Introduction to soil erosion The Universal Soil ... time

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BAEN 460 / 672 Oct 30, 2012

• Todays topics
• Introduction to soil erosion
• The Universal Soil Loss Equation (USLE)
• The Revised Universal Soil Loss Equation (RUSLE)
• Assignment
• Read Chapter 8 in the Haan et al. text
• on reserve at the library
• Class on Thur (11/1) at 11 am
• USGS webinar Improving Access to Water
  Information NWIS Web Services Snapshot Tool for
  ArcGIS

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• Soil Erosion and Control Practices

• Soil erosion
• natural processgtGrand Canyon/Niagara Fall
• agricultural land gt economic incentive
• mining/constructiongt no economic incentive
• regulatory penalties
• soil exposed to rainfall / flowing water
• Erosion gt sediment yield gt deposition
• soil detachment
• rainfall impact / shearing force of water flow
• sediment transport gt flowing water
• sediment yield gt surface runoff vel. decreases
• sediment deposited gt large particles first

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• Terminology

• Upland areas near watershed divide
• rills gt small channels in soil
• location of rills is controlled by micro-relief
  of the land
• flow concentrated in micro-channels
• erosion due to shearing forces of channelized
  flow
• tilling can remove rills
• new rills not in the same location
• interrill area gt area between rills
• sheet flow overland
• erosion due to raindrop energy on soil

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• Terminology (cont.)

• Downslope away from watershed divide
• land micro-relief control ceases
• land marco-relief starts to control erosion
• ephemeral gullies
• tillage can remove ephemeral gullies but new
  gullies will occur in the same locations
• classic gullies
• tillage can not remove classic gullies
• erosion due to gt shearing forces in sro flow
• Farther downslope from divide
• channels gt stable banks gt permanent land feature

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Ephemeral Gullies
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Gully
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• Detachment

• Interrill areas
• raindrop impact and overland sheet flow
• independent of slope length
• linear function of slope steepness
• Rill areas
• shearing forces of concentrated flow
• function of slope length and steepness
• increase depth gt increase rill erosion
• increase velocity gt increase rill erosion
• sro depth and velocity gt increase downslope
• shear increases w/ slope steepness

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• Deposition

• Can happen anywhere downslope when
• sro transport capacity lt soil available for
  transport
• direct function of flow velocity
• reduce velocity gt increase deposition
• vegetative filters / terrace channels / ponds
• Soil erosion - deposition gt interrelated
• Modeling erosion and sediment yield
• 50 years ago gt early soil models
• National Runoff and Soil Loss Data Center
• Purdue U., 1954
• Universal Soil Loss Equation (USLE)

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• USLE

• Describes erosion as a function of
• rainfall energy and intensity
• soil erodibility
• slope length and steepness
• soil cover
• conservation practices
• based on 10,000 plot years of data
• Revised USLE gt RUSLE
• computerized version w/ new parameters
• revised slope and slope length factors
• new data to define cover and management
• freeze - thaw effects / new rill factors

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• Rill and Inter-rill Erosion Modeling using USLE

• Soil erosion by water
• from a given slope
• on a per unit area basis
• not all soil erosion is lost from field
• deposition in flatter / vegetated spots
• Sediment yield
• volume of sediment that passes a given pt.
• USLE
• predict soil erosion not sediment yield
• planning tool
• impact of land use on soil erosion

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• Prediction of Soil Loss
• The USLE Equation

• Originally developed for annual loss
• Modified Universal Soil Loss Equation (MUSLE)
• Modified for monthly / single storm loss
• A R K LS C P
• A average soil loss per unit area
• typical units tons / ac / yr
• R rainfall / runoff erosivity index for a given
  geographic location ? Fig. 8.6 Haan text
• accounts for rainfall energy and runoff
• includes a factor for runoff from snowmelt
• K soil erodibility factor ? Fig. 8.9 Haan text
• rate of soil loss

