Chapter 5. Equilibrium Diagrams

System dynamics is the study of how systems change over time. Our focus is on dynamics, but dynamics are often difficult to understand. In watching students and professionals work with dynamic models, I have noticed that the moments of equilibrium provide an opportunity for great progress. If we see a system reach equilibrium at some point in a simulation, it is tremendously helpful to study the counterbalancing forces that produce that equilibrium. An equilibrium diagram is a simple, but effective technique to help us pay closer attention to the these key moments in a simulation.

An equilibrium diagram is the same as a stock-and-flow diagram except you write the numerical values and the units for each variable directly onto the diagram. The numerical values are found by simulating the model over time and picking a year in which the system is in equilibrium. Simply take note of the numerical values in that year and write them onto the diagram. The diagram will help you verify that the flows in and out of each stock are in balance. The diagram will help us to become more familiar with the numbers and to spot obvious inconsistencies in the model.