Stock and Delay Elements are specialized function elements with the unique property that their outputs are influenced by what has happened in the past. That is, unlike standard function elements, whose outputs at any given time are computed based solely on the current (instantaneous) values of their inputs, the outputs of dynamic elements are determined by the previous values of their inputs. Such elements accumulate past events and provide systems with inertia and memory, and hence are responsible for internally generating the dynamic behavior of a system.
An example of a stock element is the Reservoir. A Reservoir accumulates materials, and is useful for representing things like bank accounts and quantities of materials or items (e.g., water, soil, salmon). In its simplest form, a Reservoir requires as inputs an Initial Value, a Rate of Addition and a Rate of Withdrawal, and outputs a Current Value using the following equation:
An example of a Delay element is the Material Delay. Delays allow you to represent processes in which the output lags the input. The Material Delay accepts as input a flow of material (e.g., gal/day, $/yr, widgets/hr), moves it through a "conveyor" or "pipeline" (while optionally dispersing it), and then outputs the flow. Such an element can be used to represent such processes as the movement of water through soils or the movement of parts on a conveyor.