In pursuing its mission to protect human health and to safeguard the natural environment, the U.S. Environmental Protection Agency often relies on environmental models. In this guidance, a model is defined as a simplification of reality that is constructed to gain insights into select attributes of a particular physical, biological, economic, or social system. This guidance provides recommendations for the effective development, evaluation, and use of models in environmental decision making once an environmental issue has been identified. These recommendations are drawn from Agency white papers, EPA Science Advisory Board reports, the National Research Councils Models in Environmental Regulatory Decision Making, and peer-reviewed literature. For organizational simplicity, the recommendations are categorized into three sections: model development, model evaluation, and model application. Model development can be viewed as a process with three main steps: (a) specify the environmental problem (or set of issues) the model is intended to address and develop the conceptual model, (b) evaluate or develop the model framework (develop the mathematical model), and (c) parameterize the model to develop the application tool. Model evaluation is the process for generating information over the life cycle of the project that helps determine whether a model and its analytical results are of sufficient quality to serve as the basis for a decision. Model application (i.e., model-based decision making) is strengthened when the science underlying the model is transparent.

"EPA/100/K-09/003." "March 2009." "Principal authors: Noha Gaber, Gary Foley, Pasky Pascual, Neil Stiber, Elsie Sunderland."

The CREM developed this document to provide a simplified, comprehensive resource for modelers across the Agency on best modeling practices. When adhered to, the guidelines will help to ensure the quality, utility and regulatory relevance of the models that EPA develops and applies and the transparency of modeling analyses and model-based decisions.