Shelterbelts for Control of Wind Erosion
Note Number: LC0422
Published: July 2003
Shelterbelts offer many benefits to land managers. In Australia an important benefit provided by shelterbelts is the reduction in soil erosion caused by wind.
Australian soils are old, weathered and often depleted of valuable nutrients. Soil erosion can further reduce the productivity of land by removing valuable top soil and nutrients.
The productivity of land for agriculture is dependent on the characteristics of soil. Most soils within Australia have low levels of nutrients and top soils are very thin.
Top soils take a very long time to form. Over a one hundred year period only a couple of millimetres of soil may be formed. This soil can then be quickly eroded by even modest winds.
Victoria regularly experienced dust storms during the 1950s due to wind erosion. In February 1983, following the 1982 drought, a large cloud of dust estimated to carry 250,000 tonnes of dust passed over Melbourne. Large wind erosion events have occurred as recently as autumn 2003. These events demonstrate the vulnerability of our soils to wind erosion.
Shelterbelts can be used to reduce the impact of accelerated soil erosion. Shelterbelts can decrease windspeed within the sheltered zone to a level below the threshold for soil movement.
Impacts of wind erosion
The erodability of soils depends on surface texture, tilth, wetness and vegetative cover (Cremer, 1990). Wind erosion can begin at relatively low wind speeds. At 30 km/hour sand grains can be dislodged and suspended in air movement.
Wind can move soil particles and nutrients resulting in nutrient depleted soils and productivity losses. An example includes a site where crop yields in the year following a major erosion event were estimated to be reduced by 25%.
Nutrients lost due to soil erosion need to be purchased and replaced or productivity will be reduced. It is difficult to replace organic matter lost due to wind erosion.
Wind erosion can have a direct impact on crops in several ways. Soil particles carried by wind can physically damage crop plants. Crops can also be damaged by roots being exposed due to soil loss, being buried by moving soils and plants being blown out of the ground.
Australia's weathered, nutrient deprived soils are susceptible to erosion largely due to broadscale clearing of vegetation. The removal of vegetation has resulted in large areas of land that are traversed by strong winds. This land has little to protect its soils from being dislodged and moved over very large distances.
Most soil nutrients are held in the organic matter of plants and in the top layer of soils. This top layer of soils often contains fine silt or clay particles that hold valuable nutrients.
These fine particles of soil and nutrients are the first to move during a wind event. The fine nature of these particles means that once they have been dislodged, they can be carried over large distances while suspended by wind.
Soils are most vulnerable if they have been heavily grazed or cultivated. Large-scale erosion of soils by wind often occurs during dry or drought conditions. It is during dry conditions that vegetative cover is low due to slow growth and overstocking.
Re-establishment of vegetation and in particular shelterbelts can protect soils from the impacts of erosion. Establishment of vegetation reduces the vulnerability of soils in a number of ways.
The leaves and vegetative matter within and dropped by vegetation intercept raindrops and prevent the pounding of soil and dislodgment of soil particles.
The litter dropped by vegetation increases the organic matter within soils increasing the ability of soils to absorb and hold water and to reduce run-off. The roots of plants bind and hold the soil.
How Shelterbelts can reduce the impact of wind erosion
Shelterbelts can reduce and in some cases prevent wind erosion by intercepting wind and consequently reducing wind speed as wind passes through or over the belt. Even narrow or open belts can reduce wind speed for a distance many times the height of the belt and therefore have a considerable impact on wind erosion.
Moderate wind speeds can begin the process of soil erosion. Once soils are suspended the quantity of soil carried increases in proportion to the wind velocity cubed. Therefore a modest reduction in wind speed can result in a significant reduction in soil erosion. Reducing wind speed by half can reduce the rate of erosion to 1/8. Even wind approaching belts on angles that are almost parallel to the belt can decrease wind speed. Although the area over which shelter is provided will be reduced.
