Soil erosion is the wearing away of the earth's surface by the force of wind and water, and consists of soil particle dislodgement, entrainment, transport, and deposition. This sequence of events occurs over a wide range of temporal and spatial scales, from instantaneous raindrop splash moving clay particles millimeters in milliseconds to suspended sediment and bedload in rivers continuously moving an estimated 15.5 billion metric tonnes per year to the oceans of the world. As a matter of course, soil erosion begins in the uplands where soil is dislodged and moved downslope, progressing from splash saltation, to entrain-ment in microchannels, to collection in rills, concentrated flow channels, gullies, and ultimately stream channels. Deposition and re-entrainment occurs continuously from ridge top to deltas. The progression from dislodge-ment and entrainment to deposition can be accelerated or retarded through human manipulation of the soil resource, plant cover, or animal use. In this article, we examine how soil in uplands is influenced by natural and human forces acting on the process of soil erosion.
Geologic soil erosion by water, also commonly referred to as natural erosion, is the constant process of water detaching and transporting soil particles downslope to the lowest point in a basin, be it an ocean or closed basin, such as the Great Basin in the United States. Geologic erosion results from processes unaffected by humans and is often incorrectly considered to occur no faster than the processes required to build new soil. The uneven loss and accumulation (or development) of soil across a landscape is a primary factor in the shifting mosaic of ecological communities.
The ease with which a soil is eroded, or its erodibility, is determined by the physical/chemical interactions of the composite mineral soil particles combined with the biological processes of the community of organisms living in the soil, which include microbiotic organisms, vascular plants, herbivores, and predators. The products of these biophysical/biochemical interactions bind soil particles together into peds and resist slaking and physical destruction and movement by raindrop impact. Slaking is the process of water absorbing into a ped and dissolving the bonds holding soil particles together or breaking them with positive pore pressure. Plants provide cover to protect the soil surface from raindrop impact. Fallen plant parts, stems, stalks, and leaves detain water, allowing it to pond and infiltrate, or releasing it in concentrated streams when the detention structure is overtopped or fails. Herbivores preferentially feed on components of plant communities, continually causing change in the composition and diversity of plant life and thus the pathways of water to and through the soil. Insects and large herd animals are naturally capable of removing all vegetation cover by consumption or trampling, and large animals can then compound the potential for erosion by compacting the soil surface and reducing infiltration to zero. Herd animals can also create trails that further concentrate surface flow and energy. These interactions have occurred for millennia, and the impact on the soil resource has been continuous, or chronic in nature, and distributed relatively infrequently across landscapes and time.
Human activity generally accelerates erosion beyond geologic rates by changing the mechanisms that hold the soil in place by resisting gravity and the work done by water. These activities increase the frequency and duration of forces that disrupt the processes described in the previous paragraph. The manner in which humans have changed the relationship between plants and soil will serve as an example. Plants cover and protect the soil surface from direct raindrop impact and hold soil in place when water runs over the soil surface. Under conditions in which the soil develops, there is occasional loss of plant cover in small areas on an infrequent basis. Alternatively, humans regularly remove plant cover from large areas on a frequent basis, exposing the soil surface to be loosened by raindrops and washed down-slope at accelerated rates. Shifting mosaics of plant communities within grassland, perennial rangeland, and forest land have been replaced by cultivated crops and pasture, monocultures of annual plants, and short monoculture rotations of fast-growing trees, which are regularly and frequently harvested, burned, or both. These activities increase the erodibility of soil by increasing the frequency that soils are unprotected from wind and water. The product of erosion is sediment, which is composed of mineral and organic material. Sediment is generally a detrimental result of erosion, although it can replenish and benefit the soil in river bottoms and deltas.
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