From the Natural Resources Conservation Service (NRCS) National Agronomy Manual6
Wind is an erosive agent. It detaches and transports soil particles, sorts the finer from the coarser particles, and deposits them unevenly. Loss of the fertile topsoil in eroded areas reduces the rooting depth and, in many places, reduces crop yield. Abrasion by airborne soil particles damages plants and structures. Drifting soil causes extensive damage to adjacent land, roads, and drainage features. Sand and dust in the air can harm animals, humans, and equipment. Wind erosion events have caused major highway accidents.
Some wind erosion has always occurred as a natural land-forming process, but it has become detrimental as a result of human activities. This accelerated erosion is primarily caused by improper use and management of the land.
Few regions are entirely safe from wind erosion. Wherever the soil surface is loose and dry, vegetation is sparse or absent, and the wind sufficiently strong, erosion will occur unless control measures are applied. Soil erosion by wind in North America is generally most severe in the Great Plains. The NRCS annual report of wind erosion conditions in the Great Plains shows that wind erosion damages from 1 million to more than 15 million acres annually. Other major regions subject to damaging wind erosion are the Columbia River plains; some parts of the Southwest and the Colorado Basin, the muck and sandy areas of the Great Lakes region, and the sands of the Gulf, Pacific, and Atlantic seaboards. In some areas, the primary problem caused by wind erosion is crop damage. Some crops are tolerant enough to withstand or recover from erosion damage.
Other crops, including many vegetables and specialty crops, are especially vulnerable to wind erosion damage. Wind erosion may cause significant short-term economic loss in areas where erosion rates are below the soil loss tolerance (T) when the crops grown in that area are easily damaged by blowing soil. Figure 2-1 displays the relative crop tolerance to blowing soil.
|
Tolerant T |
Moderate tolerance 2 ton/a |
Low tolerance 1 ton/a |
Very low tolerance 0 to 0.5 ton/a |
|---|---|---|---|
| Barley | Alfalfa (mature) | Broccoli | Alfalfa seedlings |
| Buckwheat | Corn | Cabbage | Asparagus |
| Flax | Onions (>30 days) | Cotton | Cantaloupe |
| Grain sorghum | Orchard crops | Cucumbers | Carrots |
| Millet | Soybeans | Garlic | Celery |
| Oats | Sunflowers | Green/snap beans | Eggplant |
| Rye | Sweet corn | Lima beans | Flowers |
| Wheat | Peanuts | Kiwi fruit | |
| Peas | Lettuce | ||
| Potatoes | Muskmelons | ||
| Sweet potatoes | Onion seedlings (<30 days) | ||
| Tobacco | Peppers | ||
| Spinach | |||
| Squash | |||
| Strawberries | |||
| Sugar beets | |||
| Table beets | |||
| Tomatoes | |||
| Watermelons |
Figure 2-1. Crop tolerance to blowing soil.
The wind erosion process is complex. It involves detaching, transporting, sorting, abrading, avalanching, and depositing of soil particles. Turbulent winds blowing over erodible soils cause wind erosion. Field conditions conducive to erosion include:
• loose, dry, and finely granulated soil
• smooth soil surface that has little or no vegetation present
• sufficiently large area susceptible to erosion
• sufficient wind velocity to move soil
Winds are considered erosive when they reach 13 miles per hour at 1 foot above the ground or about 18 miles per hour at a 30 foot height. This is commonly referred to as the threshold wind velocity.
The wind transports single grain particles or stable aggregates, or both, in three ways (Figure 2-2):
Saltation — Individual particles/aggregates ranging from 0.1 to 0.5 millimeter in diameter lift off the surface at a 50- to 90-degree angle and follow distinct trajectories under the influence of air resistance and gravity. The particles/aggregates return to the surface at impact angles of 6 to 14 degrees from the horizontal. Whether they rebound or embed themselves, they initiate movement of other particles/aggregates to create the avalanching effect. Saltating particles are the abrading bullets that remove the protective soil crusts and clods. Most saltation occurs within 12 inches above the soil surface and typically, the length of a saltating particle trajectory is about 10 times the height. From 50 to 80 percent of total transport is by saltation.
Surface creep — Sand-sized particles/aggregates are set in motion by the impact of saltating particles. Under high winds, the whole soil surface appears to be creeping slowly forward as particles are pushed and rolled by the saltation flow. Surface creep may account for 7 to 25 percent of total transport.7,8
Suspension — The finer particles, less than 0.1 millimeter in diameter, are dislodged from an eroding area by saltation and remain in the air mass for an extended period. Some suspension-sized particles or aggregates are present in the soil, but many are created by abrasion of larger aggregates during erosion. From 20 percent to more than 60 percent of an eroding soil may be carried in suspension, depending on soil texture. As a general rule, suspension increases downwind, and on long fields can easily exceed the amount of soil moved in saltation and creep.
Saltation and creep particles are deposited in vegetated strips, ditches, or other areas sheltered from the wind, as long as these areas have the capacity to hold the sediment. Particles in suspension, however, may be carried a great distance. The rate of increase in soil flow along the wind direction varies directly with erodibility of field surfaces.
The increase in erosion downwind (avalanching) is associated with the following processes:
• the increased concentration of saltating particles downwind increases the frequency of impacts and the degree of breakdown of clods and crusts
• the accumulation of erodible particles and breakdown of clods tends to produce a smoother (and more erodible) surface. The distance required for soil flow to reach a maximum for a given soil is the same for any erosive wind. The more erodible the soil surface, the shorter the distance in which maximum flow is reached. Any factor that influences the erodibility of the surface influences the increase in soil flow.
References
6. Content from: National Agronomy Manual. U.S. Department of Agriculture, Natural Resources Conservation Service; 2011. https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb1043210.pdf. Accessed July 15, 2019. See original text for full citations.
7. Chepil. 1945
8. Lyles. 1980.