10 Main Factors Affecting Wind Erosion

This article throws light upon the ten main factors affecting wind erosion. The factors are: 1. Soil Cloddiness 2. Surface Roughness 3. Water Stable Aggregates and Surface Crusts 4. Wind and Soil Moisture 5. Field Length 6. Vegetative Cover 7. Organic Matter 8. Barriers 9. Topography 10. Soil.

Factor # 1. Soil Cloddiness:

The cloddiness of a given soil largely indicate whether the wind will erode it. Soil clods prevent wind erosion because they are large enough to resist the for­ces of the wind and because they shelter other erodible materials. Clods formed during tillage, their firmness and stability depend on soil moisture, compaction, organic matter, clay content, lime and microbial activity.

Clods are broken down by weathering, tillage implements and animal traffic and by abrasion. Excessive and improper tillage often causes excessive soil loosening and pulverisation and increases the hazards from erosion by wind.

Coarse-textured sandy loams, loamy sands and sands are most susceptible to erosion and breakdown. They are least likely to form stable clods, as they contain low silt, clay and organic matter. These soils form clods only when cultivated while moist and firm. Such clods are readily broken down by rainfall or by freezing and thawing.

The cloddiest and least erodible soils are the loams, silt loams, clay loams and silty clay loams, especially if they have a 20- 30% clay content and silt ranging from 0.005 to 0.01 mm in size. The size and bulk density of clods and the proportion of clods to total soil material affect the erodibility of a soil.

The proportion of soil aggregates that are more than 0.84 mm in diameter can be used as a simple index of wind erodibility of soils. It can also be adjusted to compensate for the increased erosion hazard on slopes and hill tops.

Several criteria are commonly used to specify the cloddiness required to control erosion on field soils. For example:

(1) About 50% of the soil surface ought to be covered with clods greater than 10 mm in diameter;

(2) About half the surface clods ought to be greater than 1.0 mm in diameter; and

(3) About two-third of the surface soil by weight ought to be of non-erodible size (>0.84 mm).

These criteria are approximate, but soils that meet any one of these criteria usually will resist all but the very strongest winds. Soil loss by wind varies directly as the 2.5 power of the surface drag of the wind and the 3.5 power of the per cent of soil fractions less than 0.42 mm in diameter.

Both coarse (> 0.42 mm) and fine (< 0.02 mm) water stable aggregates in­crease cloddiness and decrease erodibility by wind. A unit change in water stable particles greater than 0.84 mm influences equally the proportion of non- erodible clods and erodibility. Following relationship between the dry erodible clods B, greater than 0.84 mm, and the water stable fractions can be expressed.

B = a (y-b)…(4.8)

where, y = percentage of water stable fractions < 0.02 mm and > 0.84 mm in a soil, and

a, b = constants. The value of a and b could be 3 and 4, respectively, for the first 2.5 cm soil depth.

Similarly, the soil erodibility index I_w and the percentage of water stable fractions < 0.02 mm and > 0.84 mm are related exponentially.

I_W = ab^-y …(4.9)

where, a and b are equal to 1000 and 1.35, respectively.

It is important to remember that aggregated soil, when bare, offers resis­tance to erosion temporarily, because aggregates exposed to weather usually disintegrate to erodible particles.

Factor # 2. Surface Roughness:

In addition to clods, soil aggregates and ridges, depressions formed by tillage also alter wind speed by absorbing and deflecting part of the wind energy away from the erodible soil. Rough surfaces also trap saltating particles. This reduces abrasion and the normal build-up of eroding material downwind.

A smooth soil surface is generally more erodible by wind than a rough one (Table 4.4) because of being less effective in slowing the wind velocity near the ground. Although, a smooth surface reduces wind turbulence but its effect in reducing wind erodibility is not compensated by the increased surface velocity.

Roughening is not always effective in reducing wind erosion. If the soil is composed mostly of erodible fractions, roughening the surface does little good because the roughness elements continue to erode with the wind.

