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Land Management & Natural
Hazards Unit |
SOIL |
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Soil
Atlas of |
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Erosion
EROSION is a physical phenomenon that results in the removal of soil and rock particles by water, wind, ice and gravity.
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Severe gully erosion in Spain (JI). Erosion is a natural process, occurring over geological timescales, that has been largely responsible for shaping the physical landscape we see around us today. The action of rain, wind, ice (in the form of glaciers) and temperature (by freezing and thawing) wear down and shatter rock surfaces. Subsequently, geomorphic processes have distributed the weathered materials, produced by these agents, over the surface of the Earth. For millions of years, erosion has transformed the landscape, wearing down mountain and upland areas whilst sedimentation has filled in continental basins with the resulting debris. Thus erosion is a process that is essential for soil and landscape formation and has taken place since time began. Most present-day concerns about soil
erosion, leading to its perception as a process of degradation, are related
to accelerated erosion, where the natural rate has been significantly
increased by human activities. These activities include the stripping of
natural vegetation especially clearing of forests for cultivation, changing
land cover in other ways through cultivation, grazing, controlled burning or
wildfires, levelling of the land surface, and
varying the intensity of land management, for example through poor
maintenance of terrace structures and cultivation of steep slopes. The
resulting changes to the soil cover allow natural forces to remove soil
particles much more rapidly than normal soil-forming processes can replace
them, hence the term accelerated erosion. Soil losses > 2 t/ha/yr
are considered by experts in many parts of the world to be irreversible
within a timescale of 50-100 years. |
Soil erosion is regarded as one of the most widespread forms of soil degradation, and as such, poses potentially severe limitations to sustainable land use in Europe. Soil can be eroded away by water and wind. Erosion by water occurs due to the energy of water when it falls to the earth and flows over its surface. Strong wind, depending on its strength and duration (persistence), can blow away loose soil from flat or undulating terrain. Soil erosion by water is a widespread problem throughout Europe and the main processes are rain-splash, rain-wash, rill-wash and sheet-wash. Whilst these processes are often localised they produce distinct features such as rills, gullies and, in extreme cases, badlands. There are other forms of water erosion, for example snowmelt erosion in northern Europe which can also produce surface features such as rills and gullies, and bank erosion where streams, rivers and lake waters wash bank material into suspension to be carried downstream, or away from the shore, until the velocity of flow or movement diminishes sufficiently for deposition to take place. Several other types of soil erosion have been recognised and studied by researchers. For example, wind erosion occurs in areas where soil is dominantly sandy or silty, dry and not stabilised by plant roots; cultivation, levelling of land for example to create terraces, trampling by animals destroying surface cover and soil removal by harvesting of root crops cause further losses of soil. The most dominant effect is the loss of topsoil, which may not be conspicuous but nevertheless potentially very damaging. If soil loss by these various forms of accelerated erosion is to be reduced or eliminated, the amounts of soil removed must be quantified and this requires a different approach for each type of erosion. On-site measurements can quantify soil loss at the field scale and accumulation in reservoirs can reveal the amount of sediment removed within individual catchments or basins. Lakes and reservoirs act as large sediment traps and sedimentation rates can provide valuable comparisons between environments. However, complications arise because soil loss in one part of a catchment or basin can lead to deposition (sediment yield) in another part. Not all the material eroded on a hillslope will arrive at the outfall of the catchment or accumulates in reservoirs, a significant part may remain in intermediate storage on slopes, in alluvial fans, as colluvium along foot-slopes and as outwash on plains. Climate, topography and soil characteristics are important physical factors affecting the amount of erosion. The Mediterranean region is particularly prone to accelerated soil erosion because it is subject to long dry periods, followed by heavy bursts of erosive rain, falling on steep slopes often with shallow soil low in organic matter. The introduction of agriculture and grazing during Neolithic times (6000-8000 years ago), in and around the Mediterranean Basin, marked the start of progressive forest clearance which has continued until the present day. A well known example of one of the effects of this is the Ebro Delta, the growth of which is linked to deforestation and expanding agricultural activities that took place between the Middle Ages and the 19th century. In parts of the Mediterranean region, erosion has reached a stage of irreversibility such that, in some places, erosion has practically ceased because there is no more soil left. This contrasts with NW Europe where there is less loss of soil, because rain falls mainly on gentle slopes and is more evenly |
Key threats to soil in EuropeErosion causing a 1m deep rill in the Severn Valley UK, January 2001 (PNO).
distributed throughout the year than in the south. Consequently the area affected by erosion is less extensive than in southern Europe. However, erosion is still a serious problem, particularly off-site, in northwest and central Europe, and is on the increase largely as a result of sheet erosion on bare soil surfaces, the area of which has increased significantly in Europe since the Second World War. There are very few sites in Europe where
soil erosion has been or is still being monitored but the measurements of
soil loss that do exist show average rates varying from less than 0.5 to more
than 200t/ha/yr. The highest losses, sometimes as high as 500 t/ha/yr, have been
measured following single storm events of short duration but with heavy rain
falling on bare soil surfaces. Erosion literature commonly identifies
‘tolerable’ rates of soil erosion, but these usually exceed the
rates that can be balanced by natural weathering of parent materials to form
new soil particles. Soil loss in some places may be considered
acceptable from an economic standpoint but some modern cultivation methods
are causing overall erosion rates that are becoming increasingly unacceptable
from a long-term point of view. Two examples of this are the use of
herbicides to kill the vegetation on the ground in olive groves and
vineyards, thus reducing competition for water and nutrients, and cereal
cultivation on hilly land previously in pasture for stock rearing. Both these
systems currently receive financial support from the European Common
Agricultural Policy yet are visibly degrading the land. It is clear that on
some productive land there is an overall loss of soil material that is
becoming irreversible. By contrast, there are some ancient farming systems
that have proved to be sustainable over a long period, for example the dehesas in Spain, the montados
in Portugal and the bocage in France.
Unfortunately, these systems tend to be labour
intensive and mainly practised by the older people
in rural communities. |
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The above maps show the relationship
between erosion and organic matter levels in soil. The map on the left shows
estimated soil erosion rates for Spain, Portugal and southwestern France (see
Page 102). Areas with very low erosion rates are indicated in green while
high-risk zones are red or purple. The map clearly highlights the problems of
soil erosion in Andalucia, along the Tajo Valley and
just north of the Pyrenees. The map on the right shows the levels of organic
carbon in the topsoil. Note the close correspondence between the areas of low
organic carbon (shown in grey) and the high risk of erosion (RJ).