Soil Erosion and Conservation

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Soil Erosion and Conservation
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Erosion

• a natural leveling process that wears down high
  places fills in low places
• agents running water, ice, wind, gravity, waves

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accelerated erosion

• Process by which soil particles are removed,
  transported and deposited rate of removal of
  soil greater than rate of formation
• 500 yrs / inch topsoil
• Caused by removal of vegetation
• agents wind, water

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• Deposition or sedimentation is flip side of
  erosion.
• the soil that is removed has to go somewhere
  wetlands, lakes, streams, atmosphere

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Minnesota

• 154 million tons of topsoil / year
• 96 cropland
• water erosion 42
• wind erosion 58

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Worldwide

• 75 billion metric tons soil lost / year
• (predominantly cropland)
• 80 cropland moderate - severe erosion
• 10 cropland slight - moderate erosion
• highest rates in Asia, Africa, South America

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United States

• In past 200 yrs, 30 of US farmlands have been
  abandoned due to erosion, salinization and
  waterlogging
• wind erosion increasing
• water erosion decreasing
• 90 US cropland losing soil above sustainable
  rate
• croplands lose 17 tons/ha/yr
• pastures lose 6 tons/ha/yr

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In U.S.

• In past 50 yrs, average farm size change
• 90 to 190 ha (225 to 475 acres)
• to create larger fields remove shelterbelts,
  grass strips, hedgerows
• use of heavier machinery damages soil

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Short History of Agriculture

• Post WWII
• Increase in chemical/mechanical intensive
  production practices
• Decrease in number of farms
• Increase in size of farms
• Production of commodities/export crops
• Top 5 commodities (2003)
• Cattle, dairy, corn, soybeans, broilers
• Cheap food policy
• Over-production, cost-price squeeze,
  consolidation of farms

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Farm CrisisSince 1980s

• Falling prices
• Spiraling overproduction
• Bankruptcies, foreclosures

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1. Water erosion a. rainsplash erosion

• Raindrops accelerate as fall until they reach
  speed at which friction balances gravity
• for large raindrops 30 km / hr
• transfer kinetic energy to soil
• detach soil
• destroy structure
• transport soil (as much as 0.7 m vertically and 2
  m horizontally)
• Only in intense rain events soil stays local

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b. sheet erosion

• Water flows smoothly in a thin film over surface
  detached soil moves with the water

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c. rill erosion

• Sheet flow concentrates water into channels

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d. gully erosion

• Water cuts deeper into soil, rills coalesce into
  deep troughs
• cannot (easily) be removed by tillage
• most dramatic, but most soil loss is due to sheet
  and rill erosion

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Universal Soil Loss Equation (USLE)

• A RKLSCP
• R rainfall erosivity (intensity, quantity)
• K soil erodibility (erosion rate per unit of
  R in Soil Survey)
• L slope length
• S slope gradient
• C cover and management (ratio of soil loss
  compared to fallow)
• P erosion-control practices

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2. Wind erosion

• Arid and semi-arid climates
• Dry soil loss of structure wind can remove soil
  particles
• Damage is on-site and off-site

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• Smallest detach into suspension
• (lt0.1 mm)
• medium move by saltation
• (0.1 - 0.5 mm)
• large move by rolling and sliding (creep)
• gt 0.5 mm

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Wind Erosion Model (WEQ)

• E ƒ( ICKLV)
• I soil erodibility (slope angle, soil moisture,
  structural stability)
• C climate factor (wind speed , soil temp.,
  ppt.)
• K roughness factor
• L width of field factor
• V vegetative cover

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Colorado 1935
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S. Dakota 1936
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Rhode Island
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35 mph wind
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loess
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Soil Conservation Measures

• US govt response to Great Dust Bowl
• 1. SES---SCS---NRCS
• 2. 3000 Soil and Water Conservation Districts
• 3. Shelterbelt Program
• 218 million trees

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USDAs tolerable soil loss

• 2 - 11 metric tons / ha / yr.
• (11 5 tons/acre/yr)
• not sustainable soil formation rate
• 0.5 tons / acre/yr

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Prevention practices

• 1. Windbreaks
• Plant trees on windward side of crops
• 30 mph --gt 13 mph

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Senegal
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• 2. Contour plowing
• Cultivate with the contour of the slope (rather
  than parallel to it) lessens water runoff
• 3. Strip cropping
• Plant strips of alternating crops.
• (Contour strip cropping)

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• 4. Terracing
• on sloping land to check water flow

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Inca
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Bolivia
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• 5. Reclamation of gullies
• build dams (manure and straw, concrete, stones,
  sticks) to collect silt plant gully

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• 6. Cover crops or surface mulch
• (in orchards or vineyards)
• protects ground surface between rows or during
  non-growing season

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• 6. Conservation tillage
• eliminates or restricts tilling

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• In traditional tilling, surface soil is inverted,
  
• plant residue buried
• bare soil exposed

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• In conservation tillage
• leave plant residue on at least 30 of surface

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• No-till
• no plowing, seeds are planted in narrow slits or
  directly drilled into holes
• 17.5 US cropland in 2000
• increases need for herbicide

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• Conservation methods in construction
• schedule during low rain
• work one area at a time
• cover soil immediately (vegetation, straw,etc)
• control runoff to prevent gullies
• trap sediment

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Open-top culvert on logging roadlead runoff
water off of road
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Sedimentation pond
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Catch sediment
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bioengineering
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Riprap channelguide runoff, prevent gullies,
reduce soil loss