Erosion

Erosion- Introduction

Martha Mamo
Department of Agronomy and Horticulture at University of Nebraska-Lincoln, USA
Patricia Hain
Department of Agronomy and Horticulture at University of Nebraska-Lincoln, USA
Lesson Navigation Tips:
  • Click once on images to see enlarged versions.
  • Click on the "check it" icon within the java box to see the correct response to selected thinking questions.
  • Some thinking questions will have open ended answers with multiple solutions. Prepare to use your generated answers for small group discussions.
  • Click on the red highlighted phrases to be linked to another page.
  • Click on the red highlighted words to be linked to the definition of the word.

Abstract:

Soil erosion is a global problem. Each year, erosion costs billions of dollars in loss of land productivity, damage from soil sediment deposition and subsequent restoration costs, and harm to plant, animal, and human health due to air and water pollution. This lesson will consider the impacts of erosion at local, regional, national, and international levels. It will discuss how erosion occurs and the main factors that contribute to erosion. In addition, the different types of water and wind erosion will be discussed.

The lesson is written to target educational needs of lower level undergraduate students and is open for use by the public and educational institutions. Depending on the goals/objectives of a course, training, workshop, part or all sections of the lesson could be used.

Upon completing this lesson, a student should be able to:
  1. Describe the impacts of erosion on soil, water, and air quality.
  2. Describe the processes in water and wind erosion.
  3. Identify and analyze factors contributing to water and wind erosion of soil.
  4. Identify and describe the types of water and wind erosion.
Assessment:
Development of this lesson was supported in by USDA Cooperative State Research, Education, & Extension Service (USDA-CSREES), under Contract Number 2003-51130-02072.
Any opinions, findings, conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the USDA-CSREES.

Erosion- Think about it...

Before beginning the lesson, think about and write down answers to the following questions...

What is Erosion?

What is erosion?

 "Erosion is the wearing away of the land surface by rain or irrigation water, wind, ice or other natural or anthropogenic agents that abrade, detach and remove soil from one point on the earth’s surface and deposit it elsewhere." (Glossary of Soil Science Terms. Soil Sci. Soc. Am. 1997)

Soil erosion is a global problem. Erosion of soil can contribute to instability in a region because of inability to produce adequate food and fiber. (see article - Soil Erosion as Big a Problem as Global Warming Say Scientists. (pdf)). In the U.S., the cost of water and wind erosion each year is estimated in billions of dollars. This high cost is attributed to erosion removing the upper soil layer from lands and subsequently reducing their productivity and polluting water and air.

Impacts of Erosion on Soil, Air, and Water Quality

Objective

By the end of this section the student/user will be able to:
Describe the impacts of erosion on soil, water, and air quality.
Accompanying Exercise: (For Students to print off, complete and turn in for a grade)
Impacts of Soil Erosion- Exercise (pdf)
Impacts of erosion on soil quality:

Organic matter is a small fraction (2% to 4%) of soil mainly present on the soil surface. Organic matter contributes to productivity through its effect on the physical, chemical, and biological properties of the soil. Erosion gradually depletes organic matter and decreases soil productivity.

When organic matter is lost, soils tend to lose their physical structure. The degradation of soil structure makes the soil hard, compact and cloddy. The soil aeration, water-holding capacity and permeability are also decreased. Decreased aeration means less oxygen available for plant roots to grow. Decreased water availability also means less water available for healthy plant growth. When soil permeability decreases, less water will soak into the soil and more will run-off.

Beneficial organisms that suppress disease and break down organic residues will not function well due to reduced oxygen and water in soil. This in turn will reduce nutrient storage and supply abilities of the soil.

Impacts of erosion on air quality:

Soil particles blown by the wind into the air have a major impact on human and animal health. Particles suspended in air by wind are easily inhaled and accumulate in lung tissues causing major respiratory problems. Concentrated levels of wind blown particles can also reduce visibility and increase the risk of automobile accidents.
In the 1930’s erosion was the cause of great dust storms. Image by the (NOAA) National Oceanic and Atmospheric Administration/Department of Commerce.
In this video clip, the impacts of wind erosion are demonstrated. Video clip by Dr. John Tatarko, USDA
Impacts of erosion on water quality:

Sediment deposition in lakes and rivers increases water turbidity making it difficult for light to penetrate the water. This causes problems for aquatic plants that need sunlight for photosynthesis. Sediments are also rich in nutrients such as phosphorus and nitrogen. These nutrients promote the excessive growth of algae. This process is called eutrophication. Areas of excessive algae growth, called algae blooms, deplete oxygen in the water resulting in the death of aquatic animals from lack of oxygen.

