stockandflowdiagrammen

Stockandflowdiagrammen are a graphical modeling method used in system dynamics to represent how quantities accumulate over time and how they change through flows. The diagrams focus on stocks (or levels), which are accumulations of matter, people, money, or other quantities, and on flows, which are the rates that increase or decrease those stocks. Flows are typically labeled as inflows and outflows and can be governed by converters—auxiliary variables that capture conditions, parameters, or decisions that influence the flow rate. Delays may be included to represent time lags between a change in a cause and its effect on a stock.

In a stock-and-flow diagram, the behavior of a system emerges from feedback loops. A stock grows or shrinks according to the net flow, and the resulting change can, in turn, affect other variables that feed back to influence the same stock. Mathematically, the time derivative of a stock equals the sum of inflows minus the sum of outflows. While diagrams provide intuition, they are commonly translated into differential or difference equations for simulation.

Applications span a wide range of fields, including economics, ecology, epidemiology, population dynamics, supply chain management, energy systems, and engineering. The diagrams are used to test policies, study time delays, and compare scenarios without requiring detailed microscopic descriptions.

Software tools such as Vensim, Stella, and iThink facilitate building and simulating stock-and-flow models, allowing users to visualize outcomes over time and to perform sensitivity analyses. The approach originated with system dynamics pioneer Jay W. Forrester in the mid-20th century and remains a standard method for analyzing complex, time-dependent systems.