Definition
In System Dynamics, a Flow (or Rate) is the movement of resources into or out of a Stock (System Dynamics). Mathematically, a flow represents the derivative of a stock, indicating its rate of change per unit of time:
- How to read: “Flow at time t equals the derivative of the stock S with respect to time t.”
- Meaning: The flow is the instantaneous speed at which a stock is changing at a specific moment.
Why It Matters
Flows represent action and change. While stocks provide stability (inertia), flows are the mechanisms through which a system evolves. Understanding flows allows for the identification of leverage points where interventions can most effectively alter the system’s trajectory.
Core Concepts
- Rates of Change: Flows are always measured as a quantity over a time interval (e.g., births per year, liters per minute).
- Inflows vs. Outflows: Inflows increase the level of a stock, while outflows decrease it.
- Valve Analogy: If a stock is a bathtub, flows are the faucet and the drain.
- Information Dependency: The rate of a flow is often determined by the current level of one or more stocks (e.g., the birth rate flow is dependent on the size of the population stock).