Geology Reference
In-Depth Information
for summarizing and analysing the more than
10,000 plot-years of soil erosion and runoff data
mentioned above, which resulted in the USLE
(Wischmeier & Smith, 1965, 1978).
It has now been more than 50 years since the
first releases of erosion prediction technology
based on what have become widely known as the
factors affecting sheet and rill erosion and, ulti-
mately combining those in the USLE. Table 1 in
Laflen and Moldenhauer (2003) gives an excellent
synopsis of the published chronology of soil ero-
sion prediction technology in the US.
The USLE and its predecessors were meant
as field-level conservation planning rather than
research tools, and were therefore structured to
be 'user friendly' for USDA programmes in the
Soil Conservation Service (SCS) (now the Natural
Resources Conservation Service (NRCS) ), and
designed for tailoring erosion-control practices to
the needs of specific fields and farms. The USLE
was a 'paper-based' model where factors were
found in printed tables and charts, and calcula-
tions were done by hand.
with up-and-down hill tillage, thereby represent-
ing a condition very near the worst-case manage-
ment. Such a plot was used as a base condition to
which all other topographic, cropping, manage-
ment and conservation practices were compared.
Data from plots with different slopes, lengths and
crops were adjusted to the unit plot, and compared
across locations to establish reliable factor values.
Benchmark soil erodibility and other terms (rain-
fall, slope length, slope steepness, cover-manage-
ment and the support practice factors) used in the
USLE/RUSLE have evolved over the years from
data derived for varied conditions. Few if any unit
plots were ever actually developed, but the con-
cept was used to determine how the conditions of
actual plots related to the unit plot.
The USLE soil loss equation is:
A
=
R K L S C P
(8.1)
where
A
is the computed soil loss per unit area,
expressed in the units selected for
K
and for the
period selected for
R
(in common practice these are
usually selected such that they compute
A
,
soil
loss in US tons per acre per year);
R
, the rainfall
and runoff factor, is the number of rainfall erosion
index units, plus a factor for runoff from snowmelt
or applied water where such runoff is significant;
K
, the soil erodibility factor, is the soil loss rate per
rainfall erosion index unit for the specified soil
under Unit Plot conditions;
L
and
S
are the slope
length and steepness factors in relation to the con-
ditions on a unit plot;
C
, the cover and manage-
ment factor, is the ratio of soil loss from an area
with specified cover and management to that from
an identical area under the tilled continuous fal-
low Unit Plot conditions (
C
thus ranges from a
value of zero for completely non-erodible condi-
tions, to a value of 1.0 for the worst-case Unit Plot
conditions); and
P
,
the support practice factor, is
the ratio of soil loss with a support practice like
contouring, stripcropping, or terracing to that with
straight-row farming up and down slope.
Because the USLE was based on empirical ero-
sion data collected from relatively small plots or
subwatersheds on relatively uniform hillslopes,
the resulting erosion estimates were limited to
similar situations. In essence, these results did not
“Had digital computers been available in the
1940s when erosion became recognized as a
national problem, current prediction methods
might more closely mimic the theory contained
in Ellison's classic paper (1947) than the current
empiricisms of the USLE.” (Renard, 1985: 5)
What follows is a description of the evolution of
the USLE-RUSLE effort, beginning with the
improvements over the USLE leading to the
RUSLE1 computer program and publication of
the USDA Agriculture Handbook No. 703 (Renard
et al
., 1997). We will then describe the develop-
ment of RUSLE2, leading to its release in 2004
and its continuing documentation. The final sec-
tion of this chapter will examine continuing and
possible future developments of the technology.
8.1.2
USLE/RUSLE factor values
The fundamental concept in establishing factor
values in the USLE was the Unit Plot. This con-
ceptual plot was composed of a land parcel 72.6
feet (22.1 m) in length with a 9% slope, maintained
in a continuous, regularly tilled fallow condition
