Fire, climate and the landscape

Fire and water production

Many of the water catchments that have been harnessed to provide drinking water to the population of Western Australia occur on land managed by the Department of Environment and Conservation. There are 4450 km² of catchments on departmental land that service the metropolitan area of Perth containing 16 dams, eight groundwater schemes and 12 independent artesian bore schemes. The quantity and quality of water harvested from these catchments is directly affected by the type, structure and condition of the vegetation covering the catchments. Fire affects the type, structure and condition of vegetation and therefore has a significant role to play in water productivity from catchments.

Catchments that have a vegetation cover that protects the soil from erosion produces water that is clear and clean and requires very little treatment before it gets to the consumer’s tap. However, a heavy vegetation layer means that much of the rainfall is intercepted and used by the vegetation, significantly decreasing the amount available as runoff into dams or infiltration into ground water aquifers.

Fire can be used to manage the vegetation of catchments to ensure sufficient vegetation cover to maintain water quality whilst maximising the amount of water available for harvesting. The runoff from a forested catchment can increase by 20 per cent to 50 per cent after a prescribed fire depending on the severity of the fire. Prescribed fire applied to ground water catchments can increase production for up to five years after the prescribed fire. In the catchments around Perth, where effective runoff is only six per cent of rainfall, small increases in effective runoff or infiltration to groundwater aquifers by using fire to treat the catchments is a very cost effective means of increasing the amount of water harvested from these catchments.

Low intensity prescribed fire reduces the amount of vegetation and ground cover to provide increased runoff and infiltration without having an effect on water quality. Wildfires, particularly extensive, intense wildfires, have the potential to destabilise the soil surface. This can impact of the catchment by causing erosion and silt deposition that blocks drainage channels. It also has impacts on the water storages causing turbidity, eutrophication, algal blooms, fish kills and increased water treatment costs.

Clean water

Turbidity in runoff from catchments affected by intense wildfire

Drainage lines denuded making them vulnerable to erosion

Intense wildfire in a forested catchment