**Chapter 1. Overview
**A model is a substitute for a real system. Models
are used when it is easier to work with a substitute than with the actual
system. An architect's blueprint, an engineer's wind tunnel and an economist's
graphs are all models. They represent some aspect of a real system -- a
building, an aircraft or the nation's economy. They are useful when they
help us learn something new about the systems they represent.

Many of us have built and used models. Our first experiences might have involved physical models such as a paper airplane or a cardboard glider. These models were easy to assemble and easy to use. They made it fun to conduct experiments. We tried our experiments; watched the results; and tried again. Along the way, we learned about the dynamics of flight. If your experiences were like mine, you learned that you can't make a paper airplane fly further by simply throwing it harder. You also learned that each airplane seemed to follow a natural glide path through the air. Through experimentation, we learned the extent to which the plane's natural trajectory could be improved.

This book focuses on mathematical models of environmental systems. Mathematical models use equations to represent the interconnections in a system. We will concentrate on a special category of mathematical models which are "simulated" on the computer. They are called "computer simulation models" because the tedious calculations are turned over to the computer. Our job is to think about the best way to construct the model to describe the environmental system. If we do our job well, we can use the model to conduct experiments. We will try our experiments, watch the results and try again. Along the way, we will improve our understanding of the natural trajectories of environmental systems. Through experimentation with the computer model, we'll learn the extent to which the system's natural trajectory could be improved.