Geoscience Reference
In-Depth Information
approach involves using indexes that analyze the fire risk, by combining different
climate variables, which can either work together or cancel each other out regarding
their effects on the fire (as described in the previous section). This method is used
instead of trying to assess individually each t meteorological variable known as
being associated with fire risk.
In order for an index to work effectively, it should reduce the amount of false
alarms and missed alerts that occur. A false alarm is much less serious than a fire
alert that has been missed. However, too many false alarms means that the
emergency services may begin to question whether an alert is genuine each time one
is raised or they may even withdraw their services because of the waste of resources
that is generated each time a false alert is raised. A missed fire alert (for example, a
serious fire has occurred even though the meteorological risk index was low) must
be avoided at all costs.
8.2.3.2.
What is included in the indexes?
The aim of this section is not to give a list of all of the indexes that exist all over
the world, but to show that they have one common goal: to provide help so that the
most appropriate decision can be made to help fight a fire, or prevent it to start. A
meteorological index does not say that a fire is going to occur, all the more so as
what a fire will become is depending on “susceptibility” of the environment, such as
forest or shrubland (see earlier in this chapter). Should a fire be declared, the aim of
the index is to provide information on the easiness of fire to start, or how it will
spread. The index can sometimes be used to provide information on both of ignition
and spread, and is referred to as a mixed index. The index is used as an indicator to
describe any difficulties that firefighters and forest workers might have to face in
order to fight a fire, it also lets these service teams know what precautionary
measures need to be taken before they set out to fight the fires.
Since the 1970s, in France, the Orieux index has been used, which was named
after its main inventor. The Orieux index combines wind speed and the level of
water present in the soil water reserve [ORI 79].
In 1985 Carrega developed an index that combined the dryness of the air with
the variables of wind speed and the level of water present in the soil's water
reserves [CAR 85; CAR 91], and then in another index developed [CAR 88], the
variable of air temperature as well as a second soil water reserve, was added,
although this time the water reserve was superficial (saturated at 10 mm). The
reasons for including these two new variables were to elucidate the layer of water
present in the litter and in the first few centimeters of the soil (as a result rainfall or
by the dew point). The level of water found at this upper soil level can influence
whether a fire will develop or not.
Drouet worked on the speed at which flames spread [DRO 88], whereas Sol
based his work on the research of Carrega and Drouet and integrated it into a new
operational index [SOL 90], which was then replaced by a more complex index, the
