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Text 15C Soil erosion - Causes and Effects Erosion is a natural and continuous process. Soils are created through erosion of parent material and either local deposition else- where. Erosion, is defined as the detachment or uptake, and transport over a certain distance of material of the upper layer of the earth crust by an agent, like water, wind or ice. This mass movement of soil par- ticles, is part of the process of soil degradation. Before erosion takes effect, the degradation process often has started with qualitative changes in the soil, like loss of nutrients, loss of organic materials, re- duced soil life and loss of soil structure. Soil erosion is one of the forms of soil degradation along with soil compaction, low organic matter, loss of soil structure, poor internal drainage, salinisation, and soil acidity problems. These other forms of soil degradation, serious in themselves, usually contribute to acceler- ated soil erosion. Soil erosion may be a slow process that continues relatively unno- ticed, or it may occur at an alarming rate causing serious loss of top- soil. The loss of soil from farmland may be reflected in reduced crop production potential, lower surface water quality and damaged drain- age network. Soil erosion is a naturally occurring process on all land. The agents of soil erosion are water and wind, each contributing a significant amount of soil loss each year in different countries. Water erosion depends on four factors: rainfall, soil type, slope gradient, and soil use/vegetation cover. Loss of soil structure becomes often most visible in encrustation of the soil. Both rainfall and runoff factors must be considered in assessing a water erosion problem. The impact of raindrop on the soil surface can break down soil aggregates and disperse the aggregate material. Ligh- ter aggregate materials such as very fine sand, silt, clay and organic matter can be easily removed by the raindrop splash and runoff water; greater raindrop energy or runoff amounts might be required to move the larger sand and gravel particles. 158 Soil erosion potential is increased if the soil has no or very little vegetative cover of plants and/or crop residues. Plant and residue cov- er protects the soil from raindrop impact and splash, tends to slow down the movement of surface runoff and allows excess surface water to infiltrate. Wind erosion less common, but again takes place after vegetation has been lost and when soil particles are loosened. Early signs of wind erosion include deposition of sand particles around plants and micro- ripples on the surface of exposed areas. The final extreme is the clas- sic sand desert dune structures. The lack of windbreaks (trees, shrubs, residue, etc.) allows the wind to put soil particles into motion for greater distances thus increasing the abrasion and soil erosion. Knolls are usually exposed and suffer the most. Sheet erosion is the most common form of erosion. Unprotected soil particles are loosened by trampling, through wind erosion and by the impact of rainfall. The soil particles are then transported by rain- water surface flow to the river and stream systems. Sheet erosion is characterized by a general lowering of the soil level, leaving raised pedestals where the root mass of the remaining vegetation protects it. Sheet erosion is soil movement from raindrop splash resulting in the breakdown of soil surface structure and surface runoff; it occurs rather uniformly over the slope and may go unnoticed until most of the pro- ductive topsoil has been lost. Rill erosion results when surface runoff concentrates forming small yet well-defined channels. These channels are called rills when they are small enough to not interfere with field machinery operations. The same eroded channels are known as gullies when they become a nui- sance factor in normal tillage. Gully erosion is the most obvious and dramatic demonstration of erosion, although in most areas actually less significant in terms of to- tal land degradation. Gully erosion rarely occurs without sheet ero- sion. It can also be triggered by erosion along livestock tracks, foot- paths and road edges. The process can start with "rills" and end up with gullies that are tens of meters deep. There are farms that are loosing large quantities of topsoil and sub- soil each year due to fully erosion. Surface runoff, causing gull forma- tion or the enlarging of existing gullies, is usually the result of im- proper outlet design for local surface and subsurface drainage systems. 159 The soil instability of fully banks, usually associated with seepage of ground water, leads to sloughing and slumping of bank slopes. Such failures usually occur during spring months when the soil water condi- tions are most conductive to the problem. Poor construction, or inade- quate maintenance, of surface drainage systems, uncontrolled live- stock access, and cropping too close to both stream banks has led to bank erosion problems. The effects of soil erosion are complex. Some of the impacts may appear to be reversible by suitable soil conservation programmes and improving cultivation practices, whereas there are other types of deg- radation which are irreversible. The latter type includes land lost by gulling, or cases of severe sheet erosion where the soil cover has been removed to a great extent. In cases where degradation is reversible it is generally difficult to assess the actual extent and impact of land de- gradation as farmers do convert their land to less demanding uses or increase the level of inputs. Methodologies for prediction of soil ero- sion have been developed since the early thirties. Many farmers have already made significant progress in dealing with soil erosion problems on their farms. However, because of con- tinued advances in soil management and crop production technology that have maintained or increased yields in spite of soil erosion, others have not been aware of the increasing problem on farmland. Aware- ness usually occurs only when property is damaged and productive ar- eas of soil are lost. Certain conservation measures can reduce soil erosion by both wa- ter and wind. Tillage and cropping practices, as well as land manage- ment practices, directly affect the overall soil erosion problem and so- lutions on a farm. When crop rotation or changing tillage practices are not enough to control erosion on a field a combination of approaches or more extreme measures might be necessary. For example, contour plowing, strip cropping, or terracing may be considered. жүктеу/скачать 0.91 Mb. Достарыңызбен бөлісу:
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