Alan F. HamletAnthony L. WesterlingTim P. BarnettDennis P. Lettenmaier

•JISAO/CSES Climate Impacts Group•Dept. of Civil and Environmental Engineering

University of Washington•Scripps Institute of Oceanography•School of Engineering, University of California, Merced

Evaluating Recent 20th Century Changes in Cool Season Precipitation Variability in the Western U.S. in the Context of Paleoclimatic Reconstructions

Consensus Forecasts of Temperature and Precipitation Changes from IPCC AR4 GCMs

Pacific Northwest

°C

0.4-1.0°C0.9-2.4°C 1.2-5.5°C

Obse

rved 2

0th

centu

ry v

ari

abili

ty

+1.7°C+0.7°C

+3.2°C

Pacific Northwest

%

-1 to +3%-1 to +9% -2 to +21%

Obse

rved 2

0th

centu

ry v

ari

abili

ty

+1% +2%

+6%

-3

-2

-1

0

1

2

3

419

16

1920

1924

1928

1932

1936

1940

1944

1948

1952

1956

1960

1964

1968

1972

1976

1980

1984

1988

1992

1996

2000

Std

An

om

alie

s R

elat

ive

to 1

961-

1990

PNW

CA

CRB

Regionally Averaged Cool Season Precipitation Anomalies

PRECIP

-3

-2

-1

0

1

2

319

16

1920

1924

1928

1932

1936

1940

1944

1948

1952

1956

1960

1964

1968

1972

1976

1980

1984

1988

1992

1996

2000

Std

An

om

alie

s R

elat

ive

to 1

961-

1990

PNW

CA

CRB

Regionally Averaged Warm Season Precipitation Anomalies

PRECIP

2000

3000

4000

5000

6000

7000

8000

9000

1916

1921

1926

1931

1936

1941

1946

1951

1956

1961

1966

1971

1976

1981

1986

1991

1996

2001

Str

eam

flo

w (

cms)

OBS

REG

0

50

100

150

200

250

300

350

400

450

1916

1921

1926

1931

1936

1941

1946

1951

1956

1961

1966

1971

1976

1981

1986

1991

1996

2001

Str

eam

flo

w (

cms)

OBS

REG

R2 = 0.83

R2 = 0.91

Columbia River

Sacramento River

Cool Season Precipitation Explains Most of the Variability in Annual Flow in the PNW and CA

Relationship Between Annual Flow and

Cool Season Precip.

Relationship Between Annual Flow and

Cool Season Precip.

200

300

400

500

600

700

800

900

1000

1100

1916

1921

1926

1931

1936

1941

1946

1951

1956

1961

1966

1971

1976

1981

1986

1991

1996

2001

Str

eam

flo

w (

cms)

OBS

REG

200

300

400

500

600

700

800

900

1000

1100

1916

1921

1926

1931

1936

1941

1946

1951

1956

1961

1966

1971

1976

1981

1986

1991

1996

2001

Str

eam

flo

w (

cms)

OBS

REG

R2 = 0.56

Colorado River

R2 = 0.18

Colorado River

Cool Season Precip Explains Most of the Variability in Annual Flow in the CRB, but the Summer Monsoon Also Plays a Role

Relationship Between Annual Flow and

Cool Season Precip.

Relationship Between Annual Flow and

Warm Season Precip.

-2

-1

0

1

2

3

1917

1919

1921

1923

1925

1927

1929

1931

1933

1935

1937

1939

1941

1943

1945

1947

1949

1951

1953

1955

1957

1959

1961

1963

1965

1967

1969

1971

1973

1975

1977

1979

1981

1983

1985

1987

1989

1991

1993

1995

1997

1999

2001

Sys

tem

Wid

e H

ydro

po

wer

P

rod

uct

ion

(S

td A

no

mal

ies)

CRB

SSJ

PNW

Correlation:CRB-SSJ = 0.07CRB-PNW = 0.08SSJ-PNW = 0.36

Correlation:CRB-SSJ = 0.14CRB-PNW = -0.14SSJ-PNW = 0.06

Correlation:CRB-SSJ = 0.73CRB-PNW = 0.51SSJ-PNW = 0.65

Simulated Changes in System Wide Energy Production in the Western U.S.

DJF

Avg

Tem

p (

C)

20-year Flood for “1973-2003” Compared to “1916-2003” for a Consistent Late 20th Century Temperature Regime

X20 ’73-’03 / X20 ’16-’03

X20 ’73-’03 / X20 ’16-’03

Hamlet A.F., Lettenmaier D.P., 2007: Effects of 20th Century Warming and Climate Variability on Flood Risk in the Western U.S., Water Resour.

Res., 43, W06427

Long-Term Comparison of Annual Flow Records from Observations and Paleo Reconstructions

PNW:Observed (naturalized) annual flow in the Columbia River at The Dalles, OR

1858-1877 (reconstructed from observed peak river stage)1878-2003 (naturalized from observed monthly records)

CA:Reconstructed combined annual flow in the Sacramento/San Joaquin basin from tree-ring records.

(Overlapping period 1858-1977)

(Meko, D.M., 2001: Reconstructed Sacramento River System Runoff From Tree Rings, Report prepared

for the California Department of Water Resources, July)

Colorado River Basin:Reconstructed annual flow in the Colorado River at Lees Ferry, AZ from tree ring records.

(Overlapping period 1858-1977)

(Woodhouse, C.A., S.T. Gray, and D.M. Meko, 2006: Updated Streamflow Reconstructions for the Upper

Colorado River Basin, Water Resources Research, Vol. 42, W05415)

-1.5

-1

-0.5

0

0.5

1

1.5

1868

1874

1880

1886

1892

1898

1904

1910

1916

1922

1928

1934

1940

1946

1952

1958

1964

1970

1976

1982

1988

sum lag1 correl

sum inter-regional correl

sum CV

Changes in Streamflow Variability from Long-Term Observations and Paleo Reconstructions (1858-1977)

-1.5

-1

-0.5

0

0.5

1

1.5

1868

1874

1880

1886

1892

1898

1904

1910

1916

1922

1928

1934

1940

1946

1952

1958

1964

1970

1976

1982

1988

sum lag1 correl

sum inter-regional correl

sum CV

Changes in Streamflow Variability from VIC Simulations of Annual Flow (1916-2003)

-1.5

-1

-0.5

0

0.5

1

1.5

1868

1874

1880

1886

1892

1898

1904

1910

1916

1922

1928

1934

1940

1946

1952

1958

1964

1970

1976

1982

1988

sum lag1 correl

sum inter-regional correl

sum CV

Changes in Streamflow Variability from Combined Paleo Reconstructions and VIC Simulations of Annual Flow (1916-2003)

All three metrics high together

Conclusions

•Cool season precipitation is a major driver of annual river flow, hydropower production, and flood risk in the West.

•Substantial and persistent changes in cool season precipitation variability have emerged over the West since about 1975, including increased CV, within-region persistence, and inter-regional correlation.

•Long-term streamflow reconstructions show that the current changes in variability are very unusual in the context of natural variations over the last 150 years or so.

•Can these changes in cool season precipitation be related to changes in circulation associated with greenhouse-forced warming?