Geology Reference
In-Depth Information
where
P
g
is the
P
factor for off-grade contouring,
P
o
is the
P
factor for on-grade contouring calcu-
lated using the sequence described above,
s
f
is
the grade along the contour furrow, and
s
l
is the
slope grade.
As reflected in the data summarized in tables
in AH537, contouring tends to lose effectiveness
on very long slopes, as runoff tends to build up
behind the contour ridges and cause breakover of
the ridges, which can be assumed to make the
lower contour ridges ineffective. RUSLE esti-
mates the maximum slope length over which
contouring is effective (called the 'critical slope
length' in AH703) using a variation on a relation-
ship developed by Foster
et al
. (1982) for mulch
stability. Once again, this relationship depends
on slope steepness and runoff, and it is calibrated
against the critical slope lengths shown in the
tables in AH537. RUSLE then gives
P
-factor credit
(i.e. reduces the erosion estimate) for the area
upslope of the critical length, but not for the
downslope area.
with the amount of additional sediment added by
erosion within the segment. This requires esti-
mation of a runoff rate, which in RUSLE1 is based
on the ten-year
EI
storm erosivity.
The impact of the deposited sediment on the
P
factor is somewhat subjective, as the
P
factor is
meant primarily as a measure of soil resource
conservation, while the primary effect of deposi-
tion is on sediment delivery. Because sediment
deposition does not preserve the soil resource as
much as preventing erosion in the first place,
RUSLE1 does not give as much conservation
credit for practices that cause sediment deposi-
tion as for practices that prevent soil erosion.
RUSLE1 gives credit for deposition that occurs
based on its location on the slope, using the
relationship:
B
=
M
(1 -
x
1.5
)
(8.22)
where
B
is the benefit,
M
is the mass of sediment
deposited, and
x
is the location of the deposition
as a fraction of the total distance downslope. This
benefit is calculated into the
P
factor as:
Strip-cropping subfactor
The impact of manage-
ment on runoff and its ability to carry sediment is
probably the single factor that has changed most
in the USLE/RUSLE evolutionary process. As
described above, this has included substantial
changes in how the hillslope is defined. RUSLE1
included a process-based approach to estimating
the amount of deposition caused by changes in
management and the resulting slowing of runoff.
This started with the definition of a slope seg-
ment as being a portion of the topography with
constant soil, management, and steepness. The
approach taken was a simplified version of the
CREAMS approach (Foster
et al
., 1980), which
looks at four possible cases for each slope seg-
ment, where a segment is defined: (1) where there
is no runoff leaving the segment, so all incoming
sediment is deposited; (2) where there is erosion
throughout the segment; (3) where there is depo-
sition throughout the segment; and (4) where
deposition occurs at the top of the segment and
erosion at the bottom. These four cases are exam-
ined by calculating the increase in transport
capacity within the segment, and comparing that
P
s
=
(
g
p
−
B
)/
g
p
(8.23)
where
P
s
is the
P
factor for strip-cropping, and
g
p
is the potential sediment load that would occur if
there was no deposition.
Terracing subfactor
Within RUSLE, terraces
(or diversions on construction sites) provide two
benefits: (1) they break the hillslope profile into
a combination of multiple shorter profiles,
thereby reducing erosion; and (2) they cause
some deposition to occur up on the hillslope,
thereby providing some benefit in conserving
the soil resource. The first of these benefits is
taken into account through the
LS
topographic
factor described above. For the second benefit,
RUSLE uses sediment yield data collected on
watersheds with terraces to estimate the amount
of sediment deposition that will take place, then
gives that a credit benefit identical to that
described above for the benefit of deposition in
strip-cropping.
