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