Is there acceleration in streamflow timing trends across western North American mountains? [Iris Stewart]


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Is there acceleration in streamflow timing trends across western North American mountains? Presented by Iris Stewart at the "Perth II: Global Change and the World's Mountains" conference in Perth, Scotland in September 2010.

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Is there acceleration in streamflow timing trends across western North American mountains? [Iris Stewart]

  1. 1. Is there acceleration instreamflow timing trendsacross the western North American mountains? Iris Stewart (Santa Clara University) Holger Fritze (Universitaet Muenster) Edzer Pebesma (Universitaet Muenster)
  2. 2. Western North America: Mountainsnow storage = key for water supply • Precipitation low and seasonal • Southern and western areas projected to become drier
  3. 3. Determining streamflow timing measures for a) snow b) rain c) mixed regimesAll Gauges at:
  4. 4. Snowmelt Domination Categories (SDCs)• SDC1: clearly rain dominated – Snowmelt pulse in < 30% of years• SDC2: mostly rain dominated – Snowmelt pulse in >= 30 & < 50% of years• SDC3: mostly snow dominated – Snowmelt pulse in >= 50 & < 70% of years• SDC4: clearly snow dominated – Snowmelt pulse in > 70% of years
  5. 5. Temperature increases: Largest for Western U.S. and past decade Source:
  6. 6. Trends in thecenter timing (CT) of streamflow• Regionally coherent• Up to ~1 month• Rain vs snow regimes
  7. 7. Changes mostly a shift towards earlier in the water year
  8. 8. March temp trends in stream basins
  9. 9. Feb precip shifts – precip shifts not as large, spatially varying
  10. 10. Timing changes correlated to Spring temperatures
  11. 11. Connection to Winter precipitation depends on regime and region
  12. 12. Have changes in streamflow timing accelerated?What regime shifts are taking place?
  13. 13. Two linear regression models:1) Second order OLS - Acceleration when 2nd order term negative2) Piecewise linear - 2 connected straight lines, is there a change in slope?
  14. 14. Models appear to suggestacceleration for SDCs 3 and 4 • β2 negative => acceleration towards earlier present • β’2 negative => change in slope towards earlierBUT: if we incorporate spatial-temporalcovariance model to account forautocorrelations, results no longerstatistically significant
  15. 15. Regime changesbefore/after 1987/88WashingtonCaliforniaIdahoUtahNew Mexico
  16. 16. Regime changesbefore/after 1997/98
  17. 17. Summary• Shifts towards earlier snowmelt runoff timing continued through 2008 – Regionally coherent – Mostly connected to warmer spring temps• No statistically significant acceleration, when considering spatial and temporal correlation – Earlier timing indicated – Short time series with high interannual variability• Regime shifts taking place in several regions with high vulnerability to warmer temps – Most shifts towards greater rain domination