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Uncertainty and Potential Improvements to the Total Load Indicator

Uncertainty and Potential Improvements to the Total Load Indicator. Non-Tidal Workgroup Conference Call July 18 th , 2007 Michael Williams - UMCES/CBPO. Overview of Total Load Method. Tidal Load Non-point Source Calculations.

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Uncertainty and Potential Improvements to the Total Load Indicator

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  1. Uncertainty and Potential Improvements to the Total Load Indicator Non-Tidal Workgroup Conference Call July 18th, 2007 Michael Williams - UMCES/CBPO

  2. Overview of Total Load Method

  3. Tidal Load Non-point Source Calculations

  4. Index values for 85, 02, and 03 using long-term hydrology (1985–1994)

  5. Index values for all constituents(1985-1997)

  6. What does all this mean? • Clear that this technique raises many questions • For example, what are the reasons for the different index values (10-year hydrology and annual, and among constituents – e.g., P and Sed)? • Only by understanding the reasons for these differences and making sure that the indices are representative can we be confident that the total load indicator is accurate

  7. Patuxent case study

  8. summary of model biasdaily predictions(no flow weighting)

  9. average annual export errors (modeled vs observed) 1984 – 1995/97(*flow-weighted)

  10. Observed-Modeled errors (%)

  11. Potential Errors in Load Estimates • Sampling and analytical (consistent protocols for all RIMP stations since 1985, lab and method changes, human error?) • ESTIMATOR • Indices (HSPF) • Substantial calibration and large validation errors • Delivery factors (anybody’s guess) • 10-year hydrology • Point source loads – reported but not verified

  12. Average error estimates • Cumulative error (Reckhow and Chapra ’79) • HSPF (examples are preliminary phase 5 HSPF output) wsm underestimates by average of 11%

  13. wsm underestimates by average of 11% wsm overestimates by average of 29%

  14. Concluding Remarks • TL indicator has potentially large cumulative errors • Particularly for P and Sed (6, 22, 51% for N, P and Sed, respectively) using phase 4.3 HSPF output • Inflated from large errors associated with indices • Recommended improvements include: • Remove Susquehanna basin (53% area but with no coastal plain) and eastern shore areas (no piedmont) to calculate indices • Calculate indices on an annual basis (i.e., remove 10-year hydrology) • Use phase 5 HSPF output as well as an ensemble of other models (preferably less parameterized models such as GWLF) to determine indices • Quantify uncertainties

  15. Remove Susquehanna Basin (53% total area)

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