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

1
PROCEDURE POR
COMPUTING SHEET AND RILL EROSION ON
PROJECT AREAS
I NTRODl1CT I ON
Since the late 1940'x, SCS geologists, who are responsible for estima-
ting sediment yields, have been using the Musgrave Equation (1) to
compute the amount of sheet and rill erosion occurring in a watershed.
The Musgrave Equation has been a part of one of several procedures used
to estimate sediment yields. Additional research on erosion has resul-
ted in the development of the Universal Soil-Loss Equation (USLE) by the
Agricultural Research Service (ARS) in cooperation with the SCS and
certain state experiment stations (S). The USLE was originally devel-
oped for use only on cropland, hayland, and pastures in rotation.
Erosion factors reflecting the effect of cover on uncultivated land
areas have been lacking. Since the USLE has been used throughout much
of the country as a tool in planning land treatment on individual opera-
ting units, it was recommended that the use of this equation with its
refined data be extended to watersheds and other project areas in which
the SCS has responsibilities. In order to do this, additional plant
cover factors (C) were needed for permanent pastureland, rangeland,
woodland, and idle land to estimate the effect of these types of cover
on soil losses.
During a conference of SCS and ARS personnel in November of 1971, needed
factors for types of cover on uncultivated lands were discussed and
tentatively agreed upon. Subsequent analyses by the ARS I~rovided values
for these factors as presented in Tables 2, 3, and 4. These factors are
for use in the USLE to estimate sheet and rill erosion for SCS project
work such as watersheds, river basin studies, and resource conservation
and development (RC$D) projects.
The determination of the values of the factors to be used in the USLE
for project w~~rk will be a team effort. The state resource conserva-
tionist, agronomist, and/or district conservationist provide the geolo-
gist with C values. Information is needed not only for rotations to be
used on cropland, and management practices on pastureland, rangeland,
and woodland, but also the amount or percent of land treatment which
will be applied during the project installation period. The complete
USLE is A = RKLSCP
where A is the computed soil loss (sheet and rill erosion) in tons
per acre per year. A is not the sediment yield;
R, the rainfall factor, is the number of erosion-index units
in a normal year's rain;
K, the soil-erodibility factor, is the erosion rate per unit
of erosion index for a specific soil in cultivated continu-
ous fallow, on a 9-percent slope 72.6 feet long;
L, the slope-length factor, is the ratio of the soil loss
from the field slope length to that from a 72.6 ft. length
on the same soil type and gradient;