Mid-Range Streamflow Forecasts for River Management in the Puget Sound Region

1. Mid-Range Streamflow Forecasts for River Management in the Puget Sound Region Richard Palmer Matthew Wiley Department of Civil and Environmental Engineering March 21, 2005

2. Outline • Region’s Water Supply Basins • Models • Current Conditions • Forecast Methods • ESP, Extended Streamflow Prediction • CPC, Climate Predication Center • Current Forecasts • Water Snowpack Modeling • Streamflow Forecasts • Impacts on Storage Special Thanks to the Water Forum for Funding the Mid-Range Forecasts

3. Hydrologic Model - DHSVM • Physically based, distributed model • Calculates energy and water balance at the grid cell level • 150 meter resolution, 3hr time-step • Station based meteorology • Output streamflows are aggregated to monthly total volume • Recently incorporated an experimental groundwater component

4. Sultan River • Snohomish PUD Hydropower project • Everett Water Supply • S.F. Tolt River • Seattle Water Supply • Cedar River • Seattle Water Supply • Green River • Flood Control • Tacoma Water Supply • All have environmental flow requirements

5. Streamflow Forecasts • Initialize basin models to current conditions using recent observed meteorology • ESP (Extended Streamflow Prediction) • Project 13 months into the future using each year in the historic record as one of many possible scenarios (traces). • Forecast can be conditioned using subsets of years based on observed climatologically conditions, such as ENSO state • CPC (Climate Prediction Center) • Seasonal outlooks of the probability of deviations from normal temperature and precipitation for 13 months • Each season's outlook and probability that a temperature or precipitation quantity will be exceeded for a climate region at the given lead time of two weeks to a year.

6. Modeling Snowpack • DHSVM allows evaluation of snowpack conditions over time • Provides graphical presentation of basin state • Estimate of total water storage as snow • Essential for evaluating the future based on current conditions • Provide spatial information to complement Snotel and Snow Course information

13. Forecasts Results • USGS 12137290 • SULTAN1: Inflow to Spada • USGS 12147600 • TOLT1: Inflow to SFT • TOLT2: Below SFT • USGS 12115000 • USGS 12117000 • CEDAR1: Inflow to CM • CEDAR2: Below CM • HH_INFLOW: Inflow to Howard Hansen

14. Cedar River above Chester Morse Reservoir

16. S.F.Tolt River above S.F.Tolt Reservoir

18. Green River into Howard Hansen Reservoir

20. All Inflows into Spada Reservoir

22. Implications to Water Supplies • Factors to Consider • Current storage • Inflows to reservoirs • Inflows below reservoirs that contribute to fish requirements • Average demand and effect of conservation • Availability of supplemental groundwater supplies • “Normal” versus “Low” versus “Critical” instream flows • Examined Seattle and Tacoma systems

25. Implications for Puget Sound Water Supply Systems • Inflows to reservoirs expected to be only 40% to 50% of average, slightly higher in the Everett system. • 60% chance of the Seattle system not completely refilling this spring ( ~32 billion gal.) • 10% chance of the Tacoma system not completely refilling this spring (~10 billion gal.) • If NO action is taken: • (i.e. “Normal fish flows, average demand) • 10%-15% of shortfall by October.

26. Implications for Puget Sound Water Supply Systems • A 10% reduction in consumption reduces probability to around 5% chance of shortfall • Current snowpack is only half of the picture, spring rains can also refill reservoirs • 10-12 inches* (at Seatac) between March 1 and July 1 will get us to near average storage by September 1 *(Average precip is 9.5 inches) • In an average year there is a considerable “surplus” of water in the basins

27. Implications for Puget Sound Water Supply Systems • Actions have already being taken • Actual operations are very dynamic with considerable flexibility and continual adjustments based on evolving conditions • Minimizing ecological damage and preventing disruptions to customers are primary goals • A “serious” situation requires several events (low snow pack, below average summer flows, late return of rain)

28. Questions ?

29. CPC forecast: MAM

30. CPC forecast: MJJ

31. CPC forecast: JJA

32. CPC forecast: JAS

33. CPC forecast: ASO

34. CPC forecast: SON

35. Cedar River above Chester Morse Reservoir

36. S.F.Tolt River above S.F.Tolt Reservoir

37. All Inflows into Spada Reservoir

38. Cedar River above Chester Morse Reservoir

39. Taylor Creek below Chester Morse Reservoir

40. S.F.Tolt River above S.F.Tolt Reservoir

41. S.F.Sultan River above Spada Reservoir

43. Green River into Howard Hansen Reservoir

44. All Inflows into Spada Reservoir