The development and application of mathematical water quality models can be very useful in almost every aspect of the TMDL (Total Maximum Daily Load) process. These types of models provide insights that will help inform the decision-making process. Models are used to diagnose existing conditions (assess what happened), examine the causes that have taken place (why it happened) and forecast outcomes and future events (what will happen). Watershed models are commonly applied to provide a quantitative linkage between land use pollutant contributions and waterbody response for TMDL development.

Watershed and Water Quality Modeling Technical Support Center

The Watershed and Water Quality Modeling Technical Support Center provides assistance to EPA Regions, state and local governments and their contractors in the implementation of the Clean Water Act. The Center, which is part of EPA's Office of Research and Development (ORD) provides access to technically defensible tools and approaches that can be used in the development of TMDLs, waste load allocations and watershed protection plans.

• TMDL Modeling ToolBox Fact Sheet
• Watershed and Water Quality Modeling Technical Support Center Fact Sheet

Modeling Webinars

EPA formed the Water Quality Modeling Workgroup in 2013 to facilitate collaboration among EPA and state employees who are using water quality models for Clean Water Act regulatory purposes, primarily in the TMDL and Water Quality Standards Programs.  The workgroup steering committee hosts a series of two-hour webinars to help water quality professionals better understand modeling and how models can be used to solve the problems facing water quality regulators. The first three webinars covered modeling basics, such as selecting, developing and running hydrology and water quality models. Subsequent webinars focus on modeling specific pollutants (e.g., nutrients, sediment, metals) and other emerging issues.

Selected Resources

• EPA’s Council for Regulatory Environmental Modeling
• Watershed and Water Quality Modeling Technical Support Center
• TMDL Model Evaluation and Research Needs (PDF)(403 pp, 5.4 MB, About PDF)
• EPA Watershed Academy
• EPA Center for Exposure Assessment Modeling (CEAM)
• BASINS (Better Assessment Science Integrating point & Non-point Sources)