Layers
Fire Behaviour Index
The Fire Behaviour Index (FBI) is a numerical scale that describes the potential fire behavior based on the weather conditions, fuel availability, and topography.
The AFDRS calculates the FBI using weather and fuel components, which include temperature, humidity, wind speed, dryness of the fuel, and recent rainfall. The system assigns value on a scale of 1 to 100+, with higher values indicating greater danger. The index is divided into rating categories that reflect the potential fire behavior, including the speed and intensity of fire spread, the difficulty of suppression, and the likelihood of spot fires.
Continuous Haines
The Continuous Haines (cHaines) index is a measure of atmospheric stability. It ranges from 1-13 with a cHaines value of 13 meaning the atmosphere is likely to be highly unstable.
A high value of the cHaines index often precedes the passage of a trough or a front.
A high cHaines index coupled with a going bush fire may result in fire behaviour becoming erratic and be associated with the development of “pyro cumulus” clouds above a smoke column. This may result in fire behaviour that exceeds current fire behaviour models ability to predict fire behaviour.
Lightning Activity Levels
The Lightning Activity Level layer is actually a prediction of thunderstorm activity. It provides a classification of thunderstorm activity within a given hourly window prior to the time step, with:
- 0 – no thunderstorms
- 1 – isolated thunderstorms
- 2 – scattered thunderstorms
- 3 – widespread thunderstorms
Dry thunderstorms contribute to the LAL score, but do not result in a higher LAL than regular thunderstorms. LAL does not represent the frequency of lightning strikes predicted within a thunderstorm.
Fuel Availability
Drought Factor
Drought factor is an estimate of surface fuel moisture content derived from the McArthur mkV meter (McArthur, 1967). It is expressed as number between 1 and 10, where 0 represents no fuel available to burn (i.e. too wet) and 10 represents all the surface fuel being able to be burnt.
Drought factor is calculated using the rainfall total 75% chance layer. This means that forecast drought factor will be biased towards predicting dryer fuel conditions.
Curing
Fuel curing is a term used to describe the seasonal cycle of grasses dying and drying out.
During winter, most grasses are either dormant or slow growing. In spring they will undergo a period of growth, drawing water from the soil and “greening-up” while they develop their seeds. As the growth period is completed and the seed heads flower and mature, the grasses lose their ability to draw water from the soil and begin to dry out.
Rainfall will slow the process and lack of rainfall will speed it up.
It is difficult to estimate the curing by eye, using overall colour as the criterion. It is also difficult to objectively estimate grassland curing across the landscape. As a result, satellite derived curing is primary mechanism to used estimate grassland curing. Weekly checks as to the accuracy of the curing via field reports are provided by District Officers. For further information on satellite curing Double click on Figure 2 NSW RFS grassland curing information.
Curing rates are recorded as a percentage. Once curing rates enter the 90-100% range fires can begin to reach maximum rate of spread.
| Curing Rate % | Colour | Physiological changes |
|---|
| 0 | Green | From germination to start of seed head development |
| 10 | Green | Seed heads formed and flowering |
| 20 | Green-Yellow | Seed heads maturing and opening from top |
| 30/40 | Yellow-Green | Most seed heads mature and seeds dropping |
| 50/60 | Straw-odd | Half of all stems have dropped their seed |
| 70/80 | Straw-very little green | Some green still in 1/3 of stalks |
| 90 | Straw-odd green gully | Odd individual grass stalks showing some green |
| 100 | Bleached | All stalks fully cured – seed heads and stalks starting to break easily |
Method used to estimate grassland curing.
For a field guide to estimate curing got to: Guide to estimating grass curing
Grass Fuel Load
Grassland fuel load is estimated by field reports. Field reporters need to consider landscape trends in fuel load. Further information on the field reporting process is detailed below.
Grassland fuel quantity in the local area is the steady build up of vegetative matter, which will burn readily in a fire.
The rate of build up depends upon the balance between the rate of deposition and whether or not the fuel had been removed by fire or clearing.
