State of the Environment Report 2007
Soil erosion occurs through soil being blown or washed from the land. It is a natural process that usually occurs at low rates and is influenced by slope of landscapes, climate, soil type, vegetation cover and land use. Water erosion occurs when flowing water mobilises soil. It has the potential to redefine landforms, often contributing to landslides on hill-slopes, river bank collapses and the creation of gullies (steep incised channels). Eroded soil can enter inland waters causing sedimentation and eutrophication problems. Wind erosion occurs where wind detaches and removes bare dry soil. Erosion of fine soils by wind often contributes to dust storms and high levels of airborne particulates.
Soil erosion is accelerated by activities such as vegetation clearing, cultivation, mining, fires, earthworks and livestock grazing. Soils are more susceptible to erosion if they are not protected by vegetation cover or are left exposed to wind or water flow. This becomes exacerbated during intensive storm events, floods or drought conditions. Loss of fertile topsoils to erosion exposes less fertile subsurface soils and results in reduced agricultural productivity and health of native vegetation. It may also cause the undermining of infrastructure (such as roads, buildings, bridges, fences, etc.) or the filling of drains, water supplies and inland waters. Severe erosion leads to poor soil structure in remaining soil, reduced water infiltration and general loss of soil health.
Soil erosion data in WA are generally inadequate. Determining the extent of soil erosion across the State is difficult because of the large area of land potentially affected, the diversity of land uses and climate variability. Models can be used to predict erosion-susceptible areas, using factors such as rainfall erosivity, surface vegetation cover, soil erodability, slope of the land and soil management practices.
Reconnaissance surveys at 70 sites undertaken for the National Landcare Program in the 1990s, demonstrated potential rates of soil erosion across the State (McFarlane et al., 2000; Loughran et al., 2004). Small amounts of the radioactive isotope, caesium-137, were deposited in WA soils in the 1950s following atmospheric nuclear weapons testing. Subsequent disappearance of the isotope from soil has been used as a measure of the cumulative loss of soil from both wind and water erosion. Soil erosion rates varied enormously across the State, ranging from less than 1 tonne per hectare per year (t/ha/yr) to 46 t/ha/yr recorded at a pasture-cropping rotation site near Kendenup. Nearly two-thirds of selected sites across the State had erosion rates of less than 5 t/ha/yr. On average the highest rates of soil erosion were recorded in the Kimberley, west Pilbara and Gascoyne districts (Table L2.1). These survey results should be interpreted with some caution given the limited number and representativeness of sites across a very large area of land and land uses. Nevertheless, the survey does provide an indication of soil erosion rates possible in WA.
Modelling of water and wind erosion risk is undertaken in the South West by the Department of Agriculture and Food. A complex mix of factors contributes to wind and water erosion. Soils on steep sloping lands in high rainfall areas are most susceptible to water erosion because of higher flows and speed of runoff. Landscapes that concentrate water flow also increase potential water erosion, while adequate vegetation cover helps to slow water flow across the soil, allowing infiltration to occur. The parts of the South West most susceptible to water erosion are hilly with high rainfall, such as forested areas of the Darling Range and coastal areas of the south coast (Figure L2.1). Areas susceptible to wind erosion include those with dry soils and a high proportion of fine sands. Soils with weak structure and water repellent properties may also be susceptible. Vegetation cover and undulating landscapes help to reduce wind velocity, thereby reducing the susceptibility of soil to wind erosion. The parts of the South West most susceptible to wind erosion are coastal areas from Bunbury to Geraldton, the western Wheatbelt and south-east agricultural areas (Figure L2.2).
