Fire behaviour describes the basic physical features of fire, such as the rate of spread, the amount of energy released, and the height of the flames. These features are determined by the quantity and quality of the fuels, and the prevailing fire weather conditions. The major determinants of fire weather are temperature, humidity and wind speed. These meteorological measures may be used in conjunction with soil and fuel moisture features to derive Fire Danger Indices, such as the commonly used Forest Fire Danger Index (FFDI). Knowledge of the seasonal changes in fire weather and fire behaviour savannas is necessary so that the impact of fires on the environment may be better understood.

Seasonal variation in fire weather for the Jabiru region of Kakadu was explored using 11 years of meteorological data. The average FFDI for Jabiru increases steadily through the dry season, from less than 10 at the onset of the dry season (April), to about 40 in the late dry season (September-October). Extreme values (>50) occur occasionally, but even so the peak FFDI values for Jabiru are far less than the peak values of about 100 that have been recorded for eucalypt forests in southern Australia.

Fire behaviour is often described in terms of fire intensity, defined as the product of rate of spread, fuel load, and the heat released from the fuel during combustion. This measure describes the amount of energy released per unit length of fire front, in units of kilowatts per metre. At Kapalga, we measured the intensity of each of the Early and Late fires by measuring rate of spread and fuel loads. We then related intensity to post-fire measures of fire severity such as scorch height in tree crowns, and the amount of ground burnt.

Pre-fire fuel loads ranged from 2-5 tonnes per hectare in annually burnt sites. Fuel loads were 20-50% higher in the late dry season (as a consequence of tree leaf fall) than in the early dry season, and were also drier. Fuel loads in unburnt savannas were 5-10 t ha-1. Average fire intensity for Early fires was 2200 kWm-1, compared with 7700 kWm-1 for Late fires. This compares with potential intensities of 100,000 kWm-1 for crown fires burning under extreme conditions in south-eastern Australia.

The height of charred leaves (char height, itself a surrogate for flame height), the height of scorched leaves in the tree canopies, and the amount of the grass layer consumed by fire were good indicators of fire intensity. Char height increases by about 0.5 metre for each 1000 kWm-1. Fires above 2000 kWm-1 consume all the grass layer; above 4000 kWm-1 fires scorch the canopies to a height of about 20 metres. Scorch height and char height are therefore useful post-fire indicators of fire intensity, and can be easily used by fire researchers and managers to assess fire intensity after the event.