The fire behavior calculations for this model are the product of fire behavior libraries developed by the Missoula Fire Sciences Laboratory , Missoula, MT.
The IFTDSS Landscape Burn Probability (LBP) model is identical to Minimum Travel Time (MTT) Burn Probability in FlamMap 6.0, with some fixed modeling inputs (see below for details).
FlamMap MTT Background
FlamMap is a fire behavior mapping and analysis software application that computes potential fire behavior characteristics (such as spread rate, flame length, and fireline intensity) over an entire landscape under constant weather and fuel moisture conditions input by the user. FlamMap simulates surface and crown fire behavior characteristics using Rothermel’s 1972 surface fire model, Van Wagner’s 1977 crown fire initiation model, and Rothermel’s 1991 crown fire spread model.
The MTT part of FlamMap is a two-dimensional fire growth model. It calculates fire growth and behavior by searching for the set of pathways with minimum fire spread times from point, line or polygon ignitions. MTT simulates fire spread by Huygens’ principle where the growth and behavior of the fire edge is a vector or wave front (Richards, 1990; Finney, 2002). MTT includes heading, flanking, and backing spread. Because the MTT calculations spread under constant weather and fuel moisture conditions it enables analysis of the effects of spatial patterns in fuels and topography (Finney, 2006).
For LBP, thousands of randomly located ignitions are used and the resulting fires are spread using the MTT algorithm until 98% percent of the burnable landscape has burned. The probability of burning and resulting fire intensity is calculated from these repeated simulations. See the Landscape Burn Probability Output Overview topic for more details.
Many of the model inputs are controlled by the user (see the Input Overview topic for more details); however some model inputs are fixed in IFTDSS LBP runs and cannot be changed by the user:
- Resolution of the run: Landscapes over 250,000 acres are resampled prior to model run initiation (See the Resampling topic for more details).
- Number of fires simulated: Ignitions are randomly located. A minimum 1,000 fires is used. In the case that 1,000 fires is not sufficient, IFTDSS keeps adding ignitions until at least 98% of burnable pixels (cells) on the landscape burn or there is no significant change to the percentage burned even if the 98% target is not hit (see the Number of Fires Simulated topic for more details).
- Spotting delay: The time between ember landing and initiation of fire spread is set to 0 minutes.
- Spotting seed: The spotting seed is a randomly generated value that determines whether or not a specific spot causes an ignition. In IFTDSS, the spotting seed is set for the first model run for a given landscape family and Fire Size List (see the Fire Size List topic for more details). Subsequent runs using the same combination re-use the spotting seed. This allows for accurate comparisons between model runs using the same landscape family and ignition. Such comparisons can be useful to test the effectiveness of fuel treatment alternatives or to compare the impacts of changing weather parameters. Were spotting seed not held constant in these scenarios, you would be unable to deduce whether changes in fire behavior were due to the changes you specified or the result of differences in spotting.
There are two key differences between the risk assessment approach used by IFTDSS and FlamMap, relative to the Large Fire Simulation Model (FSim, Finney and others 2011):
- Weather over time
- Suppression efforts
Weather over time
IFTDSS and FlamMap model fire behavior with a fixed set of weather and fuel moisture that you input. FSim models fire behavior over multiple days and takes into account that past ~20 years of historic data for forecast what the weather is likely to be when these events occur.
IFTDSS and FlamMap model fire behavior do not take into account fire suppression efforts. FSim incorporates suppression events, based on the line building rates in various fuel types, when determining fire progression.