Soil erosion estimation
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Soil Erosion Estimation. TSM 352 Land and Water Management Systems. Predicting Soil Loss. Plot scale models (Net erosion, only water erosion) USLE – “Universal ” Soil Loss Equation (AH 537, 1978) MUSLE – Modified USLE (Williams, 1981) RUSLE – Revised USLE (AH 703, 1996)

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Soil Erosion Estimation

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Soil erosion estimation

Soil Erosion Estimation

TSM 352

Land and Water Management Systems


Predicting soil loss

Predicting Soil Loss

  • Plot scale models (Net erosion, only water erosion)

    • USLE – “Universal” Soil Loss Equation (AH 537, 1978)

    • MUSLE – Modified USLE (Williams, 1981)

    • RUSLE – Revised USLE (AH 703, 1996)

  • Small watershed scale model (Net erosion)

    • WEPP – water erosion prediction project

      More process-oriented, including parameters like

      biomass, plant height, canopy cover, temporal variations,…

  • Medium watershed models (Gross erosion)

    • Radioisotope tracer models - 137Cs, 210Pb, 7Be,…


Soil erosion estimation

USLE

  • Describes erosion as a function of:

    • Rainfall energy and intensity

    • Soil properties : erodibility

    • Topography: slope length and steepness

    • Soil cover

    • Conservation practices

    • Based on 10,000 plot years of data


Soil erosion estimation

USLE

  • A = RK(LS)(C).(P)

    • A = Estimated soil loss in tons/acre/yr

    • R = Rainfall erosivity factor, expressed by an average erosion index

    • K = Soil erodibility factor for specific soil horizon

    • LS = Topographic factor,

      • L = Slope length factor: ratio of loss from a given slope length to soil loss from a 72.6 ft length under the same conditions

      • S = Slope steepness factor: ratio of loss from a 9% under the same conditions

    • C = Cover management factor: soil loss relative to that from a continuously fallow area

    • P = Support practice factor: soil loss relative to straight row farming up- and downhill


Soil erosion estimation

The USLE Equation: R Factor

  • Rainfall and runoff erosivity factor (R)

    • Varies with:

      • amount of runoff

      • individual storm precipitation patterns

    • Characterizes:

      • The kinetic energy raindrop impact (E)

      • Maximum 30-min storm intensity (I)

    • An annual erosivity index for a location is determined by:

      • Summing up E x I for all storms (n)

    • The average annual rainfall and runoff erosivity index (R) = (sum of E x I) / n


Soil erosion estimation

  • Susceptibility of soil to erosion

  • soil loss measured on a series of soils on a unit plot with “worst case” conditions

    • 72.6 ft long

    • 9% slope

    • continuously tilled and fallow

    • assumed constant all year

  • Result of unit plot experiments

    • Nomograph based on:

      • soil texture / structure / permeability

  • Most erodible  soils with high silt contents

  • Least erodible  soils with high organic matter / strong subsoil structure / high permeabilities

Soil Erodibility Factor (K)


Soil erosion estimation

  • Equation to calculate K

  • K = [2.1x10-4 (12 – OM) M1.14 + 3.25(S – 2) + 2.5(P – 3)] / 100

  • K = Soil erodibility in tons/ac/unit rainfall index

  • OM = Percent organic matter

  • M = (%MS + %VFS)(100 - %CL)

    • MS = percent silt (0.002 – 0.05 mm)

    • VFS = percent of very fine sand (0.05 – 0.1 mm)

    • CL = percent clay (< 0.002 mm)

  • S = Structure index (very fine granular = 1; fine granular = 2; medium or coarse granular = 3; blocky, platy, or massive = 4)

  • P = Permeability index (rapid = 1; moderate to rapid = 2; moderate = 3; slow to moderate = 4; slow = 5; very slow = 6)

Soil Erodibility Factor (K)


Soil erosion estimation

Topographic Factor (LS)

  • Adjusts erosion rates for:

    • greater erosion on longer / steeper slopes

    • less erosion on shorter / flatter slopes

    • when compared to the the USLE standard of:

      • 9% slope

      • 72.6 length

  • Slope length measured from:

    • top of ridge to the outlet channel

    • top of ridge to where deposition begins


Soil erosion estimation

Topographic Factor (LS)


Topographic factor ls factor

Topographic Factor - LS Factor


Soil erosion estimation

Cover Management Factor (C)

  • C = integration of several factors

    • vegetation cover

    • crop rotations

    • length of growing season

    • land management (tillage practices)

      • Conventional tillage leaves the surface bare therefore susceptible to erosion

      • Conservation tillage leaves residue on surface protects the soil from rainfall impact : reduces sheet and rill erosion

    • residue management

    • soil surface

    • binding of plant roots


Types of tillage

Types of Tillage

  • Tillage: the mechanical modification of soil structure

  • Classified by amount of residue left

    • Conventional (CT): <15%

    • Conservation (CM): >30%

      • No-till (NT)

      • Alternating (AT)

University of Wisconsin, 2012.


Soil erosion estimation

Cover Management Factor

  • For forest, rangeland and other non-agricultural lands C factors based on:

    • density of vegetation

    • vegetative residue on the soil surface

  • For disturbed bare soil  C = 1.0


Cover management factor

Cover Management Factor

No-till planting into corn residue

Conservation (mulch) tillage using a chisel plow

No-till soybeans growing in wheat stubble


Soil erosion estimation

Support Practice Factor (P)

  • P = Effect of erosion control practices

    • Practices besides vegetation management

    • Practices characterized by P are:

      • strip cropping

      • contouring

      • terraces

    • P varies greatly with slope gradient

    • For many applications, no erosion control practices are used  1.0

    • No experimental data for forests and rangelands


Support practice p factor

Support Practice (P) Factor

  • Contouring

  • Terrace/Diversion, Grassed waterway

  • Strip systems


Example

Example

  • Determine the average annual soil loss for the following conditions: location is Champaign, Illinois; soil is Drummer silt clay loam (“the official state soil of Illinois”); l = 300-ft; s = 5%; spring corn-soybean rotation with conservation tillage; and the field has contoured strip cropping conservation practice.


Wind erosion

Wind erosion

  • Important especially in arid regions

  • Dependent on

    • Wind speed and exposure

    • Soil particle- and aggregate sizes

    • Surface roughness

    • Tillage

  • Surface roughness creates turbulence in the surface-near air layer →

    Under pressure sucks particles in the air

  • Form of movement:

    < 0.1 mm → Suspension (“dust storm”)

    < 1 mm → Saltation (“jumping”)

    > 1mm → Creep (“rolling”)


Questions

Questions?


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