Lake Tahoe TMDLScience Objectives

What are sources and relative contributions of “contaminants” causing clarity decline?

How much of a reduction is needed to achieve the desired conditions?

Lake Clarity Model&

What Makes Us Think It’s Right

OR

The Clarity Model History

1978 – DYRESM1996 – DLM-WQ1997 – INAUGURAL TAHOE SUMMIT

EPA WATERSHED GRANT2000 – FIRST PARTICLE SIZE DATA2002 – TMDL SCIENCE PROGRAM FUNDED MODELING/SCIENCE2004 – CLARITY MODEL PEER REVIEW2006 – REFINEMENT/CALIBRATION/VALIDATION

2007 – COMMENCING USE OF “FINAL” MODEL

CLARITY MODEL

A PROCESS-BASED NUMERICAL MODEL

SEVERAL MODELS COMBINED INTO ONE:- HYDRODYNAMIC/THERMODYNAMIC MODEL- WATER QUALITY (ECOLOGICAL) MODEL- PARTICLE FATE MODEL- OPTICAL MODEL

IN ADDITION, IT HAS “INPUTS” FROM OTHER MODELS- WATERSHED MODEL- METEOROLOGY MODEL- ATMOSPHERIC MODEL

Lake Tahoe Clarity Model

DLM Hydrodynamic/Thermodynamic Model

Tributaries

Climate,Precipitation

Land Use Atmospheric Deposition

AlgalGrowth

LakeN, P

Inorganic

Particles

LossLoss

SecchiDepth

LightScattering &Absorption

Groundwater

Tributaries

Climate,Precipitation

Land UseAtmospheric Deposition

Particle FateModel

OpticalModel

EcologicalModel

INPUT VARIABLESCLIMATE

OUTFLOW

INFLOW – Q, N, P, PSD

ATMOSPHERIC – N, P, PSDGROUNDWATER – Q, N, P

BATHYMETRY

Tetra Tech Watershed Model

L

IGH

T

NU

TR

IEN

T

CY

CL

ING

STRATIFICATION & MIXING

AL

GA

L

GR

OW

TH

PAR

TIC

LE

DY

NA

MIC

S

ABSORPTION(i)SCATTERING (i)

SECCHI DEPTH

Clarity Model

Base Case Do Nothing More in an Unchanging World

Test Case 1 – 55% Load ReductionAll Sources, All loads, 20 year Phase-in

FINES, N, P

N, P ONLY

Combined ResultsAll Sources, All loads, 20 year Phase-in

Test Case 2 – 75% Load ReductionUrban Sources, All loads, 20 year Phase-in

Combined ResultsUrban Sources, All loads, 20 year Phase-in

0

5

10

15

20

25

30

Atmosphere Urban Non-urban Stream Channelerosion

Shorelineerosion

Sec

chi d

epth

(m

)

0.5X Base case 2X

Sensitivity Analysis Loads

Particle load has largest impact on secchi depth

0.5X = half TMDL estimate 2X = double TMDL estimate

Sensitivity Analysis Model Parameters

Coagulation rate

Light scattering (b*)

Algal growth rate

Secchi Depth

Secchi Depth

Secchi Depth

Sensitivity Analysis Model Parameters

coagulation rate - light scattering – algal growth rate

0

5

10

15

20

25

300

-0-0

0-0

-L

0-0

-H

L-0

-0

H-0

-0

0-L

-0

0-H

-0

0-L

-L

0-L

-H

0-H

-L

0-L

-L

L-L

-L

L-L

-H

L-H

-L

L-H

-H

H-L

-L

H-L

-H

H-H

-L

H-H

-H

Scenario

Se

cc

hi

de

pth

(m

)

Mean/SD = 11%

L-L-H

Coagulation rateLight scattering

Algal growth rate

L = 0.75X 0 = X H = 1.25X

COV = MEAN/SD = 11%

WHAT MAKES US THINK THE MODEL IS CORRECT?

3 m in 2 years

LOOK AT THE RECORD

STILL NOT CONVINCED?

Particles per ml

Sec

chi

Dep

th (

m)

0 10 20 30 40 50 60 70 80 90100

0

10

20

30

4050

60

708090

100

0 10 20 304050 60 70 8090100% Nitrogen Reduction

% Phosphorus Reduction

% S

edim

ent

Red

uct

ion

20- 25………Red25.5-28…….Yellow28.5-32.5…..Blue33 & above..Purple

20- 25………Red25.5-28…….Yellow28.5-32.5…..Blue33 & above..Purple

Final SecchiDepth (m)

Parameters are forillustrative purposes only

There are a multitude of ways to achieve a specific clarity target

Process-based model – allows examination of the entire range of management, climate, disaster, growth etc. scenariosBuilt on an established and peer reviewed frameworkParticles dominate midlake clarity (nutrients secondary) – confirmed by dataUrban areas dominant source of particles – confirmed by dataModel results insensitive to uncertaintiesModel predicted level of pollutant load reduction to achieve clarity target is confirmed by dataThere are countless ways in which the desired load reductions can be achieved. The model can test them. The stakeholders must decide.

CONCLUSIONS

THANK YOU!

16

18

20

22

24

26

28

30 100

1000

104

1980 1985 1990 1995 2000 2005

ANNUAL AVERAGE SECCHI DEPTH and ANNUAL UPPER TRUCKEE RIVER FLOW

SECCHI (m) UT FLOW (10**8 L)

SE

CC

HI

(m)

UT

FL

OW

(10

**8 L

)

YEAR

UC Davis Tahoe Environmental Research Center

Runoff Fines Distribution Mass Land Use

Category Land Use Name or Watershed

(< 20 um) Residential_SF Residential_MF

Urban CICU 57 ± 18 % Roads_Primary Roads_Secondary General Creek

Non-Urban Blackwood Creek 13 ± 3 % Ward Creek Trout Creek

July 2006 Pathways Forum Received “Preliminary” Model Results

15

20

25

30

35

19

68

19

72

19

76

19

80

19

84

19

88

19

92

19

96

20

00

20

04

20

08

20

12

20

16

20

20

Years

Se

ch

hi

de

pth

(m

)

Measured Projected trend Base line 35% less fine and nutrients @1.75% per year

Calibration and validation years (1999-2004)

Fine Particle Numbers <20 µm5 x 1020/yr

Atmospheric Deposition

15%

Non-urban Upland

9%

Urban Upland 72%

Stream Channel Erosion

4%Shoreline Erosion

< 1%

Today there are different results – based on “Final” Model Results