Page 135 - Biosystems Engineering

116 Cha pte r T h ree

LS The L and S factors—the slope length factor and the slope steepness fac-
tor, respectively—represent the effect of topography and are often consid-
ered together. When the LS value is taken as a unit, the computed erosion
estimate corresponds to the USLE standard plot, which is 72.6 ft (22 m)
long and has a slope of 9 percent. The LS factor is used to adjust predicted
erosion for different slopes and slope lengths. Slope length here is defined
as the distance from the point where overland flow starts to the point
where flow is becoming concentrated or sediment deposition is started
being observed. According to Renard et al. (1997), slope lengths usually
do not exceed 400 ft (~122 m). Slope lengths longer than 1000 ft (~305 m)
are not recommended. Soil loss increases more rapidly with slope steep-
ness than it does with slope length.
The L factor can be calculated from
⎛ λ ⎞ β
L ⎜ ⎝ 72 6⎠ ⎟ (3.28)
.
where λ is the slope length in feet and β is a dimensionless number
given by

β = sin θ . 08 (3.29)
sin θ + . 0 269 (sin θ) + . 0 05
Similarly, the S factor is given as
.
.
S = 3(sin )θ 08 + 0 56 if L < 4 m (3.30)
.
S = 10 8.sinθ + 0 03 if L > 4m , tanθ < 9% (3.31)
.
S = 16 8.sinθ − 0 50 if L > 4 m , tanθ > 9% (3.32)
C The cover management factor is used to account for the effect of
cropping, crop rotation, management practices, length of growing
season, and tillage practices on erosion rates (Fangmeier et al. 2005).
The effect of different conservation plans on the average annual soil
loss and its time distribution during construction activities, crop rota-
tions, or other management schemes can be assessed by the cover
management factor. Similar to other factors, the C factor also mea-
sures erodibility relative to a reference condition, which is an area
under clean-tilled continuous-fallow conditions. The C values, again,
vary by geographic regions, and most local Natural Resources Con-
servation Service offices have values of C factors for common crop
types in that area. Forests have the lowest C values around 0.001 to
0.002, whereas an area with continuous corn or soybean with no
tillage has a C value of 0.1. Conventional tillage in autumn increases
C to 0.4. Cover management factors for various crop types under dif-
ferent management practices can be found in Renard et al. (1997).

P The support practice factor or the conservation practice factor is used
to assess the impacts of support practices on the average annual soil