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Approaches for Estimating the Margin of Safety (MOS)



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Approaches for Estimating the Margin of Safety MOS in a Total Maximum Daily Load TMDL Calculation Theoretical and Practical Considerations 1005473 Topical Report November 2004 EPRI 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 askepri epri com www epri com DISCLAIMER 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 iii REPORT 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 v transport 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 individual sources quantity and quality of observed data and model sensitivity to parameters A pragmatic scientifically sound approach for analyzing uncertainty and variability is proposed This approach when fully tested can lead to significant cost savings in TMDL implementation and higher probability of achieving water quality criteria EPRI Perspective Calculating allocating and implementing TMDLs will strongly influence creating and applying water resource regulation and policy for at least the next two decades It also will have major implications for air pollution and land use regulation and policy In 2002 the states identified over 52 000 impairments of water bodies each requiring a TMDL TMDLs affect the electric power industry both through the industry s role as a point source of contaminants directly discharged to water bodies and as a nonpoint source via both atmospheric deposition of substances such as nitrogen and mercury and runoff from facilities and rights of way TMDLs also affect the industry by impacting its business and residential


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