Wind erosion occurs over more than one third of the Earth’s surface. The soil dust that winds carry can affect the Earth’s atmospheric composition and contribute to climate change. It also carries herbicides, sediments, soil texture residues, damaging nutrient contents, vegetation growth, and decreasing farm productivity, as well as affecting human health. The frequency of blowing dust may be caused by natural processes and is common in desert environments, but agricultural, grazing and human activities that disturb the soil can greatly increase the frequency and amount of airborne dust. Aeolian transport of cultivated and grazed soils is a global problem that has generated many studies in Europe, Africa, Asia, Australia, and South America.
This review intends to ascertain the magnitude of the problem, explore the great variety of potential solutions, and demonstrate the progress that has been made in implementing solutions.
Erosion and transport of soil has negative effects both within and outside of source areas, and dust’s capability to be transported great distances makes the problem of both national and international scope. In the USA, the negative off-site impacts of erosion from farms are potentially greater than onsite losses in soil productivity. Society in general may, therefore, have greater incentives for reducing erosion than farmers have. For example, by the mid-1980s, off-site costs associated with wind erosion in the State of New Mexico were estimated at $466 million per year, dwarfing the $10 million per year on-site costs (1).
Wind erosion from croplands adds only a fraction to the total amount of atmospheric dust, but it is a contribution that can be reduced by careful management. Review of human actions and government programs in locations where problems of aeolian dust have been severe and have been addressed in comprehensive research and education programs. It provides insight that can be applied to locations where wind erosion is now becoming critical as local climates changes to more arid conditions.Places that marginal lands are cultivated, and as increasing mechanization changes the amount and rate of soil reworking. In such areas, wind erosion is a chronic problem and many actions have already been taken to try to control it (1).
Several factors other than wind velocity contribute to wind erosion. When the wind force exceeds the resistance of the soil surface, soil will be eroded. These mainly fall into two groups of closely interrelated elements: those inherent in the properties of the soil itself and those associated with soil cover. Soil erosion is increased by soil’s dryness factor. Moist soils generally do not blow or move, but soil moisture is seldom available at the surface in arid zones. A rough soil structure, especially at the surface, effectively reduces the movement of soil particles. Arid regions, however, are dominated by smooth, pulverized and unstructured topsoils. Soil texture also influences soil’s propensity for erosion; fine texture soils are particularly susceptible to wind erosion. Measurements of dust in the air up to three meters above the soil surface at Jodhpur, India, showed that on a stormy day the amount of blowing dust varied between 50 and 420 kg/ha. In the Jaisalmer region of India, where wind speeds generally are higher, average soil loss of 511 kg/ha was recorded (1).
Wind erosion processes:
There are three processes for wind erosion: surface creep, saltation and suspension. Characteristics of each process are outlined below.
Surface creep: in a wind erosion event, large particles ranging from 0.5-2 mm in diameter are rolled across the soil surface. This causes them to collide with and dislodge other particles. Surface creep wind erosion results in moving of these larger particles only a few meters.
Saltation: occurs among middle-sized soil particles that range from 0.05-0.5 mm in diameter. Such particles are light enough to be lifted off the surface but are too large to become suspended. These particles move through a series of low bounces over the surface causing abrasion on the soil surface and attrition (the breaking of particles into smaller particles). On agricultural lands, the most damaging action of wind erosion is saltation. This occurs when soil particles with a diameter of 0.1–0.5 mm are bounced across the soil surface by the wind abrading and eroding the surface.
Suspension: tiny particles less than 0.1 mm in diameter can be moved into the air by saltation, forming dust storms when taken further upwards by turbulence. These particles include very fine grains of sand, clay particles and organic matter. However, not all dust ejected from the surface is indefinitely carried in the air. Larger dust particles (0.05 to 0.1 mm) may be dropped within a couple of kilometers of the erosion site. Particles of 0.01 mm may travel hundreds of kilometers and 0.001 mm-sized particles may travel thousands of kilometers. Fine dust may remain suspended in the air until it is washed away by rainfall (2).
As the saltating particles crash into the surface they splash up more particles that also bounce across the surface. This bombardment of the surface causes an avalanching effect that spreads out in a fan shape, with more and more soil particles being mobilized downwind.