But if the roughness elements, such as ridges, are composed of erodible and non-erodible fractions, the erodible fractions move from the ridges into the furrows where they are trapped, and the ridges soon become stabilized with a mantle of soil aggregates too large to be moved by the wind.

In general, greater the surface roughness, the lower is the wind velocity against the ground and lower is the rate of erosion. The rate of soil blow under a wind force varies inversely with the roughness of the surface.

Surface rough­ness in turn is dependent on the height and lateral frequency of the surface obstructions. The initial rate of soil blow over cultivated lands is less when such soils are ridged than when the soil surface is smooth.

This is because ridges reduce the wind velocity for some distance above the average surface of the land and trap soil on the leeward side. Ridges, however, produce greater velocities of wind over the crest of the ridge, and also greater eddying.

This results in a more rapid rate of erosion at the crest of the ridge. The initial net effect of ridging on these two sets of forces tends to become reversed. The ridges lose their effectiveness and erosion is resumed.

The amount of soil which is erodible by wind at a given velocity depends upon the critical height (height that slows downwind velocity to 14.5 km/hr) of and distance between the non-erodible fractions that are exposed at the sur­face. The ratio of height of projections to distance between projections which will barely prevent the movement of erodible fractions is designed as the criti­cal surface roughness coefficient.

Under a given wind velocity the critical roughness constant remains the same for the whole range of size and propor­tion of the non-erodible clods. The critical surface roughness constant re­quired to assure soil stability, however, varies with other factors such as wind velocity and size and density of erodible fractions.

Since surface roughness also increases wind turbulence and exposes smaller areas to greater wind forces, excessive roughening may substantially reduce the benefits. Optimum roughness for wind erosion is 5 to 12.5 cm.

Factor # 3. Water Stable Aggregates and Surface Crusts:

The drifted particles of descrete sand grains and clay aggregates are water stable and exhibit greatest mechanical stability. Secondary aggregates falling next in the order of mechanical stability are held together in dry state by water dispersible contents forming clods.

Depending upon the quantity of silt and clay dispersible in water contents present, these clods maintain their identity for some time after repeated wetting and drying.

The greater the quantity of fine particles dispersible by water, the greater the degree of cementation among the structural units and the greater is the mechanical stability. The mechanical stability tends to reduce wind erosion by resisting the break-down of non-erodible units to smaller erodible particles.

Soil crusts formed by dispersion of surface soil due to raindrop impact and subsequent drying are more compact and mechanically stable against wind ac­tion than the soil below. Medium textured soils containing a high proportion of silt are prone to crusting. The crusts may be less than 1/6 to more than 1/2 cm in thickness. Sandy soils containing low silt and clay are less subject to crusting and more erodible by wind.

On crusted soils wind erosion is slow in the beginning but may accelerate as the surface crusts are broken, exposing a weakly consolidated soil below the crust. As shown in Table 4.5 a surface crust formed by wetting (spraying water) and drying (condition b) reduced soil erosion greater than when the crust formed by wetting and drying was sub­jected to abrasion (condition c).

The order of mechanical stability from the highest to the lowest for different structural units in a dry soil follows:

(a) Water stable aggregates;

(b) Secondary aggregates or clods;

(c) Surface crusts; and

(d) Fine materials among the clods cemented together and to the clods after the soil has been wetted and dried.

Factor # 4. Wind and Soil Moisture:

Wind speed and soil moisture both affect wind erosion. For example, the rate of erosion for a 48 km/hr wind is more than three times that for a 32 km/hr wind (Table 4.6). Wind erosion decreases, however, as soil moisture increases. Only dry soil particles are readily moved by wind. For convenience, wind speed and soil moisture are considered together as a local wind erosion climatic factor.

Wind velocity at the ground surface is the most important factor.  There is a minimum wind velocity to start erosion for any field condition. If no soil particles are in air, a higher velocity is required to initiate the movement than when the saltation particles from a neighbouring area are hitting the ground.

In general, wind erosion can only happen when the soil surface is dry or only slightly moist, because surface tension holds the soil particles together when wet. Therefore, soils that have a tendency to retain moisture and to conduct it to the surface are fairly resistant to drifting. The severity of wind erosion increases with periods of drought.

