INTRODUCTIONI. BACKGROUND ON MARGIN OF SAFETY IN TMDL CALCULATIONSII. GENERAL UNCERTAINTIES IN A TMDL CALCULATIONIII. APPROACHPROJECT PURPOSE AND OBJECTIVESI. PROJECT PURPOSEII. PROJECT OBJECTIVESTHEORETICAL CONSIDERATIONS FOR THE DETERMINATION OF MARGIN OSTOCHASTIC APPROACHES FOR UNCERTAINTY ANALYSISANALYSIS OF UNCERTAINTY AND VARIABILITY OF WATERSHED MODEL PI. POINT SOURCESII. NON-POINT SOURCESIII. MODEL PARAMETERSUNDERSTANDING MODEL UNCERTAINTYI. MODEL STRUCTUREII. MODELING RESOLUTIONTemporal ResolutionSpatial ResolutionIII. EXTRAPOLATIONIV. MODEL FORMULATIONV. MODEL BOUNDARIESVI. SUMMARY OF MODEL UNCERTAINTYA STRATEGY FOR ASSESSING PARAMETER SENSITIVITY AND UNCERTAINI. COMMENTS FROM WORKSHOP ON MARGIN OF SAFETY IN TMDL CALCULII. PROPOSED STRATEGY FOR SENSITIVITY AND UNCERTAINTY ANALYSCITATIONSWARMF INPUT PARAMETERS LISTLIST OF PARTICIPANTS AT MOS WORKSHOPEPRI Project Manager R. Goldstein EPRI • 3412 Hillview Avenue, Palo Alto, California 94304 • PO Box 10412, Palo Alto, California 94303 • USA 800.313.3774 • 650.855.2121 • [email protected] • www.epri.com Approaches for Estimating the Margin of Safety (MOS) in a Total Maximum Daily Load (TMDL) Calculation Theoretical and Practical Considerations 1005473 Topical Report, November 2004DISCLAIMER OF WARRANTIES AND LIMITATION OF LIABILITIES THIS DOCUMENT WAS PREPARED BY THE ORGANIZATION(S) NAMED BELOW AS AN ACCOUNT OF WORK SPONSORED OR COSPONSORED BY THE ELECTRIC POWER RESEARCH INSTITUTE, INC. (EPRI). NEITHER EPRI, ANY MEMBER OF EPRI, ANY COSPONSOR, THE ORGANIZATION(S) BELOW, NOR ANY PERSON ACTING ON BEHALF OF ANY OF THEM: (A) MAKES ANY WARRANTY OR REPRESENTATION WHATSOEVER, EXPRESS OR IMPLIED, (I) WITH RESPECT TO THE USE OF ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT, INCLUDING MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE, OR (II) THAT SUCH USE DOES NOT INFRINGE ON OR INTERFERE WITH PRIVATELY OWNED RIGHTS, INCLUDING ANY PARTY'S INTELLECTUAL PROPERTY, OR (III) THAT THIS DOCUMENT IS SUITABLE TO ANY PARTICULAR USER'S CIRCUMSTANCE; OR (B) ASSUMES RESPONSIBILITY FOR ANY DAMAGES OR OTHER LIABILITY WHATSOEVER (INCLUDING ANY CONSEQUENTIAL DAMAGES, EVEN IF EPRI OR ANY EPRI REPRESENTATIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES) RESULTING FROM YOUR SELECTION OR USE OF THIS DOCUMENT OR ANY INFORMATION, APPARATUS, METHOD, PROCESS, OR SIMILAR ITEM DISCLOSED IN THIS DOCUMENT. ORGANIZATION(S) THAT PREPARED THIS DOCUMENT University of California, Santa Barbara ORDERING INFORMATION Requests for copies of this report should be directed to EPRI Orders and Conferences, 1355 Willow Way, Suite 278, Concord, CA 94520, (800) 313-3774, press 2 or internally x5379, (925) 609-9169, (925) 609-1310 (fax). Electric Power Research Institute and EPRI are registered service marks of the Electric Power Research Institute, Inc. EPRI. ELECTRIFY THE WORLD is a service mark of the Electric Power Research Institute, Inc. Copyright © 2004 Electric Power Research Institute, Inc. All rights reserved.CITATIONS This report was prepared by Bren School of Environmental Science & Management University of California, Santa Barbara Santa Barbara, CA 93106 Principal Investigators A. Keller Y. Zheng P. Wang This report describes research sponsored by EPRI. The report is a corporate document that should be cited in the literature in the following manner: Approaches for Estimating the Margin of Safety (MOS) in a Total Maximum Daily Load (TMDL) Calculation: Theoretical and Practical Considerations, EPRI, Palo Alto, CA: 2004. 1005473. iiiREPORT SUMMARY This report provides theoretical and practical considerations for establishing approaches to estimate the margin of safety (MOS) in a total maximum daily load (TMDL) calculation. Background The U.S. Environmental Protection Agency (USEPA) and the states are faced with developing tens of thousands of TMDLs, each with a margin of safety (MOS). Watershed models are increasingly being used for this work. However, in almost all cases, MOS is defined arbitrarily, without consideration for the actual uncertainty in the likelihood of achieving water quality objectives. This can lead to two outcomes: (1) if MOS is too small, the TMDL has a high probability of not meeting its designated use; (2) if MOS is too large, the cost of implementing the TMDL will be much higher than necessary. Thus, a scientifically sound approach to determining MOS is required. Objectives To evaluate the sources of uncertainty in TMDL calculations and to develop and test a practical, scientifically sound framework for MOS that mitigates the complexity of traditional analytical approaches and their huge computational requirements and that also is capable of analyzing the ramification of each source of MOS uncertainty. Approach In addition to an introduction to the issues surrounding the development of MOS in a TMDL calculation, the project team presents a theoretical approach to determining MOS in Section 3 of the report. Since this theoretical approach has limited application in the typical watershed, given the number of catchments and parameters, Section 4 addresses possible approaches for determining MOS through numerical simulation. To understand the sources of variability and uncertainty in a TMDL calculation, Section 5 examines classes of parameters involved in a watershed model, their range, and variability or uncertainty. On a practical level, uncertainty in the predicted value also might come from modeling assumptions; Section 6 explores some of these through a number of modeling examples. Section 7 outlines a possible approach for a practical, systematic determination of uncertainty and MOS in a TMDL calculation. Results Point and nonpoint sources differ in contributions to overall uncertainty, with nonpoint source load estimates producing greater uncertainty; however, point sources have significant temporal variability that must be considered when determining MOS. Model formulation and parameterization can have significant influence on simulations of hydrology and water quality, even when models are calibrated against the same observed data. Water quality is much more sensitive to parameterization than hydrology—in particular, pollutants associated with sediment vtransport. Considerations that need to be taken into account in developing a pragmatic, scientifically sound MOS include choice of model, spatial and temporal model resolution, nature and magnitude of