1. 2nd Meeting Model Building: Stocks and Flows
6th System Dynamics Summer School, July Andrew Ford, Instructor, Intermediate Track
2nd Meeting Model Building: Stocks and Flows
Start with the Stocks
Then add the flows
Then add remaining variables* to explain the flows
The remaining variables are called “Auxiliaries” in Vensim, “converters” in Stella/iThink.

2. Why start with the stocks?
First the Stocks - - in plain view
Then the Flows - - may not be in view, but we have seen their impact on the stocks
Check the Units
Then Explain the Flows – this could be the most difficult because the forces may be “invisible”

3. What do you see on the Cover
Snow pack in the mountains
Leaves on the ground

4. Snow Pack: You See the Stock

5. Leaves: You See the Stocks

6. Stocks Represent the Storage in the System
Stocks remain at their current value if the flows go to zero
Stocks act as buffers
Stocks create the “momentum” in the system

7. Population Examples

8. Another Hydrological Example
Are you worried that there are no flows into or out of this system?

9. Another Example of a “Closed System”
Can you guess the
equations
for each
of the 6 flows?
This is a Vensim diagram with a “comment” inserted to visualize the spatial pattern of land use. The image is a “donut diagram” from a spreadsheet which is imported as a bmp image into the comment space.

10. Why were the equations easy?
Because I use Friendly Algebra. It requires clear variable names with each variable defined by either:
Add
Subtract
Multiply, or
Divide

11. Classroom Exercises Find the Errors in Stock and Flow Diagrams &
Find the Errors from inconsistent Units of measurement.

12. Find the error in each diagram

13. Units Exercise
The units of the 4 variables are described in the table. The model diagram will simulate the stock of insects (A) with time measured in years. Which of the four variables
in this model diagram must be wrong?

14. More on units in the next meeting
More on units in the next meeting. Further information on units is provided in the “Preview Copy” of Appendix A of the 2nd Edition of Modeling the Environment.