Problem of Wind Induced Soil Erosion
Wind erosion has been active in shifting soil materials since prehistoric days. In the earlier days wind erosion has not just created problems but also helped in soil formation in many regions. The activity of man accelerated the wind erosion process and it became more destructive. Deforestation, faulty method of land use, overgrazing, burning etc. are the human activities that accelerated the process of wind erosion. Along the Missouri and Mississippi rivers of the USA huge shifting of soil due to wind erosion took place. In India, wind erosion is said to be responsible for creation of the vast desert area of Rajasthan.
Wind erosion is a most serious problem in arid and semi-arid regions of the world. The normal annual rainfall in these regions is very low (5 to 15 cm), soil is dry and vegetation is very limited. Contrary to the general belief, wind erosion also takes place in many humid areas. The sandy soils along the rivers, lake, and coastal plains and the organic soils are removed by wind erosion. The wind erosion in such cases is more harmful as the value of the land affected is higher. Wind erosion causes several damages. It not only removes the top fertile soil but also damages crops, buildings, highways, railways, fences etc. As the finer particles are easily transported, they are removed along with organic matter and nutrients. Finally coarse textured sand particles are left and they can be more easily detached. No vegetation grows on this and the water holding capacity of soil reduces. Thus the problem multiplies. If the particles carried by the wind strike the young seedlings, they get damaged. Maintenance of channels, railways and highways become costly. Sometimes, the fertile land merges with the desert and the whole village or town may be affected due to the ingress of desert.
16.2 Types of Wind Induced Soil Erosion
Wind induced soil erosion can be classified as per the following types of soil movement.
Types of Soil Movement
Wind erosion takes place with the help of three types of soil movement. They are: (i) suspension, (ii) saltation and (iii) surface creep. All these types of movements generally take place simultaneously. The phenomenon of wind erosion is most important near the surface and major portion of the soil movement takes place within a height of about 1 m.
Suspension: Fine dust particles with diameters less than 0.05 mm are submerged in the laminar zone of air flow and therefore, they cannot be moved by direct action of the wind. The movement of these particles is generally initiated by the impact of the particles in saltation [described a little later in this Section]. Thus without saltation, the movement of the fine dust particles cannot take place. Once lifted up in the air-stream, the particles move in suspension by the turbulence of the wind.
Soils made up of very fine particles specially with diameters less than 0.01 mm are very resistant to movement. Apart from remaining submerged below the turbulent zone of wind flow, cohesive and adhesive forces are much greater for fine particles. Specially cohesive force is high at high moisture content end when dried, the adhesive force helps them to bind together. Therefore, without saltation the movement of fine textured soil generally cannot take place. However, if some objects move over the dried surface then formation of dust particles takes place in fine textured soil and it becomes susceptible to erosion by direct action of wind.
Once the particles are lifted, their movement in suspension depends on the pattern of the wind movement. Generally they are lifted to great heights and carried to long distances. Thus they are carried away to far off distances from the place of the eroding area and are complete loss to the area. In contrast, soil moved in saltation and surface creep [also described later in this Section] gets deposited in the nearby area. Particles carried in suspension are deposited only when the wind velocity completely subsides or rainwater wets them.
Saltation: The direct action of the wind on the soil particles and their collision with other particles create somersaulting soil movement known as saltation. Major portion of the soil movement takes by saltation. The particles are pushed along the ground surface due to the wind velocity in the initial stage [Fig. 16.1]. The movement continues for some time and then descends almost in a straight line with an angle of descent in the range of 6 to 12° with the horizontal After they strike the ground, they may rebound and continue their movement by the saltation process. When the particles lose their energy by repeated striking, they may sink into the ground to form part of movement through surface creep.
Fig. 16.1. Movement of Soil Particles by Saltation.
The initial angle of ascent of the particles in saltation is vertical but the final velocity is in horizontal direction. The particles rise to different heights and then descend at accelerated speed as a result of gravity. The vertical distance through which a particle rises in saltation is about one fifth to one forth the horizontal length of movement in a single leap. Fine grains of diameters ranging from 0.1 to 0.5 mm are mainly moved by the saltation process. The fraction of soil particles that most easily move has diameters between 0.1 and 0.15 mm. Particles of different diameters generally move at different heights.
