The 1986 Annaburroo experimental grassland fires: data

Experimental design

The experiments took place at Annaburroo station, NT, Australia (12°54′25″S, 131°40′02″E, Fig. 3). The site is a broad flat area in the flood plain of the Mary River. The area was divided in 10 main blocks (A–J in Fig. 3), and then subdivided into 170 plots. Of these, 121 fitted the requirements of the study (plots with trees or inadequate fuel distribution were excluded). The experimental plots had two sizes: 100 × 100 m and 200 × 200 m. Fuel breaks of bare mineral earth 1.5 or 3 m wide separated the plots. The experiments were carried out during July and August 1986. This period is characteristically the middle of the dry season at the location. Grasses are normally fully cured, and prevailing weather patterns ensure a predominance of warm and dry easterly winds over the area. Rain events during this period are infrequent. A wide buffer area was burnt around the perimeter of the experimental area to reduce the risk of escape of experimental fires even under heightened fire danger conditions. Table 3 summarises the means and ranges of weather, fuel and fire behaviour data collected during the experimental program. Table A1 lists all the experimental fires by date.

Fig. 3.

Map of the layout of the experimental blocks (A–J) and 200 × 200 m experimental plots within each block at Annaburroo Station. Some plots were further divided into 100 × 100 m plots. Each main plot was defined by a 3-m wide graded bare-earth break. Inset map indicates the location (star) of the experimental site in the Northern Territory, Australia.

Fuel condition and sampling

The selected site contained two distinct grass species, Eriachne burkittii Jansen (commonly kerosene grass) and Themeda triandra Forrsk. (previously known as Themeda australis R.Br, commonly kangaroo grass) that provided fuel beds of dissimilar characteristics. Further manipulation of the fuel bed was achieved by controlling grass height (by cutting at 50 and 25% of the undisturbed height) and fuel quantity (by removing or leaving the cut fuel). This yielded six different fuel bed treatments (see Treatment in Table 1). Treatments were randomly assigned to each plot.

Surface-area-to-volume ratio for the two major species was measured on selected plants. The diameter of stalks and thickness of leaves were measured with a micrometer and converted to a surface-area-to-volume ratio following Brown (1970). The mean surface-area-to-volume of a plant was estimated as the average value of leaves and stalk component, weighted by their length (Gould 1991).

Fuel bed structure at an experimental plot level was sampled through a 16-point grid located within the plot (Fig. 4). At each sample point, the mean height of the surrounding grass sward was measured and a destructive sample (0.3 × 0.6 m quadrat) of all standing and matted fuel collected. Each fuel sample was oven-dried at a nominal temperature of 104°C for 24 h to determine its dry weight, expressed as the mass per unit area in units of kilograms per square metre. Plots with large fuel heterogeneity or discontinuous fuel cover were excluded from burning or from analysis.

Fig. 4.

Experimental fire B111 fuel plot grid laid over the fire isochrone map.

Weather data

An automatic meteorological station (AWS) was located at a fixed site within the experimental area for the duration of the experimental program (see Fig. 3). At this AWS, measurements of wind speed were made at 10 and 2 m above ground. Wind speed measurements used UNIDATA anemometers that were calibrated in the CSIRO Division of Atmospheric Research wind tunnel in Aspendale, Victoria. Wind direction was measured at the 10-m height using a UNIDATA wind vane. Wind speed and direction data were recorded as 1-min averages.

Air temperature, relative humidity and solar radiation at the AWS were measured at 1.4-m height. Air temperature was measured using a solid-state temperature probe (UNIDATA AD537) that was shielded and aspirated with an encapsulated UNIDATA 2B57 pre-amplifier. Relative humidity was measured with a Philips Humidicap sensor. Solar radiation was measured with a pyranometer (make and model unknown), calibrated using the maximum recorded value on cloudless days. The temperature probe was calibrated in a temperature-controlled oil bath over the range 20–40°C. The relative humidity sensor was calibrated against field readings using an Assman psychrometer. Air temperature, relative humidity and solar radiation were measured and recorded in UNIDATA dataloggers at 1-min intervals.

