Laserfiche WebLink
39 <br />~~ <br /> <br />~~ <br /> <br />• <br /> <br /> <br /> <br />1 <br />• <br />sources of error with this estimation of R. First, snowfall constitutes <br />a significant proportion of the average annual precipitation for all of <br />the sampling sites and erosion from snowmelt may be significant. The <br />iso-erodent map by Clyde et al. was calculated only from rainfall data <br />because at present there is insufficient data available to make a statis- <br />tically sound modification of R based on snowmelt runoff (U. 5. Forest <br />Service, 1977). The high R factor value at the Delagua site (Table 4) <br />may reflect the large proportion of warm season rainfall at that site <br />(Table 1), even though the average annual precipitation at the Delagua <br />site is less than the average annual precipitation at most of the other <br />sampling sites. The second possible source of error in determining R <br />may be due to highly localized rainfall patterns in mountainous areas <br />(Osborn et al., 1977 Renard, 1975). Most of the sampling sites, how- <br />ever, may be located in areas where the iso-erodent information is rela- <br />tively accurate for a long-term average (U. S. Soil Conservation Service, <br />1977). <br />The soil erodibility factor, K (in English units), is quantitatively <br />predicted (Wischmeier, 1977) by: <br />K = (2.1x10-6)(12-OM)M1.14~ 0325(S-2)+0.0025(P-3) <br />where: <br />OM is percent organic matter <br />M is particle size parameter (% silt x (% sand + % silt); (very fine <br />sand is classified as silt) <br />S is soil structure index <br />P is profile permeability class <br />K is in units of tons per acre per 100 foot-tons per acre (.tonnes per <br />hectare per 100 meter-tonnes per hectare). <br />