Fuel for this purpose is defined as grass, standing crops or crop stubble or any fine flammable vegetative matter less than 6mm in diameter.
Fuel quantity is determined as one of three categories:
- Sparse (s):
Where previous fire activity, including hazard reduction or hand clearing, has removed fuel or where seasonal conditions has kept grass short and/or in short supply or resulted in the harvesting of crops. Grass height is less than 10cm (ankle height) and considered “eaten out”. - Moderate (m):
Where grass growth is considered “grazed” at or below knee height and above ankle height. Nominally, average grass height is between 10-49cm in height. - Prolific (p):
Where little fire activity has left fuels to build up over a period of time or where a good season has encouraged the growth of grasses and crops. Grasses are generally above knee height or greater than 50cm in height (natural).
Where little fire activity has left fuels to build up over a period of time or where a good season has encouraged the growth of grasses and crops. Grasses are generally above knee height or greater than 50cm in height (natural).
Fuel Continuity
The ease with which fire can spread from one area to another depends to a large degree on whether or not the fuel bed is continuous. Describing the fuel type as continuous or discontinuous gives an impression of the ease by which potential for fire spread. A discontinuous patch might be an area of 50cm2 repeated within the landscape.
Fuel Load equivalents
For the purposes of calculating the grass fire danger rating using the BoM standard a fuel load must be supplied.
This method is a relative method and is considered an interim until more accurate models or methods are introduced.
| Prolific (Natural) | Moderate (Grazed) | Sparse (Eaten Out) |
|---|
| Continuous | 6 | 4.5 | 2.0 |
|---|
| Discontinuous | 4.5 (e.g. Hummock situation) | 3 (e.g. harvested crop) | 1.5 |
|---|
Grass fuel condition
Grass fuel condition describes the height and arrangement of grass fuels. It is estimated using field observations in one of three categories:
- Eaten out: generally, less than 10 cm height.
- Grazed: between 10 and cm height.
- Natural: prolific growth over 15 cm tall.
Fuel condition is used in the AFDRS to calculate the rate of spread as part of the CSIRO grassland fire spread model.
Keetch Byram Drought Index
Keetch Byram Drought Index (KBDI) is an empirical estimate of the long term soil moisture deficiency. The index value represents the amount of rainfall, in mm, required to bring the soil to saturation. KBDI values range from 0mm, representing a completely saturated upper soil layer, to 200mm, representing the maximum possible drought conditions.
Prescribed Burning
The Prescribed burn Forecast Tool (PBFT) layer is an experimental product that is yet to be fully evaluated. It should be used with caution.
The Prescribed Burn Forecast Tool is a tool to identify the best times or windows of opportunity for low to moderate intensity prescribed burning in predominantly forested areas of NSW. It provides an hourly forecast map of prescribed burning conditions up to 4 days (96 hours) out from current time.
The intent is for the tool is to assist Prescribed Burn Supervisors with scheduling prescribed burns. It is not to be a “burn or no burn” (decision making) tool. It should be used as a prompt for a PBS to perform a detailed weather analysis (this may include requesting a Special Fire Weather Forecast from the Bureau of Meteorology) and to check conditions (fuel moisture content) in the field.
PBFT – How it Works
The tool uses a fuel moisture forecast system for prescribed burning developed by the CSIRO and Bureau of Meteorology forecast weather grids to derive flame height (McArthur leaflet 80), wind speed, and fuel availability (fuel moisture) values. A nominal score is given to each variable which is dependent on the suitability for burning. A matrix is then used to classify each 6km x 6km grid cell into the following categories:
| Classification | Prescribed Burn Index | Description | Action |
|---|
| Possible | 4-5 | Forecast conditions expected to be within prescription for a low-moderate intensity burn. | Schedule burn(s) if field measurements of fuel moisture content and local weather forecast are favourable |
|---|
| Marginal | 2.1-3.9 | Forecast conditions are expected to be outside of prescription. This could mean the conditions are either marginally too wet or windy or the FBI (flame heights) are forecast marginally too high. | Consider scheduling burn(s). Check fuel moisture content in the field and analyse local weather forecast. |
|---|
| Poor (too hot or windy) | <=2 | Forecast conditions are likely to be significantly outside of prescription. This could mean forecast conditions are either too windy or FBI or flame heights are too high. | Monitor conditions for future opportunities. |
|---|
| Poor (too wet) | <=2 | Forecast conditions are likely to be too wet (fuel availability too low) | Monitor conditions for future opportunities. |
|---|
Over-Riding Criteria:
As McArthur’s leaflet 80 is considered to perform best for FBI’s 12 and under, and has a very simple fuel moisture calculation, a number of overriding criteria have been developed:
- A forecast Fire Behaviour Index > 12 triggers automatic marginal conditions.
