Is there acceleration in streamflow timing trends across western North American mountains? [Iris Stewart]
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Streamflow timing trends across western North American mountains have shifted towards earlier in the water year, with changes up to one month earlier. These shifts are correlated with increasing spring temperatures and have continued through 2008. While models suggest acceleration towards earlier timing for some snow-dominated regimes, the results are not statistically significant when considering spatial and temporal correlations in the datasets. Some regions have experienced regime shifts towards greater rain domination since the late 1980s and late 1990s.
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Is there acceleration in streamflow timing trends across western North American mountains? [Iris Stewart]
- 1. Is there acceleration in streamflow timing trends across the western North American mountains? Iris Stewart (Santa Clara University) Holger Fritze (Universitaet Muenster) Edzer Pebesma (Universitaet Muenster)
- 2. Western North America: Mountain snow storage = key for water supply • Precipitation low and seasonal • Southern and western areas projected to become drier
- 3. Determining streamflow timing measures for a) snow b) rain c) mixed regimes All Gauges at: http://webpages.scu.edu/ftp/streamflowtiming/
- 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. Temperature increases: Largest for Western U.S. and past decade Source: www.globalchange.gov
- 6. Trends in the center timing (CT) of streamflow • Regionally coherent • Up to ~1 month • Rain vs snow regimes
- 7. Changes mostly a shift towards earlier in the water year
- 8. March temp trends in stream basins
- 9. Feb precip shifts – precip shifts not as large, spatially varying
- 10. Timing changes correlated to Spring temperatures
- 11. Connection to Winter precipitation depends on regime and region
- 12. Have changes in streamflow timing accelerated? What regime shifts are taking place?
- 13. Two linear regression models: 1) Second order OLS - Acceleration when 2nd order term negative 2) Piecewise linear - 2 connected straight lines, is there a change in slope?
- 14. Models appear to suggest acceleration for SDCs 3 and 4 • β2 negative => acceleration towards earlier present • β’2 negative => change in slope towards earlier BUT: if we incorporate spatial-temporal covariance model to account for autocorrelations, results no longer statistically significant
- 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