Under steady state, erosion rates are equal to soil formation rates.
The fastest rates of soil formation occur in dune sands in moist environments, where weakly developed soils can develop over decades or centuries. For example, in Macquarie River, alluvium soils classified as Dermosols, Rudosols and Vertosols (see Table LAN3) have formed in less than 5000 years.
Soil formation from weathering rock is slower, and varies with the environment and rock type. An average of about 10 millimetres per 1000 years is typical in New South Wales, increasing to about 75 millimetres per 1000 years in Arnhem Land, Northern Territory. These soil formation rates are low compared with the estimated global average of 114 millimetres per 1000 years (Stockmann et al. 2014).
In Victoria, soil erosion declined between 1990 and 2010, probably as a consequence of the widespread adoption of soil conservation measures during the previous 30 years (Chappell et al. 2012). Similarly, in South Australia, soil protection increased between 2003 and 2013 in parallel with the increasing adoption of no-till methods (South Australian Government 2013b).
Water erosion
Current rates of soil erosion by water across much of Australia now exceed soil formation rates by an order of magnitude or more. As a result, the expected half-life of soils (the time for half the soil to be eroded) in some upland areas used for agriculture has declined to merely decades.
The latest assessment (Bui et al. 2010) concluded that soil erosion by water in Australian cropping regions is still at unsustainable rates, but there are large uncertainties about the time until soil loss will have a critical impact on agricultural productivity. Environmental impacts of excessive sedimentation and nutrient delivery on inland waters, estuaries and coasts are already occurring.
Up to 10 million hectares of land have less than 500 years until the soil’s A horizon (the more fertile ‘topsoil’) will be lost to erosion. Most of this land is in humid, subtropical Queensland.
A recent study (Bartley et al. 2015) compared long-term (between 100 and more than 10,000 years) erosion rates with contemporary erosion rates obtained by monitoring sediment fluxes over about 5–10 years in Queensland’s Burdekin River Basin. The ratio of these 2 erosion rates provides a measure of the accelerated erosion factor (AEF), which can be used to identify erosion hotspots and prioritise investment in land remediation at the subcatchment scale. All but 2 of the Burdekin subcatchments had an AEF greater than 1.0, indicating higher contemporary erosion rates than estimated long-term averages. Within the context of the Reef Water Quality Protection Plan, these results justify the setting of water quality targets at the subcatchment scale. More integrated studies of soil formation and erosion using a variety of techniques will be needed to better understand the extent, severity and significance of the problem. However, it is clear that a concerted program of soil conservation is essential to control this chronic form of land degradation across large areas of Australia.
The key to controlling soil erosion by water is maintenance of a protective cover (e.g. living plants, litter, mulch) on the soil surface. Other soil conservation practices—such as contour banks, filter strips and controlled traffic—are important, but secondary to the maintenance of cover.
Land management practices have improved significantly during the past few decades, as a result of better grazing practices, adoption of conservation tillage, enforcement of forestry codes and soil conservation measures in engineering (e.g. relating to road construction and urban development).
The ability to monitor land cover provides a key input to assessments of erosion risk across the landscape (Yang 2014). Remotely sensed monitoring of land cover and land use is now routinely used to identify trends (Guerschman et al. 2009, Malthus et al. 2013). Together with data on land management practices available from the Australian Bureau of Statistics, these trends reveal:
- a pattern of more careful grazing and maintenance of effective land cover at critical times of the year
- improved adoption of conservation practices, especially across the cropping lands of southern Australia
- an associated large decline in the amount of tillage in farming systems (Figure LAN21).
Dr Dan Metcalfe
Dr Elisabeth Bui
