Effects of Fire on Soils and Erosion

Soils and Infiltration

Different soil types vary markedly in their ability to absorb and store water. For example, many soils derived from volcanic rock have high clay content, and tend to shed surface water even in low intensity storms, producing flash floods in streams. Other well-structured soils are able to absorb the most intense rainfalls, and slowly release water to streams as a steady baseflow. In forests, accumulated litter provides a protective cover for all soil types, and this cover assists rain infiltration by maintaining a crumbly texture at the soil surface. The litter also impedes and traps soil particles dislodged by rainfall and surface washoff. After fire, destruction of the litter layer leads to reduced infiltration and much higher washoff of soil particles, organic matter, nutrients and ash into streams.

Rates of washoff are also increased if the soil surface layer is water repellent. Water repellence (hydrophobicity) commonly develops in soils with high organic matter content, and is associated with biological (probably fungal) activity. It tends to occur during drought, and some soils are always hydrophobic when they are dry. In unburnt forests, we rarely see the effects of hydrophobicity on surface washoff because root-holes, large holes associated with soil fauna, stoney ground and variations in micro-topography maintain easy entry of rainfall into the soil. After fire, hydrophobicity effects sometimes become evident, and may effectively prevent local infiltration. This behaviour may persist for weeks or months after the fire, and disappear only after the soil profile becomes wetted by prolonged rain or from below. Fire may not cause hydrophobicity, but it allows its effects to be seen in the form of lower infiltration and higher rates of surface material washoff. In particular, if a water-repellent layer exists below the soil surface, the first rainstorms after fire often produce rills that erode material down to the depth of that layer. In flood-producing rains, hydrophobicity probably has little effect on stream peak flows, because other runoff processes are overwhelming.

Key reference


soil properties View Frequently Asked Questions     View Bibliography
infiltration View Frequently Asked Questions     View Bibliography
water repellence View Frequently Asked Questions     View Bibliography
runoff View Frequently Asked Questions     View Bibliography

Erosion and Mass Soil Movement

Wildfire sets up conditions that may lead to increased soil washoff, soil slumping and streambank collapse, though there are several documented cases where no significant erosion occurred following fire. The primary causes are destruction of the forest litter layer, and wetting-up of the catchment after the vegetation canopy dies. As well, the forest floor is usually covered with a fluffy layer of charcoal and ash that easily washes off.

The most important factors that determine the occurrence and severity of washoff are the timing and intensity of rain that follows the fire. Low intensity rains at any time probably will have a benign impact on washoff, and help to stabilise the friable material on the forest floor. But thunderstorms produce high intensity rains that can lead to washoff rates tens or hundreds of times higher than in moderate storms. In southern Australia, thunderstorms occur most frequently in summer and early autumn, so it is probable that for some streams massive quantities of ash, charcoal, nutrients and sediment will be washed into streams during the very early months after bushfire. These materials can more readily enter streams if the streamside "buffer" vegetation has been destroyed.

Local earthslips, streambank collapse and gully erosion are more likely to occur in landscapes with unstable soils as a result of the fires. These will be the result of wetter (and thus weaker) soils, and the destruction of stabilizing streamside vegetation. We know already that most of the sediment deposits in watercourses originate from bank or gullyhead collapse, caused mostly by poor land management practices in the past. Many firebreaks, tracks and stream crossings have been hastily constructed during fire control operations. These will be sources of sediment inflows to streams, and will possibly encourage gullying and streambank collapse. The fires in forests and national parks could therefore lead to long-lasting changes in the character of pristine watercourses and their aquatic habitat value where machinery has disturbed streams or their nearby soils. Landowners and catchment managers will need to rehabilitate these sites as a matter of urgency

Key reference


fire View Frequently Asked Questions     View Bibliography
soil properties View Frequently Asked Questions     View Bibliography
debris-flow View Frequently Asked Questions     View Bibliography
bio-transferred sediment View Frequently Asked Questions     View Bibliography
erosion View Frequently Asked Questions     View Bibliography
rainfall View Frequently Asked Questions     View Bibliography
El Niño View Frequently Asked Questions     View Bibliography

Ecological viewpoints on soil (sediment) movement after bushfire

From the monitoring experience of staff of EPA Victoria and University of Canberra after the 2003 bushfires, it appears that the effects of sediment are most apparent immediately after fire, and largely complete a year later, dependent on sources, quality, and flushing. In catchments near Sydney, managed by the Sydney Catchment Authority (SCA), observations suggest that erosion could continue for up to 4 years after wildfire.