With the continual bombardment of the surface, dust particles of less than 0.1 mm diameter become suspended in the air and are carried far away by the wind.
With constant wind speed, the rate of erosion changes with the distance downwind from the point where the erosion starts, as well as by time, over an erosion period lasting several hours. The soil surface is continually modified as it is eroded. The amount of soil eroded increases with distance downwind until it reaches a maximum. For sandy soils, erosion can start within about 0.2 m and reach the maximum erosion rate within about 5 m from where the erosion started, e.g. at the edge of a paddock. This has implications for erosion control. Any method of erosion control should be intended to stop saltation before it reaches its maximum rate; thus, the non-erodible roughness (clods or vegetation) should be no more than a few meters apart (3).
Influencing Factors on Wind Erosion:
Aridity of climate: Wind erosion demonstrates that regional climatic factors such as effective soil moisture and wind power have significant influence upon wind erosion rates measured by dust storm frequencies (4). Wind erosion and blowing dust are particular problems during droughts, which have increased over the past century (5). Seth M. Munson et al have projected that increases in aridity throughout the southwestern United States is due to anthropogenic climate change. It is likely to cause reductions in perennial vegetation cover, which leaves soil surfaces exposed to erosion. Accelerated rates of dust emission from wind erosion have large implications for ecosystems and human wellbeing, yet there is poor understanding of the sources and magnitude of dust emission in the 21st century’s hotter and drier global climate (6). The causes of major aridity are to be sought in greatly intensified atmospheric circulation aided by increased continental extent corresponding to glacial low sea levels and reduced seasonal precipitation (7). The aridity of an environment is often evaluated by the Budyko dryness ratio. The dryness ratio at a given site indicates the number of times the net radiative energy could evaporate the mean annual precipitation. Semi-arid zones where wind erosion is likely to be a serious problem have a dryness ratio of between 2 and 7. Areas with dryness ratios larger than 7 are in the desert and desert margin zones. Most of the Great Plains region of the USA has a dryness ratio between 2 and 5. The Sahara Desert in North Africa has a maximum dryness ratio as high as 200 (8). Aridity can indicate a considerable water deficit in the top layer of the soil. Wind can easily remove and transfer soil particles when the top layer of the soil is totally dry and has lost the water content necessary for bonding. Aridity is significantly related to precipitation level and temperature (9). Wind erosion can also take place in high-rainfall climates during particularly dry months of the year (but only if the soil is tilled with techniques that crush the soil surface to fine particles). 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; or when the vegetation shifts from savannah to steppe, with patches of bare soil.
Wind erosion phenomena will increase proportionately in the presence of strong, regular prevailing winds or gusts. Wind speed also has to exceed about 20 km/in or 6 m/s over dry soils(10).
Soil texture: Most vulnerable soil is loamy sand, rich in particles between 10 and 100 microns in size. More clayey soil is much stickier, better-structured and thus 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 soil particles to be transferred by wind is about 80 microns. Biological soil crusts, consisting of cyanobacteria, green algae, lichens, and mosses, are important in stabilizing soils in semi-arid and arid lands. Integrity of these crusts is compromised by compressional disturbances such as foot, vehicle, or livestock traffic (11). The effectiveness of the surface crust in preventing wind erosion seemed to be related to the modulus of rupture of the soil (12).
Soil water retention consists of molecular adsorption on the soil grain surface and capillary forces between the grains. Inter-particle capillary forces (characterized by the moisture tension) are the main factor responsible for the increase of the wind erosion threshold observed when the soil moisture increases. When the soil moisture content is close to but smaller than the maximum amount of adsorbed water (depending on the soil texture), these capillary forces are considered not strong enough to significantly increase the erosion threshold (13).
