objects/systems-principles.html

Systems Approach - Principles

This page describes the beginning steps in using a systems approach and puts this into the icon language of STELLA. There are other, less specific or constrained ways to approach a problem with systems thinking.

A model might look like this below, with a tub of water flowing into a drain and the rate controlled by the amount of water in the tub.

 

Objects and processes that are described in a systems model
stock or reservoir: a quantifiable amount of something

source or sink: a non-measured amount of something that feeds into or leaves the boundary of the model.

 

flow: the movement of a measurable quantity of anything between stocks, sources and sinks. Whatever is flowing is the same as in the stocks. Flows don't contain any measurable amount of whatever is being followed.

 
information link: information from a stock or converter can flow to a flow-valve or another converter. Information can't flow to a stock.
converter: a modeling device that shows that the information from sources is being combined. Also used to calculate ratios, differences, etc. for model output and analysis.
   

Principles of systems models

The left column will be the description and the right column will describe this in systems icons,

1. Set the boundary of the system

What processes that you will follow. Any input or output from those processes that come from the larger, untracked environment will be outside your system.

example: If you are just studying the flow of carbon in metabolism of a deer, all the food and environment are outside of the system. If you are studying the community, then the grass and the excretion are in the system.

The system is just the deer. The "clouds" represent the source for food from the environment and the sink of excretion into the environment.

 

 

The system in this case includes the amount of carbon in the grass and the amount of carbon
in the feces. The source is the carbon in the atmosphere for grass growth and soil carbon as
the fate for excreted carbon.

 

2. Determine the material or energy that you will be following.

In this class we will follow quantities such as:

  • carbon
  • energy (as embedded and heat)
  • water
  • nitrogen
  • phosphorus
  • dollars

A sub-model might look like this:

3. Establish the mass or energy balance and construct the flows.

 

4. Study the control over the flows that is determined by information links to the flow valves.

last modified on Jan 11, 2014 by John Rueter