Chapter 10. Introduction to Material Flow

System dynamics may be used to simulate material flow through a system. The material may take many forms -- people, animals, plants, water, cash, cars, heat or pollutants. The flow of material is represented by flow variables. The accumulation of material is represented by the stocks. This chapter looks at standard combinations of stocks and flows that will prove useful throughout the book. It then describes the conveyor, a special type of stock that will prove useful in simulating tightly controlled flows of material. Environmental systems sometimes exhibit tightly controlled flows, as when a school of fish migrate to the ocean. Conveyors are used to simulate animal populations that move or grow in tightly controlled patterns. Examples include the salmon in the Columbia River (Chapters 13 and 14) and the brine shrimp in Mono Lake (exercise 7, chapter 4).