An extremely important parameter is the threshold velocity for dust production. This parameter is dependent on effects of vegetative residue, roughness of the soil, live standing plants, soil texture and the effect of atmospheric precipitation (14). Silt and clay fractions are removed by wind first, leaving coarser minerals like sand and gravel. This sorting action over many years makes soil progressively coarser until nothing remains but infertile skeletal material that forms shifting sand dunes and gravelly pavements (15). Soil erodibility is an estimate of the ability of soils to resist erosion upon the physical characteristics of each soil. Soil texture is the principal characteristic affecting erodibility, but structure, organic matter and permeability also contribute. Generally, soils with faster infiltration rates, higher levels of organic matter and improved soil structure have a greater resistance to erosion. Sand, sandy loam and loam-textured soils tend to be less erodible than silt, very fine sand and certain clay-textured soils (16).
Soil structure: The less structure of a soil is improved on the surface 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, fostering wind erosion. The loss of natural nutrients and possible fertilizers directly affect crop establishment, growth, and yield. Seeds can be disturbed or removed and pesticides can be carried away. The soil quality, structure, stability, and texture are also affected, which in turn affect the water holding capacity of the soil. Particles of fine sand, silt and clay join together to form aggregates. The soil property that describes the character and formation of these aggregates is called soil structure. The glue that joins the soil particles together includes organic matter, clays, iron oxides, aluminum oxides, and lime. Aggregate formation in clay-textured soil improves water infiltration into the soil and drainage because it increases the number of large pores (larger pipes). In sandy textured soil, aggregate formation reduces the excessively fast drainage of water by increasing the number of small pores (narrow pipes)(17).
Condition of soil surface: If the soil surface is stony, forming what is sometimes called 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 direction, slows down the wind at ground level, thus reducing saltation. The amount of erosion, E, expressed in tons per acre per annum, that will occur from a given agricultural field can be expressed in terms of equivalent variables as: E = f(I, K, C, L, V) where “I” is a soil erodibility index, “K” is a soil ridge roughness factor, “C” is a climatic factor, “L” is field length along the prevailing wind erosion direction, and V is equivalent quantity of vegetative cover. These five equivalent variables are obtained by the grouping and conversion of the 11 primary variables now known to govern wind erodibility. Relations among variables are extremely complex. The equation is designed to serve the twofold purpose of providing a tool to (i) determine the potential erosion from a particular field, and (ii) determine field conditions of soil cloddiness, roughness, vegetative cover, sheltering by barriers, or width and orientation of field necessary to reduce potential erosion to a tolerable amount (18). Crusts and immobile aggregates serve as strong modulators of wind erosion. The strength of crusts and aggregates depends on both the formation processes and the soil composition. The breakdown of crusts and aggregates by abrasion depends on their strength and the abrader discharge rate (19).
Soil texture, organic matter content, water content and exterior factors like precipitation and soil management have to be taken into consideration in their manifold combinations in time and space. The surface properties can change within a short period of time and with differences in location. Therefore, predictions of the actual erosion risk are difficult and often inaccurate. An accurate conclusion can be derived for a dynamic description of the soil surface properties to prevent wind erosion as well as environmental pollution or damages to young plants (20).
Erodibility of soils depend primarily on the soil texture or relative proportion of sand, silt and clay, water-absorbing structure and degree of water dispersion within the soil grains. Other factors such as surface roughness, fetch distances and wind speeds, size and stability of soil aggregates and crusting are also important for soil’s susceptibility to wind erosion. Potential susceptibility of soil to wind erosion is mainly due to soil conditions, but is also impacted by the climatic conditions of the area. Wind erosion occurs mainly in areas where the weather is characterized by low and variable rainfall, variable and high wind speeds, the frequent occurrence of drought, rapid and extreme changes in temperature and high evaporation (21).
Vegetation: Vegetation plays an important role in determining the dynamics and morphology of desert and coastal sand dune environments by its influence on the entrainment and transport of sand by the wind. Quantification of the effect of vegetation on sediment transport can be used to assess the effects of climatic change and human disturbance on such areas, as well as aiding sand stabilization and environmental restoration efforts. Vegetation protects the soil surface, trapping dust particles and, more importantly, slowing down the momentum of the wind. also, it additionally protects the soil surface by direct coverage, trapping of particles, and most importantly by extracting momentum from the air-flow (23). Length of the ground surface acts as the function of vegetation characteristic parameters. It increases by power function with an increase in vegetation coverage linearly. It is proportional to the square root of the angle between vegetation alienation and wind direction. Given the same coverage, the evenly distributed vegetation is more effective than the unevenly distributed ones in wind erosion control. Erosion rate increases exponentially with vegetation coverage reduction. According to its influences on wind erosion intensity, vegetation coverage falls into three intensity categories: 60% is non-erodible to slightly erodible; 60% 20% is moderately erodible; 20% is severely erodible (22).
