Using a submodel to specify different time bases for different parts of a model

By default, all parts of your model tick at the same rate, as specified by the Update every... value in the run control dialogue window. However, you will sometimes want to get parts of the model updated less (or more) frequently than others. For example, you may have a model containing both trees and a crop. The crop you want to grow on a weekly basis, so you can capture its response to rainfall patterns, pest outbreaks etc. The trees grow slowly, and there is no point at all in calculating tiny increments on a week-by-week basis. Conversely, your model may include a fire spread submodel, which is triggered only very occasionally but then needs to simulate the spread of fire at very short time steps. In order to specify this, you need to ensure that the component of the model with a separate time base is in a separate submodel. You then specify the time base for this submodel (and, by default, any inside it) in the submodel properties box. When you come to run the model, Simile then realises that there is more than one time base for the model, and adds one or more extra Update every... in the run control dialogue window. The user of the model then needs to specify each one separately, in terms of the model's unit of time. In the above example, the value 1 (year) and 0.02 (years, equals roughly 1 week) could be specified for the two Update every... values.