A Fire Spread Model Predicts Rate, Perimeter, Arrival Time and Intensity
A fire model does not answer "how big will the fire be" with one number. It produces four different things, and confusing them is the most common mistake made when reading model output. Rate of spread is a speed, usually metres per minute, and it is directional: the head, the flanks and the back all move at different speeds. The perimeter is a closed curve at one instant. The arrival-time field gives, for every point on the landscape, the clock time the flaming front reaches it, so a map of contours is a map of the fire at many instants at once. Fireline intensity is a power per unit length of fire edge and is what a suppression decision actually keys on. The usable test is: name the quantity, its units, and whether it is a scalar at a point, a curve, or a field over the whole landscape. Any claim like "the model says the fire will be 4000 hectares" is an integral of the arrival-time field, not a prediction the model made directly. After this Concept you can read a model output and say exactly which of the four quantities is on the screen.
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A fire model does not answer "how big will the fire be" with one number. It produces four different things, and confusing them is the most common mistake made when reading model output. Rate of spread is a speed, usually metres per minute, and it is directional: the head, the flanks and the back all move at different speeds. The perimeter is a closed curve at one instant. The arrival-time field gives, for every point on the landscape, the clock time the flaming front reaches it, so a map of contours is a map of the fire at many instants at once. Fireline intensity is a power per unit length of fire edge and is what a suppression decision actually keys on. The usable test is: name the quantity, its units, and whether it is a scalar at a point, a curve, or a field over the whole landscape. Any claim like "the model says the fire will be 4000 hectares" is an integral of the arrival-time field, not a prediction the model made directly. After this Concept you can read a model output and say exactly which of the four quantities is on the screen.
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