Agriculture Reference
In-Depth Information
Rainfall erosivity, level 2
Combined
effect of
erosivity
and
erodibility
Rainfall
erosivity,
level 1
Soil
erodibility
scale
The interaction between rainfall erosivity and soil erodibility (redrawn from Hudson
1995).
Figure 7.15
Box 7.8
Estimating Erosion Loss with the Universal Soil Loss Equation
The
USLE
is an empirical model developed from many erosion studies in the
United States. The main factors influencing soil loss are multipled to give an
estimate of the average annual soil loss (
A
) from arable land under various
cropping systems, according to the equation
A
P
(B7.8.1)
In this equation,
R
is a measure of the erosive force of rainfall and runoff, and
K
is the soil erodibility factor. The length factor
L
, slope factor
S
, crop
management factor
C
, and conservation practice factor
P
are all ratios of the soil
loss from the field under observation relative to the loss from a field under
specified standard conditions, namely,
• a length of 22.6 m,
• a slope of 9%,
• cultivated bare fallow, and
• bare soil that is plowed up and down the slope.
Under the standard conditions, the product of
L
,
S
,
C
, and
P
is 1, and the
annual soil loss is given by
R
R
K
L
S
C
K
. For a given site, the land manager has little
control over
R
and
K
. Thus, to reduce the average annual soil loss to an acceptable
value, the values of
L
,
S
,
C
, and
P
must be reduced.
The
USLE
has been a successful predictive tool in the United States, as well
as the stimulus for similar empirical model development in other countries.
However,
USLE
has important limitations: it applies only to cropland, and it
cannot be used in catchments to predict stream sediment yields and the off-site
effects of erosion. With the availability of additional erosion data since its first
development,
USLE
has been updated to the
Revised Universal Soil Loss Equation
(
RUSLE
). Additionally, process-based models of erosion applicable to a wide range
of scales and land uses are being developed.
