## The Stella II modeling Environment

Stella II software is an object-oriented programming environment. Models can be created with Stella II by connecting four or five different icons together in different ways into a model framework so that the structure of the model is very transparent. Shown at left is a simple population model with a limited carrying capacity. The model is constructed from Stocks (Population), flows (Births and Deaths), modifiers (BirthRate and DeathRate), and connectors. The software automatically creates the difference equations based on user input.The mathematical details of the numerical solution to the underlying model equations are accessible for those that want this detail. Additional control over the time step used and the numerical solution technique is provided for the user.

The Stella II environment makes it easy for introductory geoscience students to visualize the conceptual framework of a model, manipulate the model to explore its dynamic behavior, and with a little experience create their own models. At left the graph of population over time is shown with a slider/text box input control for BirthRate.

Here is a well developed on-line tour showing ISSE Systems: STELLA Presentations (more info) and some of the basics behind using Stella.

The latest version of Stella (2003) is Stella II (vs. 7.0.3) and is available from ISEE Systems: Why STELLA for Education? (more info) Stella II runs on the PC or Mac and is very reasonably priced via educational discount. Stella II save-disabled versions (Run-time version) allow one to run and experiment with pre-made models and are available for free to educational institutions.