The following 3 case studies address different scopes of erosion on water quality; local, national, and world.
For more information:
http://www.sciencemag.org/cgi/reprint/304/5677/1614.pdf

 http://www.sciencemag.org/cgi/reprint/304/5677/1627.pdf

 http://www.guardian.co.uk/international/story/0,,1148009,00.html

Case Study on a Local Level

Case Study 1 - Local: HOLMES LAKE, Lincoln, NE
Map of sedimentation depths at Holmes Lake, Lincoln, NE. Image by Olsson Associates
The following case study demonstrates the secondary effect of soil erosion on surface water bodies. The case demonstrates how soil eroded from land makes its way to recreational lakes affecting the lakes storage capacity and aesthetic quality.

Holmes Lake is located in a 5.2 square mile watershed in Lancaster County, NE. Current use of watershed land is 80% to 90% residential and commercial. Since 1962, the bottom of the lake has risen eight feet and the lake has lost greater than 25% of its volume due to sedimentation. Increased lake sedimentation reduces the aesthetic appearance of the lake for recreation purposes, decreases aquatic and wildlife habitats, and most importantly it significantly reduces the lake’s ability to receive runoff from the watershed and control floods.

Much of the sediment that entered Holmes Lake came from development of land into residential and commercial housing.

Dredging and transporting 300,000m3 of sediment to an area near the lake will cost approximately 2 to 3 million dollars. These photos show a part of the Holmes Lake restoration project that began in 2003. (Source: Lincoln City Planner, 2003, personal communication)
Urban soil erosion. Rain water erodes exposed soil from construction sites. Image by R. Sutton Aerial view of Holmes Lake. Image by Olsson Associates Dredging removes several feet of sediment that was carried in from water erosion. Image by Olsson Associates
Restored Holmes Lake July 2005 Image by M. Mamo
For more information:
History of Holmes Lake Project (pdf)

Holmes Lake Project Information (pdf)
Thinking Question:

Without restoration and dredging, what do you think will happen to Holmes Lake in Lincoln, NE in 10 to 15 years?

 What to do...
  • Develop model plans the city of Lincoln can take to prevent future damage to Holmes Lake.
  • Develop model plans developers can take to prevent future damage to Holmes Lake.
  • Develop model plans homeowners and businesses can take to reduce future damage to Holmes Lake.

Case Study on a National Level

Case Study 2 - National: HYPOXIA, A National Level Erosion Problem
The following case study demonstrates the regional effects of soil erosion and land management on surface water bodies. The case demonstrates how sediments and nutrients associated with sediments make their way to major receiving water bodies, affecting quality of water, survival of aquatic animals and impacting the economy of the fishing and tourism industries.

In the Gulf of Mexico, approximately five thousand square miles of coastal waters are characterized by low levels of oxygen (less than 2 milligrams per liter). This zone has excessive nutrients, primarily nitrogen, carried to the Gulf by the Mississippi and Atchafalaya Rivers. Nutrients and sediments loaded into these rivers are partly contributed from soil erosion induced by different land management practices, including agricultural production and urban development. The overabundance of these nutrients has triggered excessive algal growth (or eutrophication) which results in reduced sunlight penetration, a decrease in oxygen in the water, and death of aquatic animals such as fish. This has also negatively impacted the economy of the fishing industry and tourism. Gulf fisheries provide an estimated $2.8 billion each year to the nation’s economy, while Gulf tourism accounts for approximately $20 billion annually.
Mississippi River Basin erosion contributes to hypoxia in the Gulf of Mexico. Image by the EPA Office of Water NASA Satellite image of hypoxia in the Gulf of Mexico. Image by NASA
Thinking Question:

With such a large area of the US bread basket draining into the Mississippi River, how will it be possible to create balance between food production and environmental protection?

