NRES 201 Lectures 20 21 Fall 2014 Soil Management Erosion NRES 201 Soil Management Erosion 1 Soil Management Erosion Today s lecture topics Significance of soil erosion Effects of accelerated soil erosion Mechanics of water erosion Predicting water erosion Factors affecting water erosion 2 1 NRES 201 Lectures 20 21 Fall 2014 Soil Management Erosion Significance of soil erosion Land degradation 5 billion hectares 12 billion acres degraded in the past half century About 43 of the Earth s vegetated land Caused by Desertification due to overgrazing Deforestation Soil degradation Acidification Salinization Erosion 84 of soil Source Brady and Weil 2008 degradation 3 A downward spiral Begins with a loss of natural vegetation Intensifies as bare soil erodes with the loss of Productivity Residues Organic matter Culminates in Environmental degradation Social decline Source Brady and Weil 2008 4 2 NRES 201 Lectures 20 21 Fall 2014 Soil Management Erosion Geological 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 semiarid regions Enough rainfall to be erosive But not enough to support dense Source Brady and Weil 2008 protective vegetation 5 Extreme examples of geological erosion Badlands of South Dakota Badlands National Park Clay and silt deposits subject to infrequent but intense rainstorms Loess Plateau of China Thick silt deposits subject to heavy summer rainfall Source http www legendsofamerica com Loess Plateau Linxian Shanxi Source http www emw21 com 6 3 NRES 201 Lectures 20 21 Fall 2014 Soil Management Erosion Human accelerated erosion Humankind moves over twice as much soil per year as global geologic processes Intentional soil movement for construction and excavation Unintentional soil movement due to agricultural activities such as Land clearing Tillage Overgrazing Dryland fallow Source Brady and Weil 2008 7 Effects of accelerated soil erosion On site damages The C horizon is exposed on this eroded ridgetop Loss of surface soil Decline in surface soil quality due to Selective removal of clay and organic matter The most active Source http wps prenhall com fractions for chemical physical and biological processes Structural deterioration Dense crusting reduces infiltration and increases runoff Gully formation 8 4 NRES 201 Lectures 20 21 Fall 2014 Soil Management Erosion Off site damages Sediment deposition that may Smother crops Fill ditches Cover roadways Sediment discharge into waterways Upsets the aquatic food chain Turbidity reduces sunlight penetration Interferes with fish spawning Increases flooding By reducing channel depth Source http wps prenhall com 9 Requires major public investment for Dredging to keep harbors and shipping channels open An example is the Mud to Parks project See slide 31 of the Introduction Ongoing expansion of flood control levees Source https www floodsmart gov 10 5 NRES 201 Lectures 20 21 Fall 2014 Soil Management Erosion Windblown sand and dust Restricted visibility can cause traffic accidents Dust deposits can cover roads and fill ditches Sandblasting can damage Foliage and fruits Vehicles and buildings Inhaled dust can cause respiratory irritation or disease Dust can carry pathogens This scene was part of a huge pileup on I 35 during a 2012 dust storm in Oklahoma Source http www usatoday com 11 Mechanics of water erosion Phases of occurrence Detachment Separation of particles from the soil mass Transportation Downhill movement of the detached particles by floating rolling dragging and splashing Deposition Accumulation of the transported particles at some place lower in elevation Source Brady and Weil 2008 12 6 NRES 201 Lectures 20 21 Fall 2014 Soil Management Erosion Detachment A raindrop accelerates as it falls To a terminal velocity of 20 mph Larger raindrops fall faster Raindrops impact the soil with an explosive force from their kinetic energy This impact is far more important than flowing rain water for Detaching soil Destroying granulation Splash mediated transport Rainfall impact reduced by vegetation or other Source http www soilerosion net surface cover 13 Transportation 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 Source http www soilerosion net In runoff The main transporting process Becomes channelized by irregularities in the soil surface Increasing flow velocity and turbulence Cuts into the soil mass Carries soil particles downslope 14 7 NRES 201 Lectures 20 21 Fall 2014 Soil Management Erosion Deposition 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 from North America Source Brady and Weil 2008 Types of water erosion Sheet erosion Relatively uniform soil loss from the entire land surface Except under perched stones and pebbles Rill erosion Forms small channels easily filled by tillage Interrill erosion is sheet erosion between rills Gully erosion Forms large channels still present after tillage 15 Source Brady and Weil 2008 16 8 NRES 201 Lectures 20 21 Fall 2014 Soil Management Erosion Predicting water erosion The Universal Soil Loss Equation USLE Provides long term average estimates of accelerated water erosion Empirical not theoretical A R K LS C P where A predicted annual soil loss R rainfall erosivity K soil erodibility L slope length S slope gradient or steepness C cover and management P erosion control practices Rain related factor Soil related factors Land management factors 17 Tolerable 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 conservation practices In practice erosion losses are far higher Typically 10 tons acre per year 18 9 NRES 201 Lectures 20 21 Fall 2014 Soil Management Erosion How much soil depth is lost by erosion of 5 tons per acre per year Assume 2 000 000 lb soil per acre 6 See slides 23 24 of Soil Physical Properties In tons this would be 2 000 000 lb 2 000 lb ton 1 000 tons acre 6 5 tons acre represents 5 tons 1 000 tons 0 005 of the