The movement of soil by wind is not only dependent on the force of the wind acting on the particles but also on the velocity distribution of the wind to the height of saltation. The height of movement is limited and therefore the wind velocity above certain height has no influence on the soil movement. Soil structure, surface residues, stability of structure, crusting, puddling, grading of the materials on the surface by raindrops etc., influence the soil movement.
Surface Creep: Coarser soil particles having a diameter range of 0.5 to 2 mm are too heavy and cannot be lifted up by wind action. Therefore, they can move neither by saltation nor by suspension. When the particles moving due to saltation strike them, they are pushed along the ground surface. This type of rolling or sliding of heavy particles along the ground surface is known as surface creep. Particles in saltation receive their impact energy from the direct action of the wind pressure, whereas, in surface creep the particles derive the kinetic energy from the impact of other particles moving in saltation.
Major portion of the soil erosion by wind takes place in saltation. It may vary
between 50 to 75 per cent of the total weight of the soil eroded depending upon the relative size of the particles, wind velocity etc. Suspension may erode between 3 and 40 per cent; whereas, the percentage for surface creep may be between 5 to 25. Also it may be noted that suspension and surface creep are mainly initiated by saltation. Therefore, if it is possible to prevent soil movement by saltation, the other two will automatically be controlled.
16.3 Measurement for Wind Induced Soil Erosion Control
Any practice or measure that reduces the wind velocity or improves the soil characteristics is helpful to control wind erosion. Improved soil characteristics should have better structure, improved cohesive property and good moisture holding capacity. Some of the measures may provide both the requirements. Vegetation improves the soil structure and at the same time retards the surface wind velocity. In general the following practices may be adopted to control the wind erosion:
The soil should be covered with vegetation or crop residues as far as possible.
Limited cultivation should be done.
Dry soils should not be tilled.
Permanent vegetation may be established on unproductive soils.
After the rains, the soil may be tilled so that clod formation takes place.
Tillage implements should be selected in such a manner that rough surface is formed and crop residue is not buried.
Overgrazing should be avoided.
Principal methods of reducing surface wind velocity are vegetative control, tillage practices and mechanical methods. Vegetative control consists of cultivated crops, field and strip cropping, stubble mulching. Shrubs and trees although form part of the vegetation act as mechanical barrier to wind. Other mechanical barriers or windbreaks also may he used.
Among the cultivated crops, close growing crops provide better protection when compared to the row of crops. Their effectiveness depends upon (i) type of crop grown, (ii) stage of growth, (iii) density of cover, (iv) row direction, (v) climatic condition etc. Vegetation also helps to deposit the soil that is eroded from the neighboring areas. Specially, during the dry months when the soil is most susceptible to erosion, the field should be covered with vegetation.
Row crops such as maize, cotton, jowar, bajra etc., provide only partial protection. Seeding should be done in a way to provide crop rows normal to the direction of general wind direction. Crop rotation should be suitably decided to improve the soil structure and conserve the moisture. Crops suitable for such soil and climatic conditions and also capable of providing protection should be selected.
16.3.1 Stubble Mulching
When row crops like maize, bajra etc. are grown at the time of harvesting the stubble –i.e., lower portion of the stem, should be left to a certain height and the whole crop should not be harvested from the bottom. At least 10% of the rows shou1d be left standing. In case the crops are used for pasturing, the stock should be removed leaving enough stalks along with leaves to provide the necessary protection. Thus stubble mulching is the practice of maintaining crop residues at the ground surface during harvesting to resist the soil erosion. The benefits derived are:
(i) Wind velocity is retarded.
(ii) Soil blowing is physically obstructed.
(iii) Raindrops lose their energy before striking the soil.
(iv) Better absorption of rainfall takes place due to longer retention period and permeable soil structure.
(v) Evaporation loss is reduced.
(vi) Crop yield increases.
(vii) By reducing wind velocity, they can trap eroding soil from neighbouring areas.
However, the benefits accrued from stubble mulching depend upon the size of the field, velocity and relative direction of wind, quality and quantity of stubble mulching left in the field Narrow fields separated by windbreaks will be more easily protected compared to a large open field. If the crop residues can be left in vertical position, better protection can be provided. Most erodible soil may require about 10 tonnes of stubble per hectare for protection and this much crop residue may not be available from one hectare of land. During the period of fallowing, stubble mulching is most effective to provide a cover to the soil.
