Here are a couple of more questions from an ST-Sim user:
As a fire spreads one pixel to another, does what information about the state class of the recipient affects whether or not fire spread is successful? Is it a binary function (i.e., if fire is wired to the state class, then the cell will burn)? Or, is the spread probability modified by the fire probabilities associated with the target state class?
Fire spread in ST-Sim is a stochastic process that incorporates information provided by the user about topography "slope multipliers", wind "direction multipliers" and fire probability. The fire grows to its target size by adding cells at the perimeter according to relative travel time based on topography and wind. Cells to be added are then selected randomly with those that have lower travel times and higher fire probabilities being more likely to be added to the fire than those with longer travel time and lower probabilities. We are currently working on a publication that will include the full details of the transition spread algorithm.
One other fire-spread-related question that I have is whether there’s a way to tune fire spread to the various fire severity transitions we have wired in the ILAP models. Seems like the probability of a stand replacing wildfire spreading to the adjacent cell ought to be higher than that for a nonlethal wildfire, especially in the context of suppression. However, I am currently driving fire spread with an allWildFire transition group so that nonlethal wildfires can spawn stand replacing fires in adjacent cells, and vice versa. Will I get wonky results if I add size distributions for stand replacing, mixed severity and nonlethal wildfire transitions on top of the existing allWildFire size distributions?
Good question… You can either specify fire size distributions to fire as a whole, or individually by type. You can’t do both. If you do the latter, individual fires will always be of only one type. Also, make sure that in this case you set the primary group to be for the individual fire types not for fire as a whole. Note that when a fire is spreading, if you are using the AllFire group as the primary group, the model selects which fire type to apply to a cell based on the relative probabilites of the differetn fire types that are possible.