2nd Meeting Model Building: Stocks and Flows

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Presentation transcript:

2nd Meeting Model Building: Stocks and Flows 6th System Dynamics Summer School, July 2019 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.

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”

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

Snow Pack: You See the Stock

Leaves: You See the Stocks

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

Population Examples

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

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.

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

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

Find the error in each diagram

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?

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.