Alan F. Hamlet,Phil Mote, Martyn Clark,

Dennis P. Lettenmaier

Center for Science in the Earth System Climate Impacts Group

and Department of Civil and Environmental EngineeringUniversity of Washington

October, 2005

Effects of Climate Change on Snowpack, Runoff, Soil Moisture and Evaporation in the Western U.S. from

1916-2003

Background

DJF Temp (°C) NDJFM Precip (mm)

PNW

CA CRB

GB

Winter Climate of the Western U.S.

Trends in Temperature and Precipitation

Natural Climate Influence Human Climate Influence

All Climate Influences

Natural AND human influences explain the observations of global warming best.

Rel. Trend %/yr Trend (°C/yr) Trend (°C/yr)

Precipitation Tmax Tmin

DJF

Avg

Tem

pera

ture

DJF

Avg

Tem

pera

ture

Rel. Trend %/yr Trend (°C/yr) Trend (°C/yr)

1916-2003

1947-2003

Trends in Winter (Oct-Mar) Precipitation and Temperature

Rel. Trend %/yr Trend (°C/yr) Trend (°C/yr)

Precipitation Tmax Tmin

DJF

Avg

Tem

pera

ture

DJF

Avg

Tem

pera

ture

Rel. Trend %/yr Trend (°C/yr) Trend (°C/yr)

1916-2003

1947-2003

Trends in Summer (Apr-Sept) Precipitation and Temperature

In temperature sensitive areas of the West, we should be able to see the effects of observed global warming in the historic snow and streamflow records.

Using models we should be able to more fully analyze these changes, as well as other hydrologic effects which are not typically measured (evaporation and soil moisture).

Snow Model

Schematic of VIC Hydrologic Model and Energy Balance Snow Model

PNW

CACRB

GB

Trends in April 1 Snowpack

Mote P.W.,Hamlet A.F., Clark M.P., Lettenmaier D.P., 2005, Declining mountain snowpack in western North America, BAMS (in press)

Trends in April 1 SWE 1950-1997

Trend %/yr

DJF

avg

T (

C)

Trend %/yr

Overall Trends in April 1 SWE from 1947-2003

Trend %/yr

DJF

avg

T (

C)

Trend %/yr

Temperature Related Trends in April 1 SWE from 1947-2003

Trend %/yr

DJF

avg

T (

C)

Trend %/yr

Precipitation Related Trends in April 1 SWE from 1947-2003

y = -0.5851x + 295.29

0

100

200

300

400

500

600

19

16

19

21

19

26

19

31

19

36

19

41

19

46

19

51

19

56

19

61

19

66

19

71

19

76

19

81

19

86

19

91

19

96

20

01

1-Apr

Linear (1-Apr)

y = -0.7553x + 301.86

0

50

100

150

200

250

300

350

400

450

500

19

16

19

21

19

26

19

31

19

36

19

41

19

46

19

51

19

56

19

61

19

66

19

71

19

76

19

81

19

86

19

91

19

96

20

01

1-Apr

Linear (1-Apr)

Effects of TemperatureAnd Precipitation

Effects of TemperatureAlone

-19%

-25%

Trends in April 1 SWE for the WA and OR Cascades

-2.84% per decade

-2.15% per decade

y = -1.739x + 313.96

0

100

200

300

400

500

600

19

50

19

53

19

56

19

59

19

62

19

65

19

68

19

71

19

74

19

77

19

80

19

83

19

86

19

89

19

92

19

95

19

98

20

01

1-Apr

Linear (1-Apr)

y = -1.1264x + 288.63

0

50

100

150

200

250

300

350

400

19

50

19

53

19

56

19

59

19

62

19

65

19

68

19

71

19

74

19

77

19

80

19

83

19

86

19

89

19

92

19

95

19

98

20

01

1-Apr

Linear (1-Apr)

Effects of TemperatureAnd Precipitation

Effects of TemperatureAlone

Trends in April 1 SWE for the WA and OR Cascades

-35%

-23%

-4.25% per decade

-6.48% per decade

Cedar River: -30.7%

y = -0.0020x + 4.3416

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

1945 1955 1965 1975 1985 1995 2005

May

-Sep

t fr

acti

on

of

ann

ual

flo

w

May-Sept frac

Linear (May-Septfrac)

SFTolt River: -15.7%

y = -0.0010x + 2.2890

0

0.1

0.2

0.3

0.4

0.5

0.6

1945 1955 1965 1975 1985 1995 2005

May

-Sep

t fr

acti

on

of

ann

ual

flo

w

May-Sept frac

Linear (May-Septfrac)

Obs. Summer Water Availability is Declining

55 years

Figures courtesy of Matt Wiley and Richard Palmer at CEE, UW

b) Max Accumulation c) 90 % Melta) 10 % Accumulation

Change in Date

Change in Date

Change in Date

Change in Date

DJF

Tem

p (C

)

