USLE Universal Soil Loss Equation

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USLEUniversal Soil Loss Equation

• To guide methodical decision making in
  conservation planning on a site basis
• To predict longtime average soil losses and
  runoff from specific areas in specified cropping
  and management systems.

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USLEUniversal Soil Loss Equation

• To enable planners to project limited erosion
  data to many locations and conditions not
  directly represented by research
• For estimating average annual soil loss from
  sheet and rill erosion only.

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USLEUniversal Soil Loss Equation

• A R K LS C P
• A is the average annual soil loss in tons per
  acre
• The equation can be used to estimate A or to
  determine the value of other parameters to meet a
  desired value for A

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USLEUniversal Soil Loss Equation

• A R K LS C P
• R is the rainfall factor
• R SEI
• E is the Energy in the Rainfall
• I is the maximum half-hour rainfall intensity for
  the storm.
• R varies with the climate at a particular
  location. (See Fig. 6.4, p.94)

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Average Annual Rainfall Factor (R)
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USLEUniversal Soil Loss Equation

• A R K LS C P
• K is the soil erodibility factor
• (tons/acre/unit of R)
• K depends on the type of soil
• Texture
• Clay and Organic Matter Content
• Structure, Permeability, Drainage

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USLEUniversal Soil Loss Equation

• A R K LS C P
• LS is the field topography factor
• L is the slope length factor
• S is the slope degree factor
• L 1 for a field length of 72.6 feet
• S 1 for a field slope of 9
• LS is a ratio of erosion for the given condition
  to erosion for the standard

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USLEUniversal Soil Loss Equation

• A R K LS C P
• C is the Cropping and management factor
• C is a ratio of the erosion rate for the given
  condition to the erosion rate for the standard
  condition
• The standard condition is a bare soil
• All other conditions will have Clt1
• C also depends on rainfall timing

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USLE C-Factor

• Continuous Fallow 1.00
• Fresh Clean-Tilled Seedbed 0.80
• Corn at Full Canopy 0.25
• Established Thick Meadow 0.004
• Established Meadow Poor Cover 0.1
• Typical Rowcrop Annual Value 0.40

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Crop Residue C-factor
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USLE C-factors for CornSpring Plow (residue
left)from table 6.2, page 98

• Fallow (rough plow) 0.36
• SB to 10 cover 0.60
• To 50 cover 0.52
• To 75 cover 0.41
• To harvest (90 cover) 0.24
• Harvest to Plowing (RdL) 0.30
• average 0.405

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USLEUniversal Soil Loss Equation

• A R K LS C P
• P is the factor for supporting conservation
  practices.
• The standard condition for P is direct
  up-and-down the slope cultivation.
• P will be less than one for all other conditions.
• P depends on field slope

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CONSERVATION PRACTICE FACTOR P

• The P-factor is the ratio of soil loss under the
  given condition to soil loss from
  up-and-down-slope farming. Therefore it is a
  value between 0 and 1

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USLE P-factor
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CONSERVATION PRACTICE FACTOR P

• On nearly level land contouring has little
  effect, so the ratio is 1.0
• On very steep land contouring has little effect,
  so the ratio is 1.0
• The greatest effect of contouring on erosion is
  on slopes between 3 and 8 percent.

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USLE WHEN TERRACES ARE USED

• Farming will be on the contour, so use the
  P-factor for contouring.
• The LS-factor will reflect the terrace spacing as
  the length of slope.
• If the concern is with off-field damages from
  sediment use the Sediment Delivery Factor of 0.2
  as well as the contouring P-factor

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USLE with Strip-Cropping

• Use the contouring P-factor as well as the
  Strip-Cropping Factor

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USLE Equation Terms
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USLE Example 1

• Estimate the average annual soil loss for a field
  located in central Iowa with a Silt Loam soil
  containing 2 organic matter and a 7 field slope
  for a 300-ft. slope length if the annual crop
  management factor is 0.42 and farming is parallel
  to the field boundaries.

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Example 1 Solution

• Solution Use the Universal Soil Loss Equation
  A RKLSCP
• From Fig. 6.4, page 94 in the text, for Central
  Iowa, R 170
• From Table 6.1, page 95 in the text, for Silt
  Loam Soil with 2OM, K 0.42 t/a

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Example 1 Solution

• From Fig. 6.5, page 95, for 7 slope with 300-ft
  length, LS 1.3
• Given C 0.42
• From Table 7.1, page 108 in the text, for a 7
  slope with contouring P 0.5 and for farming
  up-and-down the slope P 1.0. Parallel to field
  boundaries will be a mix. Use an average value
  of 0.8.
• A 170 x 0.42 x 1.3 x 0.42 x 0.8 31.19 t/a/yr.

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USLE Example 2

• If a change was made in Example 1 to farming on
  the contour, what would be the annual soil loss
  rate?
• Solution This would only change P in the above
  solution. Use the ration A2/A1 P2/P1
• As shown above, the value of P for this condition
  is 0.5. Thus
• A2 31.19 (0.5/0.8) 19.49 t/a/yr.

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USLE Example 3

• If the field in example 1 was altered by
  installing terraces at a 150-foot spacing, what
  would be the annual soil loss rate?
• Solution With terraces, the farming would be on
  the contour, so this solution just involves
  changing the topography factor, LS, from Example
  2. Use the ratio A2/A1 LS2/LS1. From Fig.
  6.5, page 95 in the text, the new factor, LS2 for
  7 slope and 150-foot slope length is 1.01. From
  Example 1, the old factor, LS1 is 1.3. Thus, the
  new estimate for the annual soil loss rate is
• A2 19.49 (1.01/1.3) 15.14 t/a/yr.

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USLE Example 4

• With terraces installed some of the sediment
  removed from the soil surface by the erosion
  process will be deposited in the terrace channel.
  From example 3, above, estimate how much will be
  delivered off the field if the terraces have
  graded channel outlets.
• Solution . From Table 7.1, page 108 in the
  text, for terraces with graded channel outlets,
  about 80 of the eroded sediment will be trapped
  in the terrace channel and 20 will be delivered
  to the outlet ditch or stream.
• Total off-site delivery 0.2 x 15.14 3.03
  t/a/yr.