5.3.2 Soil Erodibility Factor

  The soil erodibility factor (K-factor) is a quantitative description of the inherent erodibility of a particular soil; it is a measure of the susceptibility of soil particles to detachment and transport by rainfall and runoff. For a particular soil, the soil erodibility factor is the rate of erosion per unit erosion index from a standard plot. The factor reflects the fact that different soils erode at different rates when the other factors that affect erosion (e.g., infiltration rate, permeability, total water capacity, dispersion, rain splash, and abrasion) are the same. Texture is the principal factor affecting K_fact, but structure, organic matter, and permeability also contribute. The soil erodibility factor ranges in value from 0.02 to 0.69 (Goldman et al. 1986; Mitchell and Bubenzer 1980).

  Goldman et al. (1986) note that several methods can be used to estimate the K-factor. The most frequently used are 1) SCS County Soil Survey reports compiled for many counties in the United States and 2) nomographs relating K-factors to topsoil conditions. The SCS county soil surveys contain soil maps superimposed on aerial photographs. The maps permit easy location of sites and tentative determination of soil series. Recent surveys list K-factors for the soil series in the table outlining the soil's physical and chemical properties. Goldman et al. (1986) note that this method of determining K-factors should only be used if minimal soil disturbance at the site is anticipated and a site analysis is unavailable.

The preferred method, according to Goldman et al. (1986), for determining K-factors is the nomograph method based on the work by Wischmeier et al. (1971) and is mathematically represented as follows:



in which



where

f_p is the particle size parameter (unitless)
P_om is the percent organic matter (unitless)
S_struc is the soil structure index (unitless)
f_perm is the profile-permeability class factor (unitless)
P_clay is the percent clay (unitless).

In Equation 5.51 the factor (1.292) is needed to convert K_fact from the English units used in Golman et al. (1986) to the metric units used in this report. The soil structure index, S_struc, is equal to: 1 for very fine granular soil; 2 for fine granular soil; 3 for medium or coarse granular soil; 4 for blocky, platy, or massive soil. The profile-permeability class factor, f_perm, is equal to: 1 for very slow infiltration; 2 for slow infiltration; 3 for slow to moderate infiltration; 4 for moderate infiltration; 5 for moderate to rapid infiltration; 6 for rapid infiltration. Erickson (1977), as reported by Goldman et al. (1986), used the information from the nomograph and superimposed K-factors for 2% organic matter on a U.S. Department of Agriculture (USDA) soil textural classification triangle. Goldman et al. (1986) also presents tables to modify the results to account for

• soils with greater than 15% very fine sand
• soils with organic matter content different from that of 2%
• soils with rock (i.e., soil particle size greater than 2 mm) content greater than 14% by volume
• permeability
• structure.

  Stewart et al. (1975), as reported by Mills et al. (1985), Mitchell and Bubenzer (1980), and Novotny and Chesters (1981), also developed a table indicating the general magnitude of the K-factor as a function of organic matter content and soil textural class. Their results are presented in Table 5.10.

  Goldman et al. (1986) note that if site inspection or data analyses indicate significant variations in the soil erodibility, different K-factors can be assigned to different areas of the site. They also note that a simpler and more conservative approach is to use the highest value obtained for all parts of the site, because it may not be possible to know exactly what soils will be exposed or how varied the soils are.

Table 5.10. Soil Erodibility Factor K_fact (after Stewart et al. 1975)^(a)

	Pom(%)
Textural Class	<0.5	2	4
Sand	0.05	0.03	0.02
Fine sand	0.16	0.14	0.10
Very finesand	0.42	0.36	0.28
Loamy sand	0.12	0.10	0.08
Loamy finesand	0.24	0.20	0.16
Loamy veryfine sand	0.44	0.38	0.30
Sandy loam	0.27	0.24	0.19
Fine sandyloam	0.35	0.30	0.24
Very fine sandy loam	0.47	0.41	0.33
Loam	0.38	0.34	0.29
Silt loam	0.48	0.42	0.33
Silt	0.60	0.52	0.42
Sandy clayloam	0.27	0.25	0.21
Clay loam	0.28	0.25	0.21
Silty clayloam	0.37	0.32	0.26
Sandy clay	0.14	0.13	0.12
Silty clay	0.25	0.23	0.19
Clay		0.13-0.2
(a) The values shown are estimated averages of broad ranges of specific soil values. When a texture is near the border line of two texture classes, use the average of the two Kfact values. In addition, the values shown are commensurate with the English units used in the cited reference (and as used in the source-term module input files). To obtain analagous values in the metric units used in this report, the above values should be multiplied by 1.292.