Soil Erosion: The Basics

Soil Erosion is the detachment and transport of soil particles by water or wind.

The basic principles of erosion control are:

·         Protect the soil from raindrops, which break soils into small, easily transported particles,

·         Minimise the velocity of overland flow; and

·         Minimise the concentration of overland flow.

Erosion risk depends on:

·         Rainfall erosivity - a function of the amount of rainfall and its intensity. Rainfall erosivity in Australia is highest in northern coastal regions.

·         Slope – Greater slope increases the velocity of runoff and its ability to scour and move detached soil. The relationship between slope gradient and erosion is stronger under heavy tropical rainfall compared to temperate regions.

·         Slope length – Indicates catchment area above the site. The risk of erosion increases with slope length due to increased risk of flow concentration and velocity.

·         Soil erodibility (susceptibility to erosion) - a function of its structural stability.

o   Silty and/or fine sand surfaces are generally the most erodible because particles are both easily detached by raindrop impact and easily moved (coarse sand is less mobile and clay is harder to detach).

o   Increased soil organic matter reduces erodibility. Most NT soils have low organic content so are considered erodible.

o   The soil profile also influences erodibility. Soils with a layer of low permeability near the surface tend to be more erodible.

o   The more erodible soils are often characterised by gullies with vertical sides as deep or deeper than their width.

·         Groundcover – increased groundcover: reduces soil particle detachment by raindrops; reduces the velocity of overland flow and its ability to transport sediment; improves soil structure and infiltration; and reduces runoff volumes and the degree of concentrated flow. Increased groundcover also increases the capture of solar energy and productivity.