Climate Change and Early Warning Systems for Wildland Fire - Reducing Disaster: Early Warning Systems for Climate Change - page 135

Box 7.3 The Canadian Forest Fire Weather Index (FWI) System

The FWI System is based on the effects of weather parameters on forest fl oor
fuel moisture conditions and generalized fi re behavior that can be expected in
a typical jack pine ( Pinus banksiana Lamb.) forest stand (Van Wagner 1987 ).
Fire weather observations are used as inputs for three Fuel Moisture Codes,
which represent three classes of forest fuel with different drying rates, nomi-
nal fuel depths, and nominal fuel loads (see properties Table below). The Fine
Fuel Moisture Code (FFMC) represents the moisture content of dead fi ne
fuels and litter on the forest fl oor. The Duff Moisture Code (DMC) represents
the moisture content of loosely compacted decomposing organic matter. The
Drought Code (DC) represents the moisture content of deep compact organic
matter of moderate depth. The three Fuel Moisture Codes are each calculated
with a daily time-step and include their previous day's value as an input to the
current day's value. It is through this feedback mechanism that antecedent
information is incorporated into the FWI System and each new day's moisture
level is determined. A measure of the fuel drying speed is the time lag at
which the fuel loses 1 − e −1 (about two-thirds) of its free moisture content
above equilibrium.
The Fuel Moisture Codes provide input to the Fire Behavior Indices. The
Initial Spread Index (ISI) estimates the combined infl uence of wind speed and
the FFMC on fi re spread. The Buildup Index (BUI) is a combination of the
DMC and the DC, representing the combustibility of heavier fuels, such as
deeper organic forest fl oor layers or larger-sized dead roundwood fuels
(branches, logs). The ISI and the BUI are combined to determine the value of
the Fire Weather Index (FWI), representing the head fi re intensity of a spread-
ing fi re during the peak burning period of the day.
The FWI System is used operationally as an indicator of landscape-level
fi re danger, as infl uenced by weather. Although it was originally designed
using jack pine as a baseline fuel type, it has been calibrated using local fi re
and weather data to represent general fuel moisture and fi re behavior condi-
tions for fi re regimes in many different global regions.

General properties of the three Fuel Moisture Codes (exact rates vary with
environmental conditions)
Fuel
Moisture Code

Time
lag (days)

Water
capacity (mm)

Nominal
fuel depth (cm)

Nominal fuel
load (kg m −2 )

FFMC 2/3 0.6 1.2 0.25

DMC 15 15 7 5

DC 53 100 18 25