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A CONCEPTUAL APPROACH FOR ESTIMATING MARGIN OF SAFETY VALUES FOR TMDL MODEL SIMULATIONS

Published by the American Society of Agricultural and Biological Engineers, St. Joseph, Michigan www.asabe.org

Citation:  Watershed Management to Meet Water Quality Standards and Emerging TMDL (Total Maximum Daily Load) Proceedings of the Third Conference 5-9 March 2005 (Atlanta, Georgia USA) Publication Date 5 March 2005  701P0105.(doi:10.13031/2013.18059)
Authors:   A.M. Sexton, A. Shirmohammadi, and H.J. Montas
Keywords:   Error propagation, MFORM, modeling, nutrients, uncertainty analysis, water resources management, water quality

Hydrologic and water quality models used for Total Maximum Daily Load (TMDL) analysis have considerable amounts of uncertainty associated with their predictions. These uncertainties result from circumstances such as natural variability (spatial and temporal), incorrect model structure, incorrect model parameterization, and measurement errors. The portion of TMDL allocations known as the margin of safety (MOS) was developed to account for uncertainties involved in TMDL development, including those listed above; however the methods of deriving MOS have been criticized as being arbitrary and highly subjective. The majority of States are currently using conservative assumptions or assigning numeric safety factors to represent MOS. Based on the review of strategies associated with uncertainty in Sexton et al. (2003), we discuss a conceptual approach for deriving MOS using a formal uncertainty analysis scheme known as the Mean-Value First-Order Reliability Method (MFORM). This approach can be applied to the mechanistic type models that are recommended by EPA for TMDL development. The mathematical framework of the concepts used for MOS computation is introduced in this presentation.

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