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Hydrologic extremes in a changing climate -- modeling and observations. Dennis P. Lettenmaier Department of Civil and Environmental Engineering University of Washington University of Washington Program on Climate Change Summer Institute The water cycle in a changing climate Friday Harbor - PowerPoint PPT Presentation
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Hydrologic extremes in a changing climate -- modeling and observations
Dennis P. LettenmaierDepartment of Civil and Environmental Engineering
University of Washington
University of WashingtonProgram on Climate Change Summer Institute
The water cycle in a changing climate
Friday Harbor
September 16, 2011
A warmer climate, with its increased climate variability, will increase the risk of both floods and droughts
IPCC WG2, 2007
Most climate scientists agree that global warming will result in
an intensification, acceleration or enhancement of the global
hydrologic cycle, and there is some observational evidence thatthis is already happening.
UNESCO World Water Development Report Water in a Changing Climate, 2009
Total U.S. flood damages, 1934-2000
from Pielke et al., 2000
Extreme precipitation should be increasing as the climate warms
Is extreme precipitation increasing?
Trends in annual maximum daily (left column) and 5-day (right column) precipitation, 1951-99. Upper row from observations.
from Min et al., Nature 2011
Trends in U.S. daily precipitation by frequency interval, 1910-1996
From Karl and Knight, BAMS 1998
From Karl and Knight, 1998
Continental U.S. weighted contribution of upper 10th percentile to annual precipitation, 1910-96
Trends in annual precipitation maxima in 100 largest U.S. urban areas, 1950-2009
from Mishra and Lettenmaier, GRL 2011
Study Locations
7. Changes in design storms calculated for various return periods.
+37%
+30%
23
Can RCMs reproduce the timing of precipitation maxima ?
Winter Summer
24
Can RCMs reproduce the timing of precipitation maxima ?
Winter Summer
Are extreme floods increasing?
JANUARY FLOODS
JANUARY 12, 2009
When disaster becomes routineCrisis repeats as nature’s buffers disappear
Disaster DeclarationsFederal Emergency
Management Agency disaster declarations in King County in
connection with flooding:January 1990
November 1990December 1990November 1995February 1996
December 1996March 1997
November 2003December 2006December 2007
Mapes 2009
Pecos River flood frequency distribution (from Kochel et al, 1988)
Issues in the historical record
What causes a flood?• Heavy precipitation• Antecedent soil moisture and/or snow• Interaction of storm characteristics
(geometry, duration, intensity) with catchment geometry and characteristics (topography, channel network density, geology/soils)
• Storm orientation and movement relative to catchment/channel orientation
Other important flood characteristics
• Hydrologists usually think in terms of the annual maximum flood, which is the series of the largest floods each year (usually their peak discharge)
• Bankfull capacity corresponds roughly to 2-year return period (median annual maximum flood), which also is very roughly the mean annual flood
• Damages due to “floods” below bankfull capacity usually are minimal; damages increase rapidly (sometimes characterized as a power law) above bankfull discharge
• Flood risk is usually estimated by fitting a probability distribution to the annual maximum series, this distribution may be extrapolated to the T-year (e.g., 100-year, often used for flood plain planning) flood
• The T-year return period precipitation event (of specified duration) generally doesn’t cause the T-year flood (due to factors indicated above)
Number of statistically significant increasing and decreasing trends in U.S. streamflow (of 395 stations) by quantile (from Lins and Slack, 1999)
About 10% of the 400 sites show an increase in annual maximum flow from 1941-71 to 1971-99
Maximum flowIncreaseNo changeDecrease
Visual courtesy Bob Hirsch, figure from McCabe & Wolock, GRL, 2002
Tufts University
Decadal Magnification Factors of Floods – Sites w/ no regulation
1,642 of 14,893 USGS Gage Sites with M>1 and p>0.9
From Yaindl and Vogel, 2009
visual courtesy Rich Vogel
Tufts University
Decadal Flood Magnification Factors - HCDN Sites
208 of 1,588 HCDN Gage Sites with M>1 and p>0.9
From Yaindl and Vogel, 2011
visual courtesy Rich Vogel
Tufts University
Decadal Flood Magnification Factors Sites With No Regulation
From Yaindl and Vogel, 2011
visual courtesy Rich Vogel
Tufts University
ResultsDecadal Flood Magnification Factors
From Yaindl and Vogel, 2011
3 Groups of USGS Gages
Group Of Sites
Total
Number of Sites
Number of Sites with Significant
Positive Trends
Percentage of Sites With Significant
Positive Trends
Unregulated 14,893 1,642 11% Regulated 4,537 481 11%
HCDN 1,588 208 13%
visual courtesy Rich Vogel
Paradox: Given increases in precipitation and runoff, why are there so few significant trends in
floods?
