Erosion of Soil by Wind

W. S. Chepil.

Soil erosion by wind in North America has been worst in the Great Plains, the area that extends almost from the Mississippi River to the Rocky Mountains and from the Gulf of Mexico into the Prairie Provinces of Canada.

Other major regions subject to this costly, needless damage are the Columbia River plains, some parts of the Pacific Southwest and the Colorado Basin, the muck and sandy areas in the Great Lakes region, and the sands of the Gulf and Atlantic seaboards.

Few regions are entirely safe from wind erosion. Wherever the soil is finely divided and bare, the surface of the ground loose and dry, and the wind strong, erosion may be expected unless control measures are adopted.

The most dangerous seasons are late winter and early spring, when the wind usually blows strongest, the land is clothed with the least vegetation, and the soil is most susceptible to movement by wind.

The main cause of wind erosion is depletion of vegetation on the land. Drought is the obvious cause of that, but drought alone does not cause wind erosion. Little erosion occurred when natural vegetation covered the land.

The problem is associated with the way we use the land. Vegetative cover is Nature's way of protecting the earth's surface from erosion. Man has not been able to devise a better way.

Fallowing, a practice used to conserve moisture in dry-farming regions, leaves large acreages of bare or partly denuded land. The fallowed land in the northern sections is seeded in spring about 20 months after the previous crop was harvested. The ground must be kept free of plant growth during those 20 months if moisture is to be conserved.

Fallow is sown usually to wheat in the fall in the southern sections. If germination and growth are favorable, a good protective cover against the next spring's winds is almost assured. But if the land is dry and wheat fails to germinate or make enough growth, the danger of wind erosion becomes acute.

Some lands are so highly susceptible to wind erosion that fallowing has had to be abandoned in favor of continuous cropping or returned permanently to grass or other vegetation. In some places where fallowing was attempted and wind erosion broke loose, drastic emergency measures had to be utilized to check the spread of erosion to more valuable lands.

Large acreages suited only to permanent grass or forests are still devoted to cultivated crops. In the Great Plains alone about 14 million acres not suited for permanent cultivation were cultivated in 1955. Much of this land offers low returns and is subject to severe erosion even in average years.

The growing of cultivated crops incapable of providing sufficient cover on the land has contributed to land denudation and erosion. Cotton, tobacco, sugar beets, peas, beans, potatoes, peanuts, asparagus, and some truck crops leave too little cover on the land and contribute to erosion. When those crops are grown, special farming systems (such as stripcropping) are almost imperative to control erosion and conserve moisture.

Another cause of depletion of a vegetative cover and consequent erosion by wind has been the improper choice and use of tillage implements. Frequent cultivation, often necessary to control weeds, increases the hazard.

We have had some irregular cycles of wind erosion, which reflect the cycles of the weather. Recurring periods of drought and high winds have worsened the erosion.

High temperatures also have been a factor. High temperature usually is linked with dry years and low temperatures with wet years. The variations in the amount of precipitation and intensity of winds are based on a probability law like the one concerning floods. We can predict the general frequency of occurrence of periods of high wind and low precipitation from past records, but we cannot predict the time when they will occur. Constant preparedness for periods of high winds and low precipitation therefore is essential if erosion by wind is to be controlled.

Essential also is the establishment of practices that prevent wind erosion. Too many of us begin to worry about erosion after it has started. Then it is usually too late to prevent damage to crops and soil. It is much more practical to adopt permanent measures than it is to delay and then depend on emergency methods.

The wind erosion process has several major phases: Initiation of movement of the soil and its transportation, sorting, abrasion, and deposition. Each phase is influenced by the condition of the air, the ground surface, and the soil.

THE WEATHER causes considerable loosening and structural disintegration of the surface soil. Alternating wetting and drying and freezing and thawing tend to break down the soil aggregates to granules that are highly erodible by wind. The effects of weathering are greatest at the surface and diminish rapidly with depth. Vegetation and vegetative residues usually protect this granulated surface material from erosion by wind. Tillage tends to bury both the erodible surface material and the residues. We have no implements that can bury the surface soil and at the same time retain the crop residue on the surface. Implements for this dual purpose need to be developed.

