Why diagnose wind erosion risk?
Risk is the likelihood of something happening multiplied by the impact. The likelihood of erosion occurring is higher when all the hazards (see below) are operating at once, and the impact depends on the scale and intensity of the hazards.
Diagnosing and monitoring risk over time, enables you to implement strategies that lower the likelihood of damage by wind erosion.
Reducing wind erosion risk can start years ahead for susceptible paddocks
- by planting tree windbreaks
- claying
- by retaining enough stable groundcover.
Factors (hazards) contributing to wind erosion risk
Erosive winds
Wind speeds greater than 28 kilometres per hour are needed to move soil particles and lift dust for significant distances, when windspeed stays above that threshold for many hours or even days it can be very destructive. The Bureau of Meteorology provides wind speed and direction during the year from all Australian weather stations.
Exposure of the soil surface
Erosive winds must be able to reach the soil surface to cause erosion. Exposure can be prevented in several ways including:
- landscape position: sheltered areas include valleys, gullies, downwind of ridges and hills
- tree and shrub windbreaks: native vegetation, tree windbreaks, alley systems, tagasaste plantations
- crop and stubble groundcover: retain at least 50% groundcover with at least a third of that anchored. Less than 30% ground cover is high hazard rating for wind erosion, and more than 70% ground cover has a negligible hazard rating.
- pasture groundcover: retaining at least 50% groundcover of annual pasture or perennial pasture.
Erodible soil surface
To be eroded, the soil surface needs to be detached (disturbed or loose) at the surface. Gravelly or rocky surfaces are less erodible, and large stable clods also provide protection. The soil surface can be disturbed by:
- cultivation: mouldboard ploughing, spading, tyne cultivation, disc cultivation, deep ripping
- livestock movement: sheep tend to loosen more soil than an equivalent number of cattle
- vehicle movement: harvesting, seeding.
Assessing the current risk
All soils can be eroded by wind in certain conditions, depending on the level of protective cover (exposure) and whether the soil is loose and dry (erodible). The decision support tree is a helpful tool in assessing a paddock’s wind erosion risk.
Conditions that contribute to a high risk of wind erosion
Conditions that lead to a high risk of wind erosion are obvious – after the event. Being aware of developing conditions and having a plan can help reduce wind erosion:
- Poor seasonal conditions: dry conditions in the previous winter and spring, followed by a long, dry summer usually mean there is inadequate groundcover in late summer and autumn. Dry conditions and strong winds after seeding are hard to treat at the time.
- Grazing stubbles or pastures as the groundcover approaches critical dry matter levels or 50% groundcover; by the time that it looks like time to remove livestock, it is probably too late. Bare patches around watering points, gateways and camping pads can lead to blowouts.
- Paddock burning: full paddock burning, especially if the area has been recently grazed, will leave the soil exposed and erodible.
- Cultivation: this reduces groundcover and loosens the soil surface. Soil inversion — mouldboard ploughing, delving, spading — is the most extreme form of cultivation leading to exposure. Large stable clods or clay brought from the subsoil may reduce erodibility.
- Combinations of the conditions above.