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Hydrologic Implications of Climate
Change for the Western U.S.
Alan F. Hamlet,
Philip W. Mote,
Dennis P. Lettenmaier
•JISAO/CSES Climate Impacts Group
•Dept. of Civil and Environmental Engineering
University of Washington
Recession of the Muir Glacier
Aug, 13, 1941
Aug, 31, 2004
Image Credit: National Snow and Ice Data Center, W. O. Field, B. F. Molnia
http://nsidc.org/data/glacier_photo/special_high_res.html
Example of a flawed water planning study:
The Colorado River Compact of 1922
The Colorado River Compact of 1922 divided the
use of waters of the Colorado River System
between the Upper and Lower Colorado River
Basin. It apportioned **in perpetuity** to the
Upper and Lower Basin, respectively, the
beneficial consumptive use of 7.5 million acre feet
(maf) of water per annum. It also provided that the
Upper Basin will not cause the flow of the river at
Lee Ferry to be depleted below an aggregate of
7.5 maf for any period of ten consecutive years.
The Mexican Treaty of 1944 allotted to Mexico a
guaranteed annual quantity of 1.5 maf. **These
amounts, when combined, exceed the river's
long-term average annual flow**.
What’s the Problem?
Despite a general awareness of these issues in the water
planning community, there is growing evidence that future
climate variability will not look like the past and that current
planning activities, which frequently use a limited observed
streamflow record to represent climate variability, are in
danger of repeating the same kind of mistakes made more
than 80 years ago in forging the Colorado River Compact.
Long-term planning and specific agreements influenced by
this planning (such as long-term transboundary agreements)
should be informed by the best and most complete climate
information available, but frequently they are not.
Cool Season Climate of the Western U.S.
PNW
GB
CA CRB
DJF Temp (°C)
NDJFM Precip (mm)
Global Climate Change Scenarios
and Hydrologic Impacts for the PNW
Observed 20th century variability
°C
+3.2°C
+1.7°C
+0.7°C
0.9-2.4°C
0.4-1.0°C
Pacific Northwest
1.2-5.5°C
Observed 20th century variability
%
-1 to +3%
+1%
+6%
+2%
-1 to +9%
Pacific Northwest
-2 to +21%
Will Global Warming be “Warm and
Wet” or “Warm and Dry”?
Answer:
450000
Probably BOTH!
Natural Flow Columbia River at The Dalles
350000
300000
250000
200000
2000
1990
1980
1970
1960
1950
1940
1930
1920
1910
150000
1900
Apr-Sept Flow (cfs)
400000
2000
1996
1992
1988
1984
1980
1976
1972
1968
1964
1960
1956
1952
1948
1944
1940
CRB
1936
CA
1932
1928
3
1924
1920
1916
Std Anomalies Relative to 1961-1990
Regionally Averaged Cool Season Precipitation Anomalies
4
PNW
PRECIP
GB
2
1
0
-1
-2
-3
Schematic of VIC Hydrologic Model and
Energy Balance Snow Model
Snow Model
The warmer locations are most
sensitive to warming
2060s
+2.3C,
+6.8%
winter
precip
Changes in Simulated April 1
Snowpack for the Canadian
and U.S. portions of the
Columbia River basin
(% change relative to current climate)
20th Century Climate
“2040s” (+1.7 C)
-3.6%
-21.4%
April 1 SWE (mm)
“2060s” (+ 2.25 C)
-11.5%
-34.8%
Trends in April 1 SWE 1950-1997
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
DJF avg T (C)
Overall Trends in April 1 SWE from 1947-2003
Trend %/yr
Trend %/yr
DJF avg T (C)
Temperature Related Trends in April 1 SWE from 1947-2003
Trend %/yr
Trend %/yr
DJF avg T (C)
Precipitation Related Trends in April 1 SWE from 1947-2003
Trend %/yr
Trend %/yr
Simulated Changes in Natural Runoff Timing in the Naches
River Basin Associated with 2 C Warming
120
Simulated Basin Avg Runoff (mm)
100
80
Impacts:
•Increased winter flow
•Earlier and reduced peak flows
•Reduced summer flow volume
•Reduced late summer low flow
1950
60
plus2c
40
20
0
oct
nov
dec
jan
feb
mar
apr
may
jun
jul
aug
sep
250
Simulated Basin Avg Runoff (mm)
Chehalis River
200
150
1950
plus2c
100
50
0
oct
nov
dec
jan
feb
mar
apr
may
jun
jul
aug
sep
500
Hoh River
Simulated Basin Avg Runoff (mm)
450
400
350
300
1950
250
plus2c
200
150
100
50
0
oct
nov
dec
jan
feb
mar
apr
may
jun
jul
aug
sep
200
Nooksack
River
Simulated Basin Avg Runoff (mm)
180
160
140
120
1950
100
plus2c
80
60
40
20
0
oct
nov
dec
jan
feb
mar
apr
may
jun
jul
aug
sep
450
Skagit River
Simulated Basin Avg Runoff (mm)
400
350
300
250
1950
plus2c
200
150
100
50
0
oct
nov
dec
jan
feb
mar
apr
may
jun
jul
aug
sep
Mapping of Sensitive Areas in the PNW by Fraction of
Precipitation Stored as Peak Snowpack
HUC 4 Scale Watersheds in the PNW
Changes in Flood Risk in the Western U.S.
