Agriculture Reference
In-Depth Information
(B) Empirical Equations for Estimating Sediment Yield
The Universal Soil Loss Equation (USLE)
The most widely used and successful model to predict soil loss from upland areas is
the Universal Soil Loss Equation (USLE). The USLE is an erosion model designed
to predict the long-term average soil losses in runoff from specific field areas in
specific cropping and management systems. The procedure of USLE is founded on
an empirical soil loss equation that is believed to be applicable wherever numerical
values of its factors are available.
The USLE was originally derived and presented in English units. Conversion to
SI units was accomplished thereafter. The USLE is
A
=
R
×
K
×
LS
×
C
×
P
(6.14)
where
A
=
Soil loss per unit area per unit time (t/ha/yr)
R
=
rainfall and runoff factor (MJ-mm/ha-h-y)
K
=
soil erodibility factor (MJ-mm/ha-h-y)
LS
=
slope length and steepness factor (-)
C
=
cover and management factor (-)
P
=
support practice factor (-)
Rainfall and Runoff Factor (R)
The factor R is computed as the product of rain-
fall storm energy (
E
) and the maximum 30-min rainfall intensity (
I
30
). The product
that reflects how total energy and peak intensity are combined in each particular
storm.
It indicates how particle detachment is combined with transport capacity. An
average annual value of
R
is about 100-90,000 SI unit.
0.417
P
2.17
R
=
(6.15)
where
R
is in MJ-mm/ha-h-y, and
P
is the 2-yr, 6-h rainfall amount in millimeters.
Soil Erodibility Factor (K)
The
K
is the soil loss rate per erosion index unit for
a specified soil as measured on a unit plot. Unit plot is defined as 22.1 m length
of uniform 9% slope continuously clean-tilled fallow condition. The value of
K
for
agricultural soil usually ranges from 0.013 to 0.053 MJ-mm/ha-h-y.
Slope length and steepness factor (LS)
The factor LS is the ratio of soil loss per
unit area of a field slope to that from a unit plot. The value of LS normally ranges
from 0.2 to 6.0.
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