- A forecast Fire Behaviour Index > 25 triggers automatic poor conditions.
- A forecast Fuel Availability (Matthews) of 0-0.3 triggers automatic poor conditions.
Other Assumptions
The product assumes the hazard reduction is to be lit on flat ground or gently sloping.
Fuel availability has been calibrated for ridge top or driest conditions. Conditions outside of a ridge top may vary (be wetter) from the value used.
Temperature
Air temperature at forecast 2 meters above ground level, in degrees Celsius.
Max/Min temperature
Min’ indicates the forecast overnight minimum temperature expected between 6 pm and 9 am. ‘Max’ indicates the forecast daytime maximum temperature expected between 6 am and 9 pm
Humidity
Humidity
Relative humidity is forecast at 2m above ground level, expressed as a value between 0 and 100%.
Dew Point Temperature
Dew point temperature is forecast at 2m above ground level, in degrees Celsius. This represents the temperature to which the air would need to be cooled (at constant pressure), in order for condensation to begin. This is a more absolute measure of the air moisture content than Relative Humidity as it does not depend on the air temperature.
Wind
Average wind speed and direction at 10m above ground level, in km/h. Averages are calculated over a 10 minute interval.
he map displays wind speeds in colours and the fixed length arrows show the direction the wind is blowing towards.
Mixing Height
Displays the forecast depth of mixing through the lower atmosphere in meters
Wind at 1500m AMSL
Displays wind speeds at 1500 meters above mean sea level in colours and the fixed length arrows show the direction the wind is blowing towards
Rainfall
Total rainfall is forecast, in mm, in the three hours prior to the grid time step. This is provided in four confidence levels; rainfall total 10%, 25%, 50% and 75% chance.
Rainfall 75% Chance
Displays the amount of rainfall in the 24 hour day that there is a 75% probability of receiving. In other words, the amount that will occur in 3 in 4 days with similar weather conditions. The rainfall total 75% chance displays a high confidence that the rainfall shown will be reached.
Rainfall 50% Chance
Displays the amount of rainfall in the 24 hour day that there is a 50% probability of receiving. In other words, the amount that will occur in 1 in 2 days with similar weather conditions. The rainfall total 50% chance displays a moderate confidence that the rainfall shown will be reached.
Rainfall 25% chance
The rainfall total 25% chance displays a possible higher rainfall total. It provides a 24 hour forecast of "possible" rainfall totals in millimetres, based on a 25% chance of exceeding this amount (covers the 24 hour period from 15 to 15 UTC or 1am to 1am EST).
Rainfall 10% chance
The rainfall total 10% chance displays a possible higher rainfall total. It provides a 24 hour forecast of "possible" rainfall totals in millimetres, based on a 1-in-10 chance of exceeding this amount (covers the 24 hour period from 15 to 15 UTC or 1am to 1am EST).
Observations
PAWS
Only the observations from PAWS depicted as Deployed and Functional should be used. PAWS are depicted as "stowed" to denote that they are either stored at this location or are planned to be transported to this location. The observations from PAWS depicted as requiring maintenance should never be used. This symbology denotes that there is an issue with this station that is effecting its data. This data could be dangerously erroneous. These PAWS will be demobilised as soon as possible.
More Help
If you are experiencing difficulty with the FWV, contact the NSW RFS ICT Service desk on 02 8741 5123.