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Estimating Changes in Flood Risk due to 20th Century Warming and Climate Variability in the Western U.S. Alan F. Hamlet Dennis P. Lettenmaier. Cool Season Climate of the Western U.S. PNW. GB. CA. CRB. DJF Temp ( °C). NDJFM Precip ( mm). Pacific Decadal Oscillation.

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Estimating Changes in Flood Risk due to 20th Century Warming and Climate Variability in the Western U.S.

Alan F. Hamlet

Dennis P. Lettenmaier


Cool Season Climate of the Western U.S. and Climate Variability in the Western U.S.

PNW

GB

CA

CRB

DJF Temp (°C)

NDJFM Precip (mm)


Pacific Decadal Oscillation and Climate Variability in the Western U.S.

El Niño Southern Oscillation

A history of the PDO

A history of ENSO

warm

warm

cool

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000


Effects of the PDO and ENSO on Columbia River and Climate Variability in the Western U.S.

Summer Streamflows

PDO

Cool

Cool

Warm

Warm

Red=warm ENSO Green=ENSO neutral Blue=cool ENSO


April-September Naturalized Flow for the Columbia River at The Dalles, OR from Historic Water Years vs January Nino 3.4 Anomalies.

1916-2002




Trends in April 1 SWE 1950-1997 West

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, West

winter 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



Predominant Mechanisms Associated with Climate Related Changes in Flood Risk

Snowmelt Dominant Rivers:

Changes in spring peak snowpacks

Transient Snow Rivers:

Changes in antecedent snowpack at the time of storm

Change in effective basin area due to rain/snow area distribution during storm

Changes in storm intensity

Rain Dominant Rivers:

Changes in antecedent soil moisture

Changes in storm intensity


PNW Changes in Flood Risk

GB

CA

CRB

Schematic of VIC Hydrologic Model and Energy Balance Snow Model

Snow Model


Evaluating the Hydrologic Model Simulations in the Context of Reproducing Flood Characteristics

Ln (X100 / Xmean) OBS

Avg WY Date of Flooding OBS

Avg WY Date of Flooding VIC

Ln (X100 / Xmean) VIC

Red = PNW, Blue = CA, Green = Colo, Black = GB


100-yr of Reproducing Flood Characteristics

Red = VIC

Blue = OBS

50-yr

X100 GEV flood/mean flood

20-yr

10-yr

5-yr

Zp


Detrended Temperature Driving Data for Flood Risk Experiments

“Pivot 2003” Data Set

Temperature

Historic temperature trend

in each calendar month

“Pivot 1915” Data Set

2003

1915



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


Simulated Changes in the 20-year Flood Associated with 20 Temperature Recordsth Century Warming

DJF Avg Temp (C)

X20 2003 / X20 1915

DJF Avg Temp (C)

X20 2003 / X20 1915

X20 2003 / X20 1915


X Temperature Records100 2003 / X100 1915

X100 2003 / X100 1915

X100 2003 / X100 1915

DJF Avg Temp (C)

DJF Avg Temp (C)

DJF Avg Temp (C)

X100 2003 / X100 1915

X100 2003 / X100 1915

X100 2003 / X100 1915


X Temperature Records100 wPDO / X100 2003

X100 nPDO / X100 2003

X100 cPDO / X100 2003

DJF Avg Temp (C)

DJF Avg Temp (C)

DJF Avg Temp (C)

X100 wPDO / X100 2003

X100 nPDO / X100 2003

X100 cPDO / X100 2003


X Temperature Records100 wENSO / X100 2003

X100 nENSO / X100 2003

X100 cENSO / X100 2003

DJF Avg Temp (C)

DJF Avg Temp (C)

DJF Avg Temp (C)

X100 wENSO / X100 2003

X100 nENSO / X100 2003

X100 cENSO / X100 2003


Effects of Cool ENSO on Flood Risks in Larger Basins Temperature Records

X100 cENSO / X100 2003



20-year Flood for “1973-2003” Compared to “1916-2003” for a Constant Late 20th Century Temperature Regime

DJF Avg Temp (C)

X20 ’73-’03 / X20 ’16-’03

X20 ’73-’03 / X20 ’16-’03


  • Summary and Conclusions “1916-2003” for a Constant Late 20

  • Flood risks appear to be declining overall in the West due to systematic warming, but the simulations suggests that flood risks are increasing in many moderate elevation areas where tradeoffs between loss of antecedent snow and increasing basin size favor increasing basin size (typically basins in near-coastal mountains).

  • Flood risks are affected by interannual and decadal climate variability. During warm ENSO and warm PDO years, flood risks are generally lower in the PNW and the highest flood risks typically occur in either cool ENSO or ENSO neutral years. In phase PDO signals increase the strength of these relationships overall.

  • Changes in cool season precipitation variability since the mid 1970s have resulted in substantial increases in flood risk in many areas of the West. Are these related to warming?


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