The change in moisture content of dead and downed woody surface fuels throughout the day is critical to calculating the changes in fire behavior. In general, drier fuels increase fire spread rate, fireline intensity, and fuel consumption. FARSITE uses a model to calculate fuel moisture during the simulation in response to changing weather conditions. Moisture contents of live fuels are assumed to remain constant in FARSITE throughout the simulation (but these can be changed in the .FMS file during the simulation). Dead fuel moisture is an important input to two sub-models used in FARSITE:
the surface fire behavior model (Rothermel 1972) for determining spread rate and intensity of surface fires, and
the "Burnup" model (Albini and Reinhardt 1995) for determining fuel consumption and emissions (during flaming and after the passage of the flaming front).
The rate of change of the moisture content is dependent on the diameter of the woody fuel particle and the amount of change in environment conditions. Historically, the diameters of the woody fuel particles have been classified according to their "time lag". Time lag refers to the length of time that a particle responds to within 63.2% (1-1/e) of the new equilibrium moisture content (either drying or wetting). Larger diameter fuels generally have longer time-lags, meaning they respond more slowly to changes in environmental conditions. The time lag categories traditionally used for fire behavior and fire danger rating are specified as: 1hr, 10hr, 100hr, and 1000hr and correspond to round woody fuels in the size range of: 0-¼", ¼"-1", 1"-3", and 3"-8" (0-.635cm, 0.635-2.54cm, 2.54-7.62cm, and 7.62-20.32cm) respectively. Loadings (weight/area) of dead fuels in these size-classes are required to describe surface fuels for fire modeling (Anderson 1982).
The environmental factors that determine the fuel moisture of a fuel particle are:
Moisture content of the air (measured as relative humidity)
Solar radiation (as modified by cloud cover)
Rainfall (amount and duration)
These are heavily dependent on the local topographic and environmental site factors for the fuel particle:
Forest canopy cover
As a FARSITE simulation progresses, the moisture contents of the four fuel size classes are adjusted for the changing weather conditions over time at the local site. To do this, FARSITE uses the dead fuel moistures supplied as "initial fuel moistures" from the .FMS file (for 1hr, 10hr, and 100hr time lag categories) and modifies them according to the changes in temperature, humidity, and rainfall in the weather (.WTR) file, cloud cover in the wind (.WND) file, and the local site conditions (elevation, slope, aspect, canopy cover) from the landscape (.LCP) file.
Graph of fuel moisture content over time (about 3 days) for three time-lags of dead woody fuel sticks. The fuel moistures were calculated using the model from Nelson (2000). Greater moisture response amplitude is produced for the smaller time-lags.
Since the moisture content of 1000hr fuels are only used for fuel consumption and post-frontal combustion, their initial moisture content is obtained from the coarse woody profile (.CWD) file. The moisture content of fuels larger than the 1000hr size class (specified in a coarse woody profile (.CWD) file for fuel consumption and emissions) are assumed to remain constant throughout the simulation because of their long time lags.
The air temperature and relative humidity are modified for elevation assuming a fixed lapse rate (Rothermel et al. 1986). Solar radiation is modified for slope steepness and orientation and reduced by the percentage of canopy cover and cloud cover specified in the wind (.WND) file. Rainfall is assumed constant across the coverage of the weather (.WTR) file.
To account for the spatial variation in local site conditions (from the landscape (.LCP) file), FARSITE generates a catalogue of representative fuel "particles" for all combinations of categories of each site factor (elevation, slope, aspect, canopy cover) and initial moisture content for each size class of fuel. This is necessary because the fuel particles have a state "memory" of moisture and temperature at all internal positions (within the particle) that are unique to a given local site. The size ranges for categories for each site factor can be adjusted at run time (see Models > Dead Fuel Moisture). The moisture model is used to update the moisture content of each representative particle in the catalogue at each simulation time step. Actual values used for calculating fire behavior at an arbitrary time and point on the fire front are obtained from the catalogue by interpolation.
FARSITE version 4.0 has implemented a new model for calculating dead fuel moisture content of 10hr fuels (Nelson 2000). As used here, it also handles other fuel size-classes (1hr, 100hr, and 1000hr) with modifications by Nelson. This model replaces those developed by Rothermel et al. (1986) for BEHAVE (Andrews 1986) and Deeming et al. (1977) for the NFDRS (Bradshaw et al. 1983) as used in previous versions of FARSITE (Finney 1998). The new fuel moisture model (Nelson 2000) calculates the exchange of water between the environment and the surface of a round wooden stick and transport of water within the stick itself. The stick is assumed to be without bark and located above ground. For computational efficiency, the 1hr fuel moisture in FARSITE is obtained from a calculation involving the equilibrium moisture content (Bradshaw et al. 1983) instead of Nelson's 1hr calculation. There is yet no general duff moisture model so the empirical relationship developed by Harrington (1982) is used to predict duff moisture content from the 100hr fuel moisture value.
As implemented in FARSITE, Nelsons (2000) model is used to calculate fuel moistures a each time step in the simulation or to "precalculate" fuel moistures for the range of dates and times specified. Precalculation takes place before the fire simulation begins and allows more rapid display and calculation of the fire behavior. In either case, the moistures do not need to be recalculated for repeated simulations unless modifications are made to the .WTR or .FMS files, the duration settings (Simulate > Duration), or the options for dead fuel moisture (Model > Dead Fuel Moisture). All fuel moistures will be saved when a bookmark is created or updated.
At the beginning of a FARSITE simulation, the dead fuels of all size classes in each fuel model will not reflect the influences of the local site conditions. Fuels for a particular fuel model all have identical moisture contents obtained from the .FMS file regardless of where on the landscape the fire behavior is calculated. As the simulation progresses for a few hours or a day, however, the moisture content of the finer fuels (1hr, 10hr) will increasingly reflect the local site conditions because of their short time-lag. The influence of the constant initial conditions on fire simulations varies by the size and growth rate of the fire, spatial variation in topography and canopy cover, and the length of the simulation. The effect will be minor for long simulations (more than a few days) for slow moving fires ignited from a point source. The effect will be most significant for fires starting from an existing large perimeter in complex topography and simulated for only few hours.
To minimize the effect of this period of fuel "conditioning" on a fire simulation, the Simulate > Duration setting allows for a "conditioning period" to be inserted before the start of the simulation. The conditioning period can be set for several days to allow the catalogue of fuel moistures to reflect the range of local site conditions before the fire is simulated. When the fire simulation begins after the conditioning period, the landscape will contain spatial variation in dead fuel moisture with less influence of the initial fuel moisture input conditions.