The role of the postaudit in model validation

Abstract Most researchers agree that validation is a demonstration that a model is capable of making accurate predictions at a site-specific field setting. A successful demonstration of validation requires completion of a series of steps that form a modeling protocol. These steps include model design and calibration, and verification of the governing equation, the computer code, and the model itself. The strictest form of validation is to demonstrate that the model can accurately predict the future. This type of validation test has been called a postaudit. Results of five postaudits suggest that it will be difficult and probably impossible to validate groundwater models by means of a postaudit because it is impossible to characterize the field setting in sufficient detail. Attention should instead be focused on good modeling protocol including providing a complete description of model design, a thorough assessment of model calibration, and an uncertainty analysis.