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

Roadmap for Technical Review MonthTopic Sep-07Technical comparison of existing control regulations Oct-07Existing chlorophyll target, incl magnitude, frequency, duration Nov-07Evaluation and discussion of concentration translator Dec-07Water budget and appropriate concentrations for each flow source as precursor to common set of phosphorus loads Jan-08Phosphorus load estimates; produce common set by source Feb-08Evaluation and discussion of load translator Mar-08Hydrologic considerations for TMAL Apr-08Discuss chlorophyll-phosphorus-load linkages as basis for proposal Jun-08WQCD to finalize proposal and circulate Jul-08Notice due Nov-08WQCC RMH

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

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

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

Phosphorus in Outflow

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

Annual Distributions

Seasonal Differences

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)

Phosphorus Balance

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

Vollenweider Model  TP, phosphorus concentration in lake (mg/m 3 )  L, external phosphorus load (mg/m 2 /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

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

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

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

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

P Retention (and Extreme Flows)

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

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

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

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

Plum Creek and the “Odd” Years 2002

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?

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