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Effects of wind erosion
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.
The most spectacular forms are dunes - mounds of more or less sterile sand - which move as the wind takes them, even burying oases and ancient cities.
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.
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 increases cohesion of sand and loam, temporarily preventing their erosion by wind.
Wind erosion control
Wind erosion control is carried out on two fronts: reducing wind-speed at ground level, and increasing soil cohesion, thus improving soil resistance to wind.
INCREASING SOIL COHESION
Applications of organic matter in the surface horizons improve soil structure.
Spraying the soil with refinery sludge, heavy oil or bitumen and plastics industry waste (for example, diluted glue) binds particles to the soil surface making it difficult for the wind to remove them (Gand University experiments).
Where there is enough water, supplementary irrigation can be an effective and financially viable way of reducing erosion problems. Irrigating the soil prior to the normal rainy season is enough to allow favourable tillage conditions and establish plant cover before the destructive tornadoes which generally come at the start of the rainy season.
INCREASING ROUGHNESS OF THE SOIL SURFACE
This entails cropping techniques that leave large clods on the soil surface or ridges perpendicular to the direction of the prevailing wind - although ridges must not be more than 40 cm high or the wind will lop off their tops, thus speeding up erosion.
Another very effective control method is that of leaving crop residues in the fields. In Burkina Faso, for example, when millet and sorghum stubble is cut at a height of 1 metre and left vertical to the soil surface, it traps a large amount of dust, together with leaves that tornado winds have blown off the trees.
INCREASING PLANT COVER
Wind-speed can also be cut by increasing plant density. Since this is clearly not easy in arid zones, it is particularly important to ensure sound crop residue management, keeping residues on the ground so as to increase roughness and protect the soil surface, rather than ploughing in, which would only slightly improve soil structure and resistance to wind. In the semi-arid tropical conditions of West Africa, the large natural stands of Acacia albida so prevalent in cultivated zones generally provide fairly effective protection against wind erosion in these fragile areas by cutting wind-speed at ground level, and also shedding leaves onto the ground. Unfortunately, most of these stands are made up of between 25 and 40 very old trees per hectare and are in urgent need of regeneration. Planting 100 to 150 young trees along the defence lines against water erosion would give a good density of adult trees. In areas subject to violent blows from a regular direction, hedges and wind-breaks are well-known methods.
Their role is twofold: they cut wind-speed to reduce both evaporation and wind erosion. The effect of cutting wind-speed by 20% is operative over an area 10 to 12 times the height of the barrier before and behind it.
This protection depends on the permeability of the wind-break, for relative impermeability reduced speed more, but over a smaller area. According to Heusch (1988), if the speed is cut too much by very close planting, the temperature rises and crops are scorched along the wind-break. It would be better to regenerate a stand of about 40 adult trees to cut the wind-speed more regularly.
In principle; wind-breaks reduce evapotranspiration by up to 20% (although the water consumption of the wind-break itself can offset this positive effect), hence the attraction of windbreaks around irrigated crops. In the Keita Valley in Niger, a marked rise in yields (+27%) is seen except in the direct vicinity of the wind-breaks, where the millet suffers from root competition with the roots of the wind-break (shade and competition for water).
The best arrangement would be two rows of tall trees surrounded by two rows of low trees, making up a 10-metre strip (Figure 59b), half of which is logged at a time. The cropped area between wind-breaks can be as wide as 100 metres if the tall trees are over 5 metres. Root competition is reduced by breaking the young horizontal roots of the trees from the first year onwards by raking the tillage furrow. It is particularly important to repair breaches in a hedge to keep the wind from pouring through at these points (the Venturi effect) and considerably reducing effectiveness.
a. Influence of a wind-break on the wind (cf. Combeau 1977)
b. Influence of a wind-break on production of a cereal grain (cf. Guyot 1963)
When the trees are cut, it is best to leave two metres above the ground so that livestock do not destroy new shoots.
The most commonly grown tree species in Africa are eucalyptus, casuarina, neem, various acacias, tamarisk and cypress - although cypress is susceptible to a serious disease. Reeds of various kinds can also be useful.
A wind-break does not have to be very thick: the thicker it is, in fact, the more impervious and thus the less effective - it becomes. At a distance of 10 times the height of the barrier, the wind-speed on the leeward side is 56% the speed of the local wind behind a line of reeds, 72% behind a 20-m thick wind-break, and 83% behind a 600-m wide forest strip (Combeau 1977).
