Regional Analysis of LowFlow in Western WA

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Regional Analysis of Low-Flow in Western WA

• Presented by Ken Eng1 and Chris Curran2

1USGS, NRP, WRD, BRR-ER Reston, VA2USGS, WAWSC,
Tacoma, WA May 23, 2008
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Regional Analysis of Low-flowin Western
Washington

• Decide on a list of candidate index sites.
• Select relevant basin characteristics.
• Choose a flow characteristic to estimate (Q7,10).
• Run correlation analysis (MOVE, etc) to estimate
  Q7,10 at simulated misc. sites.
• Test different scenarios for index-site
  selection.
• Develop regression model to estimate Q7,10 at
  ungaged or future miscellaneous sites.

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Index site
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Index site
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Index-site selection

• Natural, unaltered flow.
• No significant storage (lakes, ponds).
• No significant contribution from snowmelt.
• Q7,10 cannot equal zero (correlation analysis).
• More than 10 years of continuous daily-streamflow
  record.
• Must have overlapping record (correlation only).
• Only USGS gages considered.

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USGS index gages used in the correlation analysis
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Basin characteristics

• Drainage area (StreamStats).
• Geographic proximity.
• Mean-annual precipitation (StreamStats).
• Surficial geology ( coarse material).
• Base-flow recession time constant, t.

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Base-flow Recession Time Constant, t

• Tau describes how fast base-flow recedes

Large t
Small t
Time

• Many ways to estimate t e.g., Brutsaert and
  Nieber, 1977 Eng and Brutsaert, 1999 Eng and
  Milly, 2007, Brutsaert, 2008.

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Base-flow Recession Time Constant, t

• Estimated t from pair of measurements

Single
Paired
Day 8
No t can be estimated for a single value.
Qt
QtDt
Dt5 days
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Regional low-flow correlation analysis

• Done using a Matlab script written by Ken
• Uses the daily-streamflow records for all index
  gages.
• Simulates a miscellaneous site by using an index
  site.
• Extracts the recession segments of hydrograph.
• Randomly picks a daily flow (or pairs) and treats
  it as a miscellaneous measurement.

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Regional low-flow correlation analysis

• Runs each scenario with a user-defined
  combination of basin characteristics and number
  of miscellaneous measurements.
• Determines best index site (similar A,G,P,t,S)
• Performs Q-ratio, MOVE, base-flow correlation
• Calculate estimates of Q7,10 and compares this
  with the known value of Q7,10, records error.
• Repeated 500 times.

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Low-flow correlation analysis
73
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Low-flow correlation analysis
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Summary for correlation analysis

• Minimum 4 measurements for Q-Ratio.
• Using t reduces RMSE by roughly 10.
• MOVE and base-flow correlation will underestimate
  (more conservative).
• Q-Ratio may overestimate (undesirable for low
  flow).

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Recommendations based on correlation analysis in
western Washington

• Q-Ratio (lt8) and/or MOVE (gt8) methods.
• Minimum 4 measurements (paired for t).
• 3 day minimum wait between paired meas.
• Wait 6 days until after peaks, check a nearby
  gage hydrograph to ensure recession
• Make measurements during base-flow season

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Regional regression analysis

• Different from correlation analysis
• Uses basin characteristics.
• Uses index gages
• Can use discontinued sites.
• Miscellaneous measurements can be used.
• More accurate Q7,10 benefits regional regression
  model.

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USGS index gages used in analyses
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Regional regression models using A, P, t
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Regional regression analysis
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Regional regression analysis

• Summary for regression models
• 8 pairs - 60 of potential reduction in RMSE.
• Diminishing returns after 8 pairs.
• Recommendations
• Use A, P, and t.
• Maximum of 8 paired measurements.
• Minimum of 4 paired measurements.

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Optimizing the Low-flow Networkin Western
Washington
What types of miscellaneous sites do we still
need?
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Optimizing the Low-flow Networkin Western
Washington

• Fill spatial gaps in the existing network.
• Use StreamStats to identify potential locations
  for miscellaneous sites.
• Balance location with operational concerns.

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Future Miscellaneous Sites

• Correlation analysis versus regional regression
  to estimate Q7,10 for miscellaneous sites
• Correlation
• Gambling, based on best single index site
• Lengthy, need measurements
• Regression
• Robust
• Simple
• Recommendation regional regression models to
  estimate Q7,10 at future miscellaneous sites.

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Summary of What to do for a Study

• For estimating Q7,10 at current miscellaneous
  sites
• Run a correlation analysis to estimate Q7,10 at
  current miscellaneous sites.
• Use A, P and t as criteria to select index site.
• Use Q-ratio for lt 8 pairs of measurements or MOVE
  (gt 8) to estimate Q7,10.
• Need minimum of 4 singles/pairs.

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Summary of What to do for a Study

• Develop regional regression model to estimate
  Q7,10 at future sites.
• Use A, P, and t.
• Use all continuous and current miscellaneous
  sites.
• Use t (at least 4 pairs).
• Application of regional regression model.
• Make measurements at a stream of interest.
• Compute t from all pairs and average.
• Find A and P for the site (StreamStats).
• Place into regression model to calculate Q7,10.

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References

• Brutsaert, W. and J. L. Nieber (1977),
  Regionalized drought flow hydrograph from a
  mature glaciated plateau, Water Resour. Res.,
  13(3), 637-643.
• Brutsaert (2005), Hydrology An Introduction,
  Cambridge University Press, 605.
• Brutsaert (2008), Long-term groundwater storage
  trends estimated from streamflow records
  Climatic perspective, Water Resour. Res., 44,
  W02410.
• Eng K. and W. Brutsaert (1999), Generality of
  drought flow characteristics within the Arkansas
  River basin, J. Geophys. Res., 104(D16),
  19,435-19,441.
• Eng, K. and P.C. D. Milly (2007), Relating
  low-flow characteristics to the base flow
  recession time constant at partial record stream
  gauges, Water Resour. Res.,43, W01201.