Soil erosion is a natural process, occurring over geological time, and indeed it is a process that is essential for soil formation in the first place. With respect to soil degradation, most concerns about erosion are related to accelerated erosion, where the natural rate has been significantly increased mostly by human activity. Soil erosion by water is a widespread problem throughout Europe. Soil is naturally removed by the action of water or wind: such 'background' (or 'geological') soil erosion has been occurring for some 450 million years, since the first land plants formed the first soil. Even before this, natural processes moved loose rock, or regolith, off the Earth's surface, just as has happened on the planet Mars. In general, background erosion removes soil at roughly the same rate as soil is formed. But 'accelerated' soil erosion — loss of soil at a much faster rate than it is formed — is a far more recent problem. It is always a result of mankind's unwise actions, such as overgrazing or unsuitable cultivation practices. These leave the land unprotected and vulnerable. Then, during times of erosive rainfall or windstorms, soil may be detached, transported, and (possibly travelling a long distance) deposited. Tillage erosion
More recently still, the use of powerful agricultural implements has, in some parts of the world, led to damaging amounts of soil moving downslope merely under the action of gravity: this is so-called tillage erosion. Erosion processes
The processes of soil erosion involve detachment of material by two processes, raindrop impact and flow traction; and transported either by saltation through the air or by overland water flow. Runoff is the most important direct driver of severe soil erosion by water and therefore processes that influence runoff play an important role in any analysis of soil erosion intensity. Soil may be detached and moved by
These three however differ greatly in terms of:
• where and when they occur
• what happens to the area that is being eroded (on-site impacts) • how far the eroded soil is moved, and
• if the soil is moved away from the place where it was eroded, what happens as a result (off-site impacts). Water erosion
Soil erosion by water is the result of rain detaching and transporting vulnerable soil, either directly by means of rain splash or indirectly by rill and gully erosion.
Water and soil splashed following a single raindrop impact.
Rain may move soil directly: this is known as 'rainsplash erosion' (or just 'splash erosion'). Spash is only effective if the rain falls with sufficient intensity. If it does, then as the raindrops hit bare soil, their kinetic energy is able to detach and move soil particles a short distance. Because soil particles can only be moved a few centimetres at most by this process, its effects are solely on-site. Although considerable quantities of soil may be moved by rainsplash, it is all merely redistributed back over the surface of the soil (on steep slopes, however, there will be a modest net downslope movement of splashed soil). Thus a more descriptive term might be 'rainsplash redistribution'. Because rainsplash requires high rainfall intensities, it is most effective under convective rainstorms in the world’s equatorial regions. Rainsplash is relatively ineffective where rain falls with a low intensity (e.g. because the rainfall is of frontal origin), such as in the north-west of the USA or in northern Europe. Rill and gully erosion
Rainfall may also move soil indirectly, by means of runoff in rills (small channels) or gullies (larger channels, too big to be removed by tillage). In many parts of the world, rill and gully erosion is the dominant form of water erosion. That fraction of the rainfall which does not infiltrate (soak into) the soil will flow downhill...