![Figure L2.1: Modelled water erosion risk for the South West. [Data source: Department of Agriculture [ver. 2005]; Presentation: Department of Agriculture.]](/site/files/images/Figure-L2.1_Large.jpg)
![Figure L2.2: Modelled wind erosion risk for the South West. [Data source: Department of Agriculture [ver. 2005]; Presentation: Department of Agriculture.]](/site/files/images/Figure-L2.2_Large.jpg)
The Western Australian Rangelands Monitoring System monitors changes in vegetation and soil at sites in the pastoral rangelands (Watson & Novelly, 2004). Its soil stability index shows the ability of soil to withstand erosive forces and reform after disturbance (Figure L2.3). A greater proportion of sites in the Goldfields and Nullarbor districts show stable or improved soil stability compared to previous assessments. In contrast, a greater proportion of sites in parts of the Gascoyne, Murchison, Pilbara and Kimberley districts show some decline in soil stability. These areas may be susceptible to increased soil erosion.
![Figure L2.3: Soil stability in the pastoral rangelands, by bioregions. [Data source: Department of Agriculture – WARMS [ver. 2005]; Analysis: EPA; Presentation: EPA. Note: Data derived from WARMS Landscape Function Analysis – Soil Stability Ratio.]](/site/files/images/Figure-L2.3_Large.jpg)
The most significant pressures leading to erosion are agricultural or pastoral practices that increase exposure and vulnerability of soils. These include removal of protective vegetation cover through clearing or grazing, cultivation of crops and chemical changes to the soil (e.g. salinisation or increased water repellence). Density of livestock or feral and native animals on pastoral land can have a significant influence on soil erosion. Comparing actual livestock stocking rates with the land's carrying capacity is useful for tracking sustainable use of pastoral land (see 'Pastoralism'). Stocking rates in the Pilbara district have been increasing over the past decade in areas known to have naturally erosive soils. In contrast, stocking rates in the Gascoyne and Murchison districts have fallen in recent years, more than likely due to drought conditions.
Retention of native vegetation minimises wind and water erosion, although this is not always achievable for developed land. Soil erosion in the rangelands can be a significant problem due to low vegetation cover. Monitoring results from WARMS sites were analysed at the bioregion scale to determine general change in perennial vegetation density (in the shrublands) and frequency (in the grasslands). A majority of sites in bioregions had stable or improving vegetation cover over the past decade (see 'Loss or degradation of native vegetation'). Only a few bioregions in the Murchison and Pilbara had a majority of sites with declining vegetation density or grassland frequency.
Vegetation cover in the South West has generally declined in the period 1996-2004, and by more than 8% in some areas according to Land Monitor satellite imagery (See 'Loss or degradation of native vegetation'). Significant potential for soil erosion is likely in coastal areas from Busselton to Geraldton, where significant loss of vegetation cover has already occurred. Many Wheatbelt areas have a low rate of vegetation loss over this period because many areas have already been cleared with few areas of remnant vegetation.
Several studies of agricultural land show that 20-30% vegetation cover reduces soil erosion by as much as 80-90% (Findlater et al., 1990; Freebairn, 2004). In the largest erosion events, water runoff and soil movement are greatest for bare soil but reduce where minimal tillage (i.e. retaining crop stubble after harvesting) soil management is employed. Stubble or vegetation reduces the speed of water runoff by creating a more meandering pathway for water flow and allows more water to infiltrate into the soil. Eastern states studies show a 50% reduction in maximum speed of water runoff and a ten-fold reduction in soil movement in paddocks where stubble is retained, compared to bare soil (Freebairn & Wockner, 1986). Recent surveys (Department of Agriculture, 2006) indicate that about 60% of farmers in the Wheatbelt currently use stubble retention or mulching practices to reduce soil erosion.
Erosion can be caused by water repellence of soils, where soil particles become hydrophobic ('water-hating'). Dry coatings of organic matter can build up around soil particles, forming a waxy, water-repellent barrier. In heavy rains this can result in significant runoff and erosion on sloping land. Rills and gullies have been observed on soils that would have negligible runoff if they were not water repelling. Modelling by the Department of Agriculture and Food show the most water repellent soils in the South West are found on coastal soils between Bunbury and Geraldton, scattered parts of the central Wheatbelt, and coastal soils along the south coast.
