Most hydrologic models are used to simulate and forecast streamflow. A variety of model approaches are used to accomplish this purpose. One simple method is to use an accounting approach where inflow, outflow, and change in storage are treated like a budget. A more scientific approach may use the physics and equations for conservation of mass and momentum. Yet another approach may be to use statistical relationships based on observed data. This approach has the drawback of being specific to only one basin. Empirical relationships are often used as a way to simply complex processes and use a minimum of data.
In most all cases, a particular model may be classified as either deterministic or stochastic. The next few slides will allow us to explore these terms in more detail.