Soil properties: soil erosion by water
Indicator Status: For Advice
Department of the Environment and Heritage
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Why do we need to monitor soil erosion by water?
Soil erosion by water is a major issue for Australian agriculture and catchment management. It causes unsustainable losses of soil for agriculture that far exceed rates of soil development (Edwards and Zierholz 2000). Water-borne erosion impacts on river, estuary and marine resources and reduces the life of water storages.
The National Land and Water Resources Audit (NLWRA 2001) assessed soil erosion by water across Australia. Its findings are as follows.
- Hillslope erosion (sheetwash and rill erosion) remains high in Australia's tropical northern regions during the wet season, and especially in the semi-arid woodlands and arid interior. Maintaining vegetative cover, minimising soil disturbance and building sediment-trapping wetlands remain imperatives.
- In southern and eastern Australia sheet and rill erosion are still a problem in many cropping areas. Although reduced from the severe erosion rates before the advent of conservation farming, erosive storms occurring at critical times in the cropping cycle or on heavily grazed land still cause major erosion losses (Hairsine et al. 1993, Murphy & Flewin 1993). Continued vigilance in the adoption of conservation farming practices is required (see Edwards & Zierholz 2000). Although erosion is inactive in many relict gullies, it persists as a major soil erosion process affecting river condition, as many gullies associated with salinity outbreaks are still very active. Erosion affected rivers have extensive coarse sand accumulations in their stream beds, which exacerbate flooding and smother the habitat of native organisms.
- Active gully erosion is still occurring in some parts of Australia. Changes to agricultural practices that minimise gully erosion are an imperative, both from an on-farm and off-farm perspective.
The direct or 'on-site' consequences of accelerated erosion impact on the sustainability and productivity of Australian agriculture. The major consequences are:
- loss of nutrients required for plants to grow;
- loss of organic matter, which plays a vital role in sustaining the desirable physical and chemical characteristics of the soil;
- decrease in soil depth, which reduces water storage capacity;
- burial of topsoil in low lying areas;
- reduced trafficability caused by gullies;
- spread of weed seed;
- bringing subsoil problems, such as sodicity, closer to the surface;
- damage to infra-structure such as fences, roads and buildings (Scott 2001); and
- siltation and pollution of farm dams.
Soil erosion also has the potential for downstream impacts on creeks, rivers, reservoirs, lakes, and estuarine and marine environments. Water-borne erosion increases the supply of sediment to rivers. High concentrations of suspended sediments in rivers can:
- reduce stream clarity;
- inhibit respiration and feeding of stream biota;
- diminish light needed for plant photosynthesis;
- eutrophy rivers and wetlands;
- make water unsuitable for irrigation;
- require treatment of water for human use;
- smother the stream bed; and
- increase land flooding (NLWRA 2001).
The effects of soil erosion by water on-farm are irreversible on shallow or texture contrast soils. Uniform textured soils, where the soil depth is much greater, may suffer little although the off-farm impacts continue for many generations. The implementation of effective catchment management and industry priorities are essential, particularly in terms of improved practice. Management approaches will differ across Australia's catchments as the processes that supply sediment to rivers also differ.
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