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C = The cropping-management factor: is the ratio of soil <br />• loss from a field with specified cropping and management <br />to that from the fallow condition on which the factor K <br />is evaluated. <br />P The erosion-control Dractice factor: is the ratio of <br />soil loss with contouring, strip cropping, or terracing <br />to that with straight row farming up and down slope. <br />The rainfall factor (R) was calculated (Wischmeier, 1974) by <br />using the 2-year, 6-hour rainfall (0.9 inches) for Somerset, <br />Colorado. This calculation yielded a rainfall factor (R) of <br />30. The soil erodibility nomograph for farmland and construction <br />sites (Wischmeier, 1971). The Absarokee 1o am, which had a per- <br />cent sand of 37, a percent silt of 30, a percent organic matter <br />of 5.4, a very low permeability and a medium granular structure, <br />was used to find the soil erodibility factor (k). The nomograph <br />• yielded a soil erodibility factor of 0.22. The slope-length <br />factor (L) and the slope-gradient factor (S) were determined <br />using Figure 5.15 in Barfield, Warner and Haan (1981). An LS <br />Factor of 2.7 was derived for the 400 ft. slope Length and lOX <br />slope. The factors C and P were assumed to be equal to 1.0 to <br />represent no cover and no onsite erosion control. <br />Using the values above the USLE sediment yield is: <br />A = RKLSCP <br />A (30)(.22)(2.7)(1)(1) ~ 17.8 tons/acre/yr. <br />Therefore, sediment yield from the 5.5 acres of disturbed area is <br />98 tons/yr. or 2063 ft3/yr. Thus an additional 4126 ft3 of pond <br />storage will be needed for two years sediment storage. <br />Total pond capacity below the riser exceeds 19465 ft3 of storage <br />required for sediment yield and runoff from the 10 year, 24 hour <br />. precipitation event. <br />D-7 <br />