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Soil Management:

3. Soil Erosion

3.1 Overview of processes

3.1.1 Geologic vs. Accelerated Soil Erosion  
3.1.2 Soils Susceptible to Erosion  
3.1.3 Effects/Consequences of Soil Erosion

3.2 Water Erosion

3.2.1 Mechanics  
3.2.2 Types of Water Erosion  
3.2.3 Models to predict water erosion

3.3 Wind Erosion

3.3.1 Mechanics  
3.3.2 Models to predict wind erosion

3.4 Erosion Control

3.1 Overview of processes

Soil erosion is the detachment and transportation of soil particles by the forces of water and/or wind.  It is a process that transforms soil into sediment. Sediment consists of transported and deposited particles or aggregates derived from rock, soil, or biological material.

Source: Photo Courtesy of USDA NRCS http://photogallery.nrcs.usda.gov/

 

3.1.1 Geologic vs. Accelerated Soil Erosion

Geological erosion is a natural process that wears down topographic highs (hills and mountains) and fills in topographic lows (valleys, lakes, and bays) through the deposition of eroded sediments.

RELATED LINK: 

Exploring Earth. TERC

  • Animation: Examine a landscape formed erosion website

3) Dissolution (or solution) water is capable of dissolving

Erosion that exceeds normal geologic erosion becomes destructive and is called accelerated erosion.  This type of erosion occurs when the soil and natural vegetation are disturbed by human activity.  Accelerated erosion is often 10 to 1000 times as destructive as geological erosion.

 

3.1.2 Soils Susceptible to Erosion

Soils that are susceptible to erosion include:

  • soils with low water infiltration capability

  • soils with low organic matter content

  • soils with poor (unstable) structure

  • soils on steep hills (especially in regions of high rainfall)

  • soils with hydrophobic characteristics (usually intensified by fire)

 

3.1.3 Effects/Consequences of Soil Erosion

Soil erosion causes two main problems: 

  • Loss of soil productivity: When topsoil erodes, the less fertile and harder B horizon becomes exposed.  This leads to lower forage production, lower water infiltration and greater runoff.

  • Sediment pollution: when eroded sediment is rich in fertilizers or pesticides, it can upset the ecosystem at its point of deposition.  In addition, sediments may damage machinery, agitate respiratory problems and reduce visibility.

3.2 Water Erosion

3.2.1 Mechanics

There are three steps to accelerated erosion by water:

1)    detachment or loosening of soil particles caused by flowing water, freezing and thawing of the topsoil, and/or the impact of falling raindrops.

2)    transportation of soil particles by floating, rolling, dragging, and/or splashing.

3)    deposition of transported particles at some place lower in elevation.

Mechanics: detachment, transportation, deposition

The three-step process of soil erosion by water starts with the impact of raindrops on soil.

Source: Lesley Dampier

Rain enhances the translocation of soil through the process of splashing.  Individual raindrops detach soil aggregates and redeposit particles. The dispersed particles may then plug soil pores, reducing water intake. Once the soil dries, these particles develop into a crust at the soil surface and runoff is further increased.

Mechanics: Water Drop

The splash that results when the drop strikes a wet, bare soil

Source: Photo by Lynn Betts, USDA NRCS http://photogallery.nrcs.usda.gov/

3.2.2 Types of Water Erosion  

Click on the image to review images and text regarding types of water erosion

Source: Photo by Lynn Betts, USDA NRCS http://photogallery.nrcs.usda.gov/

Source: Maja Krzic

3.2.3 Models to predict water erosion

The Universal Soil Loss Equation (USLE) was originally developed to estimate sheet and rill erosion losses from cultivated fields in the United States.  It is now also applied to regions outside the US, and to rangelands and to forestlands.  The equation is used to show how different soil and management factors influence soil erosion.

A = R K LS C P

A

Predicted soil loss due to water erosion

Results in t/acre/year or t/ha/yr

R

Erosivity factor

Quantifies the erosive force of rainfall and runoff.  It takes into account both total amount of rainfall and its intensity.