Case Study on a World Level

Case Study 3 - World: Erosion in the Amazon Basin

The following case study demonstrates the secondary effect of soil erosion on human health. The case demonstrates how soil eroded from land brings with it undesirable chemicals to rivers where it contaminates fish that are consumed by humans.

Soil erosion is contributing to mercury contamination of rivers in the Amazon. In the Amazon, soil is rich in mercury primarily because of volcanic activity that deposited mercury rich dust onto the land. In particular, villagers living on the banks of the Tapajos River in central Brazil have been adversely affected by mercury through the fish they consume. Existing vegetation and forests are cleared through slashing and burning. Lack of good soil management practices in these agricultural lands has contributed to mercury rich soil being eroded into the Tapajos River. International agencies are working to educate villagers in their fish eating habit as well as implement management practices to reduce soil erosion.
Map of the Amazon River Basin.
Image by http://www.peacockbassanglers.com/images/map1.jpg
For more information:
http://web.idrc.ca/en/ev-29124-201-1-DO_TOPIC.html
Thinking Question:

Much of the agriculture occurs along the bank of the Tapajos River. You are part of the international agencies working to educate villagers on soil erosion. Before you guide them with erosion control practices, you must first lay the foundation with information to establish the importance of soil. Describe the information about soil that you think is important for the villagers to know along the Tapajos River.

Impacts of Erosion on Air Quality

The following case study is an example of the effects of soil erosion on air quality.

Case Study 4 - Wind Erosion in the Columbia Plateau

The Columbia Plateau of eastern Washington, northeast Oregon, and the Idaho panhandle includes soils that are susceptible to wind erosion. A high concentration of aerosol-size particles (less than 10 micrometers) have contributed to respiratory health problems. Urban areas of the Columbia Plateau have exceeded the allowable limit of airborne particles as defined by the 1990 Clean Air Act. In this area, research has demonstrated that less frequent traffic and/or disturbance especially in crop lands can reduce dust emissions by up-to 94% during severe wind events. In addition, manure, plant residue, and minimizing the fallow period have proven to reduce wind erosion.
Dust blowing over a farmstead from a bare field. Image by the (NOAA) National Oceanic and Atmospheric Administration/Department of Commerce.
Map of the Columbia Plateau. Image by the United States Geological Survey
For more information:
http://pnw-winderosion.wsu.edu/

 http://topsoil.nserl.purdue.edu/nserlweb/isco99/pdf/ISCOdisc/SustainingTheGlobalFarm/P183Saxton.pdf
Thinking Question:

Discuss how wind erosion impacts water quality.

How Water and Wind Erosion Occur

Objective

By the end of this section the student/user will be able to:
Describe the mechanisms/processes of how water and wind erosion occur.
Accompanying Exercise: (For Students to print off, complete and turn in for a grade)
How Water and Wind Erosion Occur- Exercise (pdf)
The three steps common to both water and wind erosion:
The impact of a water droplet dislodges and scatters soil particles. Image by NRCS
The impact of raindrops shatters surface aggregates and detaches soil particles from them. Raindrop impact is the primary cause of particle detachment. Raindrops can splash soil particles, moving them up to three feet away. Some of the detached particles float into soil pore spaces. This can clog and seal soil pores and result in reduced water entry (infiltration) into the soil. If the rainfall rate exceeds the rate at which water can infiltrate the soil, the excess water runs off and often carries the detached soil particles with it.

Detached particles (sediment) are carried with flowing water down the slope. How many particles and how far they are transported depends on the velocity and volume of the running water. As the water velocity slows down it loses the energy needed to continue carrying the detached suspended soil particles, and the soil particles are then deposited in their new location.

