This module introduces the topic of water quality modeling to support watershed studies. Let's start with the simplest question: what are models? In general, models are representations of systems or processes. Some of the oldest forms of models were actual miniature physical representations of natural systems. Mathematical models are also representations of systems, but use a series of mathematical equations. The number, form, and interconnections of these equations in a model can range from very simple to highly sophisticated.

**The graphic to the left shows an example of a collection of processes that might be included in a watershed nonpoint source model.** As you can see, watershed nonpoint source modeling can be rather complex and require knowledge of several ongoing processes. The processes are divided into those applicable into pervious and impervious
areas. The linkages describe how the model handles the simulation of several state variables, which are the basic components for which mass is conserved. In this model, the state variables are water, sediment, and a generic pollutant (such as phosphorus). The model first represents the movement of water, starting with precipitation and snow melt. In pervious areas (the left hand side) water may evaporate, run off on the surface, or infiltrate and move through the soil or ground water. In impervious areas water does not infiltrate, so only evaporation and surface runoff are considered. The model next simulates the movement of particles, which often carry pollutants. This is done through erosion in the pervious area, and washoff of particulate matter in the impervious area. Finally, the model represents movement of pollutants, in association with either water or particulate matter.

There are two points to remember as we discuss models:

- Models are a type of tool, and are used in combination with many other assessment techniques.

- Models are a reflection of our understanding of watershed systems. As with any tool, the answers they give are dependent on how we apply them, and the quality of these answers is no better than the quality of our understanding of the system.