Improper tillage is another cause. When he tries to eradicate weeds or create a favorable seedbed, a farmer often unavoidably loosens and pulverizes the soil so that wind can carry it away. Light rains may end temporarily the danger of soil drifting because the fine particles (if the soil is not very sandy) tend to cement the soil mass together to form clods and a surface crust that resist the force of wind. But heavy rains tend to smoothen the soil surface and to leave a few loose grains of sand or water-stable soil grains on the surface. The topmost grains, as soon as they are dry, may be moved the first - stage of erosion.

THE MOVEMENT BEGINS with the most erodible grains on the most exposed positions of the surface. The direct force of the wind against the soil particles dislodges them from their perches. They move a short distance along the surface and then suddenly shoot upwards in a jumping movement known as saltation. The height of the jumps varies with the size and density of the soil particles, the roughness of the soil surface, and the velocity of the wind. The largest particles do not jump at all, but roll and slide surface creep along the surface.

Some particles jump a short distance. Others jump a foot or several feet, depending on the initial velocity of rise from the ground. As they rise and fall through the air, they gain considerable momentum from the pressure of the wind against them and continue to gain velocity until they strike the ground, when they either rebound and continue their movement in saltation or lose most of their energy by striking other particles, which they cause to rise upward while they themselves come to rest or form part of the movement in surface creep.

Movement of grains by surface creep is induced primarily by impacts of particles in saltation. The interchange of movement in saltation and surface creep is constant.

Most of the particles kicked up by jumping grains are fine grains of dust. They rise high, travel far, and drop to earth only when rain washes them down or when the wind subsides.

The movement of fine dust in suspension is the most spectacular mode of transport. But fine dust itself is extremely resistant to movement by direct force of wind against the ground partly because it coheres to the surface and partly because it is submerged below the turbulent flow of air. The dust clouds are only a show.

Dust clouds are a result of impacts of grains moving in saltation, the force that primarily brings them into the airstream. Dust is raised in a like manner by vehicles, animals, and other objects traveling along the ground.

Once kicked up in the air, dust is lifted high in the atmosphere by the upward velocity of eddies of turbulent wind. The upward velocity of eddies of erosive wind is at least 2 or 3 miles an hour enough to lift particles of the size of clay, silt, and very fine sand.

The erosive wind is turbulent at all heights except in a paper-thin zone among the surface projections. Its average forward velocity is zero somewhere among the irregularities of the surface. From this level upward, the average forward velocity increases rapidly. The rate of increase of average velocity with height the drag velocity--is governed by the driving force of atmospheric wind.

The actual velocity and turbulence of the wind near the ground depend on the nature and height of irregularities of the surface.

The degree of turbulence varies directly with the roughness of the ground and inversely with height. The wind near the ground, in fact, is characterized by eddies of extremely variable velocity moving in all directions. On the surface of a sand dune, for example, the maximum eddy velocity is about twice the average forward velocity.

This eddying, or turbulence, is what makes the soil erode. Erosion of soil by nonturbulent flow of air has never been recorded.

Turbulence of the atmosphere has a considerable tendency to increase the surface velocity and hence the momentary frictional force of the wind against the ground. Atmospheric turbulence cannot be controlled by presently known means. The surface velocity and the force of wind against the ground can be modified substantially, however.

AFTER SOIL MOVEMENT has started, the impacts of saltating particles against the ground provide the bulk of the driving force in the erosion process.

That is because the velocity of wind some distance above the ground is much greater than the velocity at the ground. The magnitude of movement of soil by wind therefore depends primarily on the velocity of the wind up to the height of saltation.

Natural variations in air density, as affected by variations in temperature, pressure, and humidity, have little effect on the rate of soil movement. Natural changes in air viscosity stickiness also have little effect on the erosive force of wind.

Little can be done to reduce turbulence, but the surface velocity can be reduced by various measures. Even a slight reduction in wind velocity near the ground produces a relatively great reduction in the possible amount of soil erosion, because the force of the wind varies as the square of its velocity. Reduction of surface velocity therefore should be one of the main principles of control of wind erosion.