16.3.2 Field Strip Cropping and Contour Strip Cropping
Field and contour strip cropping consists of alternate strips of row (i.e., erosion-susceptible) crops and close growing (i.e., erosion-resistant) crops in the same field. The strip cropping is laid out generally parallel to the field boundary or perpendicular to the erosive wind direction. The main benefits of strip cropping are:
(i) Vegetation provides physical protection against blowing of soil.
(ii) Soil erosion is limited to a distance equal to the width of the erosion susceptible crop.
(iii) Better conservation of moisture takes place.
(iv) Particles carried in saltation are trapped.
Main problems in strip cropping are:
(i) In a mechanized farm, movement of machinery becomes difficult due to narrow strips.
(ii) In case of attack by insects, there is more number of edges for protection.
The width of the strips should be selected in a way such that the farming operation is not hampered and at the same time much erosion does not take place. For example, in a sandy soil the width of the erosion susceptible crop should be limited to 6 m. But for movement of farm machinery, the width may have to be increased. In a sandy loam soil the width can be increased up to 30 m. Among the erosion resistant crops groundnut, legumes, grasses, berseem etc., that cover the ground are preferred. Row crops that permit erosion are maize, cotton, potato, bajra, jowar,.etc. Fig. 16.2 shows the field and contour strip cropping for protection of a field from wind erosion.
Fig. 16.2. Field and Contour Strip Cropping.
A windbreak is defined as any type of barrier for protection from winds and refers to any mechanical or vegetative barriers consisting of buildings, gardens, orchards and feed lots. Windbreaks made up of just mechanical barriers are not very useful for field crops. However, they are frequently used for the protection of farm sheds and small areas. The mechanical barriers include brush fences, board walls, vertical burlap or paper strips. Brush matting, rock or gravel barriers are also used as windbreaks. Some of these barriers are impermeable and others are semi-impermeable. Generally the semi-impermeable barrier are more useful as they provide better diffusion and eddying effects on the leeward side of the barrier. When vegetable crops in organic soils are required to be protected, vertical burlap or paper strips are often used. Brush matting, debris, rock, gravel etc., are more useful for stabilizing sand dune areas.
Studies of wind tunnel on the flow pattern of wind over model barriers and windbreaks indicate that the sharper barrier provides better protection compared to other shapes. The zone of influence of a rounded shape is much less than the narrow vertical shapes. The porosity of the barrier helps to extend its zone of influence downwards but may decrease the degree of protection. The wind velocity at the ground is much lower than the standard open velocity; their ratio is of the order of 0.07. Even the standard velocity may be about twice as higher compared to the surface velocity over mowed grasses. Thus the frictional drag on even vegetation reduces the wind velocity. The pull of free moving winds that pass the ends of the windbreak, can act on the sides of the stilled air mass. Thus the protection provided by the windbreak is not of rectangular shape but tends to be narrowed towards the outer limit. In addition to providing protection to the soil from wind, windbreaks have other commercial values. The tree bunches and leaves may be used as fodder and fuel.
A shelterbelt, usually consisting of shrubs and trees is a longer barrier than a windbreak. It is primarily used for protection of field crops, soils and conservation of soil moisture. The shelterbelt is not only useful for wind erosion control, but also saves fuel like windbreak, increases livestock production, reduces evaporation, prevents firing of crops from hot winds. In addition, it may provide better fruiting in orchards, make spraying of trees for insect control more effective.
To achieve better result in controlling the wind velocity, shelterbelts should be moderately dense from ground level to tree tops. A study on the distribution of wind velocity around the shelterbelt has shown that the wind velocity reduces significantly on the leeward side of the shelterbelt immediately after the barrier and at the central portion. At a distance of 15 to 20 times the height of shelterbelt, the wind velocity is almost equal to the velocity in the open. The wind velocity at the two ends of the barrier may be about 20 per cent greater than the velocity in the open. Therefore, long shelterbelts always provide better protection than a short one and no opening should be provided in a continuous long shelterbelt. An opening shortens the length of the belt and near it the velocity as usual becomes higher than the normal velocity. In case, it is essential to provide a road through the shelterbelt, it should be made curved. Another important point to be remembered for establishment of a shelterbelt is that it should be made as far as possible perpendicular to the direction of the most erosive wind.
