Erosivity is the term used to describe the potential of raindrop impact, runoff from snowmelt, or water applied with an irrigation system rainstorm to detach and erode soil. At the smallest scale, erosion begins by one of two processes: raindrop impact and detachment, where a soil particle is physically dislodged and often broken into smaller pieces, or by slaking. The result from these two processes, which generally occur simultaneously, is that these smaller soil particles move downhill, and fill pores through the soil surface and low-detention areas. The result is decreased infiltration rates for water and loss of sites where water might pond and infiltrate.
Water that does not infiltrate collects in depressions. When the depression fills to the level of the lowest crack or opening between roughness elements (clods), water will flow to the next downslope depression. As volume builds, a small rivulet will form.
A joining of these rivulets forms a larger continuous channel called a rill. Rills are very efficient conduits for removing water and soil from hillslopes. Although the next step in this process depends on hillslope topography and the amount of rainfall, rills generally tend to connect geometrically downslope to become concentrated flow channels. Ephemeral gullies are concentrated flow channels that are small enough that subsequent tillage or management practices fill them with soil from the surrounding field. This type of erosion is common in fields cultivated for crop or pasture production, where the soil surface is bare for periods of a few weeks to several months.
The next type of erosion is the classic gully, which is an excavation that might start from an ephemeral gully or a decrease in the base elevation of an existing waterway. Full development is dependent on a lowered base elevation and the resulting over-fall and head-cutting processes to excavate large volumes of soil that can be washed away. Classic gullies are difficult and expensive to control or rehabilitate. They can divide cultivated lands into such small fields that they cannot be cultivated. Large areas of cropland abandoned in the Piedmont in the southeastern United States from 1860 through 1920
suffered extreme gully erosion. In the worst case, gullies can cause displacement of individual farmsteads and entire villages. Examples can be found in China and India. Smaller gullies are commonly seen where restrictive layers are found in timberland that has been converted to cropland.
Mass wasting processes are associated with hill and mountain slopes. Examples of erosion by mass wasting or mass movement occur as debris or mud flows, rotational slumps, and soil creep, descriptions of which can be found elsewhere. The impact these events have on ecological processes depends on their magnitude, extent, and frequency. Large events, such as fall of debris and mudflows associated with volcanic eruptions, essentially change the ecological trajectory of the plant and animal communities in the areas affected. At smaller scales, large storms can create a multitude of small debris and mudflows or rotational slumps that cause local disturbance or destruction of plants and animals, but with the only lasting or chronic effect being the removal of the debris or mud deposited at the foot of a slope and in or near an active stream channel. The process of soil creep brought on by the process of soil saturation in conjunction with soil over impeding layers of soil profile (physical feature or frozen soil), while chronic, tends to occur at a rate that influences but does not disrupt plant and animal communities.
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