Evaluation of Load Translator for Chatfield Reservoir

Jim SaundersWQCD Standards Unit14 February 2008

Roadmap for Technical Review

Month Topic

Sep-07 Technical comparison of existing control regulations

Oct-07 Existing chlorophyll target, incl magnitude, frequency, duration

Nov-07 Evaluation and discussion of concentration translator

Dec-07 Water budget and appropriate concentrations for each flow source as precursor to common set of phosphorus loads

Jan-08 Phosphorus load estimates; produce common set by source

Feb-08 Evaluation and discussion of load translator

Mar-08 Hydrologic considerations for TMAL

Apr-08 Discuss chlorophyll-phosphorus-load linkages as basis for proposal

Jun-08 WQCD to finalize proposal and circulate

Jul-08 Notice due

Nov-08 WQCC 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

0.001

0.010

0.100

1.000

Jan-87 Sep-89 Jun-92 Mar-95 Dec-97 Sep-00 Jun-03 Mar-06

Ou

tflo

w T

ota

l P

ho

sph

oru

s, m

g/L

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

0.001

0.01

0.1

1

1987 1989 1991 1993 1995 1997 1999 2001 2003 2005

To

tal P

ho

sp

ho

rus

, mg

/L

Seasonal Differences

0.001

0.01

0.1

1

1-Jan 20-Feb 10-Apr 30-May 19-Jul 7-Sep 27-Oct 16-Dec

Ordinal Day

Lak

e B

ott

om

Ph

osp

ho

rus,

mg

/L

1987-1991 1992-1996 1997-2001 2002-2006

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

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

50000

1988 1990 1992 1994 1996 1998 2000 2002 2004 2006

Ph

osp

ho

rus,

lbs/

y

Retained Export

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/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

z

LTP

Estimating P Retention Coefficient

Canfield-Bachmann Artificial lakes To match 1982 data,

multiply σ by 3.6 Yields modified

Canfield-Bachmann

589.0

114.0

z

L

589.0

410.0

z

L

Derive “new” C-B from data?

Plot σ vs. L/z; No real pattern Influential extreme flows

y = 0.0284x1.0249

R2 = 0.5963

0

10

20

30

40

50

60

0 200 400 600 800

R atio of External Load to Mean D epth

Sig

ma

fro

m O

bs

erv

ed

Ex

po

rt

Does it Predict Well?

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

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

0.045

0.050

0.000 0.010 0.020 0.030 0.040 0.050

Predicted TP, mg/L

Ob

se

rve

d E

xp

ort

TP

, m

g/L

Try Another Model: Dillon-Rigler

Load

ExportLoadR

)(

zRL

TP)1(

Commonly used

for reservoirs Retention is

fraction of load Median R=0.64

P Retention (and Extreme Flows)

y = 0.7147x

R2 = 0.9594

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

0 10000 20000 30000 40000 50000 60000

External P Load, lbs/y

Re

tain

ed

P, l

bs

/y

Can We Predict R?

Not encouraging based on common approach (OECD for shallow lakes)

Probably better to use a constant

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0.0 0.2 0.4 0.6 0.8

OECD R

Ob

se

rve

d R

Performance of Dillon-Rigler?

Use constant R=0.64 (median) Alignment OK, but precision not so good

0.000

0.005

0.010

0.015

0.020

0.025

0.030

0.035

0.040

0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035

Predicted TP, mg/L

Ob

se

rve

d E

xp

ort

TP

, m

g/L

Alternate View of Precision

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

0.000

0.010

0.020

0.030

0.040

0.050

0.060

0.070

1988 1990 1992 1994 1996 1998 2000 2002 2004 2006

Su

mm

er M

edia

n T

ota

l P

ho

sph

oru

s, m

g/L

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

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

1000 10000 100000 1000000

Flow, AF/y

Res

iden

ce T

ime,

y

Total Inflow Waterton Plum Full Pool

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