Ash (short term) and sediment (short to medium term) washes into streams during heavy rain on burnt areas that are no longer stabilised by groundcover or protected by tree and shrub canopies. The extent is also dependent on the depth to which the fire sterilises the soil. The result is streamwater that is turbid and contains high concentrations of nutrients relative to pre-fire conditions (e.g. ammonium, nitrate, soluble reactive phosphate, potassium) and major cations. Depending on their location in the catchment, river beds change from consisting variably of gravel, cobbles, pools and riffles to being uniformly sediment-covered.

Sediment slugs form, depending on soil type/geology, slope, rainfall patterns and intensity and fire intensity. (These slugs are banks of sediment that slowly progress downstream.) When the slug is largely coarse sand (e.g. from granitic areas), the effects on small streams can last for decades (e.g. Granite Creeks, Victoria), though with good flushing rains and stabilisation of sources in steep upland creeks the sediment can clear in about a year (e.g. tributaries of the Cotter River, ACT).

As the catchment vegetation recovers over the year or two after fire, the amount of sediment washing into the streams generally decreases, except perhaps where there is much catchment activity on tracks. Even where there have been pine plantations in the catchment, groundcover can return in the first year or two, especially if manually planted, seeded or of a species that is stimulated by fire. Groundcover roots help stabilise the soils.

Flow regulation can interfere with sediment transport downstream of a dam; probably as a result, communities of river-bottom algae and insect-larvae downstream of a dam can take much longer to recover than in free-flowing streams.

In Sydney's water supply catchments

Research since 2002 has found that:

  • The potential impact of wildfire on soils can be coarsely assessed using inference from remote sensing analyses and limited field work. Depth of heating penetration is related to fire severity and intensity.
  • Removal of vegetation and surface litter, coupled with higher fire severity, reduces soil wettability and increases runoff.
  • In areas of extreme severity, energy levels can reach at least 70,000 KW/m, soil temperatures exceed 350°C at depths of 1.5-2cm, and large volumes of burnt topsoil can potentially be redistributed locally and to the valley footslopes. Further transport downstream can be limited by extensive bioturbation (mixing of soil by soil fauna such as ants) in the footslopes.
  • Sediment and nutrient budgets (measured using environmental radionuclides (as tracers) and mineral magnetics), show significant sediment mobilisation and redistribution is possible in conjunction with burnt organic material and ash. Movement is from plateau-top to footslope, with downstream movement through the drainage network culminating in deposition in sediment fans or storages. Although redistribution of sediments can be locally extensive, post-fire bioturbation and litter dams reduce the potential distance transported by erosion and overland flow events, especially in areas affected by low to moderate fire severity.
  • After high severity of wildfire through a particular catchment (the Little River), large amounts of surface material were delivered to the drainage network, and pulses of sediment were distributed distally from the source during several moderate rainfall events. As vegetation cover re-established, the delivery of coarse and fine sediment and nutrients declined.
  • As expected, the delivery of sediment and nutrient loads through the drainage network is event driven. The larger the rainfall event, the greater the erosion and delivery of loads to the drainage network and transport downstream. The highest loads originated in areas that experienced the highest fire severities.
  • Following severe wildfire events, the post-fire recovery period of soil stability in these catchments is 1-4 years. During this phase of recovery, above-average rainfall events lead to erosion and subsequent downstream deposition of sediments and nutrients.
  • In the eucalypt forests in the SCA water-supply catchments, there is a naturally high water-repellence in the soils of most forest types. In unburnt areas, relatively deep litter layers absorb rainfall and this maintains soil stability during higher rainfall events, reducing erosion. Low to moderate wildfire severity reduces the litter layer, providing conditions favouring higher overland flows once soil saturation is achieved, though erosion is still minimal. In areas of higher fire severity the surface water-repellence is broken down and the soil becomes wettable, but a ubiquitous subsurface layer remains water-repellent at around 2 cm depth. Thus, once the wettable layer becomes over saturated, overland flow may occur and entrain soil material leading to enhanced erosion events. Such events appear to occur when local rainfall exceeds 40-60 mm/day.

Key references


streamflow View Frequently Asked Questions     View Bibliography
fire View Frequently Asked Questions     View Bibliography
forest View Frequently Asked Questions     View Bibliography
water yeild View Frequently Asked Questions     View Bibliography
eucalypts View Frequently Asked Questions     View Bibliography
pines View Frequently Asked Questions     View Bibliography
rangeland View Frequently Asked Questions     View Bibliography