1. FIRE WEATHER TRAINING – 2 NT Fire Environment
Bureau of Meteorology
Severe Weather Section, Darwin
FIRE WEATHER TRAINING – 2 NT Fire Environment
Updated
May 2011

2. Learning Outcomes NT Fire Environment
After this session you should:
know the distribution of grassy fuel types in the NT and the seasonal variation in fuel curing
understand the impact on fire behaviour of high fuel loads associated with exotic grasses
understand the reasons for an increased number of fire weather warnings in areas infested with gamba grass

3. Eucalypt open forest, annual and perennial grasses in understorey
Mostly perennial tussock grasses.
Perennial spinifex under open eucalypt woodland or acacia (mulga)
Grass is the most common fuel type in Australia (nearly 75% - with half this area used for grazing)
72% of bushfire damage costs to property is due to grassfires
Under extreme weather conditions, grassfires can spread at > 20 km/h
Savannah vegetation types are determined by annual rainfall and soil types.
Annual grass: Plant that germinates, flowers and sets seed and dies within a year.
Perennial grass: A plant that usually takes more than one growing season to mature and usually lives for more than one year.

4. CURING OF TOP END SPEAR GRASS DURING LATE WET SEASON
SOURCE: ‘Grassfires – fuel, weather and fire behaviour’. Cheney and Sullivan CSIRO Publishing, Melbourne
Curing of Top End Speargrass during late wet season
Annual grasses in the Top End such as speargrass (sorghum) germinate after the early wet season rains, grow rapidly and set seed late in the wet season. Following flowering, senescence (or grass curing) occurs over a period of 6-10 weeks during which the grass progressively dries out to a fully cured (100% - brown) state.
Perennial grasses cure more slowly than annual grasses.

5. Curing estimates supplied by
FUEL STATE ASSESSMENT
Curing estimates supplied by
Bushfires NT
Region Curing Rate _
Jan Aug 2010
Vernon % %
Arafura % %
Arnhem % %
Savanna % %
Barkly % %
Alice Springs % %
Note: in central Australia, fuel load and curing is dependent on antecedent rainfall
Fuel State Assessment
Curing/Fuel load estimates for Fire Control Regions as supplied by Bushfires Council staff
– entered on Curing Data Entry Form as basis for GFDI calculations (fuel curing only – fuel load is fixed)
- actual values are an area average: there are likely to be patches with higher curing on elevated or rocky ground separated by strips of lower curing along watercourses or swamps

6. Native grass fuel
Low fuel bed ( 1m) 6

7. Gamba Grass Mission Grass Buffel Grass
Introduced grasses in the Top End –
Gamba grass and Mission grass:
Tall perennial grasses - high fuel loads late in dry season Established on roads and disturbed areas
Spread assisted by grading, slashing and hay transport
Introduced grasses in central Australia –
Buffel grass: Perennial. Very high fuel load causing hot, intensive burns. Displacing native grasses over arid areas of central Australia.
Buffel Grass

8. Gamba grass fuel
Tall fuel bed ( 4m) 8

9. Early dry season native grass fire9

10. Early dry season gamba grass fire
Photo: BFNT 10

11. Early dry season gamba grass fire11

12. Fuel and Fire Characteristics
Native
Gamba
Fuel load (t ha-1)
1 - 7
4 - 25
Intensity (MWm-1)
2 - 42
Char height (m)
3 -13
As you can see here, the fuel load of the gamba grass invaded sites is higher, but the moisture content is also typically higher in the fires measured in gamba grass compared to native grasses. This reflects the later curing of the gamba grass compared to all native grass species.
The intensity of the fires was substantially (x times) higher in all gamba grass fires. Higher intensities were recorded in gamba fires than any native fire in the northern savannas.
The consequence of the fire intensity and fuel load shown thru the scorch and char heights. These are post-fire indicators of fire behaviour. As you can see, the scorch heights for native grass fires are between 4 and 10 m. By comparison, the gamba grass fires were at least 13 m, and for many fires, this was not a useful measure because the entire canopy was scorched and so an estimate can’t be obtained.
Char height was also significantly increased, from ~1.5 m to between 3 and 10m.
Rossiter et al 2003, Setterfield et al 12

13. The gamba grass aerial survey

14. Gamba grass distribution
Litchfield NP
We just completed mapping of gamba grass across Litchfield and Coomalie. We’ve mapped major invasion in Litchfield, which is one of our most highly visited NPs in the NT. There was no GG in the park 15 years ago, and yet its now invaded from plantings outside, along riparian areas and expanding outwards. The cost of management to prevent further spread will be huge, but need to be weighed against the cost of environmental, social and economic costs of further degradation within this highly valuable national park. Similarly, further expansion in neighbouring Coomalie Shire will cause major costs, particularly thru fire management and fire losses to rural communities. 14

15. Gamba invasion in Litchfield National Park

16. Gamba grass
Photos: Bushfires NT

17. Batchelor (current fuel load 13 t/ha)16
May Jun Jul Aug Sept Oct
Fire Danger Index (FDI) 77 35

18. Middle Point (current fuel load 11 t/ha)
Fire Danger Index (GFDI)
111
May Jun Jul Aug Sept Oct

19. Darwin (current fuel load 9 t/ha)10
May Jun Jul Aug Sept Oct
Fire Danger Index (GFDI)
124
May Jun Jul Aug Sept Oct

20. Fire Weather Warnings vs Fuel Load Vernon region* 2008
*Batchelor, Middle Point and Darwin AWSs

21. Fire Weather Warnings vs Fuel Load Vernon region* 2008
*Batchelor, Middle Point and Darwin AWSs

22. Fire Weather Warnings vs Fuel Load Vernon region* 2008
Future scenario without extensive gamba control?
*Batchelor, Middle Point and Darwin AWSs

23. Summary NT Fire Environment
Grassy Fuels in the NT
Alice Springs District:
spinifex, buffel grass, mulga scrub: 2-3 t/ha
Barkly, Katherine, VRD and Arnhem:
savanna, spear grass, tussock grasses: 4-6 t/ha
Litchfield and Coomalie Shires:
spear grass, gamba grass: 9-15 t/ha
Fuel Load and Curing
annual cycle in Top End savanna
antecedent rainfall dependence in central Australia
Fire Weather Warnings
issued when GFDI is forecast to reach or exceed 50
increasing fuel load in NW Top End due to Gamba grass
increased number of fire weather warnings this season

24. SOURCES OF INFORMATION IN THIS PRESENTATION
‘Impact of gamba on the Fire Danger Index (FDI)’. Setterfield & Rossiter-Rachor Draft report from CDU to BFNT.
‘Grassfires – fuel, weather and fire behaviour’. Cheney and Sullivan CSIRO Publishing, Melbourne.
‘Savanna Burning – Understanding and using fire in northern Australia’. Dyer et al 2001 (Ed.) Tropical Savannas CRC, Darwin.
‘Rural Fire in the Northern Territory’. BFNT, NTFRS, Bushcare & NT Government brochure.
Bushfires NT web page -