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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. |
The change in moisture content of dead and downed woody surface fuels throughout time and topography is often critical to calculating fire behavior characteristics. In general, drier fuels increase fire rate of spread, fireline intensity, and fuel consumption. FlamMap uses a model to calculate dead fuel moisture during a conditioning period in response to changing weather conditions and topography. Moisture contents of live fuels are assumed to remain constant in FlamMap throughout the conditioning period. Dead fuel moisture is an important input to the surface fire behavior model (Rothermel 1972) for determining rate of spread and intensity of surface fires.
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: 1 hour, 10 hour, 100 hour, and 1000 hour 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:
Air Temperature
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:
Elevation
Slope
Aspect
Forest canopy cover
As a FlamMap conditioning period 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 from the Initial Fuel Moistures (.FMS) File (for 1 hour, 10 hour, 100 hour, and 1000 hour time lag categories. Then modifies them according to the changes in temperature, humidity, rainfall, and cloud cover from the Weather Stream (.WXS) File, combined with the local site conditions (elevation, slope, aspect, canopy cover) from the Landscape (.LCP) File.
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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. |
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 for the time period specified in the Weather Stream (.WXS) File. Rainfall is assumed constant throughout the time period specified in the Weather Stream (.WXS) File.
To account for the spatial variation in local site conditions (from the landscape), FlamMap 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 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.
FlamMap 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 versions 1 ,2, & 3 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 1 hour fuel moisture in FlamMap is obtained from a calculation involving the equilibrium moisture content (Bradshaw et al. 1983) instead of Nelson's 1 hour calculation.
As implemented in FlamMap, Nelsons (2000) model is used to calculate 10 hour and 100 hour fuel moistures with time steps of 0.1 hour and 0.2 hour respectively for the range of dates and times specified in the conditioning period. The dead fuel moistures used for the fire behavior calculations are those at the end of the conditioning period.
At the beginning of a FlamMap conditioning period, 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 will use the moisture contents obtained from the Initial Fuel Moistures (.FMS) File regardless of where on the landscape the fire behavior is calculated. The conditioning period can be set to allow the catalogue of fuel moistures to reflect the range of local site conditions before calculating fire behavior characteristics.
As the conditioning period progresses for a few hours or a day 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 behavior varies by the spatial variation in topography and canopy cover, and the length of the conditioning period. The effects will be minor for landscapes with little topographic variation and canopy cover. Because the short time-lag fuels (1hr, 10hr) have the most influence on calculated fire behavior, conditioning periods longer than a few days do not change results significantly for weather streams with low variability. On the other hand the longer time-lag fuels (100hr, 1000hr) change slowly and require longer conditioning periods to reflect the local topographic and shading effects. When these 100hr and 1000hr time-lag fuels are of interest (i.e. for a SpatialFOFEM run) then using a conditioning period ranging from a week to a month or more will be appropriate.
At the end of the conditioning period, the landscape will contain spatial variation in dead fuel moisture with less influence of the initial fuel moisture input conditions.
