Transcript hamlet_carpe_diem_nov_2007

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
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 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.
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
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
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
Projections for the Future Using
Global Climate Models
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
%
+6%
+1%
+2%
-1 to +3%-1 to +9%
Pacific Northwest
-2 to +21%
Hydrologic Impacts for the PNW
Schematic of VIC Hydrologic Model and
Energy Balance Snow Model
6 km
6 km
1/16th
Deg.
PNW
Snow Model
The warmest locations that accumulate
snowpack are most sensitive to warming
+2.3C,
+6.8%
winter
precip
Simulated Changes in Natural Runoff Timing in the Naches
River Basin Associated with 2 C Warming
120
Simulated Basin Avg Runoff (mm)
100
•Increased winter flow
•Earlier and reduced peak flows
•Reduced summer flow volume
•Reduced late summer low flow
80
1950
60
plus2c
40
20
0
oct
nov
dec
jan
feb
mar
apr
may
jun
jul
aug
sep
Mixed Rain and Snow
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
Rain Dominant
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
Decadal Climate Variability and Climate
Change
Will Global Warming be “Warm and
Wet” or “Warm and Dry”?
Answer:
Probably BOTH!
450000
350000
300000
250000
200000
2000
1990
1980
1970
1960
1950
1940
1930
1920
1910
150000
1900
Apr-Sept Flow (cfs)
400000
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
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
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.
Consensus Forecasts of Temperature and Precipitation Changes from IPCC AR4 GCMs
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
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
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
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.
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.