Proper land use and moisture conservation practices help in the conservation of wind erosion in agricultural lands. In areas which are subjected to severe wind erosion, especially in the arid and semi- arid areas additional protective measures in the form of wind breaks and shelter belts are needed.
The wind erosion control measures may broadly be divided into two categories.
There are –
(1) Measures to reduce surface wind velocities, and
(2) Measures to improve soil characteristics.
Some vegetative methods may achieve both the objectives as vegetation reduces surface wind velocities and at the same time improves soil aggregation.
Reduction of Surface Wind Velocity:
There are three principal methods by which surface wind velocities can be reduced.
These are –
(1) Vegetative measures,
(2) Tillage practices, and
(3) Structural or mechanical methods.
Vegetative measures retard wind velocities near the surface and generally the most effective means of controlling wind erosion. Vegetative measures could again be temporary or permanent. The temporary measures are essentially crop management practices to provide a cover to the soil.
Close growing crops provide a good protection. The root system of harvested crops holds the soil in place, while the stalks or stubbles left in the field break the force of wind currents.
Permanent vegetative measures consist of planting trees, shrubs and grasses for protecting the lands from wind erosion. Stabilization of sand dunes can be accomplished by establishing grasses and subsequently shrubs and trees.
Wind breaks and shelter belts are permanent vegetative measures for controlling wind erosion. The term wind break is associated with any type of barrier, either mechanical or vegetative, for protecting from winds areas like buildings, orchards or farmsteads. A shelter belt is a longer barrier than the wind break and consists of shrubs and trees. They exert a wider influence than wind breaks.
In addition to helping in controlling wind erosion, vegetative wind breaks and shelter belts provide other advantages. They provide fuel, reduce evaporation, protect orchards from hot and cold winds and make sprayings or trees for insect control more effective. They however have their disadvantages too.
They may affect crop yields in the adjoining fields by their shade, root competition and also by harbouring bird population. The closely planted shelter belts create still-air conditions which increases the susceptibility to fungus diseases in some fruits.
The width of the shelter belt may vary from one to five rows or even more. In American practice, five rows are adopted with the tallest trees in the middle. Long breaks are advocated as the value of the protection increases with the square of the length until the latter is about 24 times the trees’ height. Table 24.2 shows the relation between length of shelter belt and area protected. Fig. 24.9 shows the velocity distribution near a wind break.
The wind breaks or shelter belts should be located at right angles to the direction of the wind against which they are designed to protect the area. If the prevailing winds are from more than one direction, secondary breaks may be put at right angles to these directions. Spacing between successive belts depends upon the area protected by individual belts. It will be desirable to locate the shelter belts successively such that the entire area is protected.
The species to be planted for shelter belts should be selected taking into consideration their adaptability to climate and soil, value as shelter belt, rate of growth, ease of establishment and possibility of serving the dual role of wood production and shelter belt.
Species should be selected to suit the local conditions and needs. Best results are obtained where mixed plantations consisting of grasses, shrubs and trees are raised. A typical shelter belt should have the shape of a conical roof in cross-section.
The tall trees in the center should be flanked by short trees, conifers and tall shrubs and grasses with low shrubs on the outside. Shelter belt plantations should be protected from cattle and wild fire. Casualties should be replaced regularly.
Some common vegetation species used in soil conservation and for shelter belts are as follows:
Assessment of the wind action system indicates the areas from where soil is eroded and areas where soil is deposited. It is generally effective to stabilize the areas of origin of shifting sands. Prevailing wind directions help to align the wind breaks or shelter belts.
The selected vegetation species should be adaptable to local rainfall and soil conditions. Wind barriers should not obstruct access to roads, water channels, pathways for livestock etc., and should not interfere with any local traditions.
The following factors need to be considered before establishing wind breaks or shelterbelts in a given area:
1. Assessment of the wind action system in the given area;
2. Assessment of the wind regime and the prevailing wind direction;
3. Assessment of the most exposed area of wind erosion;
4. Selection of vegetation species suitable to the local environment; and
5. Analysis of local conditions.
Mechanical Methods:
Mechanical barriers consist of some structures like fences, walls, stone packings etc. which serve as wind breaks. These type of structures are used for protecting farmsteads and are not economical for protecting large cropped areas. Terraces and bunds have some effect in controlling wind erosion and may also be considered as mechanical barriers.
Tillage Practices:
Some tillage practices, though temporary, could be adopted for controlling wind erosion. These practices should be carried out before the start of the wind erosion. The practices in general used for moisture conservation are also helpful in controlling wind erosion.
These are –
(1) Strip cropping,
(2) Primary and secondary tillage, and
(3) Use of crop residues.
Primary and secondary tillage aims to develop a rough, cloddy surface to resist wind erosion. The land should be cultivated as soon as the rains and large clods after tillage are desirable.
A system of ridges and furrows, normal to the direction of the prevailing winds, reduce the wind velocities and helps in soil deposition. Crop residues left on the soil surface reduce wind velocities and trap eroded soil. Crop residues in combination with ridges and furrows are more effective in controlling wind erosion.
Controlling Soil Factors:
The principal ways of controlling wind erosion using soil factors are –
(1) Conservation of moisture to improve vegetative growth, and
(2) Conditioning of the surface soil to improve aggregation.
All the moisture conservation measures either biological or engineering measures have an indirect effect on reducing wind erosion. Among all these measures, mulching is specially used for controlling wind erosion. Crop residues are a common form of mulches and their use conserves moisture and help in reducing wind erosion.
The condition of the top soil influences wind erosion to a large extent. Wind erosion is considerably reduced if the top soil consists of large clods or non-erosive soil aggregates. Crop management practices like tillage, crop rotations and additions of organic matter should be developed to maintain good soil structure which in turn helps in the development of non-erosive soil aggregates.
Stabilization of Sand Dunes:
Formation and shifting of sand dunes are problems in areas subjected to wind erosion. Stabilization of sand dunes is desirable as movement of sand dunes could cause damage to productive agricultural lands, roads, water courses etc. Stabilization of sand dunes requires the development of permanent vegetative cover.
This may be achieved by one or a combination of the following:
1. Natural regeneration of the vegetation cover.
2. Artificial creation of a vegetation cover.
3. Chemical measures.
In order to accelerate natural regeneration or the vegetal cover, the degraded area is protected from the negative impacts and allowed to return to its natural state. However, this could be a slow process and will also depend largely on climatic and soil parameters.
Artificial creation of the vegetative cover involves seeding or planting selected drought-resistant grasses, shrubs or trees. In case of these plantations, watering in the first stages of the growth may be necessary for their establishment. In addition, protection of the seedlings with low fences or spiny branches and protection of the entire area from animal trampling and foraging is required.
Use of chemicals for sand dune stabilization has been found to be successful at many places especially in China.
The chemicals used are:
1. Asphalt emulsion;
2. Polyacrylamide (PAM);
3. Polyvinyl alcohol (PVA);
4. Polyvinyl acetate (PVAc);
5. Hydrolytical polyacrylonitrile (HPAN); and
6. Sodium silicate.
These chemicals are sprayed on the soil surface resulting the formation of a thin stable layer preventing the movement of soil particles.
In extremely dry regions, where planting of vegetation is difficult, chemicals are well suited for sand stabilization. Chemical methods can effectively be combined with vegetative methods for dune stabilization and controlling wind erosion.