Research Models Fire Physics

We've been using the same fire behavior model for over twenty years. It does a pretty good job, but it relies on extrapolating between measurements of a small number of fires. That doesn't account for the wide range of conditions and combinations of variables that make every fire unique.

So I'm working on a fire behavior model based on the fundamental physics of fire. It will then be tested against data collected from detailed measurements of real fires.

The new model will let us predict two important variables that the current model can't do: 1) how the fire responds to convection -- moving hot air; and 2) radiation -- the flame's heat energy that reaches the fuel.

This will let us model exactly when the fuel will reach its ignition point, which will make simulations of fire effects on real terrain much more accurate.

The new model will help managers better decide whether to light prescribed burns or choose the safest way to fight wildfires. It will provide the basic inputs for the following tools, which now require a lot of manipulation to get a good answer.

Forest Vegetation Simulator

Foresters and planners already use computer models like this one to see how a stand will look after a treatment.

In this case, there's a set of models that work together to create realistic pictures of possible stand structures and fire behavior. It all starts with the Forest Vegetation Simulator (FVS), which incorporates data on an area's climate, soils, terrain, and plant ecology.

The FVS model lets the user start with a particular stand, including the kinds of trees, their location in a one-acre plot, and their sizes. The model then "grows" the stand to show how it will look over several hundred years. Different thinning, planting, and prescribed burn treatments can be applied and the results observed.

To show what happens when the stand burns, the current fire behavior model is used. This model runs from information the user supplies on weather and fuels (what kinds of stuff there is to burn and how much of it there is). It calculates flame height and predicts which trees will be killed by burning and scorching. Finally, the FireFX model uses the fire behavior predictions to draw pictures of the actual burn in the stand created by FVS. This model does not predict how far the fire will spread.

The simulated stands shown here in "Living with Fire" were created from real data gathered at research plots near Missoula, Montana. The old growth stand comes from the west face of the Bitterroot Valley in the Bitterroot National Forest, and the second growth stand comes from the Sixmile Creek Drainage in the Lolo National Forest.

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FARSITE

The Fire Area Simulator (FARSITE) is a model that lets the user predict the spread of a fire over an entire landscape. It works with a Geographic Information System (GIS) to map the fire onto a real area. It shows the rate of a fire's spread over time (the concentric white lines) and the flame height (yellow = longer, hotter, flames). FARSITE uses the current fire behavior model to predict burning based on the temperature, humidity, wind, slope, and fuel type and amount.

A user can model a planned prescribed fire or an already-burning wildland fire by working with an existing database of terrain maps (digital versions of the USGS topographical maps) and vegetation maps (created from satellite images and ground surveys). Real or predicted weather observations are added, and FARSITE will use the current fire behavior model to calculate where and how fast the fire will go, including crown and spot fires.

Then suppression actions can be simulated to determine the most effective use of resources such as ground crews and air tankers.

The simulations shown here in "Living with Fire" used the outputs from the SVS model (above). They have been mapped onto the Lost Horse Creek area of the west slope of the Bitterroot Valley, in western Montana.

Bret Butler
fire researcher