Freshwater Flow to Puget Sound is Declining Why? 2009 American Water Resources Association Annual Water Resources Conference Curtis DeGasperi King County.

Freshwater Flow to Puget Sound is Declining

Why?2009 American Water Resources Association

Annual Water Resources Conference

Curtis DeGasperiKing County Water and Land Resources Division

November 10, 2009

Mean Annual Flow to Puget Sound

• Total annual inflow has declined 13% owing partly to changes in precipitation (-5% over 1948-2003)

• …both may be related to the change in phase of the Pacific Decadal Oscillation (PDO) in 1977

Mote et al. 2005

Puget Sound Basin

CANADA

WASHINGTON

Seattle

Puget Sound Basin

WASHINGTON

Seattle

CANADA

Why care about declining river flows?

• Upland Concerns– Water supply for human uses– Water to support aquatic ecosystem

• Puget Sound– Affect on circulation and stratification– Potential affect on biological processes

Selected Data

• Daily average discharge from 8 major gauged rivers discharging to Puget Sound– 1948 to 2003

General Approach

• Conduct trend analysis on annual average flow

• Include consideration of trends in precipitation and evapotranspiration

• Attempt to provide some explanation for observed trends

Trend Analysis• Non-parametric “distribution free” method

– Mann-Kendall Trend test

http://pubs.usgs.gov/sir/2005/5275/

Trend Analysis• Non-parametric “distribution free” method

– Wilcoxon-Mann-Whitney Step Trend

http://wqhydro.com/

Trend Analysis

Does a trend exist?

Yes No

Has a trend been detected?

Yes + Type I Error: false trend detected when none exists

No Type II Error: failure to detect an existing trend due to weakness of the trend, of the methodology, or the shortness of the record

+

“Nothing is certain, but I think some things are more nearly

certain than others”- Bertrand Russell (paraphrased)

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Total Puget Sound freshwater inflow

Mean Annual Flow to Puget

Sound

site_no description12200500 SKAGIT RIVER NEAR MOUNT VERNON, WA12134500 SKYKOMISH RIVER NEAR GOLD BAR, WA12149000 SNOQUALMIE RIVER NEAR CARNATION, WA

12101500 PUYALLUP RIVER AT PUYALLUP, WA

12167000 NF STILLAGUAMISH RIVER NEAR ARLINGTON, WA

12113000 GREEN RIVER NEAR AUBURN, WA

12119000 CEDAR RIVER AT RENTON, WA

12054000 DUCKABUSH RIVER NEAR BRINNON, WA

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Total Puget Sound freshwater inflow Regression Line

1948 - 2003MAF = -90.62 (YEAR) + 211190

tau = -0.151 p = 0.1026

Decline of ~5,000 cfs or a 14% decline 1948-2003

Mean Annual Flow to Puget Sound

• Wilcoxon Step Trend Significantp = 0.003

• Difference in Mean Annual Flow

– 3,350 cfs (1948-1976 vs 1978-2003)

– or about 10% of 1948-1976 Flow

Mean Annual Flow to Puget Sound

• Precipitation

• Evapotranspiration

• Human Influences– Land Cover– Consumptive Uses– Wastewater Diversion

Puget Sound Precipitation

Hamlet and Lettenmaier, 2005

Puget Sound Precipitation

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VIC Puget Sound Precip

Puget Sound Precipitation

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VIC Puget Sound Precip Regression Line

1948 - 2003P = -0.1185 (YEAR) + 319.64

tau = -0.077 p = 0.4042

Decline of 6.5 inches or a 7% decline 1948-2003

Puget Sound Precipitation

• Wilcoxon Step Trend not significantp = 0.33

• Difference in Total Annual Precipitation– 7.5 inches (1948-1976 vs 1978-2003)

– 3,360 cfs based on gauged basin area

– or about 10% of 1948-1976 Flow

Evapotranspiration

• HSPF model unit flow predictions by Hydrologic Response Unit– Forest, Till– Forest, Outwash– Disturbed, Till– Disturbed, Outwash

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1940 1960 1980 2000 2020

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FOREST, OUTWASH

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GRASS, OUTWASH

Puget Sound Evapotranspiration

– Presume minimal impact for now

FOREST

DISTURBED

Puget Sound Land Cover Change

• Presume minimal impact (on annual average discharge of major rivers) for now

Alberti et al.

Consumptive Uses

• Not directly tabulated– USGS Total Fresh

Water Withdrawals by Puget Sound Counties, 2005

• 1,200 cfs

Wastewater Diversion

• Wastewater discharge to Puget Sound– Ecology data

• 630 cfs

Conclusions• Climate change/variability-related shift in

precipitation appears to explain most of the decline in annual flow to Puget Sound

• Human-induced changes likely significant in developed/developing areas in lowlands surrounding Puget Sound

Puget Sound Basin Population Growth

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4,000,000

1940 1950 1960 1970 1980 1990 2000 2010

Recommendations

• Well designed long-term gauging network

• Compilation and access to up-to-date water resource and water management data

• Good models needed to synthesize information and enhance understanding of causes and effects

“No amount of sophistication is going to allay the fact that all your knowledge is about the past and all your decisions are about the

future”- Ian E. Wilson (former chairman of GE)