is there acceleration in streamflow timing trends across western north american mountains? [iris...
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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.TRANSCRIPT
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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)
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Western North America: Mountain
snow storage = key for water supply
• Precipitation
low and
seasonal
• Southern
and western
areas
projected to
become
drier
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Determining
streamflow
timing
measures
for a) snow
b) rain
c) mixed
regimes
All Gauges at: http://webpages.scu.edu/ftp/streamflowtiming/
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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
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Source: www.globalchange.gov
Temperature increases: Largest for
Western U.S. and past decade
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Trends in the
center timing
(CT) of
streamflow
• Regionally
coherent
• Up to ~1 month
• Rain vs snow
regimes
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Changes mostly a shift towards
earlier in the water year
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March temp
trends in
stream
basins
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Feb precip
shifts –
precip
shifts not
as large,
spatially
varying
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Timing changes correlated to
Spring temperatures
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Connection to Winter precipitation
depends on regime and region
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Have changes in streamflow
timing accelerated?
What regime shifts are taking
place?
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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?
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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
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Regime
changes
before/after
1987/88
Washington
California
Idaho
Utah
New Mexico
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Regime
changes
before/after
1997/98
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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