WATERSHEDS - Water Quality Engineering in Natural Systems - page 229

TABLE 6.9. Exponent Parameter in
Estimating Slope Length Factor

TABLE 6.10. Values of C for Cropland, Pasture, and
Woodland

Slope (%) m Land Cover or land Use C

<1 0.2 Continuous fallow tilled up and down slope 1.0

1-3.5 0.3 Shortly after seeding and harvesting 0.3-0.8

3.5-4.5 0.4 For crops during main part of growing season

Corn 0.1-0.3
Wheat 0.05-0.15
Cotton 0.4
Soybean 0.2-0.3
Meadow 0.01-0.02
For permanent pasture, idle land, unmanaged woodland
Ground cover 85-100%

≥4.5 0.5

the smaller LS factor will control the amount
eroded. Experimental results have shown that
maximum rill flow velocities occur on slopes in the
range of 58-84%, regardless of the plot size and
degree of overland flow concentration (Shao et al.,
2005).
C. The cover management factor , C (dimensionless),
also called the vegetation cover factor , in Equation
(6.23) estimates the effect of groundcover condi-
tions, soil conditions, and general management
practices on erosion rates. It is a quantity that
ranges from 0.001 for well-managed woodland to
1.0 for continuous fallow ground, which is defined
as land that has been tilled up and down the slope
and maintained free of vegetation and surface
crusting. The effect of vegetation on erosion rates
results from canopy protection, reduction of rain-
fall energy, and protection of soil by plant residues,
roots, and mulches. Typical magnitudes of C for
agricultural land, permanent pasture, and idle
rural land are shown in Table 6.10. Generally, C
reflects the protection of the soil surface against
the impact of rain droplets and subsequent loss of
soil particles. Grassed and urban areas have C
factors similar to those for permanent pasture.
P. The erosion control practice factor , P (dimension-
less), in the RUSLE (Eq. 6.23) accounts for the
erosion control effectiveness of such land treat-
ment as contouring, compacting, established sedi-
mentation basins, and other control practices.
Values of P for various agricultural practices are
shown in Table 6.11. These values of P are highly
empirical and should be used only as a first
approximation. On land slopes with more than a
15% gradient, it is commonly recommended that
an agricultural field operation be shifted to contour
strip cropping, with proper selection of crops (Jør-
gensen et al., 2005). With no support practices,
P = 1.0.

As grass 0.003

As weeds 0.01

Ground cover 80%

As grass 0.01

As weeds 0.04

Ground cover 60%

As grass 0.04

As weeds 0.09

For managed woodland
Tree canopy

75-100% 0.001

75-100% to 40-75% 0.002-0.004

75-100% to 20-40% 0.003-0.01

Other
Paddy rice and vegetable

0.60

Bush, shrub, grassland 0.18

Natural forest 0.003

Sources of data : Ranzi et al. (2012); Stewart et al. (1975); Wischmeier
and Smith (1965); Wischmeier (1972).

TABLE 6.11. Values of P for Agricultural Land Uses

Strip Cropping and Terracing

Alternate
Meadows

Close-Grown
Crops

Slope (%) Contouring

0-2.0 0.6 0.3 0.45

2.1-7.0 0.5 0.25 0.40

7.1-12.0 0.6 0.30 0.45

12.1-18.0 0.8 0.40 0.60

18.1-24.0 0.9 0.45 0.70

>24.0 1.0 1.0 1.0

Source : Wischmeier and Smith (1965).

• Predicting average annual soil movement from a
given field slope under specified land use and man-
agement conditions
• Guiding the selection of conservation practices for
specific sites
• Estimating the reduction of soil loss attainable
from various changes that farmers might make in
their cropping system or cultural practices

The RUSLE should be applied with due caution
since it was specifically designed for the following appli-
cations (Wischmeier, 1976):