Wind erosion is the removal of soil sediments by the wind. It can leave soil resources degraded and unable to support grasses and vegetation.
In Australia, wind erosion is a serious problem. Our country is the driest inhabited continent on Earth, and is built on an ancient landscape of fragile soils. Together with a 'boom and bust' climate of extremes, our soil resources are easily prone to suffering damage.
When the conditions are such that wind can blow dust, this process of wind erosion is actually a cost to our soil resources. The dust that you see is the fine, light type of soil particles that have dried, become loose and blown away. These fine particles are actually the most important sediments in terms of soil fertility; they contain vital nutrients such as iron, as well as organic matter.
Unfortunately, we can't control where the wind will carry the nutrient-rich dust - sometimes it ends up in the ocean, or other countries.
By minimising wind erosion, we are keeping the nutrient-rich soil particles in the ground and maintaining the best possible soil resources for Australia.
Soil, the source of dust:
Soils are formed by the weathering of rocks, and are also influenced by climate, organisms, the sloping of the land and the effect of time. Soils are generally classified into groups, but the exact combination of the above factors makes each soil unique.
As the source of dust, knowing about soils is helpful in learning whereabouts dust comes from – transported dust matches the same characteristics as the dust component of the soil it came from. Scientists examine the different soils across Australia and use this information to work out whereabouts particular dust events originated.
Soil erodibility by wind can be measured by testing soil strength, crust cover, the amount of erodible material on the soil surface, aggregation levels, particle-size characteristics, soil chemistry, and by simulating wind erosion (using a wind tunnel measurement device).
Each soil type has a different susceptibility to wind erosion. Sandy soils common to the Mallee region or desert loam soils are highly vulnerable to wind erosion. This is attributed to their low aggregation, variable soil moisture and the tendency to disaggregate with disturbance. In contrast, the highly aggregated soils such as black earths and ferrosols have a low susceptibility.
Wind erosion and sediment loss:
As the name suggests, wind is the transport agent in wind erosion. The speed of the wind must surpass a threshold in order to move sediments (called the Friction Threshold Velocity). Large particles (>90µm) are usually the first particles to move, hopping along the soil surface. Each time a large particle bounces off the surface, more sediment is disturbed and released into the airstream, a bit like an avalanche.
The smaller particles are lighter and therefore can be held a lot higher aloft. It is this fine fraction (particles <20µm) which is called ‘dust’ and can travel a great distance in the air. Australian dust has been tracked to New Zealand and Antarctica. Dust also contains a lot of soil nutrients (organic matter and clay) thus depleting the nutrients at the source (where the dust originated) and enriching nutrients at the sink (where the dust is deposited). For example, oceanic dust deposits are an important source of nutrient iron for marine organisms.
Measuring sediment loss is problematic. A dust event could encompass one paddock, or paddocks across the entire eastern Australia. Measurement of how ‘deep’ (high) the dust plume extends is difficult, relying on meteorological studies or observations from airplane pilots.
The role of water in wind erosion:
The greater the rainfall, the less wind erosion. This is due to an increase in soil moisture which in
turn promotes vegetation growth – two factors that decrease a soils susceptibility to wind erosion. Soil moisture increases the binding capacity of soil particles and encourages the growth of microorganisms which produce protective soil crusts.
Continental wind erosion frequency can be related to the ENSO cycle following highs and lows of rainfall patterning. Through geological history, high periods of dust activity correlate with the arid phases.
On a smaller scale, new research suggests that a little rain may in fact increase wind erosion rates. The disruption of soil particles from raindrop impact, without their redistribution (sealing effect) as with larger amounts of water, results in a soil with more loose particles. The loose particles may later be picked up by the wind.
Another role that water plays in wind erosion is sediment supply. Water erosion in the internally draining river systems of central Australia carries vast quantities of sediment downstream. The shallow floodplains of central Australia receive a supply of very fine sediments which, when dry, are often picked up by the wind and redistributed back across the continent.
The role of vegetation:
Plants protect the soil against wind. Vegetation slows the wind velocity close to the soil surface, increasing the boundary layer and increasing the difference between the surface velocities and the threshold velocity.
An important measurement is vegetation resistance to wind- a dense bush will produce a turbulent flow behind the bush, whilst a porous bush will allow wind through but slow its velocity. In the Mesquite (small prickly bush) country of southwest USA, the mesquite bushes are dense and regularly spaced. Despite the overall high vegetation cover, wind erosion occurs from the accelerated wind velocities between the bushes. Long rows of bare patches of earth will form between the bushes, making these areas unsuitable for new bushes to grow. This process has wind directions directing the vegetation patterning.
Monitoring vegetation can be done in the field (using quadrat or transect surveying methods) or by remote sensing (using satellite imagery). Remote sensing of vegetation cover is frequently used for a large number of natural resource management issues. Attempts have been made to relate these measures to frequency of dust events, but desert vegetation (where a lot of wind erosion occurs) is difficult to monitor with these techniques. This is because the satellite technology uses colour (green) to detect vegetation; unfortunately, arid vegetation is often grey, silver, or brown, which means they escape detection by these instruments.