Transcript Soil Erosion Estimation

Soil Erosion Estimation
TSM 352
Land and Water
Management Systems
Soil Erosion
Predicting Soil Loss
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Plot scale models (Net erosion, only water erosion)
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Small watershed scale model (Net erosion)
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USLE – “Universal” Soil Loss Equation (AH 537, 1978)
MUSLE – Modified USLE (Williams, 1981)
RUSLE – Revised USLE (AH 703, 1996)
WEPP – water erosion prediction project
More process-oriented, including parameters like
biomass, plant height, canopy cover, temporal variations,…
Medium watershed models (Gross erosion)
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Radioisotope tracer models - 137Cs, 210Pb, 7Be,…
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USLE
Describes erosion as a function of:
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Rainfall energy and intensity
Soil properties : erodibility
Topography: slope length and steepness
Soil cover
Conservation practices
Based on 10,000 plot years of data
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USLE
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A = RK(LS)(C).(P)
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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,
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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
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The USLE Equation: R Factor
 Rainfall and runoff erosivity factor (R)
 Varies with:
o amount of runoff
o individual storm precipitation patterns
 Characterizes:
o The kinetic energy raindrop impact (E)
o 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
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Soil Erodibility Factor (K)
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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
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Soil Erodibility Factor (K)
 Equation to calculate K
 K = [2.1x10-4 (12 – OM) M1.14 + 3.25(S – 2) + 2.5(P – 3)] / 100
o K = Soil erodibility in tons/ac/unit rainfall index
o OM = Percent organic matter
o M = (%MS + %VFS)(100 - %CL)
o MS = percent silt (0.002 – 0.05 mm)
o VFS = percent of very fine sand (0.05 – 0.1 mm)
o CL = percent clay (< 0.002 mm)
o S = Structure index (very fine granular = 1; fine granular = 2; medium or coarse granular
= 3; blocky, platy, or massive = 4)
o P = Permeability index (rapid = 1; moderate to rapid = 2; moderate = 3; slow to
moderate = 4; slow = 5; very slow = 6)
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Topographic Factor (LS)
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Adjusts erosion rates for:
o greater erosion on longer / steeper slopes
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less erosion on shorter / flatter slopes
when compared to the the USLE standard of:
o 9% slope
o 72.6 length
 Slope length measured from:
o top of ridge to the outlet channel
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top of ridge to where deposition begins
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Topographic Factor (LS)
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Topographic Factor - LS Factor
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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
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Types of Tillage
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T i l l a g e : 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.
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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
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Cover Management Factor
No-till planting into corn residue
Conservation (mulch) tillage using a chisel plow
No-till soybeans growing in wheat stubble
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Support Practice Factor (P)
 P = Effect of erosion control practices
 Practices besides vegetation management
 Practices characterized by P are:
o strip cropping
o contouring
o 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
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Support Practice (P) Factor
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Contouring
 Terrace/Diversion,
Grassed waterway
 Strip systems
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Example
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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.
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Wind erosion
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Important especially in arid regions
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Dependent on
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Wind speed and exposure
Soil particle- and aggregate sizes
Surface roughness
Tillage
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Surface roughness creates turbulence in
the surface-near air layer →
Under pressure sucks particles in the air
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Form of movement:
< 0.1 mm → Suspension (“dust storm”)
< 1 mm → Saltation (“jumping”)
> 1mm → Creep (“rolling”)
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Questions?