In this article we will discuss about:- 1. Introduction to Wind Erosion 2. Factors Influencing Wind Erosion 3. Mechanics of Wind Erosion 4. Estimating Soil Losses by Wind.
Introduction to Wind Erosion:
Wind erosion occurs when winds blow across cultivated bare fields, especially if the soil is loose, dry and finely divided. Wind erosion results in loss of soil fertility, formation of sand dunes and extension of deserts.
In addition, wind erosion causes high dust concentrations in atmosphere resulting in environmental health hazards. The effects of wind erosion in general are as significant as erosion by water.
Even though wind erosion is predominant mainly in the arid and semi-arid regions, it occurs even in humid areas during the dry parts of the year and could be as serious as in arid and semi-arid regions.
Some of the conditions favouring wind erosion are –
(1) A dry period,
(2) Absence of a protective cover on the ground, and
(3) A broad, flat or undulating topography over which wind can move unchecked.
The various indicators of wind erosion in a region are –
(1) Changes in soil texture resulting from deflation and winnowing,
(2) Removal of top soil,
(3) Scalding of the soil surface,
(4) Exposure of root system of trees and shrubs, and
(5) Sand deposits.
Climate, soil and vegetation are the major factors affecting wind erosion at any particular location. The climatic factors that influence wind erosion are the characteristics of the wind itself in addition to the precipitation, humidity and temperature. The climatic factors influence the soil moisture status which in turn influences the susceptibility of the soil for erosion by wind.
Textures, structure, density of particles, organic matter content are the soil characteristics that influence erosion by wind. Soil moisture content and surface roughness are the other factors which have an important bearing on wind erosion. Soil moisture content is important as relatively dry soil is subject to wind erosion. Surface crusts when formed have a retarding influence on wind erosion.
Vegetation influences wind erosion directly when the area is under vegetation or indirectly by protecting the adjoining areas. Type of vegetation, its height, density and seasonal distribution are the factors influencing wind erosion.
The overall occurrence of wind erosion could be described in three distinct phases.
(1) Initiation of movement,
(2) Transportation, and
The movement of the soil particles is of three distinct types and occurs depending upon the size of the soil particles.
The three types of movement are –
(2) Saltation, and
(3) Surface creep.
Very fine particles, mainly less than 0.1 mm in diameter is moved through suspension. Dust storms are an example of soil movement through suspension. The turbulence of the air keeps these particles in suspension.
The amount of soil transported through this process could be considerable. Saltation is the movement of soil particles by a series of low bounces over the surface. It occurs among the middle size of the particles, light enough to be lifted off the surface, but too large to go into suspension.
Among the three types of movements mentioned above, saltation is responsible for transporting the maximum portion of the soil along the surface of the ground by rolling and pushing by the force of the wind. The dividing lines are not precise but give the general limits.
The initiation of the movement of the soil particles is caused by several factors acting separately or in combination. In the course of collision of grains rolling and bumping on the surface, some particles may be bounced up.
Wind effects, such as an increase in the velocity reduces the pressure resulting in a net upward thrust on the particles. Moving winds which are turbulent in nature (except at low velocities of less than 3 kmh–1) imparts vibration energy to the soil particles.
After the initiation of the movement of the soil particles, they are transported across the land surface. The quantity of soil moved is influenced by particles, size, gradation of particles, wind velocity patterns and the distance on the land surface.
An empirical formula mentioned by Hudson (1971) for estimating the quantity of soil moved is –
Deposition of the particles occurs when the gravitational force is greater than the forces holding the particles in the air. Deposition could occur when the wind velocity is decreased due to surface obstructions or other natural causes.
Velocity patterns of the wind near the ground directly influence the movement of soil by wind. A wind strong enough to produce soil movement is turbulent i.e., its flow is characterized by eddies moving at variable velocities and in all directions.
The average forward velocity near the ground increases with height according to an exponential law. Zero velocity is somewhere above the average roughness elements of the surface. The taller the roughness elements of the ground, or the taller and less air-permeable the vegetative cover, the higher level at which zero velocity is found.
