Climate-driven changes in
snowmelt hydrology in the
mountainous West
Philip Mote, Alan Hamlet, Dennis Lettenmaier
University of Washington
With thanks to NRCS and Iris Stewart
ftp://ftp.atmos.washington.edu/philip/SNOWPAPER/
Main point
Over large areas and long times, the dominant
influence on snowmelt hydrology is
temperature, which has been inexorably
increasing
Warming has produced trends in
• spring snow water equivalent (SWE),
• spring snowmelt timing,
• timing of peak SWE,
• winter and spring runoff
Schematic of VIC Hydrologic Model and Energy Balance Snow Model
PN
W
GB
CA
Snow
Model
CR
B
VIC simulation
•! 1/8° long x 1/8° lat, west of Continental
Divide
•! Daily weather data, 1/1/1915 - 9/30/03
interpolated to VIC grid points from Coop
stations (earlier run ended 1997)
•! Long-term trends interpolated from
USHCN stations
•! Three simulations: T+p, T only, p only
Trends 1950-1997, relative to 1950 value
Declines in SWE are largely
temperature-driven
Declines in April 1 SWE depend on proximity to snowline
Trends in precipitation increase the range but not the
mean
aP<P
>
April 1 SWE trends,
1950-2002
aT<T
>
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
500
Effects of Temperature
And Precipitation
-6.48% per
decade
350
Effects of Temperature
Alone
-4.25% per
decade
1950
1951
1952
1953
1954
1955
1956
1957
1958
1959
1960
1961
1962
1963
1964
1965
1966
1967
1968
1969
1970
1971
1972
1973
1974
1975
1976
1977
1978
1979
1980
1981
1982
1983
1984
1985
1986
1987
1988
1989
1990
1991
1992
1993
1994
1995
1996
1997
1998
1999
2000
2001
2002
2003
Trends in April 1 SWE for the WA and OR Cascades
600
-35%
400
300
1-Apr
200
100
0
400
-23%
300
250
1-Apr
200
150
100
50
0
DJF AVG T (C)
1916-1997
Effects of
Precip
Relative Trend in April 1 SWE
(% per year)
Shifts in timing
a) 10 %
Accumulation
b) Max Accumulation
c) 90 % Melt
Trends
in
SWE
19161997
Change in
Date
Change in
Date
(C)
Change in
Date
Change in
Date
DJF Temp
(C)
DJF Temp
(C)
DJF Temp (C)
Change in
Date
Change in
Date
Western Cordillera trends in Apr SWE/Mar SWE
By several measures,
Western snowfed
streamflow has been
arriving earlier in the
year in recent decades
Spring-pulse dates
Center
time
Spring
pulse
Centers of Mass
Stewart et al., 2004; Stewart et al., in press, J. Clim.
Shifts in flow from
late spring and summer to
winter and early spring
As the West warms,
winter flows rise and
summer flows drop
Figure by Iris Stewart,
Scripps Inst. of Oceanog.
(UC San Diego)
Trends in fraction of annual runoff 1947-2003 (cells > 50 mm of SWE on April
1)
March
Relative Trend (% per
year)
June
Miscellaneous points
Soil Moisture
Effects of temp and precip
Sep 1
Effects of temp only
Sep 1
19252003
Relative Trend (% per
year)
Decadal Climate Variability Doesn’t Explain the Loss of
SWE Due to Warming
1916-97
1947-97
1925-46
with 1977
-95
Relative SWE Trends Due to Temperature Effects Alone (% per year)
HADCM3
simulation
Conclusions
The dominant influence on snowmelt
hydrology is temperature, so we’re in
trouble
ftp://ftp.atmos.washington.edu/philip/SNOWPAPER/