Visual courtesy Tim Cohn, USGS
[Lins and Cohn, 2002]
Possible explanation
Visual courtesy Tim Cohn, USGS
Predicting urban flooding in a future climate – Thornton Creek example
Global Climate Models
ECHAM5
• Developed at Max Planck Institute for Meteorology (Hamburg, Germany)
• Used to simulate the A1B scenario in our study
CCSM3
• Developed at National Center for AtmosphericResearch (NCAR; Boulder, Colorado)
• Used to simulate the A2 scenario in our study
Global Climate Models
Mote et al 2005
ECHAM5
CCSM3
Dynamical Downscaling
Courtesy Eric Salathé
Global Model Regional Model
Results of Future Analysis
SeaTac Spokane Portland
1-hour +16% +10% +11%
24-hour +19% +4% +5%
1-hour -5% -7% +2%
24-hour +15% +22% +2%
* Statistically significant for difference in means and distributions, and non-zero temporal trends
EC
HA
M5/
WR
FC
CS
M3/
WR
F
* *
* *
Changes in average annual maximum precipitation between 1970–2000 and 2020–2050:
Results of Bias Correction -- SeaTac
Raw Change Corrected Change
1-hour +16% +14%
24-hour +19% +28%
1-hour -5% -6%
24-hour +15% +14%
EC
HA
M5
CC
SM
3
Comparison of changes in average annual maximum between 1970–2000 and 2020–2050:
*
* Statistically significant for difference in means and distributions, and non-zero temporal trends
*
* *
Thornton Creek
Bypass PipeBypass Pipe
Thornton Creek
Historical to Future Change in Peak Flow
-10%
0%
10%
20%
30%
40%
50%
60%
Avg
. Cha
nge
2-yr
to 5
0-yr
CCSM3-WRFECHAM5-WRF
Kramer Ck135 ac
South Branch 2294 ac
North Branch4143 ac
Thornton Ck7140 ac
Changes in Average Streamflow Annual Maxima (1970-2000 to 2020-2050)
Results of Hydrologic Modeling
Changes in average annual maxima streamflow at outlet of watershed between 1970-2000 and 2020-2050:
Juanita Creek Thornton Creek
CCSM3 +25% +55%
ECHAM5 +11% +28%
* Statistically significant for difference in means
**
The November Surprise
JAN FEB MAR APR
MAY JUN JUL AUG
SEP OCT NOV DEC
Courtesy Eric Salathé
NOV
Drought trends
from Andreadis and Lettenmaier, GRL 2006
Reconstructed U.S. soil moisture trends, 1915-2003
Trends in U.S. drought duration, 2915-2003
from Andreadis and Lettenmaier, GRL 2006
Trends in U.S. drought severity, 1915-2003
from Andreadis and Lettenmaier, GRL 2006
Trends in number of global droughts, 1950-2000
from Sheffield and Wood, J Clim, 2008
from Sheffield and Wood, J Clim, 2008
Trends in global drought duration, 1950-2000
Soil moisture trends in China, 1950-2006
from Wang et al., J Clim, 2011
Trends in soil moisture, area in drought, and drought severity over China, 1950-2006
from Wang et al., J Clim, 2011
Trends in number of droughts by area covered over China, 1950-2006
from Wang et al., J Clim, 2011
ConclusionsOver the last 50-100 years, based on instrumental record:
• Extreme precipitation seems to be increasing (although certainly not everywhere, not surprising given detectability issues)
• Evidence for increases in floods is much sketchier (trends in low flows are clearly pervasive across the U.S. however)
• Trends in droughts also not obvious -- U.S. has experienced a general wetting trend over the last century, and drought trends essentially represent a balance between increasing precipitation and increased evaporative demand – some evidence balance is towards increasing droughts in parts of the western U.S., decreasing elsewhere