Natural Heritage Trust/National Action Plan for Salinity and Water Quality (NHT/NAP): are two Commonwealth Government programs that aim to ensure environmental (on-ground) improvements occur via a targeted strategic approach at the regional level. All regional natural resource management groups have recognised soil erosion as a threat to natural resources and have identified projects for the remediation of eroded land and protection of valued natural assets at risk from erosion.
Agriculture extension program: The Department of Agriculture and Food promotes application of best practice by farmers to minimise soil erosion, including use of grade banks, stubble retention, soil testing, and retention and protection of native vegetation. Recent surveys (Department of Agriculture, 2006) indicate that 64% of farmers in the Wheatbelt currently use stubble retention or mulching practices to reduce soil erosion. The surveys also show 58% of farmers in the Wheatbelt undertake regular monitoring of pasture and vegetation cover, and 41% of farmers undertake regular monitoring in high rainfall South West agricultural areas. Up to 73% of farmers in the Wheatbelt regularly utilise erosion and surface runoff controls such as grade banks compared to 36% in high rainfall parts of the South West. About two thirds of pastoralists deliberately excluded stock from areas impacted by land degradation and about 80% undertake formal monitoring of vegetation and pasture conditions.
Ecosystem Management Unit (EMU): Although this project has recently ceased, it provided many pastoralists with tools for the ecologically-based management of leases which brought together production, productive capacity and biodiversity management. The process utilised both the pastoralist's local knowledge and the technical knowledge of ecologists to plan and implement on-ground actions for pastoral sustainability, including minimising soil erosion.
Monitoring: A number of initiatives are underway to measure erosion potential and the effects of erosion. The Western Australian Rangelands Monitoring System provides regional scale assessments of trends in perennial vegetation and soil surface condition. The system is made up of 1628 sites where changes in vegetation, soil and landscape are monitored. Satellite imagery has also been used to identify areas of wind erosion in the South West. Limited areas have been surveyed by this technique, which has proven cheap and effective compared to on-ground survey techniques.
Total grazing management yards: Pastoral grazing can now be managed primarily through the distribution of watering yards on stations and by controlling animal access to them, thereby limiting uncontrolled grazing of vegetation and consequently limiting the extent and severity of erosion. The number of yards is difficult to ascertain, but estimates indicate approximately 1000 yards have been installed across 8 million hectares of pastoral land in the Gascoyne-Murchison region (Rangelands Natural Resource Management Coordinating Group, 2005).
Soil erosion leads to land degradation and may also contribute to problems in inland waters and the atmosphere. It results in a loss of topsoil, often resulting in reduced soil fertility and structure and poor soil health. Erosion and corresponding deposition of soil can affect biodiversity by stripping native vegetation seed banks from topsoil, spreading weed seed and smothering habitat. Loss of valuable topsoil can dramatically reduce productivity of agricultural land, requiring additional soil treatments. It may also result in significant cost to the community through damaged infrastructure (e.g. collapsed fences and roads, filling of dams and reservoirs, clogging of irrigation systems, and the undermining of buildings). Eroded soil may accumulate on roads and railways affecting transportation, and cause increased flooding by filling drains and blocking outlets. Increased community effort may be required for restoration works, bank stabilisation and revegetation. Soil erosion can cause other environmental problems such as contributing to eutrophication problems in inland waters and contributing to fine particle air pollution (i.e. dust storms and high levels of particulates). Although the actual cost associated with soil erosion is difficult to quantify, it is undoubtedly substantial.
3.4 Enhance agricultural and pastoral programs addressing soil erosion and provide incentives to farmers and pastoralists to promote high adoption rates of best practice in managing soil erosion.
3.5 Ensure integration of erosion control measures into farm and pastoral management plans.
3.6 Develop an agreed baseline of the extent and severity of soil erosion in WA.
