Overview and Discussion of Integrated Watershed Modeling Systems for Use in TMDLs

Modeling of pollutant generation, transport, and fate plays an important role in the TMDL process. Models are used to evaluate cumulative impacts on the listed waterbody from the different contributing sources, and to develop and validate load reductions that will achieve compliance with water quality standards.The increasing availability of complex models and technology, combined with the need for effective solutions to TMDL and watershed management issues, has led to the development of integrated watershed modeling systems that include both watershed and receiving water processes. These systems may very well match up well to the problem at hand and available resources, but also may be completely inappropriate to the problem or require unavailable data. In addition, TMDL development often has unique challenges that are not characteristic to other applications of these models. These unique challenges include issues of defensibility, limitations in data, estimating effectivenss of control measures and calcuating margin of safety.This paper will provide a brief overview of four such integrated modeling systems that integrate one or more watershed and waterbody models for TMDL development. Processes reflected in these models include pollutant generation, fate, and transport and links to databases and other information are embedded in these systems. The paper describes the main capabilities of these four leading systems, and provides some discussions of model strength, limitations, and data needs. The modeling systems are:BASINS - US EPA's multipurpose environmental analysis system for use by regional, state, and local agencies in performing watershed and water quality based studies.MIKE SHE and MIKE BASIN - MIKE SHE is a dynamic modeling tool from DHI that simulates the entire land phase of the hydrologic cycle, while MIKE BASIN is an ArcView-based comprehensive water resources model.WAM - Watershed assessment model for agricultural and urban landscapes developed by SWET.WARMF - Watershed Analysis Risk Management Framework, a decision support system for watershed management developed by Systech Engineering.