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Declining Freshwater Flow to Puget Sound: Causes and Impacts

This article discusses the reasons behind the decline in freshwater flow to Puget Sound and its potential impacts on the ecosystem and human water supply. It highlights the role of climate change and human-induced changes in the lowlands surrounding Puget Sound. The article also emphasizes the need for long-term gauging networks, up-to-date water resource data, and enhanced models to better understand the causes and effects of the decline.

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Declining Freshwater Flow to Puget Sound: Causes and Impacts

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  1. Freshwater Flow to Puget Sound is Declining Why? 2009 American Water Resources Association Annual Water Resources Conference Curtis DeGasperi King County Water and Land Resources Division November 10, 2009

  2. 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

  3. Puget Sound Basin CANADA Seattle WASHINGTON

  4. Puget Sound Basin CANADA Seattle WASHINGTON

  5. 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

  6. Selected Data • Daily average discharge from 8 major gauged rivers discharging to Puget Sound • 1948 to 2003

  7. 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

  8. Trend Analysis • Non-parametric “distribution free” method • Mann-Kendall Trend test http://pubs.usgs.gov/sir/2005/5275/

  9. Trend Analysis • Non-parametric “distribution free” method • Wilcoxon-Mann-Whitney Step Trend http://wqhydro.com/

  10. Trend Analysis

  11. “Nothing is certain, but I think some things are more nearly certain than others” - Bertrand Russell (paraphrased)

  12. Mean Annual Flow to Puget Sound Decline of ~5,000 cfs or a 14% decline 1948-2003

  13. Mean Annual Flow to Puget Sound • Wilcoxon Step Trend Significant p = 0.003 • Difference in Mean Annual Flow • 3,350 cfs (1948-1976 vs 1978-2003) • or about 10% of 1948-1976 Flow

  14. Mean Annual Flow to Puget Sound • Precipitation • Evapotranspiration • Human Influences • Land Cover • Consumptive Uses • Wastewater Diversion

  15. Puget Sound Precipitation Hamlet and Lettenmaier, 2005

  16. Puget Sound Precipitation

  17. Puget Sound Precipitation Decline of 6.5 inches or a 7% decline 1948-2003

  18. Puget Sound Precipitation • Wilcoxon Step Trend not significant p = 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

  19. Evapotranspiration • HSPF model unit flow predictions by Hydrologic Response Unit • Forest, Till • Forest, Outwash • Disturbed, Till • Disturbed, Outwash

  20. Puget Sound Evapotranspiration • Presume minimal impact for now FOREST DISTURBED

  21. Puget Sound Land Cover Change • Presume minimal impact (on annual average discharge of major rivers) for now Alberti et al.

  22. Consumptive Uses • Not directly tabulated • USGS Total Fresh Water Withdrawals by Puget Sound Counties, 2005 • 1,200 cfs

  23. Wastewater Diversion • Wastewater discharge to Puget Sound • Ecology data • 630 cfs

  24. 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

  25. 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

  26. “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)

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