Geology Reference
In-Depth Information
In RUSLE1, this definition was expanded
slightly to include areas of deposition caused by
management changes on the hillslope, which was
accomplished by including some of the more
process-based routines used in CREAMS (Foster
et al
., 1980).
gram automatically and continuously adjusts the
m
value based on slope steepness, soil type and
management impacts.
Slope steepness factor (S)
Soil loss increases
more with steepness than with slope length. In
RUSLE, the slope steepness has changed from
that used by the USLE, and is evaluated with the
relationship (McCool
et al
., 1987):
Slope length factor (L)
Plot data used to derive
slope length (
L
) show that erosion for slope length
λ
(ft) varies as:
S
=
10.8 sin
q
+
0.03
S
<
9%
(8.4)
L
=
(
λ
/72.6)
m
(8.2)
S
=
16.8 sin
q
−
0.50
S
>
9%
(8.5)
where 72.6
the RUSLE unit plot length (ft) and
m
is a variable slope length exponent. The slope
length
=
The relationship is based on the assumption that
runoff is not a function of slope steepness for
slopes greater than 9%. Slope effect on runoff and
erosion as a result of mechanical disturbance,
cover and vegetation is considered in the cover-
management (
C
) or support practice factor (
P
). For
slopes shorter than 4.6 m (15 ft), use:
is the horizontal projection. The value
for
m
can be found from
m
λ
b
), where the
slope-length exponent
b
is related to the ratio of
rill erosion (caused by overland flow) to inter-
rill erosion (principally caused by raindrop
impact). The ratio of rill to inter-rill erosion
when the soil is susceptible to both rill and
inter-rill erosion is:
=
b
/(1
+
S
=
3.0 (sin
q
)
0.8
+
0.56
(8.6)
b
=
(sin
q
/0.0896) / [3.0(sin
q
)
0.8
+
0.56]
(8.3)
Equation (8.6) applies to conditions where the
water drains freely from the slope end. For the
slope steepness factor above, it is assumed that
rill erosion is insignificant on slopes shorter than
4.6 m (15 ft), and that inter-rill erosion is inde-
pendent of slope length.
When freshly tilled soil is thawing, in a weak-
ened state and primarily subjected to surface
flow, use the following (McCool
et al
., 1993):
where
q
is the slope angle. For a value of
b
, the
slope-length exponent
m
is calculated using the
relation above. When runoff, soil, cover, and man-
agement conditions indicate that the soil is highly
susceptible to rill erosion, the exponent should
be increased (see AH703, Chapter 4). These con-
ditions are expected, for example, for steep,
freshly prepared construction slopes. In such
cases where the soil is highly susceptible to rill-
ing, AH703 recommended doubling the value of
b
resulting from Equation (8.3). When conditions
favour more inter-rill and less rill erosion, as in
cases of consolidated soils like those found in no-
till agriculture,
m
should be decreased by halving
the
b
value. A low rill to inter-rill erosion ratio is
typical of conditions on rangelands. With thaw-
ing, and cultivated soils dominated by surface
flow, a constant value of 0.5 should be used
(McCool
et al
., 1989, 1993). In RUSLE1 the choice
between these alternatives was made by selecting
a general land-use category; in RUSLE2 the pro-
S
=
10.8 sin
q
+
0.03
S
<
9%
(8.7)
S
=
(sin
q
/0.0896)
0.6
S
>
9%
(8.8)
In most practical applications, a single plane or
uniform slope can be a poor representation of the
hillslope topography, and erosion can vary greatly
between concave or convex slopes of equal aver-
age steepness. Users are cautioned and encour-
aged to use the complex slope calculations,
because differences can be significant when con-
trasted with a uniform plane.
