Erosion could be defined as displacement of solids (e.g., soil, mud, rock) by the agents of currents such as wind, water, or ice by downward movement in response to gravity or by living organisms (i.e., bioerosion) (http://en.wikipedia.org/ wiki/Erosion). The degree of erosion is accelerated by various sources. Rainfall (i.e., the amount and intensity of precipitation) is the primary cause of erosion and plays a key role for the other agents to intensify the erosion impact on ecosystems. Rate of erosion depends on various environmental factors such as soil texture, gradient of slope, ground cover (e.g., vegetation, land use), and current velocity of streams (http://en.wikipedia.org/wiki/Erosion).
Due to rapid agricultural/industrial development and mismanagement of natural ecosystems (e.g., overuse of trails in parks), erosion (especially soil erosion) has been a global issue in achieving sustainable ecosystem management. Soil erosion is accelerated by water (e.g., rain detaching, transporting soil; Figure 1), wind (Figure 2), or tillage, and affects greatly agricultural areas and the natural environment. Soil erosion is one of the most serious environmental problems and has both on-site and off-site impacts (Favis-Mortlock 2005 in http:// www.soilerosion.net/).
The main 'on-site' impact occurs at the place where the soil is detached and is presented as the reduction in soil quality resulting from the loss of the nutrient-rich upper layers, and the reduced water-holding capacity. Loss of soil quality is presently a global problem. Soil erosion's most serious impact may well be its threat to the long-term sustainability of agricultural productivity. The 'off-site' problem is caused from soil detachment by accelerated water or wind erosion and occurs wherever the eroded soil ends up. The soils could be transported considerable distances (see Figure 3) and may give rise to the 'off-site problems'. The transportation of soil will result in accumulation of sediments and agricultural pollutants in watercourses, leading to the silting up of dams, disruption of the ecosystems of lakes, and contamination of drinking water and downstream watercourses (Favis-Mortlock 2005 in http:// www.soilerosion.net/).
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