Introduction to STELLA and Modeling Dynamic Systems

The goal of this section is to introduce you to some basic concepts of dynamic systems before plunging in to modeling these systems with STELLA. We will consider some very important questions about the process of modeling including why we model these systems on a computer, how to understand the meaning and significance of a model that is a simplification of the real world. I think you will see that computer modeling can be relatively easy, even fun, but it is not so easy to figure out how to interpret the results of these models - how much faith or skepticism is appropriate in considering the results. This chapter will also provide our first experience with the program STELLA as we construct and experiment with a very simple model.

What is a Dynamic System?

Throughout this book, we will be exploring things like the global water cycle and the global carbon cycle; key components or subsystems of the global climate system. Before going any further, we need to discuss what is meant by a system; it is a somewhat vague term that means different things to different people. In this book, we will be talking about dynamic systems - systems that involve change - because change at all time scales is a major theme in the study of the Earth. Dynamic systems are related sets of processes and reservoirs (places where things can reside or forms in which matter or energy exists) through which material or energy flows, characterized by continual change. This is a simple, fairly precise definition that is worth thinking about carefully. A few examples will help.

A bathtub is a simple example of a dynamic system. Water flows into the tub through a faucet and leaves the tub through a drain. The faucet and the drain represent processes that are related because they both involve water moving into and out of the same reservoir, which is the tub itself. The balance between the inflow through the faucet and the outflow through the drain determines how much water accumulates in the tub. If the inflow and outflow rates are the same, then there will be no change in the amount of water in the tub, so the system would appear to be unchanging and not a true dynamic system. But of course, individual water molecules are moving through the system; material is flowing through this system.

Another example of a dynamic system is a pot of water set on a burner. In this case, energy, rather than matter, flows through the system. Energy is added to the pot via the burner and it is absorbed by the water in the pot, raising the temperature of the water. Energy escapes the system via infrared radiation (heat waves) and through vapor loss. When water changes from a liquid to a vapor, a process called evaporation, it requires a good deal of energy and this energy comes from the body of liquid water. This is why evaporation from a film of water covering your skin cools you. So in this system, there is one process adding energy to the system and two processes removing energy to the system and a reservoir of water through which the energy passes.

Our world is filled with dynamic systems; all populations, human and otherwise, are dynamic systems, epidemics are dynamic systems, economies at all scales are dynamic systems. The more you look at the world around you with an understanding of what dynamic systems are, the more examples you see. Clearly, it is important to understand how these systems work and especially how they respond to changes. One of the major goals of this book is to help you develop a good sense of how dynamic systems behave - to develop a kind of intuition that will allow you to understand these systems