Wind Erosion

General Definition: Wind erosion is the process of soil particles being removed from the surface of the earth and transported by wind, especially in dry, loose, and bare areas where vegetation cover is minimal.

Soil Erosion by Wind (Agricultural Definition): Wind erosion refers to the detachment and transport of soil particles, particularly fine particles such as sand, silt, and clay, by wind forces, which leads to the degradation of soil, reduction in fertility, and damage to crops.

US Department of Agriculture (USDA) Definition: Wind erosion is the process of soil loss from the land surface due to the action of wind, which removes the most fertile topsoil and deposits it elsewhere, negatively impacting agriculture and the environment.

Environmental Impact Definition: Wind erosion is the removal of the topsoil from the surface due to wind forces, often leading to the degradation of soil quality, desertification, and the reduction of agricultural productivity, especially in arid and semi-arid regions.

Mechanics of wind erosion

1. Initiation of Movement:

This is the first phase of wind erosion, where soil particles are detached from the surface. The movement is initiated by the wind’s force acting on the soil.

• Wind Force: The wind exerts a shear force on the surface of the soil. When the wind velocity exceeds the threshold velocity (the minimum speed required to move the particles), soil particles are lifted off the surface.
• Threshold Velocity: This is the key parameter in determining when soil particles will start to move. The threshold velocity varies with soil type and surface conditions, but typically, for fine, erodible particles (0.1 mm), the threshold wind speed is around 16 km/h at a height of 30 cm from the ground. The finer and looser the soil particles, the lower the wind speed required to initiate movement.
• Impact and Cutting Action: Wind can also lift soil particles through impact and cutting action. The wind creates turbulence, which collides with and detaches soil particles from the ground. Once the particles are detached, they are available for transportation.

2. Transportation of Soil Particles:

Once the soil particles are detached, they are transported by the wind. There are three primary mechanisms for this transportation:

a) Suspension:

• Process: Fine soil particles, typically smaller than 0.1 mm in diameter (e.g., silt and clay), are lifted high into the air. The wind carries them away for long distances, sometimes hundreds of kilometers.
• Distance & Height: Particles suspended in the atmosphere can travel great distances and remain airborne for extended periods. These particles often contribute to dust storms and visibility problems in affected areas.
• Effect: Suspension is the most visible and dramatic form of wind erosion, as it results in visible dust storms. These particles can be deposited in new areas, affecting soil and agricultural productivity far from the original site of erosion.

b) Saltation:

• Process: This is the most common form of soil transport in wind erosion. It occurs when medium-sized soil particles (0.1 to 0.5 mm in diameter) are lifted into the air by wind and travel in a hopping or bouncing motion near the soil surface.
• Mechanism: Saltation occurs when the wind’s force is strong enough to lift the particles off the ground but not enough to keep them suspended in the air. These particles rise briefly before falling back to the surface, only to be lifted again by the wind. During their flight, they can cause further erosion by colliding with other particles and knocking them loose.
• Contribution: Saltation is responsible for the majority of soil movement during wind erosion (approximately 50-75% of the total soil movement). These bouncing particles can travel several times their height in distance, and they often cause significant damage to the soil surface, breaking down aggregates and creating a rougher, less fertile soil surface.

c) Surface Creep:

• Process: Larger soil particles (typically 0.5 mm to 2 mm in diameter) are too heavy to be lifted into the air, so they roll or creep along the surface.
• Mechanism: Surface creep occurs when particles move by colliding with other particles that have been lifted by the wind. These larger particles are pushed across the soil surface in a rolling motion, gradually moving forward in the direction of the wind.
• Contribution: Surface creep accounts for around 5-25% of the total soil movement during wind erosion. Although the amount of material moved by surface creep is relatively small compared to saltation and suspension, it can still cause significant damage to soil structure and vegetation, especially in areas where wind velocity is high.

Interaction Between the Mechanisms:

These three mechanisms (suspension, saltation, and surface creep) interact with each other. For example:

• Saltation can cause surface creep by hitting larger particles and rolling them along the ground.
• Suspended particles can also fall back to the ground and contribute to saltation or surface creep.

Overall, the majority of the soil movement occurs within the first meter of the surface, where the wind velocity is strongest, and it diminishes rapidly with height.

3. Deposition:

The final phase of wind erosion is deposition, where the transported soil particles settle out of the air and accumulate in new areas. Deposition occurs when the wind velocity decreases, or when barriers (like vegetation, ditches, or artificial structures) reduce wind speed.

• Decrease in Wind Velocity: When the wind velocity falls below the threshold velocity needed to keep particles suspended, the particles will fall to the ground due to gravity. This can happen when the wind encounters physical barriers like vegetation, buildings, or even changes in terrain.
• Vegetation & Barriers: Vegetation, windbreaks, or artificial barriers increase surface roughness, which helps reduce the wind’s velocity at the surface. This causes the suspended particles to settle down, depositing them in new locations. Similarly, soil particles are deposited by rain or when dust clouds encounter moist conditions that cause particles to settle.
• Effect: Deposition can have both positive and negative effects. In some cases, deposition can improve soil fertility if the wind transports nutrient-rich soil from one area to another. However, in other cases, it may lead to the accumulation of fine particles (dust) on crops, roads, and structures, leading to smothering or clogging of surfaces.

Summary of Key Factors:

• Wind Velocity: Higher wind speeds are essential to initiate movement and maintain the transport of soil particles. The threshold wind speed is critical in determining when erosion will start.
• Soil Texture: The size of the particles determines which transportation mechanism will dominate. Fine particles are more likely to be suspended, medium particles move through saltation, and larger particles roll on the surface.
• Vegetative Cover: Vegetation provides surface roughness, slowing wind velocity and reducing soil loss.
• Soil Moisture & Structure: Moisture content affects the soil’s cohesion, which can reduce susceptibility to erosion, whereas dry, loose soil is more easily eroded.