"A nation that destroys its soils, destroys itself."

– President Franklin D.

Roosevelt, from a letter sent to governors on

February 26, 1937.

Wind Erosion

Concern for soil erosion in the United States dates back to Presidents Washington and Jefferson. The tobacco plantations in Virginia in the eighteenth century were managed in a way that caused tremendous soil erosion, and one impetus for westward expansion by the European settlers was the need for "fresh" land. The problem of soil erosion was first addressed by federal legislation in the 1930s under the presidency of Franklin Roosevelt. The drought and dust storms of the 1930s, particularly in Texas, Oklahoma, Colorado, and Kansas, resulted in tremendous crop and farm losses and left thousands of farmers unemployed. This brought presidential and congressional attention to the issue.

Some progress was made to arrest cropland erosion since the 1930s using new techniques like crop rotation, contour plowing, and reserving marginal lands (not good cropland) as pastureland.

But in the 1970s an increase in worldwide demand for farm commodities led the U.S. government to create new crop-subsidy incentives to encourage farmers to plant "fencerow to fencerow." Many farmers, eager to increase production, put marginal lands into production and discontinued the practice of crop rotation.

But the need to combat the nutrient depletion of overused soils required farmers to increase their use of chemical fertilizers and other inputs which were costly.

Wind moves ~40% of soil transported by erosion in US.

12% of continental US is affected.

8% moderately

2-3% greatly

6 Great Plains states Wind>water

Which state is the worst?Nebraska –

4t/ha/yrNew Mexico –

29t/ha/yr

Soil may be detached and moved by water, wind or tillage. These three however differ greatly in terms of:

•where and when they occur

•what happens to the area that is being eroded

•how far the eroded soil is moved, and

•if the soil is moved away from the place where it was eroded, what happens as a result.

A good test question would be to compare water and wind erosion

Wind erosion occurs when soils bared of vegetation are exposed to high-velocity wind. When its

velocity overcomes the gravitational and cohesive forces

of the soil particles, wind will move soil and carry it away in

suspension.

Wind erosion is most visible during the suspension stage, as dust storms, or subsequently as deposition along fencelines and

across roads.

When are the gravitational and cohesive forces of a soil their lowest?

Where/when do you encounter these conditions?

General rule would be that: wind erosion tends to be serious where

• annual rainfall is less than 50 cm,

• along beaches or other sandy regions

• or in organic soils of humid regions.

Soil movement is initiated as a result of wind forces exerted against the surface of the ground. For each specific soil type and surface condition there is a minimum velocity required to move soil particles. This is called the threshold velocity. Once this velocity is reached, the quantity of soil moved is dependent upon the particle size, the cloddiness of particles, and wind velocity itself.

Detachment versus transport (or actual loss)

Well aggregated soil versus non-coherent sand

Erosion is a process that involves the

detachment of soil particles from within

the soil surface

FOLLOWED by the

transport of these

detached particles away from the site

of detachment.

Suspension, saltation, and surface creep are the three types of soil

movement which occur during wind erosion.

Wind moves soil particles 0.1-0.5 mm in size in hopping or bouncing fashion

(known as saltation) and those greater than 0.5 mm by rolling

(known as soil creep). The finest particles (less than 0.1 mm) detach

into suspension.

Suspension occurs when very fine dirt and dust particles are lifted into the

wind. They can be thrown into the air through impact with other particles or by the wind itself. Once in the atmosphere, these particles can be carried very high and be transported over extremely long distances. Soil moved by suspension is

the most spectacular and easiest to recognize of the three forms of

movement.

While soil can be blown away at virtually any height, the majority (over 93%) of soil movement takes place at or below

one meter.

Less than 40% of total movement. Usually ~15%.

Saltation – Most wind erosion occurs through the process of saltation, where fine soil particles are lifted into the air and drift horizontally across the surface increasing in velocity as

they go. Saltation can cause severe damage to the soil surface and vegetation. Particles travel approximately four times

longer in distance than in height. When they strike the surface again they either rebound back into the air or knock other

particles into the air.

50-70% of total movement. Movement is only about 30cm, max. per bounce.

Creep - The large particles which are too heavy to be lifted into the air are moved

through a process called surface creep. In this process, the particles are rolled across the surface after coming into contact with

the soil particles in saltation.

5-25% of total movement.

How has the soil texture changed?

How has the soil texture changed?

E = f(I’,K’, C’, L’, V’)

I’ = soil erodibility

K’ = surface roughness

C’ = climate

L’ = width of unprotected soil surface

V’ = vegetative cover

Soil erodibility

• Ease of detachment• Once detached, ease of transport• Cohesiveness • Clod stability, surface crusts• Essentially based on texture and

structure.

There are 8 soil erodibility groups based on texture.

WEG 1 = vf to m sn (480 t/ha/yr)WEG 2 = l vf to m sn (300)WEG 3 = vf to c sn l (195)WEG 4 = c to sl c (montmorillonite) (195)WEG 5 = l, sn cl l, sn cl (125)WEG 6 = sl l, cl l (105)WEG 7 = sl cl l (85)…potential loss from wide unprotected smooth surface with no climatic factor

C’ = Climatic factor :

• Intensity and frequency of windstorms

• Rainfall and evaporation (especially during windstorms)

• 12 mph is a minimum needed for significant erosion to occur.

• Highest values (100%) relate to Garden City, Kansas – the heart of the Dust Bowl.K’ = surface roughness

• Roughness causes particle entrapment

•Cloddiness – entrapment is independent of wind direction.

•Ridges – only entrap if they are normal to wind

Vegetation –

• Residues act similarly to ridges

• Slow down wind velocity

• Provide shelter for settling sediment

• Intact vegetation (don’t want it to blow away)

Field width –

•The more times particles hit the ground the greater the chances for detachment

•Total amount of soil set in motion = f(unobstructed distance over which particles can move – up to a maximum)

•The downwind end of a field erodes the most

Surface texture is the best key to wind erosion protection. Properly managed crop residues, carefully timed soil tillage, and accurately placed crop strips and crop barriers can all effectively reduce wind

erosion.

Mulch - a protective covering in direct contact with the ground provides more protection than canopy cover. It can vary from straw, compost, wood chips to saw dust. Spread evenly onto the

ground, mulch helps to reduce water evaporation, control weeds, enrich the soil, and protect soil

where vegetation has not yet established itself. By absorbing the destructive forces of raindrops and

wind, mulch reduces erosion until the the seedlings mature enough to provide their own protective

cover.

Soil Moisture - Wind erosion is directly related to how wet or saturated a soil is. A moist soil is much more stable than a dry

one because water cements the soil particles together increasing their effective size, and increases adhesive and gravitational forces.

Reducing Field Width - Wind erosion increases directly with field length or width. As soil particles bounce and skip along the surface their erosive capabilities seem to compound, or increase with distance. Thus, by decreasing or breaking up the

length of a field, one can cut down on wind erosion.

Strip Cropping – is a very effective and inexpensive way to reduce field width. Strip

cropping is a combination of contouring and crop rotation in which alternate strips of row crops and

soil conserving crops (sods) are grown perpendicular to the wind (or water flow). When soil

is detached from the row crops, the dense soil conserving crops trap the soil particles and reduce

wind translation (and/or runoff).

Wind Breaks - can be both living or inanimate objects. They can range from trees and shrubs, to

fences and concrete walls. When positioned perpendicular to the wind, wind breaks will deflect

the wind stream and reduce wind velocity. By reducing wind velocities, evaporation will be

slowed and a more favorable environment will be provided for the crops.

What happens to all of that eroded sediment? Where does it end up?