Woodruff and Zingg (1952) conducted wind tunnel studies for estimation of the distance of full protection from a windbreak or shelterbelt and gave the following formula.
where d = distance of full protection, m
h = height of the barrier, m
Vm = actual wind velocity at 15 m height, m/s, and
q = the angle of deviation of prevailing wind direction from the perpendicular to the barrier.
From the wind erodibility of farm fields, Chepil (1959) concluded that the velocity (Vm) at 15 m height required to move the most erodible soil fraction was about 9.6 m/s. This is valid for a smooth bare surface after the initiation of erosion and before formation of surface crust by rainfall. In fact, Equation 16.1 is valid for wind velocities up to 65 km/h. While deciding the width of crop strips, the same equation may be used by substituting crop height as the height of barrier.
A shelterbelt will be more effective if a combination of low, medium and, tall trees is used as shown in Fig. 16.3. This helps to provide a compact and dense barrier. Generally shrubs of low height should be grown on the windward side. Tree species of low branches may be placed at the middle and tall trees with high branches on the leeward side. But such a multiple row shelterbelt occupies large land area. Suitable varieties of trees should be selected for the specific location. For the desert areas of Rajasthan trees like Neem (Azadirachta Indica), Anacardium Occidentale, shrubs like Sisal (Agave Americana) etc. are commonly used.
Fig. 16.3. Side View of Tree Arrangement in Shelterbelts.
16.3.5 Other Tillage Practices
Other tillage practices -if properly adopted, can reduce the wind induced soil blowing to a great extent. Similarly, faulty tillage operations increase the soil erosion by wind. If the soil is pulverized and the crop residues are buried due to tillage operations, erosion problem increases. The effective way of prevention of wind erosion is by producing a rough, cloddy surface and exposing the crop residues on the surface. If the land is ploughed at optimum moisture content after the rains, big clod and large aggregate formation takes place.
If by tillage practices, small ridges perpendicular to the direction of the wind can be formed then significant control is possible. In case the surface soil consists of mainly sandy soil underlain by fine textured clayey soil, tillage may give some immediate benefit. The sand being more erodible should be buried and resistant clayey soil be brought to the surface. Efforts should be made to grow vegetation at the earliest. Otherwise this may not be effective for a long period. The clay may also provide cloddy structure on the surface. Generally, these types of tillage operations are Very costly and should be taken up only when other better alternatives are not immediately available. If vegetation is grown, the organic matter produced in the soil by the vegetal cover can serve the same purpose in addition to providing other benefits.
As discussed earlier, stubble mulching provides a good control for wind erosion. This is specially important in a year of crop failure or when sufficient vegetative cover cannot be produced. Sweep furrow openers which can cut under the material, leaving it in almost standing position are very effective implements. One-way disc plough also leaves the crop residues in partially standing position. Mould board plough turns the soil and buries the crop residues and is not therefore, suitable for this purpose. Again when it is required to produce a rough cloddy surface, mould board plough is suitable under an optimum moisture condition. Vertical disc plough or harrows are suitable neither for retaining crop residues nor for creating cloddiness.
Other important implements used for ridging and clodding are the lister plough, shovel or sweep cultivator, deep-furrow drill, spring-tooth harrow etc. Mould board plough, subsoiler, lister, disc plough and grading machines can bring the subsoil to the surface. When straight or V-shaped blades or rods are used as subsurface tillage implements, they can undercut without disturbing the surface or the residues. Obviously, the clods cannot be formed on the surface by these tillage implements.
It should however, he remembered that tillage practices offer only temporary and urgent controls and may have to be repeated. They are quite costly and should not be used as a general practice. They cannot act as substitutes for the vegetative covers which provide long term and multiple benefits. Therefore, other tillage practices should be used only as emergency measures when no other method is immediately effective.
Keywords: Wind induced soil erosion, Stubble mulching, Field strip cropping, Contour strip cropping, Windbreaks, Shelterbelts.