Background

Other Physical Changes Due to Fire

Soils and Infiltration

Different soil types vary markedly in their ability to infiltrate 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 repellant. Water repellancy (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 rootholes, 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-repellant 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

• Prosser and Williams, 1998

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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 of 2003. These materials can more readily enter streams if the streamside ‘buffer’ vegetation has been destroyed. Data and climate model projections indicate that the El Niño is weakening and southeast Australia should experience average conditions in the coming months. The end of prolonged droughts can take various forms, ranging from average or moderately wetter conditions to widespread thunderstorms. The reader is referred to the Bureau of Meteorology’s website www.bom.gov.au/climate for weather projections.

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 references

• Brown, 1972
• Zierholz et al., 1995

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