Similarly, wind erosion is a world-wide problem that occurs when strong winds blow across dry soil on unprotected surfaces. Wind detaches soil particles from the surface. Once detached these particles are transported by either suspension into air and/or rolling along the soil surface. Fine sands, silt or clay size particles can be transported for great distances by strong winds. While larger particles rolling along the soil surface move shorter distance and also shatter other soil particles along the way. As the wind speed decreases, deposition of soil particles begins. Wind erosion most commonly occurs in arid and semi-arid regions, because of the frequent occurrence of dry and windy conditions.
This video clip explains the three steps in wind erosion (detachment, transport, and deposition). The clip also explains how threshold velocity varies depending on the nature of the soil surface. Video Clip by Dr. John Tatarko, USDA

Factors Contributing to Water and Wind Erosion of Soil

Objective

By the end of this section the student/user will be able to:
Identify and analyze factors contributing to water and wind erosion of soil
Accompanying Exercise: (For students to print off, complete, and turn in for a grade)
Factors Contributing to Water and Wind Erosion of Soil- Exercise (pdf)
Climate, soil properties, vegetation, soil cover, and land management practices are factors that influence both water and wind erosion. Soil surface roughness, unsheltered distance, and wind velocity and turbulence are additional factors influencing wind erosion, and topography is an additional factor influencing water erosion.

Factors Influencing Erosion
Water Erosion
Wind Erosion
climate
climate
soil properties: aggregation /soil moisture
soil properties
soil cover
soil cover
land management
land management
topography
soil surface roughness
 
unsheltered distance
 
wind velocity and turbulence

The following six pages will discuss each of the factors contributing to water and wind erosion.

Factors Continued...Climate

Climate   
Duration and intensity of rainfall regulates the amount of soil detachment and subsequent loss from the land. During intense storms, rainfall can detach up to 100 tons/acre.

The erosive energy of running water depends on the volume and velocity. When the erosive energy is high (i.e., high volume and velocity), water can detach and transport large particles as well as smaller particles.
Thinking Question: Assuming both reach the ground at the same velocity, which rain event would likely have more erosive power? Explain why.
  • 30 minutes large raindrops
  • 30 minutes small raindrops
SOIL FACT
The total energy for a 760 mm (30 inches) annual precipitation occurring over 2.6 square kilometers (One square mile) is equivalent to the energy of 9,100 metric tons (8,256 short ton) of TNT (Meyer and Renard, 1991)!!!

Factors Continued...Soil Properties

Soil Properties   

Soil texture (proportion of clay, silt, and sand particles in a soil) has two effects on soil erosion. The first is in its influence on infiltration or entry of water into the soil. When rainfall infiltrates rapidly, runoff is minimal. For example sandy textured soils have large pores acting like large pipes that allow much of the rainfall to soak right into the soil. Sandy soils are known to have good infiltration and drainage. Clay textured soil have small pores more like narrow pipes that do not allow water to soak into the soil fast. Clay soils are known to have poor infiltration and drainage.

Second, particles vary in their ease of detachment. Silt particles are most easily detached because they are small and do not easily form aggregates.
Image by NASA Globe program http://ltpwww.gsfc.nasa.gov/globe/pvg/texture1.htm
Aggregation
Particles of fine sand, silt and clay may join together to form aggregates. The soil property which describes the character and formation of these aggregates is called soil structure. The glue that joins the soil particles together includes organic matter, clays, iron oxides, aluminum oxides, and lime. Aggregate formation in clay-textured soil improves water infiltration into the soil and drainage because it increases the number of large pores (larger pipes). In sandy textured soil, aggregate formation reduces the excessively fast drainage of water by increasing the number of small pores (narrow pipes).

Aggregates can be described by their grade. The grade is a measure of how well the aggregate is cemented together or, conversely, how easily it is broken down by the impact of water, wind, or human activity. A soil with good structure has many aggregates present which are stable, meaning well cemented together. Such aggregates resist the forces of water, wind, and human activity. Consequently, they can maintain their porosity and allow better water and air movement.

Soil Moisture
Moist soils are less prone to wind erosion due to the cohesive or binding effects of water on soil clay and organic matter. As wind dries the soil, the risk of soil loss erosion by wind increases, especially if aggregate grades are weak. The risk peaks at or below permanent wilting point.
Thinking Questions:

What soil textural class is expected to have a low rate of water infiltration? Why?

What can be done to improve the water infiltration rates of soil?
Thinking Questions:

What change(s) in soil can cause soil to have low rate of water infiltration?

How might these changes come about?

