Fire behaviour can be described in such terms as rate of forward spread, flame height, scorch height, ground fire, crown fire, and so on. A fire may be described as “cool” or “hot” or of “low intensity” or “high intensity.” These are purely qualitative expressions and in no way indicate the damage potential of a fire. If a measure of fuel quantity was introduced, rate of spread would be the most adequate of the above descriptions.
The fire energy or fire intensity concept developed by Byram (1959), is one of the most complete descriptions of a fire which is meaningful in terms of the damage it can do under a specific set of circumstances. Fire intensity is the rate of energy release or rate of heat release per unit time, per unit length of fire front. Fire intensity can be expressed by a simple equation.
$${\rm{I}} = {\rm{Hwr}}$$
(1)
where I = fireline intensity in kW m−1 of fire front, H = heat yield in kJ kg−1 of fuel, w = weight of available fuel in kg m−2, and r = rate of forward spread in m sec−1.
Fire intensity can vary from 17 kW m−1 to around 103 000 kW m−1 in forest and grass-fires. However, the more normal intensity range for a grassfire would be 17 kW m−1 to 24 000 kW m−1.
In the intensity range of 17 kW m−1 to 350 kW m−1 little damage is done to forest trees although some significant ecological changes can occur in this range. Many Australian species, particularly members of the Proteaceae, Myrtaceae and Casuarinaceae can be killed by these low intensity fires but little seedling regeneration will result as the fire is not hot enough to induce germination. Dogwood (Cassinia aculeate [Labill.] R.Br.) is another fire weed species which can be killed by a remarkably low intensity fire. Many of these species are highly inflammable and are undesirable fire fuel components.
In the intensity range 350 kW m−1 to 1700 kW m−1 some slight physical damage to eucalypt and coniferous species occurs. Some ultimate branchlets on more fire sensitive eucalypts may be killed and occasional log degrade in the form of gum veins may occur.
Fires in the intensity range of 1700 kW m−1 to 3500 kW m−1 generally cause physical damage to the bole and crown of eucalypts and some timber degrade is likely. Loss of increment due to crown scorch will occur in the more fire sensitive species. Young regeneration up to 4.5 m in height may be killed. A fire intensity of 3500 kW m−1 is about the maximum that a Pinus radiata D.Don plantation can stand without causing some tree deaths. However, Pinus elliottii Engelm. can withstand a fire intensity of 3500 kW m−1 to 7000 kW m−1, without causing death of mature trees. A 2 yr to 3 yr loss of increment will result from a fire of this intensity.
The expression of fire intensity gives the rate of energy output of each meter of the fire front, and may at first appear a little complicated. However, the three variables in the equation are easily measured and provide a precise means of estimating the damage potential of a fire.
Heat yield or heat of combustion is fairly constant over a wide range of natural fuels and can generally be taken as 14 000 kJ kg−1 to 15 000 kJ kg−1 of fuel. It varies slightly with fuel moisture content and may be reduced due to incomplete combustion in a fast spreading, high intensity fire.
The second term, weight of fuel consumed, is difficult to estimate, and generally requires measurement. However, reasonably precise ocular estimates can be made with some practice. The range of fuel quantity is not great in natural fuels and may range from 4.5 Mg ha−1 to around 45 Mg ha−1 in exceptionally heavy, long unburnt fuels. In the absence of fuel quantity measurements, yardsticks such as time since last burn, fuel depth and fuel continuity may be used to estimate roughly fuel quantity.
Fuel availability is complicated by rain effects and the relationship between total fuel and available fuel will be discussed later.
Rate of forward spread of a fire varies considerably and may range from <1 m sec−1 to 1 m sec−1 in the case of a fire burning in eucalypt forest or from <1 m sec−1 to 6.1 m sec−1 in the case of a grassfire. However, it is the easiest of the three variables determining fire intensity to measure providing some time record of the fire is available.
Fire intensity, expressed in kW m−1 of fire front should be associated with specific fire behaviour characteristics such as flame height, scorch height, rate of spread and damage.
Beyond intensities of 3500 kW m−1, physical damage to the standing forest varies widely according to the heat tolerance of individual species. Most understory species will be killed by the fire but most will coppice or regenerate heavily from seed.
The relationship between fire intensity and damage to a 40 year-old pole stand of jarrah (E. marginata Donn. Ex. Sm.) is shown in Figure 1. Jarrah is one of the most fire resistant eucalyptus species and the data is derived from damage incurred in the Dwellingup Fire of January, 1961. The estimate of physical damage is expressed in terms of final crop potential. A damage assessment of 100 % means that no trees in the stand will make final crop trees due either to death or severe log degrade. It seldom means that all trees are killed. A 50 % assessment of physical damage indicates that sufficient trees remain for a final crop, but most will carry severe fire scars and there will be at least 20 % loss of timber due to degrade, and an absolute loss of 5 years increment.
The same study related monetary loss per acre to fire intensity and these values are shown in Figure 2. This loss is in terms of commercial timber and does not include other values such as watershed or recreation.
The fire intensity is generally calculated for the headfire region and intensity on the sides and rear of a fire will be much lower due to slower rates of spread. The headfire zone generally embraces no more than one third to one half of the total area burnt by an individual fire and the fire damage assessed for the headfire region should not apply to the whole fire area.
The fire intensity concept has an important advantage in that it can be readily calculated without having seen the fire, provided the rate of spread and fuel consumption per unit area is known.
Fire danger rating systems for Forest Fuel Types developed by McArthur (1963) are based on rate of spread derived from meteorological and fuel factors and can be related directly to fire intensity. This system is in the form of a simple slide rule and could be readily applied to ecological fire studies. Cheney (1965) has further enlarged on the fuel energy concept involved in this system of fire danger rating.