WATERSHEDS - Water Quality Engineering in Natural Systems - page 226

(a) (b)

Figure 6.18. (a) Rill; (b) gully. Sources : Natural Resources Conservation Service (2005b) and U.S. Department of Agriculture
(2005).

Figure 6.18, where the size of the rill is scaled by a
persons hand and the size of the gully is scaled by a
persons height.
Erosion is usually measured in tons/ha per unit time
(year or season) or per storm. The ease with which
surface soils give way to erosion is called erodibility , and
watersheds can typically be divided into areas where
sediment is eroded and areas where sediment is depos-
ited. The eroding process is called degradation , and sedi-
ment deposition is called aggradation . Erosion and soil
loss are typically not a major problem in flat watersheds
with slopes in the range 0-2%, however, for alisol * soils
with slopes greater that 1%, (Bermuda) grass cover has
been shown to significantly reduce erosion compared to
bare ground conditions (P.K. Mishra et al., 2006). Erosion
control implies an action to reduce soil loss and subse-
quent delivery of sediment from the source area to the
receiving water body. Erosion control is typically accom-
plished by land management, buffer strips, channel
modification, sediment traps, and other structural and
nonstructural practices. The major soil properties related
to erosion are soil texture and composition. Soil texture
determines the permeability and erodibility of soils;
and higher permeability soils are less hydrologically
active. Vegetation influences sediment yields by dissi-
pating rainfall energy, binding the soil and increasing
porosity by its root system, and reducing soil moisture
by evapotranspiration, thereby increasing infiltration.
Erosion models are broadly classified as empirical
models, conceptual models, or process-based models.
Empirical models are derived expressions that fit field
observations, conceptual models are based on spatially

lumped continuity and linear storage discharge equa-
tions of water and sediment, and process-based models
are based on mass conservation of sediment. Although
all types of models are used around the world, the
empirical universal soil loss model is most commonly
used in the United States.
The universal soil loss equation (USLE) is the most
widely used estimator of soil loss caused by upland
erosion. The USLE was originally formulated by Wisch-
meier and Smith (1965) to estimate the annual soil loss
from small plots of an average length of 22 m (72 ft),
incorporating primarily sheet and rill erosion. The
current version of the universal soil loss equation is
called the revised universal soil loss equation (RUSLE)
and is given by

A RK LS CP = ( ) (6.23)

where A is the soil loss or annual potential soil erosion
(t·ha −1 ·yr −1 ), R is the rainfall energy factor or erosivity
factor (MJ·mm·ha −1 ·h −1 ·yr −1 ), K is the soil erodibility
factor (t·h·MJ −1 ·mm −1 ), LS is the slope length and steep-
ness factor (dimensionless), C is the cover management
factor (dimensionless), and P is the supporting practice
factor (dimensionless). Guidelines for estimating the
parameters in the RUSLE are as follows:

R. The erosivity factor, R , is normally calculated as
the product of the rainfall kinetic energy ( E ) and
the maximum 30-minute rainfall intensity ( I 30 ) for
the duration of the averaging period, typically one
year. The product EI 30 is called the erosivity index,
and the erosivity factor, R , is equal to the erosivity
index. The total kinetic energy in an individual
rainstorm is defined as

* Alfisols are soils developed under temperate forests of the humid
mid-latitudes.