Regionally Averaged Temperature Trends Over the Western U.S. 1916-2003
3.00
PNW
Linear Trend (Deg. C per century)
CA
PNW
1.50
1.00
0.50
0.00
-0.50
-1.00
nov
dec
jan
feb
mar
apr
may
jun
jul
aug
sep
4.00
Linear Trend (Deg. C per century)
CA CRB
GBAS
2.00
oct
GB
CRB
Tmax
2.50
CA
3.50
CRB
Tmin
GBAS
3.00
PNW
2.50
2.00
1.50
1.00
0.50
0.00
oct
nov
dec
jan
feb
mar
apr
may
jun
jul
aug
sep
DJF Avg Temp (C)
Simulated Changes in the 20-year Flood
Associated with 20th Century Warming
DJF Avg Temp (C)
X20 2003 / X20 1915
X20 2003 / X20 1915
X20 2003 / X20 1915
DJF Avg Temp (C)
20-year Flood for “1973-2003” Compared to “1916-2003” for a Constant
Late 20th Century Temperature Regime
X20 ’73-’03 / X20 ’16-’03
X20 ’73-’03 / X20 ’16-’03
Summary of Flooding Impacts
Rain Dominant Basins:
Possible increases in flooding due to increased precipitation
variability, but no significant change from warming alone.
Mixed Rain and Snow Basins Along the Coast:
Strong increases due to warming and increased precipitation
variability (both effects increase flood risk)
Inland Snowmelt Dominant Basins:
Relatively small overall changes because effects of warming
(decreased risks) and increased precipitation variability
(increased risks) are in the opposite directions.
The Evaporation Paradox
Many people intuitively associate increasing temperatures with
higher evaporation rates, however there is evidence that potential
evaporation rates may actually be trending downwards. Longterm pan evaporation data, for example, frequently shows
downward trends in the western U.S.
Two hypotheses:
1) Pan evaporation records are influenced by increased humidity
associated with increased summer precipitation or other effects
and are therefore not an accurate representation of potential
evaporation. I.e. potential evaporation is actually going up with
warming, but pan evaporation data does not show it.
2) Increased humidity and attenuation of solar radiation is actually
lowering potential evaporation.
Simulated trends in July Avg PotET for Mature Alfalfa over the Southern Plain
Region in the Snake River Basin from 1915-2002
8.5
7.5
7
jul
Linear (jul)
6.5
6
y = -0.0061x + 7.3589
5.5
2000
1995
1990
1985
1980
1975
1970
1965
1960
1955
1950
1945
1940
1935
1930
1925
1920
5
1915
Reference Crop PotET (mm/day)
8
Conclusions
•Climate change will result in significant hydrologic changes in
the Western U.S. including reduced natural storage as
mountain snowpack, increased flow in winter, and reduced flow
in summer. Changes in extremes (droughts and floods) are
likely to occur.
•Impacts will not be equally distributed, and areas near freezing
in mid winter will be the most sensitive to warming related
losses of snowpack and streamflow timing shifts.
•A number of impact pathways related to water resources
management, water quality, and ecosystem function are likely
to be activated by these changes.
•There is a wide-spread need to incorporate expected changes
in climate into long range planning.
Water Resources Implications
•Reductions in natural storage in mountain watersheds
•Potential increases in water demand and evaporation
•Increasing drought and altered flood risks
•Increasing competition over water resources
•Tradeoffs between traditional water resources
objectives such as water supply and hydropower
production and environmental services related to
instream flow
•Need for changes in flood control evacuation and refill
schedules
•Disruption of existing water allocation agreements
•Disruption of transboundary agreements
Implications Related to Water Quality
•Increasing water temperature
•Altered sediment transport processes
•Altered chemical processes (dissolved gas, BOD)
•Altered biological and ecological processes
•Need for increased use of managed storage to
maintain equivalent dilution flows in summer.