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Quantifying Hydrologic Changes
in Pacific Northwest Watersheds
Center for Science in the Earth System
Climate Impacts Group
and Department of Civil and Environmental Engineering
University of Washington
December, 2004
Alan F. Hamlet
Dennis P. Lettenmaier
Changing PNW
Floods Risks
Summary of Past Research
In the 20th century, risks of moderate
floods have been non-stationary in
time and are strongly linked to the
variability of PDO and ENSO
Risks of extreme flooding events are
not necessarily linked to PDO and
ENSO in the same way as are
moderate events (sample size
problems)
Chehalis River 1999
(~2.5 ft above flood stage)
Challenges in Evaluating Potential Changes in Flood Risks
Severe floods by definition occur infrequently
Problems:
Sample size is very small
Streamflow measurement errors (models may be better)
Regulation effects are significant in almost all large watersheds
Fitting of probability distributions is imprecise (and inappropriate?)
Changes in climate or land use that affect flooding can be gradual, have
varying spatial extent, and affect different areas at different times.
Problems:
Flood risks are probably not stationary in time
Detection using conventional statistical procedures is problematic (need
models and larger samples)
Ability to identify cause and effect is frequently difficult
(e.g. climate or land use?)
Conclusions:
•Need hydrologic models
•Need long, homogeneous records of temperature and
precipitation
Use of a Hydrologic Model with Long Precipitation and Temperature Records
Meteorological Records from 1915-2003
•De-trended Temperatures
•Observed Precipitation Variability
VIC
Hydrology Model
Variability of Runoff
In Different
River Basin Types
for A Consistent
“Early” and “Late”
20th Century
Temperature
Regime
Research Questions:
•
Have increasing temperatures in transient snow basins systematically
increased flood risks in early winter as climate change scenarios suggest they
should?
•
Have changes in temperature and precipitation in snowmelt dominant
basins increased or reduced spring flood risks? What is the relative role of winter
temperature (loss of snow) and spring precipitation in these changes?
•
Has the variability of floods in snow melt dominant and transient snow
basins been systematically altered by the loss of snowpack due to a more direct
coupling of precipitation and runoff production?
•
How have flood risks changed in rain dominant basins? What aspects of
evolving precipitation variability are most important in these basins and what is the
role of climate and topography?
•
What role have observed changes in ENSO frequency and decadal
variability associated with the PDO and PNA played in observed changes in flood
risk? Have the influence of these climate indices remained constant over time?
What are the implications for projecting flood risks forward in time?
Effects of Climate Change on Pacific
Northwest Watersheds:
Decision Support for Pacific Northwest Watershed
Planning Units
Hydrologic Data for Watershed
Planning Units
Watershed planning units frequently have limited financial
resources and may have no access to hydrologic models or
even observed hydrologic data in some cases.
Sensitivity to regional warming will inform decisions on
whether to invest these limited resources in obtaining better
information about the impacts of climate change.
Macro-scale hydrologic simulation models can provide a
great deal of information about the potential impacts of
climate change on particular watersheds in the Pacific
Northwest which can help inform these decisions.
Example:
Hydrologic Summary for WRIA 38
Naches River Basin
1916-2003
700
Area Average Water
(depth in mm)
600
500
precipitation
400
swe
runoff+baseflow
soil storage
300
evapotranspiration
200
100
Elevation (m)
sep
aug
jul
jun
may
apr
mar
feb
jan
dec
nov
oct
0
Four Delta Method Climate Change Scenarios for the PNW
Delta T, 2020s
Delta T, 2040s
5
5
~ + 1.7 C
~ + 2.5 C
4
hadCM2
3
hadCM3
2
PCM3
ECHAM4
1
Degrees C
Degrees C
4
mean
0
hadCM2
3
hadCM3
2
PCM3
ECHAM4
1
mean
0
J
F
M
A
M
J
J
A
S
O
N
D
J
-1
F
M
A
Precipitation Fraction, 2020s
J
J
A
S
O
N
D
Precipitation Fraction, 2040s
1.75
1.75
1.5
1.5
hadCM2
hadCM3
1.25
PCM3
1
ECHAM4
Fraction
Fraction
M
-1
hadCM2
hadCM3
1.25
PCM3
1
ECHAM4
mean
0.75
mean
0.75
0.5
0.5
J
F
M
A
M
J
J
A
S
O
N
D
J
F
M
A
M
J
J
A
S
O
N
D
Somewhat wetter winters and perhaps somewhat dryer summers
Current Climate
600
Area Average Water
(depth in mm)
Seasonal
Water Balance
700
500
precipitation
400
swe
runoff+baseflow
soil storage
300
evapotranspiration
200
100
sep
aug
jul
jun
may
apr
mar
feb
jan
dec
nov
oct
0
700
500
precipitation
400
swe
runoff+baseflow
soil storage
300
evapotranspiration
200
100
sep
aug
jul
jun
may
apr
mar
feb
jan
dec
nov
0
oct
2040s Scenario
(+ 2.5 C)
Area Average Water
(depth in mm)
600
Current Climate
700
Water Balance from April-September
(depth in mm)
Summer
Water Balance
600
500
precipitation
400
snowmelt
soil drainage
300
streamflow
ET
200
100
0
1
2040s Scenario
(+ 2.5 C)
Water Balance from April-September
(depth in mm)
700
600
500
precipitation
400
snowmelt
soil drainage
300
streamflow
ET
200
100
0
1
Quantifying Changes in Irrigation Requirements
Changes in Average July PotET over the Southern Plain Region in Idaho
Current Climate vs. MPI2040 scenario (+ 4° C)
Current Climate
PotET (mm/day)
MPI2040
Extra Materials
Hydrologic Characteristics of PNW Rivers
Normalized Streamflow
3.0
2.5
Snow
Dominated
2.0
Transient Snow
1.5
Rain Dominated
1.0
0.5
0.0
10 11 12
1
2
3
4
Month
5
6
7
8
9
Pacific Northwest flood risks are not stationary in time
and are related to ENSO and PDO variability
Snow-Dominant Basins
Probability of Flood Event Above
Threshold
1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.00
EwPw
EnPw
EcPw
EwPc
Climate Category
EnPc
EcPc
Warm ENSO to Cool ENSO Transition
450000
Average Flow (cfs)
400000
350000
300000
250000
Category Year
200000
Non Category Year
150000
Category Avg
100000
Non Category Avg
50000
Category Year
0
1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000
Water Year