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Uncertainty Analysis of Margin of Safety in Nutrient TMDL Modeling and Allocation

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Abstract:

Total Maximum Daily Load (TMDL) determinations are critical to attaining water quality goals. There are significant uncertainties associated with many aspects of the TMDL development, such as linking loads to effects on water quality attainment and estimating loads from storm-driven, time-varying nonpoint sources (NPS). The selection of the “margin of safety (MOS)” is typically made by subjective decision. In light of the complexity of watershed and water quality modeling, simply assigning a specific percentage of the TMDL as the MOS or just using conservative model assumptions as suggested by USEPA's TMDL guidelines may not be sufficient to evaluate the relative significance of the contributing uncertainty sources or to assess management controls. Further, the recommendation and strategy on how to reduce uncertainty cannot be formulated due to the fact that the sources of uncertainty have not been determined. The objective of this study focuses on how to improve the current USEPA method for estimating the margin of safety by using a practical uncertainty analysis method.

The First-Order Error Analysis (FOEA) was applied for quantifying the MOS in the TMDL formulation. The case study was based on “Nitrate TMDL Development for Muddy Creek / Dry River, Virginia”, in which the state-of-the-art watershed model BASINS/HSPF was used. In this study, emphasis was given to the uncertainties resulting from precipitation data and model parameters in water quality and watershed models, particularly complex nitrogen cycle simulation. Besides computational efficiency, one of the major advantages of FOEA is its capability to determine the parameters that contribute significantly to the overall variance of the model output. Precipitation was found by far to be the most dominant uncertainty source.

By definition, the MOS is a portion of the load, while receiving water model output is usually in the form of pollutant concentration. A relationship was established to link the load with FOEA output concentration. For any given water quality target, there may be many combinations of load allocations between point source and nonpoint source. The results from testing different allocation scenarios demonstrated that with the increase of relative percentage of point source load reduction in the total load reduction, the portion needed to be reserved for MOS decreases. In summary, the use of the FOEA is considered as an improvement over the current USEPA simple explicit and implicit methods in estimating the MOS in TMDL allocation.

Document Type: Research Article

DOI: https://doi.org/10.2175/193864702785664725

Publication date: 2002-01-01

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