NRES 201 Lectures 20-21 (Fall 2014): Soil Management & Erosion11NRES 201Soil Management & Erosion2Today’s lecture topics- Significance of soil erosion- Effects of accelerated soil erosion- Mechanics of water erosion- Predicting water erosion- Factors affecting water erosionSoil Management & ErosionNRES 201 Lectures 20-21 (Fall 2014): Soil Management & Erosion23Significance of soil erosionLand degradation- 5 billion hectares (12 billion acres) degraded in the past half century- About 43% of the Earth’s vegetated land- Caused by:- Desertificationdue to overgrazing- Deforestation- Soil degradation• Acidification• Salinization• Erosion84% of soildegradationSource: Brady and Weil (2008)4- A downward spiral:- Begins with a loss ofnatural vegetation- Intensifies as baresoil erodes with theloss of:• Productivity• Residues• Organic matter- Culminates in:• Environmentaldegradation• Social declineSource: Brady and Weil (2008)NRES 201 Lectures 20-21 (Fall 2014): Soil Management & Erosion35Geological erosion- Erosion is a natural process that:- Wears down hills and mountains- Fills in valleys, lakes, and bays- Transforms soil into sediment- Erosion rates depend on:- Rainfall- Type of regolith- Highest rates in semiaridregions- Enough rainfall to beerosive- But not enough tosupport dense,protective vegetationSource: Brady and Weil (2008)6- Extreme examples of geological erosion:- Badlands of South Dakota• Clay and silt depositssubject to infrequentbut intense rainstorms- Loess Plateau of China• Thick silt depositssubject to heavysummer rainfallSource: http://www.legendsofamerica.comBadlands National ParkSource: http://www.emw21.comLoess Plateau (Linxian, Shanxi)NRES 201 Lectures 20-21 (Fall 2014): Soil Management & Erosion47Human-accelerated erosion- Humankind moves over twice as much soil peryear as global geologic processes- Intentional soil movement for construction and excavation- Unintentional soilmovement due toagriculturalactivities such as:- Land clearing- Tillage- Overgrazing- Dryland fallowSource: Brady and Weil (2008)8On-site damages- Loss of surface soil- Decline in surface soilquality due to:- Selective removal ofclay and organicmatter• The most activefractions for chemical, physical, and biological processes- Structural deterioration• Dense crusting reduces infiltration and increases runoff- Gully formationEffects of accelerated soil erosionSource: http://wps.prenhall.comThe C horizon is exposed on this eroded ridgetop.NRES 201 Lectures 20-21 (Fall 2014): Soil Management & Erosion59Off-site damages- Sediment deposition that may:- Smother crops- Fill ditches- Cover roadways- Sediment discharge intowaterways- Upsets the aquatic foodchain• Turbidity reducessunlight penetration- Interferes with fishspawning- Increases flooding• By reducing channel depthSource: http://wps.prenhall.com10- Requires major public investment for:• Dredging to keep harbors and shippingchannels open An example is the Mud to Parks project(See slide 31 of the Introduction)• Ongoing expansion of flood-control leveesSource: https://www.floodsmart.govNRES 201 Lectures 20-21 (Fall 2014): Soil Management & Erosion611- Windblown sand and dust- Restricted visibility cancause traffic accidents- Dust deposits can coverroads and fill ditches- Sandblasting can damage:• Foliage and fruits• Vehicles and buildings- Inhaled dust can causerespiratory irritation ordisease• Dust can carry pathogensSource: http://www.usatoday.comThis scene was part of a huge pileup on I-35 during a 2012 dust storm in Oklahoma.12Phases of occurrence- Detachment- Separation of particles from the soil mass- Transportation- Downhill movement of the detached particlesby floating, rolling, dragging, and splashing- Deposition- Accumulation ofthe transportedparticles at someplace lower inelevationMechanics of water erosionSource: Brady and Weil (2008)NRES 201 Lectures 20-21 (Fall 2014): Soil Management & Erosion713Detachment- A raindrop accelerates as it falls- To a terminal velocity of 20 mph- Larger raindrops fall faster- Raindrops impact the soil with an explosive forcefrom their kinetic energy - This impact is far more important than flowingrain water for:- Detaching soil- Destroying granulation- Splash-mediatedtransport- Rainfall impact reducedby vegetation or othersurface coverSource: http://www.soilerosion.net14Transportation- By raindrop splash- Detached soil particles fly in every direction• Up to 2 ft high• Up to 5 ft laterally- Becomes directional:• With a blowing wind• On sloping land- In runoff- The main transporting process- Becomes channelized by irregularities in thesoil surface- Increasing flow velocity and turbulence:• Cuts into the soil mass• Carries soil particles downslopeSource: http://www.soilerosion.netNRES 201 Lectures 20-21 (Fall 2014): Soil Management & Erosion815Deposition- Can occur after a journey of a few feet- Or more than a thousand miles- Greater delivery of eroded soil to a stream- With steep slopes- From a smaller watershed- Rivers vary greatly in sediment loading- 5-10% of eroded soil washed out to sea fromNorth AmericaSource: Brady and Weil (2008)16Types of water erosion- Sheet erosion- Relatively uniform soil lossfrom the entire land surface• Except under perchedstones and pebbles- Rill erosion- Forms small channels easilyfilled by tillage- Interrill erosion is sheeterosion between rills- Gully erosion- Forms large channels stillpresent after tillageSource: Brady and Weil (2008)NRES 201 Lectures 20-21 (Fall 2014): Soil Management & Erosion917The Universal Soil-Loss Equation (USLE)- Provides long-term average estimates ofaccelerated water erosion- Empirical, not theoreticalPredicting water erosionA = R  K  LS  C  Pwhere:A = predicted annual soil lossR = rainfall erosivityK = soil erodibilityL = slope lengthS = slope gradient or steepnessC = cover and managementP = erosion-control practicesRain-related factorSoil-related factorsLand-management factors18Tolerable soil losses- Erosion can be reduced but not prevented- Tolerable (T) levels defined as maximum erosion loss that would not affect long-term productivity- T values as of 2000:- Range from 2 to 5 tons/acre per year- Depend on:• Soil properties• Previous erosion• Climatic factors• Soil conservationpractices- In practice, erosionlosses are far higher-