Laserfiche WebLink
38 <br />• computed from rainfall data expressed in tenths of inches received from <br />a two-year frequency, six-hour duration rainstorm, then converted to R <br />factor values with a modification curve from Soil Conservation Service <br />Technical Note No. 32 (Brooks and others, 1974). The R factor values <br />were converted to metric units for Table 4. The U. S. Soil Conservation <br />Service iso-erodent map tends to duplicate Clyde et al., but with less <br />detail, and presumably less accuracy. <br />Wischmeier (1962) found that the R factors east of the Rocky <br />Mountains can be approximated as the product of the maximum two-year <br />frequency, thirty minute duration storm; maximum two-year frequency, 24 <br />hour duration storm; and the average annual precipitation, all in English <br />units. The products were calculated for the sampling sites of this study L <br />and are listed in metric units under "Calculated" in Table 4. While this <br />• short-cut method of calculating R factor values may be accurate for re- t <br />glans east of the Rocky Mountain area, the method appears to strongly <br />underestimate the rainfall erosivity factor values for the sampling sites <br />in Colorado. A similar. problem was discovered with a predecessor of the <br />Universal Sail Loss Equation, the Musgrave Equation (Musgrave, 1947). <br />Based on limited data from Wisconsin, the Musgrave Equation assumed the <br />annual erosivity varied as the 1.75 power of the maximum 2-year, 30 <br />minute rainstorm. Later research did not support the accuracy of that <br />term as a widely applicable indicator of rain erosivity (Wischmeier, <br />1959). <br />Because of lack of detail in the U. S. Soil Conservation Service <br />iso-e rodent map and the obvious error in the short-cut calculation method <br />. by Wischmeier, the iso-erodent map by Clyde et al. was selected as the <br />best method to determine R for this report. There are two possible <br />