Soil Erosion Estimation TSM 352 Land and Water Management Systems

Soil Erosion Predicting Soil Loss Plot scale models (Net erosion, only water erosion) o USLE – “Universal” Soil Loss Equation (AH 537, 1978) o MUSLE – Modified USLE (Williams, 1981) o RUSLE – Revised USLE (AH 703, 1996) Small watershed scale model (Net erosion) o WEPP – water erosion prediction project More process-oriented, including parameters like biomass, plant height, canopy cover, temporal variations,… Medium watershed models (Gross erosion) o Radioisotope tracer models Cs, 210 Pb, 7 Be,…

Soil Erosion Describes erosion as a function of: o Rainfall energy and intensity o Soil properties : erodibility o Topography: slope length and steepness o Soil cover o Conservation practices o Based on 10,000 plot years of data USLE

Soil Erosion 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

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

Soil Erosion

Soil Erodibility Factor (K)  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 Erosion

Soil Erodibility Factor (K)  Equation to calculate K  K = [2.1x10 -4 (12 – OM) M (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 (< 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)

Soil Erosion Topographic Factor (LS)  Adjusts erosion rates for: o greater erosion on longer / steeper slopes o less erosion on shorter / flatter slopes o 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 o top of ridge to where deposition begins

Soil Erosion Topographic Factor (LS)

Soil Erosion Topographic Factor - LS Factor

Soil Erosion 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

Soil Erosion 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 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

Soil Erosion

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

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

Soil Erosion

Support Practice (P) Factor Contouring Strip systems Terrace/Diversion, Grassed waterway

Soil Erosion 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.

Soil Erosion Wind erosion Important especially in arid regions Dependent on o Wind speed and exposure o Soil particle- and aggregate sizes o Surface roughness o 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”)

Soil Erosion Questions?