I finally got around to making some long-planned changes to FBP Graph, the MS Excel tool for comparing fire behaviour using the Canadian Fire Behaviour Prediction System.
The latest version has the subtle-but-significant change of showing (if the box is checked) fire type. That is, when checked, the graph lines now indicate three thresholds, in terms of ISI or wind speed: these are the crown fire initiation threshold (between surface fire and intermittent crown fire, or 10% Crown Fraction Burned, or CFB), the point at which CFB is 50%, and the threshold between intermittent and continuous crown fire (90% CFB). As with the rest of the FBP Graph tool, the point thresholds move around on the graph depending on fuel type, buildup index (BUI), and some other modifiers depending on fuel type.
These thresholds are important for two reasons. As fuel consumption and spread rate increase, conifer crowns become involved in fire spread (according to the Van Wagner crown fire model), further increasing fire intensity. However, the only fuel type in the FBP System that demonstrates a change in spread rate as crown involvement increases is the C-6 type, which is in many ways the most sophisticated fuel type in the system. So it is important to note where along the lines changes in fire type occur, as these can show where one can expect sudden changes in fire behaviour.
A second reason for the importance of these thresholds is that predicted rate of spread may well be incorrect as fire type changes. As many authors have noted, at the onset of crown fire initiation, spread rate can increase more than threefold, as the flaming front is suddenly exposed to above-canopy ambient winds. This change is not reflected in sigmoidal ROS curves that most FBP fuel types have, so fire type thresholds can indicate where these curves oversimplify, and likely underpredict, spread rate. As always, using the FBP Graph tool for operational purposes assumes expert knowledge and training in fire behaviour prediction using the CFFDRS, and I make no guarantees about the accuracy of the outputs (but you know, if you think there are errors, please let me know so I can address them). The ISI or wind speed threshold is calculated to the nearest 1 unit (ISI or km/h); so fire behaviour calculators such as RedApp are more accurate, but this is still pretty accurate given the inaccuracy of the base measurements (precipitation, wind speed, etc.). Thanks for your interest.
FireResearch.ca 