Soil erosion refers to the removal of soil materials from their original location and their subsequent transport to another location through the action of wind, water, ice, biotic processes, or human activities. Under most natural conditions, soil-forming processes and soil erosion are in equilibrium. When the balance between these two forces is upset through the removal of the natural cover, soil erosion can accelerate and result in loss of the soil's capacity to support vegetation or in the soil's complete removal.
Factors affecting soil erosion include climate, soil properties, slope gradient and length, vegetation, and soil management practices. Drier climates mean less vegetation and less plant cover to protect the ground surface from raindrop impact and from the flow of water over the surface. Drier soils are also more susceptible to wind erosion than moist soils due to lower particle cohesion. Soils with low clay content and a high content of fine to medium sand tend to be most susceptible to erosion from water and wind. Soils with high clay content tend to be less susceptible due to their ability to form stable aggregates that are resistant to erosive forces. Steeper slopes mean that runoff flows more rapidly, and with more erosive force, while longer slopes provide greater distances for runoff to build up speed. Vegetation reduces soil erosion by providing cover to protect soil surfaces against erosive forces, while organic matter from the breakdown of plant residues helps produce stable soil aggregates. Management practices that build up soil organic matter, increase groundcover, and incorporate other methods of soil conservation help lower erosion risks.
Since humans began growing crops and modifying the landscape, accelerated soil erosion has been a major problem facing humankind. Soil erosion has been among the culprits blamed for the decline of civilizations such as the Mesopotamian civilization, the Classical Mayan civilization, and the Roman Empire. Soil erosion is also linked to the mass exodus of migrants in the 1930s from the western Great Plains when drying conditions combined with severe wind erosion set off the series of events that turned the region into a “Dust Bowl.”
Humans have long been aware of the consequences of accelerated soil erosion caused by their activities. Passages in the Old Testament mention threats of streams drying up, while the Greek philosopher Plato referred to the relationship between flood damage and deforestation, and the Roman writer Virgil advocated what amounted to conservation farming. Many early civilizations were not overly concerned with erosion's impacts because most of these civilizations had developed on irrigated alluvial plains, and thus, they counted on erosion in the headwaters of rivers such as the Nile, the Tigris, and the Euphrates to provide sediments to renew soil fertility. By the late 18th century, however, more observers began to notice the consequences of accelerated erosion, partly due to the increased impacts of humans on the landscape as populations began to grow at unprecedented rates. In 1818, James Madison lectured on the evils of poor land management. In 1864, George Perkins Marsh wrote about the impacts of forest clearance on river sedimentation and flooding in New England and the Mediterranean Basin.
German soil scientists conducted the first scientific investigations of soil erosion in the 1870s. The U.S. Forest Service first conducted quantitative soil erosion experiments in 1915 in Utah. In 1928, Hugh Bennett of the Soil Conservation Service established a network of field experiment stations that enabled work on the analytical study of the erosion process. In 1954, a national study of soil erosion used modern data analysis techniques to correlate the results of all field experiments to identify and mathematically enumerate the main factors in the erosion process. This allowed for the development of equations that could be used to estimate the soil erosion in an area based on a number of known variables such as rainfall amount and intensity, slope gradient, slope length, soil erosivity, vegetation cover, and the soil management system.
There are many difficulties involved in estimating soil erosion impacts on a large scale due to the huge variations in soil type, rainfall, management practices, slope, and vegetation that occur over large areas. The use of different assumptions and methods of measurement and analysis also leads to wide variations in erosion estimates. Also, while industrialized countries such as the United States and the United Kingdom have well-funded agencies that produce annual reports on the impacts of soil erosion on croplands under their jurisdiction, similar data are harder to come by in poor countries such as Ethiopia or Guatemala.
In 2003, total annual erosion on all cropland in the United States was estimated at 1.75 billion tons, with 970.6 million tons removed by water and another 776.4 million tons removed by wind. This converts to average per-acre soil losses of 2.6 tons due to water-induced erosion and 2.1 tons due to wind-induced erosion. Although the overall soil loss figures were lower than the overall estimated soil loss rates for 1982, 102 million acres (28% of all cropland) were still losing soil at rates above the soil loss tolerance limits.
One study of the mean water-induced global soil erosion rates between 1979 and 1993 estimated the total annual erosion losses for that period to be around 4.7 tons acre-1 yr.-1 (per acre per year), with the highest overall soil losses estimated for South America (6.8 tons acre-1 yr.-1) and Asia (6.4 tons acre-1 yr.-1). However, erosion losses in mountainous regions can be as much as 100 tons acre-1 yr.-1 or more.
Land Degradation, Marsh, George Perkins, Population and Land Degradation, Rill Erosion, Soil Conservation, Soil Degradation, Soil Depletion, Soils
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