Projected Impacts of Climate Change and Implications for River
Flows in the Pacific Northwest
Presented by: Austin Polebitski and Richard Palmer
Department of Civil and Environmental EngineeringUniversity of Washington
November 7, 2006King County
Precipitation
• Anyone notice it raining more lately?
Changing Characteristics of Precipitation
• Outline– Investigate historic trends in precipitation– Explore data from global models to local
situations• Statistical Downscaling• Dynamic Downscaling
– Preliminary Results
Projected Average Monthly Flows –Snoqualmie Falls
0
1000
2000
3000
4000
5000
6000
SeptAugJulJunMayAprMarFebJanDecNovOctMonth
Ave
rage
Flo
w (c
fs)
0
5
10
15
20
25
30
Ave
rage
Pre
cipi
tatio
n (in
ches
)
PrecipitationHistoricECHAM5-2000ECHAM5-2025ECHAM5-2050ECHAM5-2075
Changing Characteristics of Precipitation
• How will precipitation patterns change in a warmed climate?– More Extreme Events?– More Winter and less Summer Precipitation?– More precipitation as Rain rather than Snow?
• How can these questions be answered?– Historic trend evaluation– Use of GCM output
• Dynamical Downscaling• Statistical Downscaling
Historic Trend Evaluation of Daily Precipitation
• Preliminary Results suggest:
– Most significant trend is in November
– Larger percentage of annual rainfall occuring in November, total annual precipitation has remained relatively constant
– Difficult to identify an increase in extreme events to date, models do forecast an increase.
Decadal Trends in November Monthly Precipitation
0
50
100
150
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250
300
350
400
450
500
Buck Cedar Everett Kent Landburg Palmer Snoq
Station
Ave
rage
Cum
ulat
ive
Prec
ipita
tion
(mm
)
1930194019501960197019801990
Evaluation of Projected Daily Precipitation
• Two methods discussed
– Dynamical Downscaling*
– Statistical Downscaling*
*Downscaling – the process of transforming coarsely-gridded global data to a regionally relevant scale for use in models.
Dynamic Downscaling
• Method under study by Cliff Mass and Eric Salathé of UW-Department of Atmospheric Sciences and the Climate Impact Group
• The method utilizes a mesoscaleforecasting model (MM5) coupled with GCM output to derive future weather and climate patterns
• Can produce output at high resolution and temporal frequency
MM5 Climate Projections –Percent Change in Precipitation
Statistical Downscaling
• Map cdf’s of future climate to historic cdf’sto produce climate impacted datasets (Wood)
• Historical records and site specific information used to downscale
• Precipitation and temperature are the two main outputs from this downscaling method
Percent Change in 20-Year Design Storm from ECHAM5 2000
10%
15%
5%
20%
25%
15%
Average Percent Change in 20 Year Storm
-5.00% 0.00% 5.00% 10.00% 15.00% 20.00% 25.00%
ABERDEEN20NNEBREMERTON
BUCKLEYCEDAR
CULMBACKCUSHMAN_POWER
DARRINGTONELMA
ELWHAEVERETT
FORKSKENT
LANDSBURGLONGMIREMCMILLINMONROE
MUDMTOLYMPIAPALMER
PARADISEPORTANGELES
PORTTOWNSENDQUILCENE
SEATACSFTOLT
SNOQUALMIESTAMPEDE
STARTUP
Percent Change relative to ECHAM5 2000
Impacts Urban Flooding
• ECHAM5 GCM projects increases ‘extreme’precipitation
• More common events (5 year, 10 year, 20 year return events) change more significantly than 100 year event
• To determine impacts to urban systems, output needs to be run through urban models to determine whether code changes might be necessary
Flooding in Natural Systems
• The Distributed-Hydrology-Soil-Vegetation-Model (DHSVM) uses physically based parameters to determine runoff and streamflow for a basin
• Downscaled ECHAM5 output can be used to drive DHSVM
Projected Daily CDF’s of Flow at Snoqualmie Falls
Projected Return Interval Flows for Natural Systems
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10000
20000
30000
40000
50000
60000
70000
1 2 5 10 20 50 100Return Interval
Flow
(cfs
) HistoricECHAM5-2000ECHAM5-2025ECHAM5-2050ECHAM5-2075
Distribution of Events Exceeding 1-year Return Interval (7000 cfs)
0
100
200
300
400
500
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700
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Num
ber o
f Eve
nts
Ove
r 700
0 cf
s
Historic2000202520502075
Next Steps
• Complete DHSVM runs for regional basins, i.e. Cedar, Green, Puyallup/White
• Identify key areas of flood inundation interest and possibly perform HEC-RAS models for those areas
• Run climate impacted datasets through Urban-basin models to examine effects of changing design-storms on conveyance