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• Prediction of Soil Loss
• The USLE Equation

• A R K LS C P
• LS slope steepness and length factor
• Fig. 8.13 Haan et al. text
• L slope length factor
• soil loss relative to that from a slope length of
  72.6 ft (22.1 m)
• S slope steepness factor
• soil loss relative to that from a slope of 9

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• Prediction of Soil Loss
• The USLE Equation

• A R K LS C P
• C cover and management factor
• soil loss relative to that from a continuously
  fallow area
• Table 8.8 Haan text
• P supporting conservation practice
• Table 8.13 / 8.14 Haan text
• 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
• 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

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• 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 (Fig. 8.9 Hann text) 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)

• Example problem
• Given
• Soil with
• 65 silt and very fine sand
• 5 sand
• 2.8 organic matter (OM)
• fine granular soil structure
• slow to moderate permeability
• Required
• Determine the K factor
• Answer ? K 0.31 (Fig. 8.9 Haan text)

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• Soil Erodibility Factor (K)

• Equation to calculate K
• K 2.1x10-4 (12 OM) M1.14 3.25(S1 2)
  2.5(P1 3) / 100 (eq. 8.36 Haan)
• K soil erodibility in tons/ac/unit rainfall
  index
• K tons ac hr / hundreds ac ft tonsf
  in
• OM percent organic matter
• P1 permeability index
• S1 structure index
• 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 (lt 0.002 mm)
• Note Equation 8.36 only valid for MS VFS lt
  70

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• Soil Erodibility Factor (K)

• K 2.1x10-4 (12 OM) M1.14 3.25(S1 2)
  2.5(P1 3) / 100 (eq. 8.36 Haan)
• Soil with
• 65 silt and very fine sand
• 5 sand
• 2.8 organic matter (OM)
• fine granular soil structure
• slow to moderate permeability
• K 2.1x10-4 (12 2.8) M1.14 3.25(S1 2)
  2.5(P1 3) / 100
• CL 100 65 5 2.8
• CL 27.2
• M 65 (100 - 27.2)
• M 4,732
• S structure index 2 (fine granular) from
  F. 8.9 Haan
• P permeability index 4 (slow to moderate)
  F. 8.9 Haan
• K 2.1x10-4 (12 2.8) 4,7321.14 3.25(2 2)
  2.5(4 3) / 100
• K 0.32

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• 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

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• Cropping Management Factor

• C integration of several factors
• vegetation cover
• crop rotations
• productivity level
• 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
• expected time distribution of erosive events
• interception drip gt splash erosion
• binding of plant roots

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• Cropping 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 gt 1.0

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• Erosion Control Practice

• 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

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Downstream Sediment Yields

• What natural resource managers need to know
• Use the USLE and a sediment delivery ratio
• USLE estimates gross sheet and rill erosion
• Does not account for sediment deposited enroute
• Does not account for gully or channel erosion
• Sediment delivery ratio ? the ratio of sediment
  delivered at a location in the stream system to
  the gross erosion from the drainage area above
  that point.

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Sediment Delivery Ratio

• Varies widely with
• The size of the area
• Steepness
• Density of drainage network
• Etc.
• Affect of watershed size on ratio
• 6.4 ac ? 65 of eroded soil delivered to stream
• 320 ac ? 33 of eroded soil delivered to stream
• 3,200 ac ? 22 of eroded soil delivered
• 64,000 ac ? 10 of eroded soil delivered

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White Sand National Monument, NM Gypsum dunes
move up to 30 ft/yr
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White Sand National Monument, NM
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White Sand National Monument, NM
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White Sand National Monument, NM Cottonwood
tree that is 50 ft tall
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White Sand National Monument, NM Roots holding
sand in-place!
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Class Wrap-up

• Assignment
• Read Chapter 8 in the Haan et al. text
• on reserve at the library
• Class on Thur (11/1) at 11 am.
• USGS Webinar