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