K

Soil erodibility factor

Represents the ease with which a soil is eroded.  It quantifies the cohesiveness of a soil and its resistance to detachment and transport. 

LS

Slope length and steepness factor

It is the ratio between the plot in question and a standard unit plot.  Steeper slopes lead to higher flow velocities; longer plots accumulate runoff from larger areas and thus also result in higher flow velocities.

C

Vegetative cover and management factor

Considers the type and density of vegetative cover as well as all related management practices such as tillage, fertilization, and irrigation.  This is the most complicated factor to calculate.

P

Erosion control practices factor

Influence of conservation practices such as contour planting, strip cropping, grassed waterways, and terracing relative to the erosion potential of simple up-down slope cultivation.

3.3 Wind Erosion

3.3.1 Mechanics

Wind erosion occurs where soil is exposed to the dislodging force of wind.  The intensity of wind erosion  varies with surface roughness, slope, type of cover on the soil surface, and wind velocity, duration, and angle of incidence.  Fine soil particles can be carried to great heights and for hundreds of kilometers.

Research Links:

Following two video clips illustrate transportation of sand particles by wind:

a) DUNE SAND MOVEMENT; from Kansas State University, the MPG movie shows wind tunnel sediments moving through creep, saltation, and suspension.
 b)DUNE SAND MOVEMENT CLOSE UP; from Kansas State University, a close up of video 6 above, the MPG movie shows sediments moving in the form of creep, saltation, and suspension in a wind tunnel. (end of new addition)

Source: Photo Courtesy of USDA NRCS http://photogallery.nrcs.usda.gov/

Source: Photo by Lynn Betts, USDA NRCS http://photogallery.nrcs.usda.gov/

Susceptibility to wind erosion is related to the following soil properties:

  • water content

  • stability of dry soil aggregates

  • stability of soil crust

  • surface roughness

  • vegetative and/or mulch cover

3.3.2 Models to predict wind erosion

The Wind Erosion Prediction Equation (WEQ) shows how wind erosion is a function of five factors and their interactions.

E = f (I C K L V)

E

Predicted soil loss due to wind erosion

Results in t/acre/year or t/ha/yr

I

Soil erodibility factor

Represents the resistance of the soil to the abrasive action of wind-carried particles.  It is dependent upon a number of factors including soil texture and aggregation.

C

Local wind erosion climate factor

Takes into account moisture, wind speed and wind direction.  Wind erosion is most common in arid and semi-arid regions.

K

Roughness factor

Describes the surface roughness of the soil.  Greater roughness indicates greater resistance to erosion.

L

Length of field factor

A measure of the unsheltered length of the field.  A longer field will have higher wind velocities and thus greater erosion potential.

V

Vegetative cover factor

Accounts for cover type and density, including cover from crop residues.

3.4 Erosion Control

Management practices that reduce erosion:

  • Increase vegetative cover, especially close to the soil surface

  • Increase the content of soil organic matter which helps improve soil structure

  • Increase soil roughness

  • Plant windbreaks and shelterbeds to reduce wind erosion

  • Use contour furrows, terraces, plowed strips, and/or ridges to reduce or deflect runoff.

 

Slide Show Images - Sources:

Contour farming and terraces

Source: Photo by Jeff Vanuga, USDA NRCS http://photogallery.nrcs.usda.gov

Contour and stripcropping

Source: Photo by Tim McCabe, USDA NRCS http://photogallery.nrcs.usda.gov/

Contour terraces and grassed waterways

Source: Photo by Jeff Vanuga, USDA NRCS http://photogallery.nrcs.usda.gov/

Buffer strips

Source: Photo by Tim McCabe, USDA NRCS http://photogallery.nrcs.usda.gov/

Windbreak

Source: Photo by Lynn Betts, USDA NRCS http://photogallery.nrcs.usda.gov/

  USDA http://soils.usda.gov/use/worldsoils/

 

Faculty of Land and Food Systems
THE UNIVERSITY OF BRITISH COLUMBIA