DISCUSSION 1

useful paper of automatic (computer assisted) model calibration using a robust optimization routine, which provides additional insights into model predictive uncertainty. The authors introduce a critically important topic for the current Total Maximum Daily Load (TMDL) crisis facing water quality managers in the U.S. While some progress is being made in addressing thousands of impaired waterbodies nationwide, a great deal still needs to be accomplished in the next decade if the ultimate goals of the Clean Water Act are to be realized. This is particularly true when the modeling tool is applied at the watershed scale in contentious stakeholder scenarios. Hence, the paper is timely and touches upon issues and concepts worthy of further attention by the modeling community, professional societies, and regulatory agencies. In general the HSPF model used to simulate nonpoint source pollutant transport and reaction is fairly robust with a growing user community. Unfortunately, however, it remains difficult to calibrate and use in spite of U.S. Environmental Protection Agency (USEPA) and other agency adoption and support in the past two decades. It is still not user friendly due to the abundance of model parameters with abstract meanings that are, for the most part, immeasurable because they lack physical significance. Even USEPA’s BASINS (Better Assessment Science Integrating Point and Nonpoint Sources) modeling system that includes HSPF as one of its nonpoint source models is not easy to use, but easier than a stand alone application because of the pre-processors and postprocessors and the packaged meteorological and STORET datasets. Hence, BASINS developers addressed only one deterrent to HSPF use – data availability – and left the user to contend with HSPF calibration issues. The HSPF starter parameter sets are grossly inadequate for many watersheds. There remains a small, select group of modelers, having a proven track record of developing credible HSPF simulations, who could not possibly address such issues in every watershed. Efforts such as this paper hold promise for more widespread credible use of HSPF (and other models) in addressing our nation’s water quality impaired waters, if allowed to mature by adequate and persistent nurturing by federal agencies and academic institutions. The purpose of this discussion is to dramatize the significance of the effort in the evolution of modeling tools, especially those significant in the TMDL domain. The particular combination of computer assisted model calibration with concurrent information about uncertainty speaks to the essence of scientifically credible TMDLs. USEPA Office of Research and Development’s Twenty Needs report (USEPA, 2002a) and the Science Advisory Board initiated Council on Regulatory Environmental Model (USEPA, 2002b) are evolving too slowly and may not impact water quality modeling and management for several more years. Further USEPA budget priorities and model development funding shortfalls may similarly delay critical modeling science research development. Parameter estimation with PEST is a powerful tool and hopefully will evolve to assist modelers with more credible point and nonpoint source models. I thank the authors and JAWRA for publishing such a timely paper for the water quality modeling field.