The amount of erosion caused by water is dependent on four factors: climate, soil, vegetation, and topography. The impact of climate is related to the amount and the intensity of rain. The greater the annual rainfall—the greater the potential for water erosion; and as the frequency of intense storms increases—the greater the potential for water erosion.
The contribution of soil to erosion relates to the size of the soil particles and the moisture content of the soil. Sandy and organic soils have the greatest potential for water erosion. These soils are not bound together, especially when wet, and the soil particles are easily moved.
Vegetation plays an important role in water erosion; it reduces the energy of the raindrops striking the soil, in turn reducing the displacement of the soil. In cultivated fields, crop residues in and on the soil provide the same function as natural vegetation. The amount of crop residues can be measured by randomly laying out a 100-ft tape across a field and counting the number of one foot marks that are over or touching any residue. This number is the residue cover as a percent. If four foot marks are over a piece of residue, then the percent is 4%. The recommended minimum percentage is based on the location and the soil type.
The influence of topography is associated with the slope of the land. The greater the slope, the greater the potential for water erosion.
These four factors predict what type of water erosion occurs and how much occurs. Water erosion usually is divided into three stages: raindrop, sheet and rill, and gully. Raindrop erosion is the soil splash resulting from the impact of water on soil particles. If the soil is covered with vegetation, raindrop erosion is almost zero. When the soil is a bare cultivated field, raindrop erosion can be significant. It is estimated that raindrops can displace soil particles 2 ft vertically and 5 ft horizontally. The effect of raindrop erosion increases as the slope increases because there is greater movement down slope than there is up slope.
Sheet and rill erosion are the next two stages. They are combined because many experts believe sheet erosion only exists in theory. As the rainfall intensity exceeds the infiltration rate of the soil, water starts to move across the soil's surface in a thin sheet. Almost as soon as the movement begins, small but well-defined channels develop, which are called rills. Rills can be easily farmed over and are easy to overlook, but they account for most of the soil loss to water erosion.
Gully erosion is the next stage. Gullies are an advanced stage of rills. Small ones may still be farmed over, but if they are not checked, they become too large and then must be farmed around.
Movement of the soil due to wind erosion is not down the slope as in water; instead the direction is determined by the wind. When the soil is completely covered with vegetation, very little wind erosion occurs because the vegetation reduces the velocity of the wind at the surface of the soil. When the wind has access to the surface, erosion may occur. The velocity of the wind, the soil moisture, and the soil particle size determine the amount of erosion and type of erosion. The greater the velocity—the greater the volume and the larger the size of particle that the wind can pick up. Fine sandy soils are affected by wind more than clay soils. Dry soils are easier to erode than moist soils.
Wind erosion is not divided into types; it only varies by degree (stages). There are three stages of movement. All three stages may occur simultaneously. The first stage, suspension, occurs when soil particles are fine enough and the velocity is high enough to keep the particles suspended in the air. Very fine particles can be transported for hundreds of miles. The second stage is skipping and bouncing, or saltation. This state of wind erosion accounts for the largest volume of soil movement. In saltation, the wind is strong enough to pick up the soil particle, but not strong enough to hold them in suspension. The wind lifts up the particles and carries them for a short distance, but then they are dropped. The third stage is rolling or creep. In the creep stage the wind moves or rolls the soil particles across the surface of the soil, but it does not pick them up.
The five factors used to estimate wind erosion are soil erodibility, climate, soil roughness, field length, and vegetation. To estimate the amount of wind erosion for a specific location, contact the National Resource Conservation Service (NRCS).
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