Section 12.21 - Identification of highly erodible lands criteria(a)Basis for identification as highly erodible. Soil map units and an erodibility index will be used as the basis for identifying highly erodible land. The erodibility index for a soil is determined by dividing the potential average annual rate of erosion for each soil by its predetermined soil loss tolerance (T) value. The T value represents the maximum annual rate of soil erosion that could occur without causing a decline in long-term productivity. The equation for measuring erosion is described below. (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 windspeed and surface soil moisture (C) and the degree to which soil resists wind erosion (I).(3) The USLE is explained in the U.S. Department of Agriculture Handbook 537, "Predicting Rainfall Erosion Losses." The WEQ is explained in the paper by Woodruff, N.P., and F. H. Siddaway, 1965, "A Wind Erosion Equation," Soil Science Society of America Proceedings, Vol. 29. No. 5, pages 602-608. Values for all the factors used in these equations are contained in the NRCS field office technical guide and the references which are a part of the guide. The Universal Soil Loss Equation, the Revised Universal Soil Loss Equation, and the Wind Erosion Equation and the rules under which NRCS uses the equations are published at §§ 610.11 through 610.15 of this title.(b)Highly erodible. A soil map unit shall be determined to be highly erodible if either the RKLS/T or the CI/T value for the map unit equals or exceeds 8.(c)Potentially highly erodible. Whenever a soil map unit description contains a range of a slope length and steepness characteristics that produce a range of LS values that 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 unit delineations will be made by an on-site investigation, or by use of Light Detection and Ranging or other elevation data of an adequate resolution to make slope length and steepness measurements. In any case where a person disagrees with an off-site determination on potentially highly erodible soils, a determination will be made on-site.61 FR 47025, Sept. 6, 1996; 61 FR 53491, Oct. 11, 1996; 83 FR 63051, Dec. 7, 2018