Water erosion is the removal of soil from the earth’s surface by water. Water erosion occurs when raindrops impact the soil surface and displace soil particles and when water flowing over the land surface mobilises soil particles. It occurs naturally at low rates, but it can become accelerated as a result of human-induced management changes to the natural landscape. In each case the same processes operate and the distinction is only a matter of degree and rate of erosion.
An acceptable erosion rate is one that is less than, or closely matches,the soil formation rate. Estimated soil formation in WA ranges from 1mm/100 years to 1 mm/1000 years, which equates to between 14 t/ha/year and 1.4 t/ha/year (McFarlane et al. 2000; State of the Environment2011 Committee 2011). Elsewhere in Australia, soil erosion of about0.5t/ha/year is regarded as natural, but in WA, “soil loss at almost any rate is unlikely to be sustainable”(George 2001).
Water erosion and sedimentation processes can be both insidious and episodic, and are largely irreversible. Once the soil has eroded and a new base level established, it is difficult or impossible to revert to the previous state. Further, the time required to form new soil is so long that soil should be considered a finite resource.
Water erosion may occur over relatively wide areas where flow has been concentrated but channels are not perceptible (sheet erosion), along numerous small channels (rill erosion), or well defined large channels (gully erosion), by the direct action of stream flow (stream bank erosion) or shallow subsurface water movement (tunnel erosion).
Water erosion is affected by those factors which influence the amount of runoff (surface and shallow subsurface) generated, as well as the inherent erodibility of the soil.
The amount of surface runoff is determined by complex interaction of factors including climate (e.g. likelihood and timing of severe rainfall), landscape (e.g. slope angle, slope length), soil properties (e.g. permeability, cohesion, organic matter content) and vegetation. Soil must be exposed to moving water, so the amount an type of ground cover (i.e. vegetation) is generally recognised as the single most important factor affecting soil loss at a site (Emerson 1991).
The inherent erodibility of an exposed soil depends on soil texture and aggregate stability. Inherent erodibility appears to increase within increasing clay. According to Evans (1980) highly erodible soils tend to have a medium texture with clay content between 9 an d 35. Structurally degraded soils tend to be more susceptible to erosion than those with good structure.
Water erosion results in the loss of valuable topsoil. More importantly, it is a selective process, which preferentially removes clays and silts and associated nutrients, especially phosphorus. The remaining soil is depleted and low in fertility.
Water erosion is the major contributor to the siltation and eutrophication of water bodies and declining stream quality through increased turbidity and salinity.
Management should focus on both decreasing the amount of runoff generated and reducing the erodibility of the soil. Runoff generated can be decreased by revegetation of slopes, the use of grade and contour banks, and by ploughing on the contour. Water sensitive design of drainage lines, fire breaks and unsealed tracks will prevent erosion along these features. Soil erodibility can be decreased through direct drilling, no-till sowing combined with improved stubble management and the prevention of overstocking. Activities that disturb the soil should bot coincide with peak rainfall periods.
Key messages across south-west Western Australia (DAFWA Report Card 2013)
Condition and trend
- Water erosion hazard across the south-west of WA – during the growing season – has diminished because of declining winter rains, increased stubble retention and adoption of reduced tillage practices.
- Water erosion events are mainly caused by intense, localised summer storms.
- There appears to be a trend towards more frequent, potentially erosive summer storms in the eastern wheatbelt and south-eastern coastal areas, and relatively little change in other areas.
- The average annual opportunity cost of lost agricultural production in the south-west of WA from water erosion is estimated at $10.1 million. This loss is cumulative because of the irreversible nature of soil erosion.
- The requirement to retain a cover of 70% or greater of plant material anchored by intact roots, means that total prevention of water erosion is difficult, particularly with livestock grazing systems.
- A minimum of 70% intact and anchored ground cover is the land managers’ target for prevention of both wind and water erosion (Coles and Moore 2004). This figure will provide an easy-to-understand DAFWA extension message.
- Programs continue to develop and extend farming systems that maximise crop and pasture biomass and provide continuous ground cover, including:
- crop-grazing to maximise early growth of pastures following the autumn feed gap.
- pasture cropping to improve ground cover and prevent erosion during seeding and early crop growth.
- pasture mixes that contain summer- and winter-active perennial components (in suitable climatic locations), and diverse swards
- increased use of confinement feeding systems to reduce grazing pressure on stubble and pasture.
Projects addressing water erosion
SWCC supports a number of landholders and partners to undertake projects addressing agricultural practices including water erosion.
Further information on these and many other projects can be found on the Agtrialsites website at www.agtrialsites.com.
The Agtrialsites website is a cross regional NRM WA initiative that aims to provide information on sustainable agriculture projects across Western Australia.
- Galloway, P and van Gool D (2013). ‘Water Erosion’. In: Report card on sustainable natural resource use in agriculture, Department of Agriculture and Food, Western Australia.
- Van Gool, D., Vernon, L. and Runge, W. (2008). Land Resources in the South-West Agricultural Region. A shire-based summary of land degradation and land capability. Department of Agriculture and Food Western Australia.