Using TST to conditionally set transition probabilities

[leonardo-frid]

I recently had the following question:
Q: I was searching for how to set a Time-Since-Transition. I see TST mentioned in the docs.stsim.net page but nothing specifically talking about how to set it. I’m trying to prevent the same pixel from burning more frequently than 20 years. I found Time Since Transition under Advanced settings but can’t seem to make sense of where to go from there.

A: It sounds like you want to create a fire transition pathway conditionally on whether at least 20 years has elapsed since a previous fire. There are three things you need to do to accomplish this:
1. In the scenario property Transition Pathways navigate to the last tab to edit the “Transitions” (http://docs.stsim.net/reference/prop_prob_transitions.html) and right click on the table to display the “TST Min” field. For all Fire pathways where you would like to wait a minimum of 20 years until another fire can occur, set the “TST Min” value to 20.
2. Navigate now to the Scenario Property “Advanced | Time Since Transition | Time Since Transition Group” (http://docs.stsim.net/reference/prop_time_since_transition_groups.html) and add a record where the Fire transition that you are using depends on itself. For example, you might enter: Transition Type: Fire; Transition Type/Group: Fire [Type]. This tells the model that the TST clock for Fire transitions is reset to zero for a cell whenever a fire occurs.
3. You need to initialize the TST for your each cell in your model somehow. Otherwise all cells will initially have the TST set to zero. and you will see no fire for the first 20 years of your model run and then a spike at year 20. Typically I recommend that if you don’t have data for initializing this variable you randomize between zero and your TST Min value of 20. You can do this by going to “Advanced | Time Since Transition | Time Since Transition Randomize” (http://docs.stsim.net/reference/prop_time_since_transition_rand.html) and setting your “Initial TST Max” for Fire [Type] to 20.

Note that if you cannot see a field such as “TST Min” in one of the scenario properties it is an optional field and you can display it by right clicking on the table itself.

[leonardo-frid]

As a follow-up to this post I would like to point out that releases of ST-Sim 3.2.18 and later now have additional options for applying TST. In fact the above can be more easily accomplished by adding two records to the “Advanced | Transition – Multipliers” Scenario property.
The first record would specify:
TST Type/Group: Fire [Type]; TST Min: 0; TST Max: 19; Transition Type/Group: Fire [Type]; Multiplier: 0.
The second record would specify:
TST Type/Group: Fire [Type]; TST Min: 20; TST Max: [BLANK]; Transition Type/Group: Fire [Type]; Multiplier: 1.

In this single screen you are now specifying that the probability of a fire occurring within 20 timesteps of a previous fire on the same pixel is multiplied by zero whereas after 20 timesteps have elapsed the probability is multiplied by one.

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