PERMFILE125190

• Permit No: C1989074
• IBM Index Class Name: Permit File
• Doc Date: 12/11/2001
• Section_Exhibit Name: Exhibit J Erosion Computation
• Media Type: D
• Archive: No

z
S, the slope-gradient factor, is the ratio of soil loss from
the field gradient to that from a 9-percent slope;
C, the cropping management factor, is the ratio of soil
loss from a field with specified cropping and management
to that from the fallow condition on which the factor K
is evaluated;
P, the erosion-control practice factor, is the ratio of soil
loss with contouring, stripcropping, or contour irrigated
furrows to that with straight-row farming, up-and-down
slope.
RAINFALL FACTOR (R)
The energy of moving water detaches and transports soil materials. The
energy-intensity (C•I) parameter measures total raindrop energy of a
storm and its relation to the maximum 30-minute intensity. Soil losses
are linearly proportional to the number of EI units. The EI values of
the storms are summed to obtain an annual rainfall-erosivity index for a
given location. Tliis annual index serves as the R factor and can be
obtained from Figure 1. This figure differs from Figure 1, Agricultural
Iandbook 282 dated December 1965 (5), and will appear in the 1977 revision
of Handbook No. 282.
A procedure for determining the effect of snowmelt on the R factor is
given in a LVest TSC Technical Note (4). Technical notes are available
which give R values for Hawaii and Puerto Rico (2) and (3). Individuals
interested in those areas should consult the appropriate technical note.
SOIL-ERODIBILITY FACTOR (K)
The capability of a soil surface to resist erosion is a function of the
soil's physical and chemical properties. The most significant soil
characteristics affecting soil erodibility are texture, organic matter
content, soil structure and permeability. The K values are assigned to
named kinds of soil and may be obtained from the soil scientist, the
technical guides, or published lists.
SLOPE LENGTH (L) AND SLOPE GRADIENT (S)
Soil loss is affected by both length and degree of slope. For conveni-
ence in the field application of these factors they are combined into a
single topographic factor, LS.
The LS factor for gradients up to 60% and slope lengths to 2000 feet is
obtained from the Slope-Effect Chart, figure 2. Similar data in tabular
form is shown in Table 1. Values shown on the chart and table for
slopes of less than 3%, greater than 20%, or longer than 400 feet,
represent extrapolations of the formula beyond the range of research
data. Computed soil loss obtained using such LS values may require
adjustment based on experience and judgment.