The development of a model requires clear agreement on the model purpose and context, involving not only the modelers but also the stakeholders in its use. The process of achieving agreement is iterative, with continual refinement of the purpose and objectives as the modelers and stakeholders learn more about the system to be modeled and the scope for using the model. From that point conceptual models can be developed to guide the choice of model features and families and to help determine how model structure and parameter values are to be found. Performance criteria can then be developed, geared to the model purpose and context, the model structure, and the available data. Once the model has been constructed, it must be subjected to calibration, quantification of uncertainty, testing ('validation', which in practice means checking whether the model is proof against attempts to falsify it), and evaluation of its effectiveness. At any point in the model development it may be necessary to revisit and revise earlier steps as new information unfolds.
This article discusses a ten-step model approach (see the section titled 'Further reading') in the context of a 3-year project that is developing an ecological model for a wetland system. It is hoped that by discussing the model development options at an early stage of the project, more of the issues and options will be drawn out.
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