Factors Continued...Soil Cover

Soil Cover   

Bare soil is exposed to the full erosive power of raindrops and runoff water. Vegetative canopy helps stabilize soil and controls runoff. The vegetative canopy intercepts raindrops and reduces the erosive energy of the raindrops. Dense canopies that cover much of the soil surface intercept a large proportion of the rainfall. The roots of vegetation, such as grass, bind soil particles together to resist erosion. Vegetation can also help lessen or deflect wind, intercept wind-borne sediment, and keep soils moist, making them less susceptible to wind-related erosion.

Crop or plant residue dissipates the energy available to cause erosion. A mulch or crop residue absorbs the energy of falling raindrop, lessens detachment, reduces sealing of pores in the soil surface, and promotes good water infiltration. The chart below shows residue cover effects from 0 to 100% on soil erosion reduction. For example, a residue cover of 20% will reduce erosion by 50% compared to field with no residue cover.


Chart of the effect of residue cover on soil erosion. Image by UNL
Thinking Question:

Using the graph above, describe the relationship of % residue cover and soil loss? Would this relationship be expected for all residue types? Why or Why not?
Crop residue at various percentages of cover
Image by Iowa State University

Soil loss due to water erosion in relation to percent residue cover for Iowa, based on the Universal Soil Loss Equation
Image by Iowa State University
Thinking Questions:

Using the graph above, describe the relationship of % soybeans residue and corn residue covers and soil loss? Is there a difference between corn and soybean residue on soil loss? Why and Why not?

What general conclusion can you make about residue (mulch) type in its effect to reduce soil loss by erosion? Justify your conclusion.

Factors Continued...Topography

Topography  

Topography, or lay of the land, is an important variable in water erosion. More specifically, the degree of steepness (percent slope), as well as the slope length, is important. Steep slopes have high runoff water velocity. This increases its erosive energy (remember that erosive energy of runoff is a function of runoff velocity and volume). When the slope is longer (length), surface area for water collection also increases and therefore increases the run-off volume.

The ’plan’ is a term used to describe the distribution of water across the slope, that is, whether water flow is evenly distributed across the slope (linear); water flow is concentrated in one area (concave); or water flow is moved away from the slope (convex).

The pattern and speed of water flow will influence the area of most intense soil erosion across the slope.

Click on the animation
Hover over a hillside area to see the water flow pattern. Animation by NC State University

Thinking Questions:

View the animation and then do the following activity.

Below is an example sketch of water flow characteristics of a concave profile with a concave plan.

Click here to view the concave profile with a concave plan.

Now find the linear profile with a convex plan within the animation. Based on the water flow characteristics, sketch a hillside and show the area of most intense erosion. (This sketch should be similar to the sketch of the concave profile with a concave plan.)  

Factors Continued...Land Management

Land Management   
Land Development projects, such as road and housing construction, can contribute to soil erosion and sedimentation both during and after the actual construction activity. Clearing, grading, and other activities disturb the soil surface, remove existing vegetation, and alter topography, thereby increasing erosion risk. Massive land clearing done by large construction equipment pulverizes the soil and clears all vegetation. This mechanical disturbance exposes the soil directly to the impact of rainfall energy. Highly disturbed soils have also lost much of the organic matter that glues them together. Also, the rate of water infiltration is usually decreased and water that does not soak into the soil runs off carrying sediment.

Construction disturbs the soil leaving it highly susceptible to erosion. Image by M. Mamo
Rill erosion on an urban construction site. Image by NRCS
SOIL FACT
Soil erosion due to urban development often exceeds 40 tons per acre per year (typical agricultural land is usually less than 10 tons per acre per year).
For more information:
http://extension.agron.iastate.edu/soilmgmt/GallarySoil.html
Image by R. Sutton
Thinking Question:

The house shown on this picture does not yet have drain gutters and downspouts. How will the presence of a housing structure and the lack of drain gutters affect soil erosion?
Irrigation practices can increase runoff by over-saturating the soil or allowing sprinklers to spray onto impervious surfaces. Under sprinkler irrigation system, water droplets can create splash erosion, if the soil is not fully covered with mulch or vegetation.