When temperature is nearly constant with height, actual observations of wind velocity with height shows that-
Where ux is known as the friction velocity and k is a constant of proportionality, approximately equal to 0.4 (known as Von Karman’s constant). The friction velocity ux is a characteristic velocity in a turbulent boundary layer and is determined using fluid mechanics principles. However, the slope of the wind profile is given by k/ux and z0 is the intercept on the z axis (where u = 0).
Where, d is called the zero plane displacement, since the wind speed goes to zero at the height z0 + d, rather than at z0 in Eq. 24.4. Sellars (1965) reports the values of z0 in Eq. 24.4 as given in Table 24.1.
Using the friction velocity, Chepil (1963) gave the equation for the surface drag as –
Where, T0 is the surface drag or shearing stress, ρ is the density of air (0.0012 g/cm3) and a is a drag coefficient depending upon the nature of surface. If ux is expressed in cm/sec then T0 is dynes/cm2. For rough vegetation covered surfaces, Chepil (1963) mentions that the mean drag T0, for a given natural wind varies significantly with surface roughness.
The rougher the surface greater is the drag. In his study Chepil (1963) reported that over a sorghum stubble 50 cm high the average drag was 8.7 dynes/cm2 whereas over wheat 22.5 cm high it was 5.4 dynes/cm2. It should not however be concluded that a rough surface is more erodible than a smooth surface. This might be true if the rough surface consists of only erodible fractions.
In order to predict the amount of wind erosion that is likely to occur under a given set of field conditions, attempts have been made to develop a numerical equation combining the factors which influence wind erosion. However, the evaluation of the constants in the equation for wind erosion is comparatively difficult than the universal soil loss equation.
The equation is of the form-
E = IRKFCWDB … (24.6)
Where, E = soil loss by wind erosion,
I = soil cloddiness factor representing the ratio of non-erodible soil aggregates,
R = surface cover factor,
K = surface roughness factor,
F = soil textural class factor,
C = factor representing local wind conditions,
W = field width factor,
D = wind direction factor, and
B = wind barrier factor.
The wind erosion model used in the USA is as follows –
E = f (I, K, C, L, V) … (24.7)
Where, E = estimated average annual soil loss,
I = soil erodibility index,
K = ridge roughness factor,
C = climate factor,
L = unsheltered length of eroding field, and
V = vegetative cover factor.
The model is not a simple product of the parameters involved, but interrelationships of the parameters need to be taken into consideration. The details of the parameters in the above model are given in Schwab et al. (1993).
Several experimental studies are needed to evaluate the factors involved in any wind erosion equation. Studies using wind tunnels of both laboratory and portable field types are useful for evaluating the variables involved in wind erosion. A wind tunnel is useful in studying the wind erosion phenomena and also establishes the required relations for use in the equation for estimating soil losses by wind.
Michael and Roy (1966) describe the design, fabrication and testing of a wind tunnel for wind erosion studies.
The wind tunnel setup described by them essentially consists of –
(1) Blower unit,
(2) Transition section,
(3) Air stream lining device,
(4) Wind tunnel section, and
(5) Devices for measurement of air flow.
The blower unit provides the wind at the desired velocity.
The transition section and air stream lining section, which consists of a honeycomb structure passages directs the air stream in straight path over the wind tunnel section. The wind tunnel section consists of detachable trays at the bottom and some glass windows on the sides for observation purposes.
The required types of surfaces are simulated in the trays. The wind velocity distribution in the tunnel can be measured using a Pitot tube arrangement. The wind tunnels are calibrated to establish the wind patterns in the, test section.
Studies using wind tunnels indicate that the surface conditions that promote or retard erosion in a tunnel act similarly in large fields. Wind tunnel is usually shorter than a cultivated field.
Because of this, the rate of soil flow in a tunnel is rapid at first but quickly diminishes. All the erodible material on the surface is blown off quickly. Because of these inherent differences suitable factors need to be developed for comparing wind erosion effects in a wind tunnel and the actual field subjected to wind erosion.
A wind tunnel study by Arika et al. (1986) showed that clods to be more effective than ridges for wind erosion control.
As compared to measurement of water erosion, measurements of wind erosion are relatively few. de Ploey and Gabriels (1980) mention a ground trap for surface collection of wind-blown material and a psammograph for recording the chronology of sand storms. It was also mentioned that satellite imagery are being used for studying sand transport and dune formation in desert areas.