Soil erosion is the removal of soil from a position on the landscape. The two main agents of soil erosion are wind and water. The type of erosion that occurs is generally related to climate. Because the climate of the Great Plains is relatively dry, and strong winds are common, wind erosion is widespread throughout the region. On May 12, 1934, for example, winds carried an estimated 200 million tons of soil from the Southern Great Plains over 1,500 miles to the Atlantic Ocean. Ideal conditions for wind erosion are loose, finely divided and dry soil on a bare, smooth surface.
Wind erosion moves soil in three ways: suspension, saltation, and surface creep. Very fine particles (less than 0.05 mm in diameter) can be blown into the air and carried in suspension for long distances. The particles fall out of suspension when the wind velocity is reduced or they are washed out by rain. Soil grains between 0.05 mm and 0.5 mm in diameter are too heavy to be suspended. These grains are lifted briefly in the air, move a short distance, and fall back to the surface. Most soil eroded by wind moves by this type of motion (saltation). Saltating grains generally bounce along the surface of the soil until the wind velocity lessens or they meet some obstruction. These grains may knock other grains into the air. Soil grains between 0.5 mm and 1 mm in diameter are too large to be lifted into the wind stream. They are bumped along the soil surface by saltating grains in a movement called surface creep. Soil grains moving by saltation are the keys to wind erosion. Saltating grains increase the number of smaller and larger particles that move in suspension or by soil creep.
Water erosion is more prominent in humid regions. It does, however, occur in the Great Plains especially on sloping landscapes. In 1992 the average annual water erosion rate on cropland was estimated at 3.1 tons per acre. The movement of soil by water is a complex process that is influenced by the amount, duration, and intensity of rainfall, as well as by the nature of the soil, ground cover, and slope of the land. Raindrops play a substantial part in the movement of soil by water. Soil grains are detached from the soil mass by the force of raindrops striking the soil grains. The soil grains may then be splashed, rolled, slid, or carried in suspension along the land surface.
There are three types of water erosion: sheet, rill, and gully. Sheet erosion is the rather uniform removal of thin layers of soil over the entire soil surface. Sheet erosion is the least recognizable type of erosion. It probably occurs rarely because minute channeling usually takes place soon after erosion begins. Rill erosion occurs in small well-defined channels or streamlets where there is a concentration of flow. These rills are large and stable enough to be seen. However, they can be removed by normal tillage operations. Most rill erosion occurs on recently cultivated soil. Gully erosion produces large channels that cannot be erased by normal tillage operations.
Mark Kuzila Conservation and Survey Division University of Nebraska-Lincoln
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