Model Desktop1 :
Variable doing fire :
doing fire = any(any([[lit]])) (boolean)
Where:
[[lit]] = Value(s) of Quads/lit
Comments:
True if any square i the grid is on fire. Prevents normal growth or spontaneous ignition from happening (because timescale is shorter)
Variable done fire :
done fire = last(doing_fire) (boolean)
Where:
doing_fire = Value(s) of doing fire
Comments:
Take the last value of doing_fire here -- because using last(doing_fire) inside the submodel will cause an intermediate variable to be created for it in each instance, which wastes memory.
Submodel Quads :
Submodel "Quads" is a fixed_membership multi-instance submodel with dimensions [200,300].
Comments:
Each instance represents a square area of forest
Enumerated types: {rect_nbr sw s se w e nw n ne}
Compartment fuel :
Initial value = if firebreak then 0 else 4 (real)
Where:
firebreak = Value(s) of firebreak
Comments:
Because growth rate is a function of current load, setting the initial value in the firebreak area to zero ensures that it remains at zero.
Flow burn :
burn = if lit then 0.5*fuel/dt() else 0 (1/day)
Where:
fuel = Value(s) of fuel
lit = Value(s) of lit
Comments:
Half the fuel in a cell will be consumed if it catches; it might catch more than once in a single fire, causing uneven burn patterns.
Flow growth :
growth = if doing_fire then 0 else 0.001*fuel*(100-fuel) (1/day)
Where:
fuel = Value(s) of fuel
doing_fire = Value(s) of ../doing fire
Comments:
This function causes the fuel load to follow a sigmoid curve levelling out at 100 in the absence of fire. Fires happen much quicker than growth, so growth is set to zero while a fire is burning because model time takes on a different meaning.
Variable firebreak :
firebreak = column_id()>148 and column_id()<=152 and row_id()>4 (boolean)
Comments:
Incomplete firebreak down the middle of the simulated area.
Variable lit :
lit = not done_fire and luck*1e8<fuel or luck*200<fuel*last(nbrs_lit) (boolean)
Where:
nbrs_lit = Value(s) of nbrs lit
fuel = Value(s) of fuel
luck = Value(s) of luck
done_fire = Value(s) of ../done fire
Comments:
A cell will burn if its neighbours were burning last time step and it is unlucky, or if nothing was burning last time step and it is very unlucky (e.g., gets hit by lightning). More neighbours alight means more chance of burning. There must be more fuel available than luck!
Variable luck :
luck = rand_var(0,1) (real)
Comments:
Adds the random element; the lower it is, the more likely a cell is to burn.
Variable nbrs lit :
nbrs lit = howmanytrue({from_8_nbrs_lit}) (int)
Where:
{from_8_nbrs_lit} = Value(s) of lit
lit = Value(s) of lit
Comments:
Counts the number of a cell's eight neighbours which are alight.
Variable show :
show = if lit then -3 else fuel (real)
Where:
fuel = Value(s) of fuel
lit = Value(s) of lit
Comments:
For display purposes, The grid display should be set up with a range of -3 to 100 and the first swatch set to orange to represent fire. This variable will then show orange if the cel is burning, or a colour from a scale representing fuel load (black -> green -> straw) if not burning.
