A spread equation needs numbers for the fuel bed — how much fuel there is in each size class, how finely divided it is, how deep the bed sits, and the moisture at which it stops burning. Nobody measures those in a live incident, so fire modelling uses standard fuel models: named, published parameter sets chosen to represent a vegetation type. The original set has thirteen, running from short grass to heavy logging slash; Scott and Burgan's later set of forty splits them into grass, grass-shrub, shrub, timber-understorey, timber-litter and slash-blowdown families with dry and humid variants. Choosing one is a modelling decision, not an observation: you are asserting that this patch of ground behaves like that published fuel bed. The confusion this resolves is that a fuel model is not a picture of the vegetation. Two visually different stands can share a fuel model if they carry fire the same way, and one photogenic forest may need a different fuel model in spring than in autumn. After this Concept you can explain what a fuel model supplies to a spread equation and why swapping one changes the forecast more than most weather changes do.
Fire behavior fuel models are standardized combinations of fuel descriptors (load, size class, moisture of extinction, live/dead status) that serve as inputs to Rothermel's fire spread equation, allo…