Evaluation of Modeling Tools for Tmdl Development and Implementation T

The Total Maximum Daily Load (TMDL) program, established by U.S. federal law, drives U.S. water quality policy and management today. Mathematical models, in combination with field monitoring data, are widely used when developing TMDLs since they can potentially save time, reduce cost, and minimize the need for experimentally evaluating management alternatives. This article introduces the collective effort of a multidisciplinary panel of experts to evaluate the current status of TMDL modeling technology available for the most common waterbody impairment factors, along with issues of proper model use, uncertainty of modeling results, and economic tools to optimize the selection and application of these tools for TMDL development. Each of these topics is developed in individual articles within this collection. The review indicates that the status of TMDL modeling tools for the most common stream impairments is inconsistent. Research must continue to advance our understanding of many of the processes leading to stream impairment, and to address many of the existing model limitations. Reviews of case studies within this collection of articles show that users must be better trained to improve the application of TMDL models. In some cases, lack of adequate data sets limits model development and application. Existing computer models are considered capable of simulating sediment and nutrients, lacking for dissolved oxygen, and grossly insufficient for biological indicators. Quantification of modeling uncertainty, communication to end users, and economic optimization of the results are suggested as indispensable components to improve the success of the TMDL program.