This means that even sparse shelterbelts can provide considerable reductions in soil erosion. An example is where windspeed at 0.5mts above the ground, at a distance of 12 times the height (12H) of a shelterbelt was measured at a modest 60% of the open windspeed. The erosive force of the wind there was estimated to be reduced to 25% of that in open areas (Bird et al. 1992).
Shelterbelt design for reductions in wind erosion
The location, density, height and length of a shelterbelt will determine its effectiveness in reducing wind erosion.
A grid of shelterbelts positioned at appropriate distances apart provides the maximum level of shelter for a property as shelter will be provided from all wind directions.
Shelterbelts provide the highest level of protection when they are located at right angles to erosive winds. Good protection from wind erosion can be maintained for up to 30H of a shelterbelt if wind is approaching the belt at right angles. Therefore consideration should be given to the direction of winds protection is required from when planning a shelterbelt network.
Although maximum protection from erosive winds is achieved by placing belts at right angles to the wind direction, soil erosion can still be substantially reduced if wind is coming from other directions. However, the area protected by a belt will be reduced during events of higher wind speeds.
Dense shelterbelts provide a high level of protection but over a reduced area when compared to less dense belts.
Moderately dense belts (~40% density) will provide a considerable reduction in wind speed to a distance of at least 20H. Dense belts generally provide good shelter to a distance of at least 15H.
To control wind erosion over a large area the height of a belt needs to be maximised. Therefore it is wise to incorporate at least one row of tall species within the belt.
It is important to incorporate shrubs into a belt where an even density is required from the ground to the top of the belt. It is also advisable to fence out a belt to prevent stock grazing on the lower limbs of vegetation and creating gaps. If gaps occur within a belt wind will tunnel through the gaps at an accelerated rate. This can result in blowouts near the gaps and also be detrimental to stock and crops.
Wind speed accelerates around the ends of shelterbelts. By increasing the length of the belts or joining them to other belts this impact is reduced. Belts can be joined where they can form a right angle for protection from a range of wind directions as illustrated in Figure 1. Stock should not be able to graze below or at the ends of a shelterbelt as these areas are very susceptible to erosion. The more effective a belt is in reducing wind erosion the more eroded the sites at the end of the belt can be, unless the ground surface is protected.
Large quantities of topsoil can be lost in very short periods of time due to wind erosion. This topsoil is almost irreplaceable as it can take hundreds of years to restore soils that can be removed in a matter of hours during erosive wind events.
Shelterbelts can provide a relatively cheap and long-term option for reducing wind erosion on farms. The decrease in wind speed due to shelterbelts can result in a considerable reduction in the removal of fertile soil from agricultural land.
The design and location of belts should take into consideration other benefits that can be achieved through establishing shelter. Shelterbelts offer a range of benefits such as improved livestock productivity, protection for horticultural and cropping enterprises, reduced salinity impacts, wildlife habitat, fire protection and improved aesthetics.
Careful consideration needs to be given to the location, height, density and composition of a shelterbelt. These factors are best considered as part of a wholefarm planning process.
See Landcare Notes:
- Shelterbelts for Livestock Protection
- Shelterbelt Design
- Shelterbelt Management
- Shelterbelts and Wildlife
- Tree planting and aftercare
- The benefits of using indigenous plants
Bird, P.R. (1998) "Tree windbreaks and shelter benefits to pastures in temperate grazing systems". Agroforestry Systems 41: 35-54.
Bird, P.R., Bicknell, D., Bulman, P.A., Burke, S.J.A., Leys, J.F., Parker, J.N., van der Sommen, F.J. and Voller, P. (1992). "The role of shelter in Australia for protecting soils, plants and livestock". Agroforestry Systems 18: 59- 86.
Brandle, J.R.,Hodges, L. and Wight, B. (2000) "Windbreak Practices". North American Agroforestry: An Integrated Science and Practice 4: 79-118.
Burke, S. (1998) Windbreaks. Inkata Press, SydneyCremer,K.W. (1990) Trees for Rural Australia. Inkata Press, Melbourne and Sydney.
This Information Note was developed by Hayley Johnson, Ararat, July 2003.