Moisture film between individual particles provide the cohesive force to hold them together. Damp and moist soil particles are, therefore, virtually stable. Wind velocities must create a force in excess of these film forces in order to cause soil movement. Force of cohesion between erodible soil particles varies directly with moisture content.

Observations indicate that wind is seldom strong enough to overcome even the cohesive force of soil water at about 15 bar tension (Table 4.6). In the table, the equivalent moisture of 1.0 designates the 15 bar tension. The equivalent moisture is a ratio of the water content in question to water content at 15 bar tension.

Factor # 5. Field Length:

Erosive winds vary highly in direction and seldom follow field boundaries. Therefore, the amount of soil lost from a given field cannot be determined by the width or length of field alone. The distance across the field along the direc­tion of the prevailing wind must be known.

Fields with their broad sides at right angles to, and their narrow sides parallel with, the prevailing wind direction will have the minimum overall rate of erosion. Field orientation is of little conse­quence where erosive winds blow equally from all directions.

If a barrier is present on the windward side of the field, the distance it fully shelters from the wind must be subtracted from the total distance across the field along the prevailing direction when computing the soil loss.

Shelterbelts and barriers provide variable lengths of shelter, depending on their porosity and shape on wind speed. A distance equal to 10 times the height of the barrier is usually subtracted from the total distance across a field, when using the wind erosion equation to calculate the amount of soil loss.

Factor # 6. Vegetative Cover:

The most important basic cause of wind erosion is destruction of vegetation or vegetative residue on the land. Good vegetative cover on the land is the most permanent and effective way to control wind erosion.

Living or dead, vegeta­tive matter protects the soil surface from wind action by reducing wind speed and by preventing much of the direct wind force from reaching erodible soil particles. It also reduces rates of erosion by trapping soil particles, which, in turn, prevents the normal avalanching of soil material downwind.

Protection depends on the quantity, size and orientation of the residue in relation to prevailing wind direction (Table 4.7). The finer and denser the residue, the more it slows the wind and the more it reduces wind erosion. For example, wheat stubble is more effective than sorghum or corn stubble. The higher the residue stands above the ground, the more it slows the wind speed and lowers the rate of erosion.

Effects of kind, quality, and orientation of vegetative cover can be expressed as a single factor. Soil loss by wind erosion varies inversely as the 0.8 power of the weight of surface residue. The threshold velocity for un-decomposed crop residues and weeds, even when these are near­ly scattered on the surface of the ground, is higher than for most of the erodible soil gains.

In Chandan, 50 km east of Jaisalmer, the removal of grass cover over an area of 2007 m² resulted in soil depletion of 1065 m³ within 3 years. There was a continuous removal of sand from a bare sandy plain, while both removal and deposition of sand occurred from grass cover and bajra stubble cover fields.

Cumulative sand removal from bare sandy plain was 9.66 cm, while it was only 0.17 cm from bajra stubble cover and almost no removal but deposition of 0.09 cm occurred on grass cover during the 75 day period from April to end of June, 1977.

Factor # 7. Organic Matter:

High organic matter content of soil is conducive to high fertility and good tilth but facilitates erosion by wind. Observations showed that wheat straw in the process of decomposition increased soil cloddiness and decreased erodibility by wind but the trends were reversed after the straw decomposed.

Numerous cementing substances responsible for binding soil particles together are produced in the initial stages by soil micro-organisms as they attack the vegeta­tive matter.

Broadly, the cementing substances may be:

(a) Lyophilic and lyophobic colloids consisting of decomposition products of plant residues;

(b) The micro-organisms themselves and their secretory products such as mucus, slime or gum; and

(c) Polysaccharides synthesised by some micro­organisms.

The aggregating effects of these initial cementing materials decline either due to loss in their sticky property or destruction and replacement by secondary materials which by wetting and drying and by freezing and thawing of soil finally break up disintegrating the coarse primary and secondary ag­gregates to a granulated condition.

