Erosion

In geology and environmental science, erosion is the displacement of solids (soil, mud, rock and other particles) by the agents of wind, water, ice, or living organisms, or by down-slope movement in response to gravity. Erosion needs to be distinguished from weathering, although the two processes may be concurrent. Weathering refers to the decomposition of rock and minerals through processes involving no movement, that is, no physical removal of material.

Erosion is a natural process, moving material and nutrients from one place to another and converting rock into soil. Thus a certain degree of erosion is healthy for the ecosystem. Excessive erosion, however, can damage the ecosystem, such as by increased water sedimentation or the outright loss of soil.

In many places, erosion is heightened by human land use. Poor land-use practices include deforestation, overgrazing, unmanaged construction activity, and road or trail building. On the other hand, improved land-use practices can limit erosion, by techniques such as tree planting and terrace-building, or leveling of the land interrupting steep slopes

Severe soil erosion in a wheat field near Washington State University, U.S.A.

Causes

Erosion is governed by a combination of factors, including the amount and intensity of precipitation (particularly rain), soil texture, gradient of the slope, ground cover (from vegetation, rocks, and so forth), and land use. Of these, the main agent of erosion is rainfall.

In general, if one compares areas with the same degree of vegetative cover, erosion tends to be higher in areas with high-intensity precipitation, sandy or silty soils, and steep slopes. On the other hand, there is less erosion of soils with a higher content of clay, situated along lower slope gradients, and receiving less intense precipitation.

Among the factors listed above, the one most likely to change over time is the amount and type of ground cover. When fires burn an area, or when vegetation is removed during timber operations or house or road construction, a soil's susceptibility to erosion is greatly increased. Road construction can increase the rate of erosion because, in addition to removing vegetative ground cover, it can significantly alter drainage patterns. However, erosion is minimized if the road has a lot of rock and is "hydrologically invisible," that is, the water flows off the road as quickly as possible, mimicking natural drainage patterns.

Bank erosion started by four-wheeler, all-terrain vehicles in Yauhanna, South Carolina.

Changes in the type of vegetation in an area can also affect erosion rates. Different kinds of vegetation have an effect on the rates of infiltration of rain into the soil. Forested areas can take up water at higher rates, so precipitation there results in less surface runoff, and subsequently, less erosion of surface materials. In these areas, much of the water goes into subsurface flows, which are generally less erosive. Leaf litter and low shrubs also contribute to the high infiltration rates of forested systems, and removing them can lead to an increase in erosion rates. Leaf litter also shelters the soil from the impact of falling raindrops, which is a significant agent of erosion. Vegetation can also change the speed of surface runoff flows, so the presence of grasses and shrubs plays an important role in this respect as well.

Many human activities, such as logging and heavy grazing by livestock, can reduce an area's vegetation, making the soil more susceptible to increased erosion. One of the main causes of erosive soil loss in the year 2006 was the result of "slash-and-burn" treatment of tropical forests. When the total ground surface is stripped of vegetation and seared of all living organisms, the upper soils are vulnerable to erosion by both wind and water. In a number of regions of the world, entire sectors of a country have been rendered unproductive. For example, on the Madagascar high central plateau, constituting approximately 10 percent of that country's land area, virtually the entire landscape is sterile of vegetation, with gully erosive furrows scattered along the landscape—typically more than 50 meters deep and one kilometer wide. Shifting cultivation is a farming system that sometimes incorporates the slash-and-burn method in some regions of the world.

When land is overused by human and animal activities, there can be mechanical erosion as well as removal of vegetation, leading to erosion. In the case of animals, one sees this effect primarily with large herd stampedes, such as the Blue Wildebeest on the Serengeti plain. Even in this case, there are broader material benefits to the ecosystem, such as continuing the survival of grasslands indigenous to that region. This effect may be viewed as a problem only when there is a significant imbalance or overpopulation of one species.

In the case of human use, the effects are also generally linked to overpopulation. For example, when large numbers of hikers use the same trails, or when there is extensive off-roading by vehicles, erosive effects often follow, arising from vegetation removal and furrowing of the soil. These effects can also accumulate from a variety of outdoor human activities, again arising from too many people using a finite land resource.

One of the most serious and long-running water erosion problems worldwide is in China, around the middle reaches of the Yellow River and the upper reaches of the Yangtze River. From the Yellow River, over 1.6 billion tons of sediment flow into the ocean each year. The sediment originates primarily from water erosion in the Loess Plateau region of northwest China.

Estimating erosion
The Americans have, for almost fifty years, pioneered a soil erosion estimating system which requires the farmer to comply with required soil management techniques, if he wishes to continue receiving government support. The Food Securities Act of 1985 requires that farmers apply conservation measures to remain eligible to participate in certain government programmes.
The erosion risk (A=annual soil loss) is calculated from a number of factors that have been measured for all climates, soil types, topography and kinds of land used in the USA. This technique helps to predict erosion and shows farmers which farming methods to use. It also identifies erosion-sensitive areas.
The factors are combined in a number of formulas of the 'Universal Soil Loss Equation', which returns a single number, the tolerance factor, equivalent to predicted erosion in ton/ha: A = RKLSCP , where:

• A = Average annual soil loss: the predicted erosion or tolerance factor in ton/ha, calculated from all others.
• R = Rainfall erosivity factor: a factor dependent on climate and likelihood of extreme events.
• K = Soil erodibility factor: an estimate made from soil properties as catalogued in the National Resources Inventory. It depends on the particle sizes and proportions of sand, silt and clay, organic matter, granularity and profile permeability to water.
• L = Slope length factor: the slope length is the length of the field in a down-slope direction. The larger slope length, the more water accumulates at the bottom of the field, increasing erosion. It also depends on the land's slope.
• S = Slope steepness factor: calculated from the slope of the land in %.
• C = Cover management factor: depends on crop growth rate in relation to the erosivity variation in the climate (?).
• P = Supporting practice factor: reflects the use of contours, strip cropping and terracing.

A ‘tolerance factor’ ( ton/ha/yr) is aimed for, which is still much larger than natural soil formation, but which does not appear to affect productivity. If the tolerance factor exceeds sustainability of productivity, different cover management practices or supporting practices are selected and enforced. It is hoped that where no erosion estimating system is available, farmers and land owners, consider the above factors in the ways they manage their soils. Remember that sustainability of cropland is very difficult to attain. (NWE)