Change in Date

DJF

Tem

p (C

)

Change in Date

DJF

Tem

p (C

)

TrendsinSWE1916-1997

Trends in Runoff Timing

As the West warms,winter flows rise and summer flows drop

Stewart IT, Cayan DR, Dettinger MD, 2004, Changes toward earlier streamflow timing across western North America, J. Climate (in review)

March June

Relative Trend (% per year)

Trends in simulated fraction of annual runoff in each month from 1947-2003 (cells > 50 mm of SWE on April 1)

Trend %/yrD

JF T

emp

(°C

)Trend %/yr

DJF

Tem

p (°

C)

Trends in March Runoff Trends in June Runoff

Trends in Soil Moisture

April 1

July 1

Trends in Simulated Soil Moisture from 1947-2003

Trend %/yr

DJF

Tem

p (°

C)

DJF

Tem

p (°

C)

Trend %/yr

Trend %/yrD

JF T

emp

(°C

)Trend %/yr

DJF

Tem

p (°

C)

Trends in April 1 SM Trends in July 1 SM

Trends in the “Runoff Ratio”(runoff/precipitation)

Trend Oct-Mar PCP

Trend Apr-Sep PCP

Tre

nd R

unof

f R

atio

Tre

nd R

unof

f R

atio

Trend %/yr

DJF

Tem

p (°

C)

Conclusions•Large-scale changes in the seasonal dynamics of snow accumulation and melt have occurred in the West as a result of increasing temperatures.

•Hydrologic changes include earlier and reduced peak snowpack, more runoff in March, less runoff in June, and corresponding increases in simulated spring soil moisture and decreases in late summer and fall soil moisture.

•Because these effects are shown to be predominantly due to temperature changes, we expect that they will both continue and increase in intensity as global warming progresses in the 21st century.

•Trends in the runoff ratio are primarily linked to winter precipitation trends, which are not necessarily related to global warming

Trends in Potential Evaporation

Average July PotET over the Southern Plain Region in the Snake River Basin Current Climate vs. MPI2040 scenario(Effects of temperature increases alone)

Current Climate MPI2040+ 4 C

PotET (mm/day)

Trends in July Avg PotET over the Southern Plain Region from 1915-2002

y = -0.0061x + 7.3589

5

5.5

6

6.5

7

7.5

8

8.5

1915

1920

1925

1930

1935

1940

1945

1950

1955

1960

1965

1970

1975

1980

1985

1990

1995

2000

Ref

eren

ce C

rop

Po

tET

(m

m/d

ay)

jul

Linear (jul)

Will Global Warming be “Warm and Wet” or “Warm and Dry”?

Answer: Probably BOTH!

150000

200000

250000

300000

350000

400000

45000019

00

1910

1920

1930

1940

1950

1960

1970

1980

1990

2000

Ap

r-S

ept F

low

(cfs

)

Implications for Water Planning

•Water resources managers and planners should expect systematically decreasing natural flows in summer in river basins with substantial winter snow accumulation.

•Hydrologic changes due to global warming are likely to exacerbate future stresses on water systems due to increasing population, and may intensify existing conflicts between water supply and other objectives such as instream flow for fish.

•There is little evidence in the historic record for systematic changes in winter precipitation associated with global warming in the West. Water managers should be prepared to cope with extended periods of both “warm and wet” and “warm and dry” conditions at different times in the future.

•Effects to potential evaporation (irrigation demand) remain unclear and appear to be strongly affected by uncertain changes in cloudiness and humidity in addition to temperature.

•Straight-forward methods to include these kinds of hydrologic changes in water planning studies are readily available.

Selected References:

Hidalgo HG, Cayan DR, Dettinger MD, 2005: Sources of variability of evapotranspiration in California, J. of 6 (1): 3-19

Hamlet A.F., Lettenmaier D.P., 2005, Production of temporally consistent gridded precipitation and temperature fields for the continental U.S., J. of Hydrometeorology, 6(3), 330-336

Hamlet A.F., Mote P.W, Clark M.P., Lettenmaier D.P., 2005, Effects of temperature and precipitation variability on snowpack trends in the western U.S., J. of Climate (in press)

Hamlet A.F., Mote P.W., Clark M.P., Lettenmaier D.P., 2005: 20th Century Trends in Runoff, Evapotranspiration, and Soil Moisture in the Western U.S. , J. of Climate (in review)

Mote P.W., Hamlet A.F., Clark M.P., Lettenmaier D.P., 2005, Declining mountain snowpack in western North America, BAMS, 86 (1), 39-49

Stewart IT, Cayan DR, Dettinger MD, 2005: Changes toward earlier streamflow timing across western North America, J. Climate, 18 (8): 1136-1155