Geology Reference
In-Depth Information
community, with attendant roughness caused by
protruding roots, soil pushing up around old
basal areas, rocks, and so on. RUSLE assumes
that the formation of this vegetative roughness
follows a typical sigmoidal growth curve, increas-
ing from the minimum roughness (
r
min
with a
default of 0.24 in.) to the total roughness when
soil is consolidated (
r
max
) over the time required
for consolidation (
t
con
).
Once the current roughness
R
u
has been
defined based on the tillage roughness and all the
roughness decay calculations described above,
the surface roughness subfactor for this time
period is then:
soil, and roughness. The
P
factor is generally seen
as reflecting the positive impacts of management
through the control of runoff, with special empha-
sis on how the management changes the direc-
tion and speed of that runoff, but also reflecting
to some degree management practices that con-
trol the amount of runoff. Traditionally the
P
fac-
tor has been used to reflect the impact of
agricultural practices such as the various forms of
strip-cropping (buffer strips, filter strips, rota-
tional strip-cropping), terraces, contour tillage,
and subsurface drainage. In other land uses,
P
would reflect the impact of analogous practices,
such as filter strips for water quality control, or
the use of diversions on construction sites.
RUSLE1 brought to the USLE structure a sub-
factor approach for the
P
factor as well as the
C
factor, with separate subfactors for contouring,
strips, terraces, and subsurface drainage. As with
the
C
factor, these subfactor values are multiplied
together to give the overall
P
factor.
SR
=
exp [
−
0.66 (
R
u
−
0.24)]
(8.20)
Soil moisture subfactor (SM)
In non-irrigated
portions of the Northwest Wheat and Range Region
(NWRR; Austin, 1981), soil moisture during criti-
cal crop periods depends upon crop rotation and
management. In such cases, the addition of a soil-
moisture subfactor (
SM
) is suggested.
SM
reflects
dry fall conditions and increasing soil moisture
over winter. The soil moisture decrease during the
growing season depends upon crop rooting depth
and soil depth, and the soil moisture replenish-
ment during the winter and spring depends upon
precipitation amount and soil depth. Research to
make such a correction is needed. In most instances
this factor is assumed to be unity, which means
that there is no substantial impact of soil moisture
extraction by the vegetation on erosion. This
assumption of
SM
Contouring subfactor
Data on the effect of con-
touring show a tremendous amount of scatter,
but there are some trends, as shown in Figure 6-2
of AH703. These indicate that higher ridges give
more benefit than lower ridges, that contouring is
more effective for areas with lower rainfall inten-
sities, and that the effectiveness reaches a peak at
about 9% slope, losing effectiveness at lower
slopes due to less inherent erosion, and at higher
slopes due to potential breakover of the ridges by
ponded runoff. In addition, contouring is most
effective when the ridges are perfectly on the con-
tour, with its impact decreasing rapidly as the
furrows have more grade.
RUSLE1 fits the scattered contouring data
with a series of equations used to describe the
base contouring
P
value for different slope steep-
nesses. It then adjusts these for climate and storm
intensity using a runoff scaling factor based on
the 10-year storm
EI
compared with a value for
the central part of the US, and finally adjusts the
results based on the contour furrow grade, using
the relationship (AH703 eqn. 6-11):
1.0 is probably valid for all
areas except those experiencing erosion caused by
light rains on frozen-thawing soils.
=
(iv) Conservation practice factor (
P
)
It is not
always clear how the conservation practice factor
(
P
) differs from the cover management factor (
C
),
because both are meant to indicate the impact of
management practices on erosion. In general
terms, the basic difference is that the
C
factor
reflects the positive impact over the larger por-
tion of the management area, through factors like
vegetation, biomass on the surface or within the
P
g
=
P
o
+
(1
−
P
o
)(
s
f
/
s
l
)
1/2
(8.21)
