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