Calculating the Grassland Fire Danger Index (GFDI)
The GFDI is a mathematical relationship between air temperature, relative humidity and wind speed and impacted by the curing factor, or greenness, of vegetation. It means a fire in conditions where the GFDI is 35 or above is unlikely to be controlled by a harvest operator's fire-fighting resources.
The GFDI is designed to be determined in the paddock and used in conjunction with other fire risk management measures, as outlined in the code. The code covers areas such as fire prevention, requirements for firefighting facilities to contain and extinguish a fire if it starts, and also conditions that harvest operations should cease (GFDI calculation).
When it comes to measuring the GFDI in real-time in the paddock, CFS and GPSA are encouraging growers to at least use a hand-held weather meter to measure wind speed and local conditions, or preferably a fixed or portable weather station, together with the Code of Practice and its Grain Harvesting Operations Table (below).
The local actual GFDI may also be determined by a local committee, in consultation with local farmers.
Table 1 calculates the average wind speed for different temperature and relative humidity combinations that equate to a GFDI of 35.
For example, if the temperature is 35C, the relative humidity is 14% (round down to 10%) then harvest must stop when the average wind speed is greater than 26km/hr.
The GFDI provides a numerical assessment of likely fire behaviour which is then grouped into categories. These are:
- Low/moderate – GFDI of 0-11
- Small fires, easily controlled, likely cool weather with high relative humidity.
- High – GFDI of 12-24
- Fires may present some threat to property but first attack fire-fighting is likely to succeed.
- Very high – GFDI of 25-49
- First attack fire-fighting will likely fail, fires may move quickly and threaten property, indirect strategies may succeed. The Kangaroo Island bushfires in December 2007 occurred on a day of very high fire danger.
- Severe – GFDI of 50-99
- Fires are wind-driven, will spread rapidly and may be uncontrollable with spot fires likely to occur. Total Fire Bans are declared for categories severe and above.
- Extreme – GFDI of 100-149
- Aggressive fire behaviour usually driven by strong winds. Fire-fighting is ineffective. The extreme category presents a significant threat to life and property.
- Catastrophic – GFDI of 150 and over
- Fires on catastrophic days are uncontrollable until conditions ease. The Pinery fire in November 2015 and the Wangary fire in January 2005 occurred on days of catastrophic fire danger.