Erosion
Introduction to soil erosion
Soil erosion is one of the most important agricultural problems in the world. Water erosion is the commonest form of erosion and is closely related to run-off. According to the FAO (1986), "about 25 000 million tonnes of soils are being washed away every year" worldwide.
Erosion occurs when soil resources are over-exploited and when farming practices do not take into account the ease with which soils can be washed or blown away. Some examples of potentially harmful farming practices are the reduction of the fallow period which normally allows the land to rest and fertility to build up, plowing the land too deeply and too frequently (2-3 times a year) to produce annual crops), the lack of crop rotations, or simply planting crops down the contour instead of along it. Some examples of the over-exploitation of the land resource are overgrazing, which has already damaged 20 percent of the world's pasture, and the destruction of many forests to provide fuel wood and new agricultural lands, exposing land which was stable when covered by vegetation but which erodes quickly when cut over.
People do not destroy their land deliberately. In developed countries, the increasing need for greater productivity drives the use of the most efficient and economical practices, sometimes regardless of their environmental impact. In developing countries, rural poverty and a desperate need for food is closely related to erosion and land degradation.
Processes of erosion
Detachment
Soil particles are detached under the force of impact of water drops directly on the soil particles or on thin water surface. If raindrops fall on crop residue or growing plants, the energy is absorbed and thus soil splash is reduced. But if raindrops fall on bare soil, they not only cause splash but also decrease aggregation and cause deterioration of soil structure. Detachability increases with increased particle size.
Transport
Soil particles are transported with run-off (refer to following section), carrying with them nutrients, particularly phosphates, to waterways. Phosphates contribute to eutrophication of lakes and streams (refer to later appropriate section). Soil transportability increases with decreased particle size.
Deposition
Eroded sediments deposit either somewhere else in the field or reach waterways and eventually deposit in reservoirs (and fill them) or river estuaries. Coarser particles deposit first and finer particles travel further.
Run-off
Run-off is the portion of precipitation that makes its way toward streams, channels, lakes or oceans, and is the primary cause of water erosion. So it is important to understand how it works and which factors influence a reduction in erosion caused by run-off.
Before run-off can occur, precipitation must satisfy the demands of evaporation, interception, infiltration, surface storage, surface and channel detention. Interception by vegetated canopy may be so great as to prevent a light rain from wetting the soil. Interception by dense covers or shrubs commonly amounts to 25 percent of the annual precipitation. Interception also has a detention storage effect, delaying the progress of precipitation that reaches the soil only after running down the plant or dropping from the leaves. Run-off will only occur when the rate of precipitation exceeds the rate at which water may infiltrate the soil. After the infiltration rate is saturated, water begins to fill the small and big depressions on the soil surface. As the depressions are filled, run-off, also called overland flow, begins.
Factors influencing the extent of erosion
Climate
- Rainfall, intensity, temperature, wind, humidity, snowmelt, etc., are all factors that influence erosion and depend on the climate.
Soil
- Affects infiltration capacity and the extent to which particles can be detached and transported.
Vegetation/land use
- Influences the interception of rainfall, slows down erosion by decreasing the water surface velocity, physically restrains soil movements, improves the aggregation and porosity of soil.
Topography
- Degree of slope, shape and length of slope, size and shape of watershed, orientation of slope are all influencing the extent of erosion.
Types of erosion
(listed by least to greatest potential for damage)
Interill (sheet) erosion
- Uniform removal of the soil in thin layers from sloping land, resulting from run-off. The beating action of raindrops combined with the surface flow causes initial microscopic rilling through which water flows.
Rill erosion
- Detachment and transport of the soil by a concentrated flow of water. Rills are small enough to be removed by normal tillage operations. Rill erosion is the predominant form of erosion under most conditions.
Gully erosion
- Produces channels larger than rills. These channels carry water during and immediately after rains, and, as distinguished from rills, gullies cannot be removed by tillage. The nuisance from having fields divided by large gullies is a great problem.
Stream bank erosion
- Soil removal from stream banks or soil movements in the channel. Stream banks erode either by run-off flowing over the side of the stream bank or by scouring and undercutting below the water surface.
Predicting erosion
It is possible to estimate the sediment loss from a given cultivated area using the Universal Soil Loss Equation (USLE). This equation takes into account many factors affecting soil erosion such as the rainfall run-off potential for erosion, the soil erodability, the slope steepness and slope length, the cover crop management and conservation practices.
This equation over predicts the amount of loss because it does not take into account the fact that some of the soil particles being eroded deposit further away in the same field. But it is a good decision-making tool to choose the best cover crop management and the best conservation practices (the only two factors that can be worked at to make changes) that will keep the overall loss below acceptable levels (less than 2 tonnes/ha in Québec).
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