On selected days, vertical temperature measurements were taken every 150 m from the ground up to ~1400 m using a helicopter with either an Assman psychrometer or the helicopter’s temperature probe. On most days, the data revealed a super adiabatic layer near the ground up to 150 m, a well-mixed layer aloft to ~800 m and an inversion above that (Durre and Beer 1989).

For each experimental fire, wind speed at 2-m height was measured at each plot corner (NW, NE, SE, SW). Purpose-built three-cup sensitive anemometers (Bradley 1969) with a start-up speed of 0.05 m s⁻¹ were used for these measurements, with the anemometers being calibrated in the wind tunnel as described above. These wind speeds were recorded on pre-synchronised UNIDATA data loggers at 5-s intervals, with the recorded value being the average of the 5-s period. Fig. 5 illustrates the variability of the wind speed measured before, during and after an experimental fire.

Fig. 5.

Wind speed variability as measured at the four anemometer locations prior to, during and after experiment B111.

Fuel moisture content and curing level

Four samples of fully cured grasses were collected in, or within a few metres of, each experimental plot for determination of fuel moisture content representative of the conditions of the experimental fire. Samples (approximately 30 g each) were collected in an undisturbed area where boundary effects, such as the existence of a fire break, were not present. Two samples were collected immediately prior to ignition and two after the experimental fire was concluded. The samples were stored in a hermetically sealed container to ensure no moisture loss occurred prior to determination of wet weight. Wet weight was determined within hours of sample collection using a sensitive balance, and then the sample was oven-dried at 104°C for 24 h and reweighed. Fuel moisture was calculated as a fraction of oven-dried weight (see Matthews 2010). The average of the four measurements expressed the dead fuel moisture content of each experimental fire.

Curing level was initially not a variable in the experimental burn program as it was conducted during the dry season in uniformly fully cured grasses. However, an unseasonal rain event just prior to commencement of the burning experiments caused germination of green shoots underneath the fully cured grass midway through the burning experiment. Subsequently, curing level for each experimental plot was visually estimated by the two senior scientists on site based on guidelines developed from previous experience in determination of grass curing through destructive sampling methods. It was felt by observers that the new growth under the taller fully cured grass had minimal effect on fire behaviour as the fire propagated through the taller dead fuels.

Fire behaviour data

Fire behaviour monitoring

Fire behaviour observations were recorded at 1-min intervals by experienced ground observers accompanying the fire progression. These observers identified features such as flame dimensions (depth, height and angle; see Fig. 6), occurrence of spot fire ignitions ahead of the main fire, wind changes and up- and down-draughts. Low-altitude, oblique aerial photographs were taken from a helicopter at 15–30 s intervals (Fig. 2a–e). The photographs were taken from a position that produced a depression angle of approximately 45° to the principal point (approximately the centre of the plot) and 60° to the base of the plot.

Fig. 6.

Cross-section of stylised wind-driven flame front depicting the three flame characteristics measured. Flame angles >90° indicate flame is leaning towards burnt ground.

Data interpretation and intermediate calculations

The process of interpreting fire propagation and averaging its behaviour over a burn observation period and for a single experimental fire requires some explanation.

A burn observation period was defined as the interval between two sequential oblique aerial photographs depicting the location of the flame front. From this, measurements of rate of fire spread and effective head fire width were taken. The 2-m wind speed measurements made at each of the four plot corners were then averaged over this observation period. Careful examination of the anemometer data was conducted to ensure they were representative of the wind field driving the fire. Data from individual anemometers were removed if the location of the anemometer was influenced by external factors such as the presence of trees, firefighting vehicles, or the approaching flame front. A vertical wind profile and estimate of atmospheric stability were calculated at the AWS for each experimental fire (see Durre and Beer (1989) for details). This wind profile, along with the measured flame characteristics and 2-m wind speeds, were used to calculate a representative 10-m open wind speed and a mid-flame height wind speed for each experiment.

The fuel sample grid was overlayed on each of the fire isochrone maps. The mean fuel load and fuel height for each fire plot was calculated from the sample points within the burnt area (Fig. 4).