The preferred method of erosion control is to maintain a vegetation coverage of more than 50% of the soil surface, when viewed straight down, after the vegetation has been flattened. If erosion does start on a paddock, then it is best to try to maintain any remaining cover. Vegetation cover helps to control erosion by acting as a blanket that prevents the wind from picking up any soil particles; absorbing the force of the wind and reducing the wind speed on the ground; and trapping eroded soil particles and reducing the amount of bombardment on the soil surface. Once there are stubble, plants, grass butts or small shrubs (higher than 10 cm) that sit up into the air, these offer even more protection. Shrubs and tussock grasses protect the soil when the spacing between the plants is less than three times their height, and when they are evenly distributed across the paddock (24).
Vegetation is known to strongly impact the erosion of soil by the wind. Lateral cover is the primary parameter used to represent the amount of vegetation in aeolian research and, in particular, shear stress partitioning research. Although lateral cover provides a simple means for representing how much vegetation is in an area, it is not capable of characterizing how vegetation is distributed (25).
Soil moisture: soil moisture increases cohesion of sand and loam, temporarily preventing their erosion by wind. Soil water retention consists of molecular adsorption on the soil grain surface and capillary forces between the grain. Inter-particle capillary forces (characterized by the moisture tension) are the main factor responsible for the increase of the wind erosion threshold observed when the soil moisture increases (26). It was shown that erodibility by wind is a function of the cohesive force of adsorbed water films surrounding the soil particles (27).
Soil moisture is one of the most important factors influencing resistance to wind erosion. The increase in intrinsic factor in soil resistance due to moisture content is the cohesive force of soil water. As soil moisture increases, it decreases the wind erosion rate. When the moisture content reaches more than 4%, the rate erosion of decreases and slows down and become almost constant with successive increments of moisture (28). Sujith Ravi and colleagues experimentally proved the hypothesis in their research that in arid regions important changes in soil moisture are due to changes in atmospheric humidity. In particular, the changes in surface soil moisture associated with variations of air humidity significantly affect soil susceptibility to wind erosion. This new result was explained through an analysis of the factors affecting (absorbed layer) wet bonding in air-dry soils: higher air humidity is associated with relatively moister surface soils (29).
Wind erosion decreases as soil moisture increases. For example, dry soil erodes about one-and-one-third times more than soil with barely enough moisture to keep plants alive (30). Numerous studies have considered that the influence of soil moisture on wind erosion rates depends on soil texture and can be explained by inter-particle cohesion forces due to soil water retention processes. Based on those, a large number of relations have been proposed in order to link the erosion threshold to the soil moisture. They consist mainly of numerical adjustments of the measured erosion threshold as a function of the soil moisture for a specific soil type or for different types of soil. In fact, such empirical parameterizations fail to reproduce other experimental data sets than those from which they have been established (31).
As it is clear from above literature review, wind erosion is an immensely serious problem. An accurate understanding of the processes that cause wind erosion is key in developing effective wind erosion control strategies. Although conservation practices can be successful in controlling erosion, droughts can be a limiting factor. Erosive winds will not always blow in a predicated direction. Land managers, engineers and technicians must, therefore, always be vigilant in consideration of innovative methods in confronting wind erosion. A combination of practices may need to be maintained when planning wind erosion control systems. The most effective method of controlling erosion by wind and water is to maintain adequate levels of vegetation cover on the soil surface. To achieve these goals, any country in the Middle East, in addition to building co-operation among each other, must develop a national policy in combating wind erosion by setting soil, water conservation and erosion control as top national priorities. Forests, grazing lands and stock numbers need to be managed to match the current land potential and expected seasonal conditions. The people who are benefiting from the land have to as well make regular decisions about how many animals to keep. Administrative institutions must implement necessary laws to control wind erosion, protect and improve land conditions for optimal use.
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