7.2 Water & Wind Erosion Mechanics, RUSLE & Conservation Practices

Soil Erosion Mechanics and Control

Soil erosion is the process of detachment, transport, and deposition of soil particles by water or wind. It is one of the most significant threats to agricultural sustainability, reducing soil productivity by stripping away the nutrient-rich topsoil and degrading water quality through sedimentation and nutrient runoff.

Mechanics of Water Erosion

Water erosion is initiated by the impact of raindrops on bare soil. The kinetic energy of falling raindrops shatters soil aggregates, detaching individual particles. This process, known as splash erosion, also seals the soil surface, reducing infiltration and increasing runoff.

Once runoff begins, it transports the detached particles downslope. Water erosion takes several forms:

• Sheet Erosion: The uniform removal of a thin layer of soil from the surface. It is often imperceptible but accounts for massive total soil loss over time.
• Rill Erosion: As runoff concentrates, it forms small, well-defined channels called rills. Rills can typically be smoothed over by standard tillage equipment.
• Gully Erosion: When rills merge and deepen, they form gullies. Gullies are large channels that cannot be crossed by farm machinery and represent severe, concentrated soil loss.

The Revised Universal Soil Loss Equation (RUSLE)

To predict average annual soil loss from sheet and rill erosion and guide conservation planning, agronomists use RUSLE. The equation is:

A = R × K × LS × C × P

Where:

• A (Average Annual Soil Loss): Estimated in tons per acre per year. This value is compared to the "T value" (Tolerable soil loss), which is the maximum rate of erosion that can occur without reducing long-term productivity (typically 1-5 tons/acre/year).
• R (Rainfall-Runoff Erosivity Factor): Quantifies the erosive force of specific rainfall events in a given geographic region. Areas with frequent, intense storms have higher R factors.
• K (Soil Erodibility Factor): Represents the inherent susceptibility of a specific soil to erosion based on its texture, organic matter, structure, and permeability. Silt soils generally have high K factors because they lack the cohesiveness of clay and the weight of sand.
• LS (Length and Steepness of Slope Factor): Topography significantly impacts erosion. Longer and steeper slopes increase runoff velocity and erosive power.
• C (Cover-Management Factor): This is the most crucial factor influenced by the farmer. It compares the soil loss from a specific cropping and management system to the loss from a bare, tilled field. Practices like no-till, cover crops, and high-residue crops drastically lower the C factor.
• P (Support Practice Factor): Accounts for physical structures or specific field operations like contour farming, strip cropping, or terracing that slow runoff and induce deposition.

Wind Erosion and the RWEQ

Wind erosion is prominent in arid and semi-arid regions with flat terrain, sparse vegetation, and loose, dry soils. It occurs in three modes:

• Saltation: The bouncing movement of medium-sized particles across the surface. This is the primary mechanism and initiates the other two.
• Suspension: Very fine particles (silt and clay) are lifted high into the air by saltating particles and can be carried for hundreds of miles.
• Surface Creep: Larger, heavier particles roll along the ground, pushed by wind and saltating impacts.

Wind erosion is modeled using the Revised Wind Erosion Equation (RWEQ), which incorporates factors like weather (wind speed, precipitation), soil erodibility, soil roughness, crusting, and vegetative cover. Unlike water erosion, which moves downhill, wind erosion can move soil in any direction and is highly dependent on wind barriers and surface roughness.

Conservation Practices

Effective erosion control relies on minimizing detachment and transport by keeping the soil covered and slowing down fluid flow.

Agronomic Practices:

• Conservation Tillage / No-Till: Leaves crop residue on the surface, absorbing raindrop impact and slowing runoff or wind speed at the soil surface.
• Cover Crops: Provide living cover during vulnerable periods between cash crops, anchoring soil with roots and protecting the surface with foliage.
• Crop Rotation: Alternating high-residue crops (like corn or wheat) with low-residue crops (like soybeans) helps maintain soil structure and cover.

Structural Practices:

• Terraces: Earth embankments constructed across slopes to intercept runoff, effectively breaking a long slope (L factor) into several shorter ones.
• Grassed Waterways: Broad, shallow, vegetated channels designed to safely carry concentrated runoff from a field without causing gully erosion.
• Windbreaks/Shelterbelts: Rows of trees or shrubs planted perpendicular to prevailing winds to reduce wind velocity across fields.