Fire Models Range from Fitted Curves to Solved Conservation Equations
Every fire spread model sits somewhere on a ladder defined by how much physics it solves versus how much it borrows from measured fires. At the empirical end, a model is a curve fitted to observed fires: the inputs go in, a regression comes out, and it is only trustworthy inside the range of fires it was fitted to. In the middle sit semi-physical models, which write a physical energy balance but close it with coefficients measured in a laboratory; the Rothermel equation is the canonical example. At the far end, physics-based models discretise the air and the fuel and solve conservation of mass, momentum and energy directly, inventing no fire-specific coefficients but paying for it in computer time. The test that places any model on the ladder is a single question: if you ran it on a fire unlike anything in its calibration set, would it still be defensible? Empirical models say no, physics-based models say in principle yes. After this Concept you can place a named fire model on this ladder and state what its position costs and buys.
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Every fire spread model sits somewhere on a ladder defined by how much physics it solves versus how much it borrows from measured fires. At the empirical end, a model is a curve fitted to observed fires: the inputs go in, a regression comes out, and it is only trustworthy inside the range of fires it was fitted to. In the middle sit semi-physical models, which write a physical energy balance but close it with coefficients measured in a laboratory; the Rothermel equation is the canonical example. At the far end, physics-based models discretise the air and the fuel and solve conservation of mass, momentum and energy directly, inventing no fire-specific coefficients but paying for it in computer time. The test that places any model on the ladder is a single question: if you ran it on a fire unlike anything in its calibration set, would it still be defensible? Empirical models say no, physics-based models say in principle yes. After this Concept you can place a named fire model on this ladder and state what its position costs and buys.
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