In Niger, Renard and Van den Beldt (1991) noticed that huge quantities of sand were trapped in the strips of Andropogon surrounding their trial plots, and they therefore suggest that farmers should surround their fields with a double row of Andropogon. Elsewhere, low crops such as groundnut and cotton are protected by interspersing them with rows of millet and sorghum, which can reach a height of 4 m. Lastly, although the initial objective of windbreaks is that of reducing evaporation caused by the wind, they also help to reduce the amount of solid wind-borne matter. The wind must be able to pass through them and not set up eddies, and they must combine species with complementary forms and heights and in sufficient numbers, so that they can be logged in succession and regularly renewed (Figure 59).
The point of dune fixation is to eliminate the source of sand and to keep the dunes in place, using both mechanical and biological methods. In places where dangerous winds come from only one direction, wind erosion can be stopped by rows perpendicular to this wind at distances of 20 times the height of the rows. So if millet or sorghum stalks 1 to 1.5 metres high are used, rows should be spaced every 20 metres, or the wind will take up sand between these lines of defence. This means that large amounts of material are needed (millet stalks, oleanders which grow in wadis, palms, or prunings from the forest trees or shrubs found in the region), and the removal of this material helps to degrade the area. If - as is often the case - the dangerous winds come from a variety of directions, the use of grids of permeable plastic sheeting with a 5- to 10-mm mesh and a height of 50 to 80 cm is indicated. The stronger the winds, the smaller the grid, ranging from 5 × 5 m to 8 × 8 m in normal conditions. Some plastic sheeting, scorched by UV rays, turns to dust after two years, and is therefore to be avoided, while some black UV-resistant plastic can be used for two years on one site and then moved to another. The main problem is to stretch it and to have solid enough stakes to hold it (12-mm reinforced concrete posts). As soon as this grid is in place and the soil surface has become more stable, a variety of grasses and shrubs must be planted inside it to restore plant cover and definitively stabilize the dune.
Another inexpensive method well suited to West Africa is that of sowing rows or grids of millet or some other fast-growing plant in the rainy season, thus giving the soil further stability. If the survival of these fragile planted plots is to be assured, it is obviously vital to protect them against grazing and fire, although after five years some light and well-supervised grazing may be possible.
In France, the first attempts to stabilize coastal dunes were made in the 16th century, when the town of Bayonne had a sand-loving plant sown on the live dunes at Cap Breton, followed by reforestation with sea pines. In 1786, Bremontier, a public works engineer, initiated measures to fix the sandy heaths near Arcachon, by having branches of broom spread over the sand, and sowing with pine. These attempts were so successful that work then continued until 1876, covering 80000 ha and costing 9.6 million "gold francs" plus 3.5 million for the creation and upkeep of a long, protective coastal dune to cut the wind-speed and allow the pines to grow.
This protective dune 50 metres behind the high water mark has a bank with a 20% slope facing the west winds and planted with Ammophila arenaria, then a flat top, with a palisade on posts along its axis, then a crumbling bank of sand. When the fence is about to be capped by deposits, it is raised again with a gin, until the dune eventually reaches a height of 10 metres (in 10 + 2 years). In front and to the sides, wattling marks off the area to be stabilized, and seeds are broadcast inside this area, after which the soil is covered with branches of pine, gorse, heather or broom laid one over the other like tiles on a roof, with the thick ends of the branches pushed into the ground towards the wind.
One hectare will take 25 kg of pine seed, 15 kg of broom seed, plus gorse and Ammophila, 120000 15-kg bundles of branches, and 120 days of work, plus the cost of creating the fence (Heusch 1988, p. 184).
It is interesting to see how similar wind and sheet water erosion are in terms of the processes involved, the effects on the soil, and the factors and control methods. Indeed, an equation very similar to the USLE has been drawn up to forecast wind erosion. Wind erosion assumes significant proportions only when the wind carries a load of sand grains which bombard the bare soil surface, and sheet erosion occurs when rain splashes on naked soil. Both forms of erosion selectively carry off fine particles from the soil surface, and both are eliminated by mulching the soil or by providing an adequate plant cover. Both processes lead to a reduction in fine particles in the surface horizon - or scouring of the whole horizon in the most extreme cases. The factors that can be brought into play are soil cover, pervious barriers that allow the medium (water or air) to filter slowly, and improvement of the structure, cohesion and roughness of the tilled horizon. The control methods are therefore very similar: hedges, windbreaks, keeping crop residues on the soil surface, thick plant cover, coarse tillage, mounding or tied ridging, reduction of the length of fields exposed to prevailing winds or runoff, organic or mineral applications (lime or gypsum), etc. This is why this publication is confined to general principles, referring readers to the many manuals giving details of plant species suited to local dry conditions.
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