Stock Block

Accumulates incoming and outgoing flows over time. Represents a level or state in a system.

Description

The Stock block is the core accumulator in a System Dynamics model. It represents a state variable that changes over time based on the net flow. It acts like a reservoir or tank that stores a quantity.

Mathematical Model

Level(t) = Level(t0) + ∫(Inflow - Outflow) dt

The stock is computed as the integral of the net flow:

$$Level(t) = Level(t_0) + \int_{t_0}^{t} (Inflow(\tau) - Outflow(\tau)) d\tau$$

In discrete time, using the Euler method: $$Level_{n} = Level_{n-1} + (NetFlow \times \Delta t)$$

Parameters

initialValue

The starting amount in the stock at the beginning of the simulation.

• Default: 0
• Tooltip: Starting value of the stock at t=0

lowerLimit

The minimum physical limit of the stock. The value will not drop below this level (saturation).

• Default: -1,000,000
• Tooltip: Minimum possible value for the stock

upperLimit

The maximum physical limit of the stock. The value will not exceed this level (saturation).

• Default: 1,000,000
• Tooltip: Maximum possible value for the stock

Ports

flow_in (Input)

The net rate of change. Usually connected to a Flow block or a subtraction of multiple flows.

reset (Input, Optional)

When this port receives a value greater than 0, the stock is immediately reset to its initialValue.

value (Output)

The current level or state of the stock.

See Also

• Flow: Used to define the rate of change for a Stock.
• Integrator: The standard continuous-time counterpart.
• Converter: Used for intermediate calculations.