Mechanical disturbance affects soil physical properties, specifically by disrupting soil aggregation, causing increase erosion potential. For example, tillage buries soil cover or residue that otherwise would serve to protect the soil from erosion.
A moldboard plow, part of a conventional tillage system, inverts the soil along with the surface residue. Most (if not all) of the residue is incorporated and/or buried, leaving a rough soil surface.
Image by M. Mamo		 Various tillage implements and the soil disturbance that results.
Image by M. Mamo
 
Soil loss associated with tillage systems used for planting corn into corn residue on a silt loam soil in northeastern, NE. Water was applied at the rate of 2.5 inches per hour.
Image by UNL
Thinking Question:

Why is there more soil loss under the moldboard plow compared to other tillage systems?

Using the pictures, make a comparison of soil surface characteristics of the different tillage systems.

Factors Continued...Soil Surface Roughness

Soil Surface Roughness  

Soil surfaces that are smooth offer little resistance to the wind. Keeping the surface rough by tilling when the soil is moist enough to form large clods will reduce wind erosion. Although, it is important to recognize that excessive and frequent tillage can gradually reduce roughness of soil by breaking clods and aggregates that resist erosion. Tilling a dry soil may cause a dry dust to form, which aggravates the erosion problem.

Unsheltered Distance  

The lack of windbreaks allows the wind to put soil particles into motion over greater distances thus increasing the abrasion and soil erosion. There is no obstacle to reduce wind velocity.

Wind Velocity and Turbulence  

Wind erosion potential begins when the wind speed increases to 25 km/h (15 mph). Soil movement increases by the cube of the wind velocity, so the amount of soil transported increases rapidly with wind speed. Turbulent winds carry soil particles into the atmosphere to greater altitude, and increases soil particle detachment.
Factors and processes of wind erosion are explained in this video clip. The clip explains how wind velocity or speed affects wind erosive energy. The video also explains how the nature of the soil surface affects erosive wind energy.
Video Clip by Dr. John Tatarko, USDA

Types of Water and Wind Erosion

Objective

By the end of this section the student/user will be able to:
Identify and describe the types of wind and water erosion.
Accompanying Exercise: (For students to print off, complete, and turn in for a grade)
Types of Water and Wind Erosion- Exercise (pdf)


Types of Water and Wind Erosion
Water
Wind
sheet erosion
suspension
rill erosion
saltation
gully erosion
soil creep
splash erosion
 

Water Erosion

Types of water erosion

Types of Water Erosion

sheet erosion
rill erosion
gully erosion
splash erosion
  1. SHEET EROSION: Removal of thin layer of soil from a large area.
    In sheet erosion a thin layer of soil is removed from a large area.
    Image by M. Mamo, Labels added by UNL
  2. RILL EROSION: A series of small channels on a slope carved by running water.
    Rill erosion in a field.
    Image by NRCS, Labels added by UNL    		 Rill erosion at a construction site.
    Image by M. Mamo, Labels added by UNL
  3. GULLY EROSION: Large, wide channels carved by running water. As a rule of thumb, a gully is large enough that it cannot be smoothed out with conventional tillage equipment.
    Gully erosion in a pasture. Image by NRCS
  4. SPLASH EROSION: Direct movement of soil by splashing. Soil particles can be thrown as far as 3 feet by raindrop splash.
    The impact of a water droplet can dislodge soil particles making them susceptible to erosion. Image by NRCS
    Click on the Splash Animation
    Animation by http://serc.carleton.edu/NAGTWorkshops/visualization/collections/soil_erosion.html
     http://www.public.asu.edu/~mschmeec/rainsplash.html

Wind Erosion

Types of wind erosion

Types of Wind Erosion

suspension
saltation
soil creep
In the following animation, you will observe the processes of saltation, surface creep, and suspension. Note that the larger particles tend to move by saltation and surface creep while finer particles are suspended and moved in the air.
Animation of saltation, soil creep, and suspension during wind erosion.
Animation by UNL
The following two video clips (part I and part II) demonstrate different types of wind erosion and the relationship of soil texture to the types of wind erosion.
Part I Part II
This video clip explains the three types of wind erosion, (saltation, surface creep, and suspension).
Video Clip by Dr. John Tatarko, USDA		 This video clip explains the relationship of soil texture to type of wind erosion (saltation, surface creep or suspension).
Video clip by Dr. John Tatarko, USDA

Summary