Geology Reference
In-Depth Information
include any impact (either erosion or deposition)
of the concentrated flow channels that form in the
natural swales at the bottom of the roughly planar
hillslopes, and certainly did not address classical
gullying processes that often occur at steep bound-
aries such as headcuts and sidewall sloughing.
Use of the plot data to establish values for the
factors above began with an analysis of rainfall
erosivity by correlating the erosion measured
under Unit Plot conditions with a whole series of
measured rainfall values. A very strong correla-
tion was found between this worst-case erosion
and a combination of two rainfall factors, namely
the total storm energy
E
and the maximum storm
30-minute intensity, or
I
30
(Wischmeier, 1959).
The
R
factor was then calculated by summing
over the calendar year the
E
internationally. Perhaps its most common use
was as one of the primary tools of the USDA Soil
Conservation Service for conservation planning
on agricultural lands. As use of the USLE expanded
and it was applied in other situations, like dis-
turbed forest lands (Dissmeyer & Foster, 1981,
1984), limitations of the technology became
apparent. At the same time, continuing soil ero-
sion research on both natural plots and under
simulated rainfall led to improved understanding
of the physical processes involved in hillslope
sheet and rill erosion. Recognized limitations and
advancements in erosion science pointed to the
need for updating the USLE.
8.2 RUSLE
I
30
values for all
storms of over 12 mm (0.5 in.) or with more than
6.5 mm (0.25 in.) falling in 15 minutes, and taking
the average of those annual values over all years
of record. The soil erodibility (
K
) values were then
determined for Unit Plot conditions (
C
⋅
8.2.1 RUSLE1 development
In 1985, scientists and engineers from the
USDA-ARS and the USDA Soil Conservation
Service and affiliated academics with expertise
in soil erosion assembled in West Lafayette,
Indiana. At that workshop, two important deci-
sions evolved, including the need to (1) develop
technology to replace the USLE with a physically-
based model (subsequently called the Water
Erosion Prediction Project or WEPP); and (2) to
computerize and update the 1978 version of the
USLE with an improved model, subsequently
called the Revised USLE or RUSLE. All subse-
quent material in this chapter is directed to a
description and analysis of the various portions
of the RUSLE effort, including both RUSLE1 and
RUSLE2.
The first version of RUSLE1, a software pro-
gram designed to operate in a DOS-based compu-
ter environment, was released in 1997. RUSLE1
was supported by USDA-ARS through Agriculture
Handbook No. 703 (AH703) (Renard
et al
., 1997).
The computer system soil erosion model described
therein was a major conversion of the factor
approach presented in AH537. Perhaps the most
significant change was the subfactor approach to
the calculation of the cover-management factor
C
, thereby allowing use of RUSLE1 for any land
use that could be adequately addressed by these
subfactors. This broke the previous bonds of the
=
P
=
LS
=
1.0) solving for
K
using measured
A
and
R
values.
With the
K
values in hand, the values for
C
,
P
and
LS
could be determined by replicated plot studies
on similar soils using different management prac-
tices or topographies.
Techniques for determining factor values to
insert in the USLE (Equation (8.1) ) were first pre-
sented for general use in the USDA's Agriculture
Handbook No. 282 (Wischmeier & Smith, 1965).
As use of this technology expanded and new
studies were carried out to fill gaps and address
weaknesses, new data were incorporated into
the USLE, resulting in the second and most
widely known release of the USLE technology in
the USDA's Agriculture Handbook No. 537
(AH537) (Wischmeier & Smith, 1978). The val-
ues for the USLE factors as presented in AH537
were generally created to represent an average
annual basis, although the form of the relation-
ship does not demand that. The exception to
this was the
C
factor, which was recognized as
changing substantially through the year, leading
to the cropping-period approach presented in
AH537.
Following the release of AH537, the USLE
became very widely used, both within the US and