The secondary cements are more brittle than the initial products and, therefore, form better granules which are suscep­tible to wind movement. Similar to decomposed organic matter, lime also decreases soil cloddiness and mechanical stability of clods and increases the erodibility by wind.

Vegetative matter spread on the surface is more effective in aggregation than when incorporated in soil because the former decomposes less rapidly and, therefore, continues to replenish the cementing products for much longer periods.

The products concentrate in and around the water stable aggregates. These cementing products are not entirely water soluble and, therefore, in­crease the size of the aggregates or clods large enough to resist wind erosion.

Factor # 8. Barriers:

Wind barriers such as shelterbelts hedges etc., reduce wind velocity near the ground surface for some distance downwind. Although, highly effective for only relatively short distances, they are most advantageous for localised control around farmsteads, fields, etc. Periodic strips of tall vegetation are also effec­tive, as are inter-planting of stable vegetation in young orchards.

Windbreaks can be used to control wind erosion where rainfall is enough to support trees (Fig. 4.8). Planting trees as windbreaks in humid areas where wind erosion constitutes a problem is relatively simple as moisture is not a limit­ing factor. In dry land areas trees should be planted at low lying areas where water tends to accumulate during rains.

The principal value of trees in wind erosion control is in the broader protection, they give to cultivated fields, which may start to blow as a result of exposure to dust, swept from bare land surfaces. Windbreaks, scattered over a wide area, help to break up the surface wind cur­rents, which otherwise would sweep the ground without obstruction.

The ef­fectiveness of windbreak in lowering the surface velocity of wind depends upon the shape, height, length and density of the barrier and the velocity of the wind. The windbreak should be planted perpendicular to the direction of the prevail­ing wind.

Factor # 9. Topography:

Level land is generally more liable to wind erosion than rolling land, because the wind encounters less resistance. Nevertheless, some of the greatest erosion hazards exist on knolls, ridges and in the lee of pockets, because the wind pres­ses against such areas instead of flowing parallel to the surface.

Small depres­sions catch saltating particles, thus protecting soil from wind erosion. Hummocks formed by deposits of saltating particles erode easily, as they stick out from the general ground level and are, therefore, exposed to stronger winds.

Factor # 10. Soil:

Sands erode easily because they contain a great proportion of particles of sal­tation size, but little binding material. High sand percentage are also not con­ducive to clod formation and generally undergo high erodibility. Soils, high in clay and silt, are rather stable because they are coherent (clay acting as binding agent) and easily form a protective crust at the surface.

Clods formed by clay and sand are harder and less subject to abrasion by wind borne sand than those formed by silt and sand. Although silt fraction encourages greater clod forma­tion, but the clods are softer and more readily abraded than those formed from clay and sand. A soil containing 20 to 30% clay, 40 to 50% silt and 20 to 40% sand produced a greater proportion of non-erodible and mechanically stable clods.

Following relationship has been observed between the soil erodibility index I_w and the amount of clay:

I_w = a G^b C^G …(4.10)

where, G = per cent of clay in soil g/g, and a, b, C are constants.

Clayey soils are highly variable with respect to wind erosion. Those contain­ing high proportion of fine water dispersible particles tend to puddle and resist erosion by wind relative to those containing less water dispersible particles. The latter remain as fine granules which are subjected to rapid erosion by wind.

Mucks erode readily because of their low density. Soils that contain ag­gregates of erodible size erode easily because aggregates have a low bulk den­sity and, therefore, are lifted by winds of fairly low velocity. Coarsely granulated soils erode mostly in saltation, and finely pulverized soils in saltation and suspension.

Soils containing larger aggregates and those with surface crusts are rather resistant to erosion, as are soils composed exclusively of fine dust, so long as no particles in saltation impinge upon them.

Deep loamy sands in areas of 45-50 cm of rainfall offer fair possibilities of maintaining fertility and stability against wind erosion when green manure crops are used in rotation. Medium textured soils do not erode easily as they do not drift readily unless improperly managed, but sandy soils and some clays are difficult to manage.s