one to directly determine <br />the peak flows and runoff volume, it d,)es not: produc,e a runoff hydro- <br />graph. If a runoff hydrograph is desired, the engin'"er should refer <br />to the SCS National Engineering Handbook, Section 4-'-Hydrology (D. S. <br />Dept. of Agriculture, Soil Conservatio:l Service, 197:2) which is the <br />basic reference for this procedure.. <br /> <br />4.4.1 Data requirements <br /> <br />The method requires as input information, the following data: <br /> <br />Watershed area in square mill~s' <br /> <br />24-hour precipitation value <br /> <br />A curve number to reflect the soil and vegetal <br />cover hydrologic response <br /> <br />Time of concentration <br /> <br />4.4.1.l Watershed areas. The watershed areas, in square miles, can <br />be determined by planimete:ring, using ,3 7. 5-rninut,e USGS quadrangle or <br />other suitable map. <br /> <br />4.4.1.2 Precipitation value. The 24-hour precipitation values for <br />Larimer County are present,ed in Seetio," 3. F:lg. 3.3.2-7 through 3.3. 2-12. <br /> <br />4.4.1.3 Curve numbers. The eoncept: of a curve number (CN) is that <br />the hydrologic response of the soil and vegetal cover to precipitation <br />can be related to runoff through the use of a curve number. The CN can <br />be ascertained by following these steps: <br /> <br />Step 1. Determine soil series (see Table 4.4.1.3-1). <br /> <br />Step 2. Classify soil series into proper hydrologic group <br />(see Table 4.4.1.3-1). <br /> <br />Step 3. Identify type or land cover and use (see Tables <br />4.4.1.3-2 and 4.4.1.3-3). ~ <br /> <br />Step 4. Estimate curve number with data in Table,s 4.4.1.3-2 <br />and 4.4.1.3-3. <br /> <br />LCS-WM Manual <br /> <br />4.4---1 <br /> <br />April 1979 <br />