Alan F. Hamlet,

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Implications of Climate Change for Freshwater
Availability in the Western U.S.

• Alan F. Hamlet,
• Philip W. Mote, Nate Mantua,
• Dennis P. Lettenmaier
• JISAO/CSES Climate Impacts Group
• Dept. of Civil and Environmental Engineering
• University of Washington

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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.       
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Whats 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.
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Global Climate Change Scenarios and Hydrologic
Impacts for the PNW
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3.2C
C
1.7C
0.7C
1.2-5.5C
0.9-2.4C
Observed 20th century variability
0.4-1.0C
Pacific Northwest
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-1 to 3
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2
1
Observed 20th century variability
-2 to 21
-1 to 9
Pacific Northwest
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The warmer locations are most sensitive to warming
2060s
2.3C, 6.8 winter precip
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Simulated Changes in Natural Runoff Timing in the
Naches River Basin Associated with 2 C Warming

• Impacts
• Increased winter flow
• Earlier and reduced peak flows
• Reduced summer flow volume
• Reduced late summer low flow

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Chehalis River
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Hoh River
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Nooksack River
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Skagit River
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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)
2060s ( 2.25 C)
-3.6
-11.5
-21.4
-34.8
April 1 SWE (mm)
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Mapping of Sensitive Areas in the PNW by Fraction
of Precipitation Stored as Peak Snowpack
HUC 4 Scale Watersheds in the PNW
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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
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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.

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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

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Water Quality Implications

• Increasing water temperature
• Altered sediment transport processes
• Altered chemical processes (dissolved gas, BOD)
• Altered biological and ecological processes
  (algae, periphyton, macrophytes)
• Need for increased use of managed storage to
  maintain equivalent dilution flows in summer.