Hotter Fires Change the Bushfire Management Equation

Accurately predicting fuel loads — the amount of vegetation available to burn — is one of the most important tools fire managers have to prepare for and respond to bushfires.

As the BNHRC’s Dr Chris Gordon explains: “We need to have the most accurate methods of determining how much fuel there is at any one point in the landscape.”

Beyond ‘time since fire’

Traditionally, government agencies like the NSW Rural Fire Service and National Parks and Wildlife Service have estimated fuel loads based on two factors: how long it’s been since the last fire, and the type of vegetation present.

But Dr Gordon, based at WSU’s Hawkesbury Institute for the Environment, was asked to test a missing piece of the equation – fire severity. This was the focus of his recently completed project, which utilised state-of-the-art fire extent and severity mapping or 'FESM'.

“Nobody has really tested that idea across very large areas and then used that information to try to optimise fuel prediction models,” Gordon says. “We wanted to determine if a hot or a cool fire burns more fuel and then leaves more or less fuel after the fire.”

What the data shows

The team analysed survey data from across New South Wales and Victoria, taken in the weeks after bushfires. The results were clear.

“Where you have a hotter high-severity fire, there is less flammable vegetation remaining after the fire than in an area where there was a cooler low-severity fire,” Gordon confirms.

Cool fires tend to burn patchily, leaving fuel behind. Hot fires with higher flame lengths consume vegetation more completely.

By factoring fire severity into the models used to predict fuel load, researchers can therefore give government agencies a far more accurate picture of the fuel load landscape.

A better model

“At the moment the equation says: time since fire plus vegetation type equals fuel load. We’ve just tweaked that to say: time since fire plus how hot it was plus vegetation type equals fuel load,” Gordon says.

It may sound simple, but that extra factor could significantly improve statewide fuel maps used for hazard reduction and emergency response.

From research to operations

The work is already moving towards practical application. “The next step is how to actually implement this into fuel prediction models,” Gordon notes.

He and others at the BNHRC are collaborating with key end-user agencies such as the National Parks and Wildlife Service to build operational systems that automatically pull in fire severity data alongside other layers.

As Gordon puts it, better data means better decisions: “It’ll give us a better estimation of what the fuel load is at any one point in the landscape, which is going to make us better at fighting fires and managing fire in the landscape.”

Chris Gordon conducting fieldwork in the Blue Mountains, Sydney (Picture supplied)

What is Fuel Load? ‘Fuel’ is all living and dead plant biomass, with small diameter fuels such as twigs and leaves most important for fire spread. Fuel load generally refers to the weight (not arrangement) of all vegetation that could potentially act as the fuel for a spreading fire. In Australia, we generally consider all leaves and small twigs < 6 mm in diameter within different vertical strata: Surface = leaf litter and twigs on the ground, Near-surface = live and dead vegetation generally between 0-50 cm height (grasses, herbs) Elevated = live and dead vegetation generally between 50-200 cm height (shrubs, small trees) Bark = thin flammable loosely held tree bark that can carry embers. Our study focused on surface fine fuels < 6mm diameter.