Geology Reference
In-Depth Information
where E is the mean annual rainfall loss, R is the rain-
fall erosivity factor, K is the soil erodibility factor, L is
the slope length factor, S is the slope steepness fac-
tor, C is the crop management factor, and P is the
erosion control practice factor. The rainfall erosivity
factor is often expressed as a rainfall erosion index,
EI 30 , where E is rainstorm energy and I is rainfall
intensity during a specified period, usually 30 min-
utes. Soil erodibility , K , is defined as the erosion rate
(per unit of erosion index, EI 30 ) on a specific soil in a
cultivated continuous fallow on a 9 per cent slope on
a plot 22.6-m-long. Slope length , L , and slope steep-
ness , S , are commonly combined to produce a single
index, LS , that represents the ratio of soil loss under
a given slope steepness and slope length to the soil
loss from a standard 9 per cent, 22.6-m-long slope.
Crop management , C , is given as the ratio of soil
loss from a field with a specific cropping-management
strategy compared with the standard continuous cul-
tivated fallow. Erosion control , P , is the ratio of soil
loss with contouring strip cultivation or terracing to
that of straight-row, up-and-down slope farming sys-
tems. The measurements of the standard plot - a slope
length of 22.6 m (72 1 / 2 feet), 9 per cent gradient, with
a bare fallow land-use ploughed up and down the slope
- seem very arbitrary and indeed are historical acci-
dents. They are derived from the condition common
at experimental field stations where measured soil losses
provided the basic data for calibrating the equation. It
was convenient to use a plot area of 1/100 acre and a
plot width of 6 feet, which meant that the plot length
must be 72 1 / 2 feet.
To use the USLE, a range of erosion measure-
ments must be made, which are usually taken on small
bounded plots. The problem here is that the plot itself
affects the erosion rate. On small plots, all material that
starts to move is collected and measured. Moreover,
the evacuation of water and sediment at the slope base
may itself trigger erosion, with rills eating back through
the plot, picking up and transporting new sources of
sediment in the process. Another difficulty lies in the
assumption that actual slopes are uniform and behave
like small plots. Natural slopes usually have a complex
topography that creates local erosion and deposition of
sediment. For these reasons, erosion plots established
to provide the empirical data needed to apply the USLE
almost always overestimate the soil-loss rate from
hillslopes by a factor twice to ten times the natural rate.
Table 7.2 Examples of physically based soil erosion models
Model
Use
References
Lumped or non-spatial models
CREAMS (Chemicals, Runoff and Erosion
from Agricultural Management
Systems)
Field-scale model for assessing
non-point-source pollution and the
effects of different agricultural practices
Knisel (1980)
WEPP (Water Erosion Prediction Project)
Designed to replace USLE in routine
assessments of soil erosion
Nearing et al. (1989)
EUROSEM (European Soil Erosion Model)
Predicts transport, erosion, and
deposition of sediment throughout a
storm event
Morgan (1994)
Distributed or spatial models
ANSWERS (Areal Nonpoint Source
Watershed Environment Response
Simulation)
Model surface runoff and soil erosion
within a catchment
Beasley et al. (1980)
LISEM (Limburg Soil Erosion Model)
Hydrological and soil erosion model,
incorporating raster GIS, that may be
used for planning and conservation
purposes
De Roo et al. (1996)
 
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