§ 704.8 - Criteria for identifying highly erodible land.

(a) Soil map units will be used as the basis for identifying highly erodible land. The erodibility of a soil is determined by dividing the potential average annual rate of erosion for each soil by the predetermined soil loss tolerance (T) value for the soil.

(1) The potential average annual rate of sheet and rill erosion is estimated by multiplying the following factors of the Universal Soil Loss Equation (USLE):

(i) Rainfall and runoff (R),

(ii) The degree to which the soil resists water erosion (K), and

(iii) The function (LS), which includes the effects of slope length (L) and steepness (S).

(2) The potential average annual rate of wind erosion is estimated by multiplying the following factors of the Wind Erosion Equation (WEQ): Climatic characterization of wind speed and surface soil moisture (C) and the degree to which soil resists wind erosion (I).

(3) The USLE is explained in U.S. Department of Agriculture Handbook 537, “Predicting Rainfall Erosion Losses.” The WEQ is explained in Agriculture Handbook 346, “Wind Erosion Forces in the United States and Their Use in Predicting Soil Loss.” Copies of the handbook may be obtained by writing: U.S. Department of Agriculture, Soil Conservation Service, Land Treatment Programs Division, Washington, D.C. 20013-2890. Values for all the factors used in these equations are contained in the SCS field office technical guide and the references which are a part of the guide.

(b) A soil map unit subject to significant erosion by water or by wind shall be determined to be highly erodible if either the RKLS/T or the CI/T value equals or exceeds 8.

(c) Whenever a soil map unit description contains a range of slope length and steepness characteristics that produce a range of LS values which result in RKLS/T quotients both above and below 8, the soil map unit will be entered on the list of highly erodible soil map units as “potentially highly erodible.” The final determination of erodibility for an individual field containing these soil map delineations shall be made by an on-site investigation.