Skip to main content

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

Buy Article:

$9.50 plus tax (Refund Policy)

Or sign up for a free trial

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.
No Reference information available - sign in for access.
No Citation information available - sign in for access.
No Supplementary Data.
No Article Media
No Metrics

Document Type: Research Article

Publication date: 2002-01-01

More about this publication?
  • Proceedings of the Water Environment Federation is an archive of papers published in the proceedings of the annual Water Environment Federation® Technical Exhibition and Conference (WEFTEC® ) and specialty conferences held since the year 2000. These proceedings are not peer reviewed.

    A subscription to the Proceedings of the Water Environment Federation includes access to most papers presented at the annual WEF Technical Exhibition and Conference (WEFTEC) and other conferences held since 2000. Subscription access begins 12 months after the event and is valid for 12 months from month of purchase. A subscription to the Proceedings of the Water Environment Federation is included in Water Environment Federation (WEF) membership.

    WEF Members: Sign in (right panel) with your IngentaConnect user name and password to receive complimentary access. Access begins 12 months after the conference or event
  • Subscribe to this Title
  • Membership Information
  • About WEF Proceedings
  • WEFTEC Conference Information
  • Learn about the many other WEF member benefits and join today
  • Ingenta Connect is not responsible for the content or availability of external websites
  • Access Key
  • Free content
  • Partial Free content
  • New content
  • Open access content
  • Partial Open access content
  • Subscribed content
  • Partial Subscribed content
  • Free trial content
Cookie Policy
Cookie Policy
Ingenta Connect website makes use of cookies so as to keep track of data that you have filled in. I am Happy with this Find out more