Pict. 10.3. Surface erosion                      Pict. 10.4. Rill erosion                             Pict

Wind erosion

Wind erosion occurs when strong winds blow over a smooth, exposed, loose, and dry soil surface. Depending on conditions, the wind speeds required to initiate erosion of mineral soils vary between 25 and 50 km/hour measured at 30 cm above the soil surface. Soil particles between 0.1 and 0.5 mm diameter are first to move. The wind pressure causes them to vibrate and, if their resonant frequency is achieved, they are ejected into the wind stream. Gravity quickly brings these particles back to earth but meanwhile they have gained considerable energy from the wind and they collide into the soil surface dislodging other particles.

The first effect is the winnowing of light particles. Wind erosion is very selective, carrying the finest particles - particularly organic matter, clay and loam - many kilometres. The build-up of this alluvial matter stripped by the wind from the periglacial steppes gave rise to the fertile loess soils that cover large areas of Europe and North America, where highly productive farming has developed. Degradation of sedimentation crusts on the surface of stripped soils, or the weathering of rocks at their base where they are in contact with the soil (abrasion). Sheets of sand travelling close to the ground (30 to 50 metres) can degrade crops (particularly millet or cotton seedlings in semi-arid zones). Lastly, wind erosion reduces the capacity of the soil to store nutrients and water, thus making the environment drier.

There are several factors affecting the extent of wind erosion:

Aridity of climate. - Wind erosion can also take place in high-rainfall climates when certain months of the year are particularly dry (but only if the soil is tilled with techniques that crush the surface fine). It tends to be slight in Africa, however, except where rainfall is less than 600 mm; there are more than six months without rain; potential evapotranspiration exceeds 2000 mm; soils have been left bare; and the vegetation shifts from savannah to steppe, with patches of bare soil. Wind-speed also has to exceed about 20 km/in or 6 m/s over dry soils. Wind erosion phenomena will increase proportionately in the presence of strong, regular prevailing winds or gusts.

Soil texture. - Loamy sand, rich in particles between 10 and 100 microns in size, is the most vulnerable soil (Bagnold 1937). More clayey soil is much stickier, better-structured, and hence more resistant. Coarse sand and gravelly or rocky soils are also more resistant, since the particles are too heavy to be removed by wind erosion. The optimum size for wind erosion is about 80 microns.

Soil structure. - The less structure-improving matter a soil has on the surface (organic matter, iron and free aluminium, lime), the more fragile it will be, while the presence of sodium or salt often leads to formation of a layer of dust on the surface, which fosters wind erosion.

State of the soil surface. - If the soil surface is stony, forming a "pavement", the risks of wind erosion are lower - as, for example, in regs.

A rough surface, left by cloddy tillage or ridges perpendicular to the prevailing wind, slows down the wind at ground level, thus reducing saltation.

Vegetation. - Stubble and crop residues in the soil cut wind-speed at ground level.

Soil moisture – It increases cohesion of sand and loam, temporarily preventing their erosion by wind (Picts. 10.7., 10.8.).