Description about Universal Soil Loss Equation

1. UNIVERSAL SOIL
LOSS EQUATION
D R . J . R A M A C H A N D R A N
T E A C H I N G A S S I S T A N T ( A G R I C U L T U R A L E N G I N E E R I N G )
D E P T . O F A G R I C U L T U R A L E N G I N E E R I N G
A G R I C U L T U R A L C O L L E G E A N D R E S E A R C H I N S T I T U T E
M A D U R A I
SWE 112 Soil and Water Conservation Engineering (1+1)
Unit 2

2. UNIVERSAL SOIL LOSS
EQUATION
• Wischmeier in 1959 presented the Universal Soil Loss Equation,
which has an adoptability to a wide range of conditions.
• The Universal Soil Loss Equation (USLE) is given by,
• A = RKLSCP
• Where,
• A – the average soil loss (tons/ha/year)
• R – Rainfall erosivity index
• K – Soil erodibility factor
• L – Length of slope
• S – Steepness of slope factor
• C – Cropping management factor
• P – Conservation Practice factor (Terracing, strip cropping and
contouring)

3. SOIL LOSS (A)
The factor A represents soil loss per unit area per unit time.
Because L,S,C and P are dimensionless.
Unit for A result from the multiplication of R and K in the
solution of the USLE.
A is denoted in metric tons per ha per year.

4. RAINFALL FACTOR (R)
It refers to the rainfall erosivity index, which expresses the ability
of rainfall to erode the soil particles from an unprotected field.
It is a numerical value.
From a long time observations, it has been obtained that the
extent of soil loss from a barren field is directly proportional to
the product of two rainfall characteristics.
(1) Kinetic energy of the storm and
(2) Its 30 minute maximum intensity.

5. SOIL ERODIBILITY FACTOR
(K)
The factor is closely related to soil properties by virtue of which, a
particular soil becomes susceptible to get erode, either by water or
wind.
The physical properties of soil play an important role on soil
erosion compared to land management practices.
Bouyoucos (1935) suggested that, the soil erodibility depends on
mechanical composition of soil, such as sand, silt, and clay.
It is presented in the ratio
The soil erodibility factor (K), is expressed as tones of soil loss per
hectare per unit rainfall erosivity index from a field of 9 percent
slope and 22 meters as field length.
clay
%
silt
%
sand
%
E
y
Erodibilit



6. LENGTH AND STEEPNESS OF
SLOPE FACTOR (LS)

7. LENGTH AND STEEPNESS OF
SLOPE FACTOR (LS)
The slope length has a direct relation with soil loss. It is
approximately equal to the square root of the slope length (L0.5).
In general as steepness of slope increases, the soil erosion also
increases, because the velocity of runoff increases with increase in
field slope, which allows more soil to detach and transport them
along with surface flow.
Length and steepness of slope factors are combined together is
termed by a specific name as topographic factor, which is
defined as the ratio of soil loss from a field having specific
steepness and length of slope (i.e. 9 percent slope and length 22 m)
to the soil loss from a continuous fallow land.

8. CROP MANAGEMENT FACTOR
(C)
• The crop management factor (C) may be defined as the
expected ratio of soil loss from a cropped land under specific
crop to the soil loss from a continuous fallow land, provided
that the soil type, slope and rainfall conditions are identical.
• The crop and cropping practices affect the soil erosion in
many ways by their various features such as the kind of crop,
quality of cover, root growth, water use by growing plants

9. CONSERVATION PRACTICE
FACTOR (P)
• It may be defined as the ratio of soil loss for a given
conservation practice to the soil loss obtained from up and
down the slope.
• The conservation practice consists of mainly contouring,
terracing and strip cropping
• From field observations, it has been found that when strip
cropping is adopted with the terracing practice, then it becomes
more effective to control erosion and soil loss.

10. APPLICATION OF USLE
• There are three main applications
– It predicts the soil loss
– It helps in selection of the agricultural practices
and
– It provides recommendations on crop
management practices to be used.

11. LIMITATIONS OF USLE
 It is empirical: the USLE is totally empirical. Mathematically it does not
illustrate the actual soil erosion process. For computing the value of factor
R many individuals have proposed several factors, exponents and other
methods too. More over these are less general and more difficult to fit them
to a specified set of data available.
 It predicts average annual soil loss. Basically, the universal soil loss equation
was developed on the basis of average annual soil loss data, hence its
applicability is limited to estimate only average annual soil loss of the given
area. This equation computes less value than the measured, especially when
rainfall occurs at high rate.
 It does not compute gully erosion. The universal soil loss equation is
employed for assessing the sheet and rill erosion and not used for
predicting the gully erosion. The gully erosion caused by concentrated
water flow is not counted by the equation and yet can produce greater
volume of eroded soil.
 It does not compute sediment deposition. This equation estimates only soil
loss, but not the soil deposition. The deposition of sediment at bottom of
the channel is being less than the total soil loss taking place from the entire

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