Cosmos Magazine: Everything you need to know about Australia's new bushfire rating system

Fire danger ratings describe the potential level of danger should a bushfire start. They provide people with information so they can take action to protect themselves and others from the potentially dangerous impacts of bushfires.

The Australian Fire Danger Rating System (or AFDRS) has in many ways been a project more than 50 years in the making - only now it's replacing the modelling originally developed in the 1960s with more precise scientific understandings of fuel sources, fire and human behaviours.

The new system will extend to the way information is delivered to the public.

While many roadside signs will be refreshed, some jurisdictions will instead use more modern communications like the internet and social media to relay information to their communities.

So how does a sixty-year-old system - and the science behind it - get an overhaul?

While the public sees a colour-coded dial on a road sign, or catches an emergency update on the radio, there is a substantial body of scientific modelling underlying a system which covers one of the world's largest countries by area.

One of the most important changes to the AFDRS science is the expansion of vegetation models used to predict fire behaviour.

Previously, the system considered just two types of vegetation - grassland and forest.

Now, it's eight.

Grassland and forest remain, but the models have been refined and expanded to include grassy woodlands, spinifex, shrubland, mallee heath, button grass and pine forest.

The left map of Australia shows how fire modelling has seen Australia for the best part of sixty years, the right indicates how the nation will be viewed from September 1.

That, says Dr Stuart Matthews, a principal project officer at NSW Rural Fire Service who led the science update for AFDRS, is because Australia is a land of more than just sweeping grassplains and expansive gum forests.

"There's very diverse range of vegetation across the whole country and pretending it's all either grasslands or open, dry sclerophyll forest just isn't an accurate representation," Dr Matthews said.

The eight new vegetation models include the ability to consider 22 different fuel types.

Creating a system that uses four times as many models is an involved process - one that has covered almost a decade of research, prototyping and public engagement. And the science goes beyond the expansion of vegetation analysis.

Working closely with the Bureau of Meteorology to understand weather and climate patterns, the atmospheric science and reporting provided to both the AFDRS and the wider public has been updated to reflect the new vegetation modelling.

Fire authorities use sophisticated systems to determine the rating for any given point in time.

These systems - which are to be used nationally - divide the Australian landmass into 2.25 square kilometre units and analyses the fuel source within them.

These units also reflect data on when the last time fuel in the region was burnt-through.

Combined with the latest forecasts for areas from the weather bureau, information is placed into the fire behaviour model appropriate for the area. The system then creates a textual description of what a burning fire in that area would do, including its rate of spread, heat intensity and flame height.

This description is then transferred to a series of tables which converts the description into a numerical value.

This value aligns to a rating category on the Fire Behaviour Index, a 'stepped' indicator that allows authorities to prepare systems and resourcing to respond in the event of a fire.

This index effectively guides local authorities' actions on days of fire risk.

For the public, it's the guide to moving the dial on road signs.

There are five categories on the behaviour index: no rating, moderate, high, extreme and catastrophic.

Until the end of August 2022, the scale includes ratings like 'low-moderate', 'very high', and 'severe' - these have been retired.

The 'no rating' acts as the "parking bay" for the roadside needle, advising the public of the lowest risk conditions.

But there's science behind that too.

'No rating' days align to conditions appropriate for a controlled, prescribed burn.

"For instance, in forest, prescribed burnings are normally done at a heat output of about 750 kilowatts per metre," Dr Matthews said.

"So, in forests, if the prediction coming out of the model was that intensity would be less than 750kW, it would be classified in the 'no rating' range."

Anything above 750kW in a forest community would force authorities to 'grade up'.

The previous fire rating system was developed by CSIRO scientist A. G. McArthur in the 1960s.

And while familiar characteristics have been transferred to the new project, the 'back end' processes and science were rigid.

Rigid enough that the system has been largely unchanged for the six decades since it was first implemented.

In contrast, the new AFDRS is designed to be updated as new science comes to hand.

It's also informed by real world, operational knowledge and experience.

This, says Dr Matthews, provides authorities with more precision in fire preparation and response.

"It's a really big change from the old system, where none of the rating levels had an established and agreed meaning," he says.

"In the McArthur work, they were set up as a suppression difficulty measure, but that was never 'tied down' and everyone had their own interpretation of what that meant, which made it really hard to assess how the system was working. It was poorly defined.

"It's taken sixty years to get to where we are and one of the obstacles, from a science point-of-view, was that the system was fixed. You couldn't make incremental changes to it.

"With the new system, we've built it in a modular way, and clearly broken down the different parts of the system so we can upgrade and change things if we need to."

An important element of the bushfire rating system is to be able to indicate the impact of a possible bushfire in words a landowner might understand.

The Country Fire Service (South Australia) describes a 'Catastrophic Level' bushfire as:

Dr Matthews points out that the messaging for consequences will continue to be based on fire behaviour and known elements like flame heights, fuel loads, wind speed and spotting.

"We are building a case for funding for more research to improve the accuracy in the future of the fire impact."

Originally published by Cosmos as Australia has a new bushfire danger rating system, here's what you need to know.