1. Wednesday After Lunch Adding Seasonal Effects to a Model

2. Remember the Flowers Model
This model had time in years and showed S-shaped growth. The flowered area reached 800 acres in 15 years
Your Client asks you to expand the model to include seasonal effects such as growth in Spring, retreat in Fall.
How would you respond?

3. We will simulate in Months rather than Years
Change time to “months” and ask for a 180 month simulation (15 yrs is enough to see S shaped growth)
If the growth rate & decay rate are discussed as fraction/yr we should retain the same inputs from before.
Add “months per year” to the model and divide the flows by “months per year” to get monthly flows.
Add nonlinear multipliers to change the monthly grow or decay based on the Month of the year
But how do we know the Month of the year?
In Stella: Month of year = MOD(time,12) “time” is understood so it does not appear in the diagram
In Vensim: Month of year = MODULO(time,12) “time” is a shadow variable on the model diagram

4. New Flowers Model (in Stella)

6. New Results
The new results are essentially the same as before. Was this a waste of time?

7. The Striga Model: A more complicated calendar (Thanks again to Timothy Silberg, Michigan State University)