Alan%20F.%20Hamlet,

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Understanding the Effects of Climate Change on
Water Resources in 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

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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
water supply planning and specific agreements
influenced by this planning (e.g. water
allocation agreements) should be informed by the
best and most complete climate information
available, but frequently they are not.
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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/gl
acier_photo/special_high_res.html
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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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As the West warms, spring flows rise and summer
flows drop Stewart IT, Cayan DR, Dettinger MD,
2005 Changes toward earlier streamflow timing
across western North America, J. Climate, 18 (8)
1136-1155
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Projections for the Future Using Global Climate
Models
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3.2C
C
1.7C
0.7C
1.2-5.5C
0.9-2.4C
0.4-1.0C
Observed 20th century variability
Pacific Northwest
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2
1
Observed 20th century variability
-1 to 3
-1 to 9
-2 to 21
Pacific Northwest
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Hydrologic Impacts for the PNW
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Schematic of VIC Hydrologic Model and Energy
Balance Snow Model
6 km
1/16th Deg.
PNW
6 km
Snow Model
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The warmest locations that accumulate snowpack
are most sensitive to warming
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

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

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Mixed Rain and Snow
Nooksack River
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Rain Dominant
Chehalis River
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Decadal Climate Variability and Climate Change
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Will Global Warming be Warm and Wet or Warm
and Dry? Answer Probably BOTH!
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Annual Naturalized Flow in the Colorado River at
Imperial Dam
Source Christensen and Lettenmaier, 2007 A
multimodel ensemble approach to assessment of
climate change impacts on the hydrology and water
resources of the Colorado River Basin, HESS, 11
1417-1434
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Regionally Averaged Cool Season Precipitation
Anomalies
PRECIP
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In Managed Systems the Storage to Flow Ratios are
Important
High Storage to Flow Ratio (e.g. the Colorado
River basin) Low sensitivity to streamflow
timing shifts, high sensitivity to systematic
changes in precipitation and multi-year
drought Low Storage to Flow Ratio (e.g. many PNW
water systems) High sensitivity to streamflow
timing shifts and changes in single year
droughts.
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Consensus Forecasts of Temperature and
Precipitation Changes from IPCC AR4 GCMs
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Projected Changes in Temperature, Precipitation,
and Runoff for the Colorado Basin
A2 2070-2090 -10
Source Christensen and Lettenmaier, 2007 A
multimodel ensemble approach to assessment of
climate change impacts on the hydrology and water
resources of the Colorado River Basin, HESS, 11
1417-1434
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Impact Pathways Associated with Hydrologic Changes

• Changes in water quantity and timing
• Reductions in summer flow and water supply
• Increases in drought frequency and severity
• Changes in hydrologic extremes
• Changing flood risk (up or down)
• Summer low flows
• Changes in groundwater supplies
• Changes in water quality
• Increasing water temperature
• Changes in sediment loading (up or down)
• Changes in nutrient loadings (up or down)
• Changes in land cover via disturbance
• Forest fire
• Insects
• Disease
• Invasive species

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Impact Pathways Associated with Hydrologic Changes

• Changes in energy resources
• hydropower
• electrical demand
• Changes in outdoor recreation
• Tourism
• Skiing
• Camping
• Boating
• Changes in engineering design standards
• Road construction
• Storm water systems
• Flood plain definitions
• Changes in transportation corridors
• Changing risk of avalanche or debris flows
• Human health risks
• Temperature and water-related health risks

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Approaches to Adaptation and Planning

• Anticipate changes. Accept that the future
  climate will be substantially different than the
  past.
• Use scenario based planning to evaluate options
  rather than the historic record.
• Expect surprises and plan for flexibility and
  robustness in the face of uncertain changes
  rather than counting on one approach.
• Plan for the long haul. Where possible, make
  adaptive responses and agreements self tending
  to avoid repetitive costs of intervention as
  impacts increase over time.

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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 at all levels of governance.