Evaluation of Load Translator for Chatfield Reservoir

1. Evaluation of Load Translator for Chatfield Reservoir Jim Saunders WQCD Standards Unit 14 February 2008

2. Roadmap for Technical Review

3. For Today…. • What is a load translator? • The data set • Review history of load translator • Evaluate performance of load translator(s) • Discuss approach and path forward

4. What is a Load Translator? • Quantitative linkage between phosphorus load to the reservoir and the resulting phosphorus concentration in the reservoir • A necessary component for: • Establishing the load consistent with a chlorophyll (or TP) standard • Predicting the response (chlorophyll) for future P load scenarios

5. Data Needs • Phosphorus concentrations in lake • Phosphorus loads • Hydraulic data (volume, area, computed inflow, outflow) • Phosphorus export • Flow * concentration in release • Must include manifold, too • Assume concentrations same in manifold and release to SP

6. Phosphorus in Outflow

7. Outflow P: Expectations and Concerns • Large volume of reservoir buffers outflow P concentration • MDL issues (esp. 2002) • Are there patterns in concentration? • Over years • Between seasons

8. Annual Distributions

9. Seasonal Differences

10. Aggregating Data • Sampling program: ~13 samples/y • How best to assign concentrations to all flows? • Can’t aggregate across years • Aggregate within seasons (just in case) • Oct-Mar • Apr-Sep (stratification season) • Assume constant concentration (median) within each season in each year • Seasonal Load = Seasonal median*Σ(flows)

11. Phosphorus Balance

12. Previous Load Translator • Clean Lakes Study used a mass balance model (Vollenweider) • Modified a component so prediction would match the datum from 1982 • No subsequent changes • Now have ample data for review

13. Vollenweider Model • TP, phosphorus concentration in lake (mg/m3) • L, external phosphorus load (mg/m2/y) • Z, mean depth (m) • σ, phosphorus sedimentation coefficient (y-1) • ρ, reservoir flushing rate (inflow/volume; y-1) • All terms known for historical data except σ

14. Estimating P Retention Coefficient • Canfield-Bachmann • Artificial lakes • To match 1982 data, multiply σ by 3.6 • Yields modified Canfield-Bachmann

15. Derive “new” C-B from data? • Plot σ vs. L/z; No real pattern • Influential extreme flows

16. Does it Predict Well? • Use constant σ=6 (NB: differs from text) • Note 5 yrs at right (’88, ’95, ’98, ’99, ’05)

17. Try Another Model: Dillon-Rigler • Commonly used for reservoirs • Retention is fraction of load • Median R=0.64

18. P Retention (and Extreme Flows)

19. Can We Predict R? • Not encouraging based on common approach (OECD for shallow lakes) • Probably better to use a constant

20. Performance of Dillon-Rigler? • Use constant R=0.64 (median) • Alignment OK, but precision not so good

21. Alternate View of Precision • Compare predictions of summer median P (bootstrap) • Five peculiar years predict very high • 1996 predicts low

22. Is Load Translator Ready to Use? • Would prefer it to be stronger • Plan to continue exploring options • Especially interested in 5 odd years • Consider two main tributaries from hydrologic perspective • Flows largely uncorrelated • SP always the dominant flow • Loads closer to being equal due to higher concentrations in Plum Creek

23. Plum Creek and the “Odd” Years 2002

24. Thinking Out Loud… • High flow years for Plum Cr (>20,000 AF) stand out: over-predict phosphorus • What’s different about Plum Cr load? • Normal stream that carries particulates at high flow • Contrast with SP that has been decanted through series of reservoirs • Can the retention be partitioned?

25. Next Steps • Refine load translator • Flows and TMAL • What scenario for inflows? • Is 261,000 AF scenario appropriate? • TMAL not exceeded even in 1995 (336K AF) • Does worst case for in-lake concentration represent highest load scenario? • Next meeting Mar 13: Hydrologic considerations for TMAL