Then it is very easy to change the parameters, the initial conditions, the equations, or to add more model components. The package is designed for non-specialist or specialist use. The above example is a bit spartan. A very clearly written accompanying book of 319 pages builds up an example of pollution in a system of two lakes connected by rivers. In Figure 2 one variant of that model is shown with compartments for the volumes of water in the two lakes and for the amount of pollution in each lake.
Associated variables are the pollution concentrations in the two lakes.
A pollution monitoring station downstream from the second lake measures pollution concentration, but after a time delay represented by the hexagonal river box. On Lake 1 there is a factory which normally processes its effluent from a store. But when the store is too full it dumps the excess into the lake. Seasonal data about the inflow to Lake 1 is represented by a look-up table, labelled L1, giving inflow rates at different times over a 365 day period. These are influences of a particular type, where a conserved quantity is moved between compartments.