Geology Reference
In-Depth Information
Actual selection of the hillslope used to repre-
sent a field can be a complicated choice, and is
best done through examples rather than verbiage.
Additional detail and guidance for field measure-
ment of the
LS
factor for varying field scenarios is
given in AH703.
the Northwest Wheat and Range Region area
(Austin, 1981), otherwise unity). Expanded details
for evaluating
C
factors are presented in AH703.
Although ground cover is known to affect ero-
sion more than the other subfactors, it is wrong
to give it exclusive attention without considering
within-soil effects such as those associated with
root mass and tillage. A 30% surface cover after
planting is the criterion frequently used for con-
servation tillage; the USLE relationships predict
that this 30% cover will reduce soil loss by about
72%. By comparison, the soil loss for a freshly
ploughed meadow is reduced by about 75% from
that for Unit Plot conditions, showing that
within-soil effects can have a substantial impact.
Although the effects are not as pronounced, the
impacts of canopy cover and surface roughness
can also provide substantial benefits, especially
in the absence of surface cover.
The structure of Equation (8.9) implies that
the effects of subfactors in reducing erosion are
multiplicative. For example, if there is a canopy
cover that reduces erosion by 45% from Unit Plot
conditions, this means that
CC
Cover-management Factor (C)
The cover-
management factor,
C
, is possibly the most
important of the RUSLE/USLE factors because it
represents the most readily managed condition
for reducing erosion. In the USLE, the
C
factor
was described as providing a measure of how ero-
sion from the current condition compares with
that for the Unit Plot condition, which is consid-
ered as nearly worst-case. The individual values
of
C
vary between 0 for a completely non-erodible
condition, to a value somewhat greater than 1.0.
Values greater than 1.0 imply conditions more
erodible than those normally experienced under
Unit Plot conditions, which can occur for condi-
tions with very extensive tillage (e.g. roto-tilling),
leaving a very smooth surface that produces much
runoff and makes the soil especially susceptible
to erosion.
C
values are weighted average soil-
loss ratios (SLRs), each of which represents the
ratio of soil loss under current conditions for a
short period of time to the expected soil loss
under Unit Plot conditions during that same
period. The SLRs vary throughout the year as soil
and cover conditions change with soil distur-
bance and plant growth. The
C
value then repre-
sents the average of the time-varying SLR values,
each weighted by the portion of rainfall erosivity
during that same time period.
In contrast to the tables of
C
factors presented
in AH282 and AH537, RUSLE1 uses a subfactor
method to compute SLRs as a function of five
factors:
0.55. If there is
also enough surface cover to reduce erosion by
60% from Unit Plot conditions (
SC
=
0.4), then
assuming all other factors are under Unit Plot
conditions (
PLU
=
=
SR
=
SM
=
1.0), the overall fac-
tor value would be
C
0.22, or a 78%
reduction in erosion from Unit Plot conditions.
The subfactor approach in RUSLE1 was
designed to break the dependence of the USLE
structure on specific land-use data. Without this
break, calculations would require separate com-
plete and expensive datasets for each possible
combination of land uses. The subfactor analyti-
cal approach was carried out under the basic
assumption that the erosion impact of various
factors such as surface cover and roughness is
really independent of the type of land manage-
ment controlling that factor. For example, the
impact of covering the surface with straw mulch
and of growing grass should be relatively inde-
pendent of whether this is done as part of normal
agricultural field operations or to control erosion
on construction sites. Under this assumption, we
start with the relationships to estimate erosion
=
0.55 × 0.4
=
C
=
PLU · CC · SC · SR · SM
(8.9)
where
C
is the overall cover-management factor,
PLU
is the prior landuse subfactor,
CC
is the can-
opy cover subfactor,
SC
is the surface cover sub-
factor,
SR
is the surface roughness subfactor, and
SM
is the soil moisture subfactor (used only in
