Geology Reference
In-Depth Information
alternative means of selecting an equivalent
R
value for these conditions was included in
RUSLE1.
tist or other local soil specialist to certify the
value to be used for their location. In other areas
of the world, users may have to resort to soil sam-
pling and the use of Fig. 8.2.
(ii) Soil erodibility factor (
K
)
The soil erodibil-
ity factor (
K
) represents the effect of soil proper-
ties and soil profile characteristics on soil loss
(see Chapter 2 in Renard
et al
., 1997). In a practi-
cal sense,
K
is a lumped parameter representing
an integrated annual average of the soil and pro-
file reaction to erosion and hydrological proc-
esses. The processes consist of soil detachment
and transport by raindrop impact and surface
flow, deposition due to topography and tillage
roughness and rain infiltration into the soil
profile.
The best erodibility factors are obtained from
long-term direct soil-loss measurement on natu-
ral plots. Rainfall simulation data has also been
used, but is recognized as being less accurate
(Römkens, 1985). Only inherent soil properties
are considered determinants of the USLE soil
erodibility factor, which means that soil erodi-
bility must be measured under the Unit Plot
conditions described earlier. The minimum ade-
quacy of the observation period for soil erodibil-
ity was usually taken as two years, but longer
periods provide better results due to the likeli-
hood of experiencing a broader range of climatic
and soil conditions. Most of the plots used in
measuring soil erodibility were in the Midwestern
cropping areas of the US (see Table 3.1 in Renard
et al
., 1997).
In most cases, US RUSLE1 users will have lit-
tle trouble in selecting specific
K
values, because
NRCS has identified values for most major soil
mapping units. Site-specific values can be
obtained from the widely available NRCS soil
surveys, or directly from USDA soil databases. If
such data are not available, the erodibility nomo-
graph (Fig. 8.2), based on a relationship fitting the
data as described above, is the most commonly
used tool to estimate
K
, although there are some
soils where it does not apply, and one of the site-
specific relationships for specific soils (Renard
et al
., 1997: 75) may be a better choice in the US.
Users should contact their NRCS state soil scien-
(iii) Topography factors (
LS
)
There are more
questions and concerns about the
LS
topographic
factor than for any other term in RUSLE. The pri-
mary reason for these concerns is that the choice
of slope length involves substantial judgment; dif-
ferent users choose different slope lengths for sim-
ilar situations. The two primary questions here
are what hillslope (downslope runoff path) to use
to represent an area, and how then to define that
hillslope in terms of specific length and steepness
values. The first question is really one of policy
rather than science (do we choose the worst-case
hillslope, or the median slope, or some other?),
while the second question is a more technical yet
qualitative one of how to define where runoff
begins, the path it takes down the slope and when
it reaches a concentrated flow channel, thus end-
ing the hillslope. The attention given to slope
length is not always warranted because soil loss is
often less sensitive to slope length than to any
other USLE/RUSLE factor. For typical slope condi-
tions, a 10% error in slope length results in a 5%
error in computed soil loss. In contrast, soil loss is
much more sensitive to changes in slope steep-
ness than to errors in slope length. In the USLE,
for example, a 10% error in slope steepness will
usually give about a 20% error in computed soil
loss. RUSLE has a more linear slope steepness
relationship than did the USLE. Improvements in
the relationship for steep slopes mean that com-
puted soil loss for slopes less than 20% are similar
in RUSLE and USLE, but on steep slopes, com-
puted soil loss in RUSLE is just over half that pre-
dicted by the USLE, whose relationship did not
include data for steep slopes. In addition, RUSLE
makes more explicit the reliance of the length
relationship on the susceptibility of the soil to
rilling, which may be influenced by the slope
steepness, soil characteristics, and management
impacts. Finally, RUSLE includes a slope relation-
ship specifically for the frozen soil region of the
Northwest Wheat and Range Region (Austin
