description

Rill erosion often occurs with sheet erosion and is commonly seen in paddocks of recently cultivated soils following high-intensity rainfall. It is easily identified as a series of little channels or rills up to 30 cm deep.

process

If rainfall exceeds infiltration, a surface film of water forms (see sheet erosion). Rill erosion results from a concentration of this surface water into deeper, faster-flowing channels which follow depressions or low points through paddocks. The shearing power of the water can detach, pick up and remove soil particles making these channels the preferred routes for sediment transport. Rill erosion is often described as the intermediate stage between sheet and gully erosion.1

impacts

The loss of topsoil and nutrients reduces productivity greatly, as the remaining subsoils are often much less fertile. Also related soil deposition off-site causes sedimentation of streams, dams and reservoirs, resulting in water-quality deterioration and damage to aquatic habitats.

potential problem areas

Rill erosion is common on agricultural land devoid of vegetation and so is often seen in cropping areas after tillage. Following intense rainfall cultivated topsoils overlying denser cohesive subsoils often exhibit rill erosion. Texture-contrast (duplex) soils are susceptible, as are poorly managed pasture areas where overgrazing occurs.2

early signs

Watch for the development of surface water flows generated from moderate rainfall events. Beware in cropping areas of leaving soil bare during periods of high-intensity rainfall. Red-brown earths on slopes are particularly predisposed to erosion.3 Rills up to 30 cm deep will be easily visible in recently cultivated soils or areas cleared of vegetation. In tilled soils, rills will often extend to the depth of the tilled layer.4

what else to look for

Muddy run-off may indicate high sediment loads; look for evidence of off-site soil deposition. In some cropping situations, jolting of harvest machinery may indicate rills.4

action

Revise land use on potential problem areas, avoid extensive summer fallow. Modify cultivation and rotation of cropping land. Control grazing pressures, improve vegetation cover and soil management to increase organic matter and promote water infiltration and evapotranspiration by plants.1

key words

fallow, rill erosion, subsoil, tillage, tilled layer, topsoil.

references

1.McDonald Holmes (1946). 2.Office of the Commissioner for the Environment (1991).
3.Gibbons and Papst (1990). 4.Hunt and Gilkes (1992).