1. Estimating Changes in Flood Risk due to 20th Century Warming and Climate Variability in the Western U.S.
Alan F. Hamlet
Dennis P. Lettenmaier

2. Cool Season Climate of the Western U.S.
PNW GB CA
CRB
DJF Temp (°C)
NDJFM Precip (mm)

3. Pacific Decadal Oscillation El Niño Southern Oscillation
A history of the PDO
A history of ENSO
warm
warm
cool

4. Effects of the PDO and ENSO on Columbia River
Summer Streamflows
PDO
Cool
Cool
Warm
Warm
Red=warm ENSO Green=ENSO neutral Blue=cool ENSO

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

6. Global T as a Predictive Variable for TMAX Trends Over the West

7. Global T as a Predictive Variable for TMIN Trends Over the West

8. Trends in April 1 SWE
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

9. winter flows rise and summer flows drop
As the West warms,
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):
Spring snowmelt timing has advanced by days in most of the West, leading to increasing flow in March (blue circles) and decreasing flow in June (red circles), especially in the Pacific Northwest.

10. Regionally Averaged Cool Season Precipitation Anomalies

11. 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

12. Schematic of VIC Hydrologic Model and Energy Balance Snow Model
PNW CA
CRB GB
Snow Model

13. Evaluating the Hydrologic Model Simulations in the Context of Reproducing Flood Characteristics
Avg WY Date of Flooding OBS
Ln (X100 / Xmean) OBS
Fig 2 Simulated vs Observed date of flooding and ratio of 100 year flood to mean annual flood
Avg WY Date of Flooding VIC
Ln (X100 / Xmean) VIC
Red = PNW, Blue = CA, Green = Colo, Black = GB

14. X100 GEV flood/mean flood Zp Red = VIC Blue = OBS 100-yr 50-yr 20-yr

15. Detrended Temperature Driving Data for Flood Risk Experiments
“Pivot 2003” Data Set
Temperature
Historic temperature trend
in each calendar month
“Pivot 1915” Data Set
1915
2003

16. Trends in January TMIN for a VIC cell in the Cascades

17. Meteorological Records from 1915-2003
Use of a Hydrologic Model with Long Precipitation and Temperature Records
Meteorological Records from
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

18. Simulated Changes in the 20-year Flood Associated with 20th Century Warming
DJF Avg Temp (C)
X / X
Fig year flood A spatial scale
DJF Avg Temp (C)
X / X
X / X

19. X / X
X / X
X / X
Fig year flood for A,C,E spatial scales
DJF Avg Temp (C)
DJF Avg Temp (C)
DJF Avg Temp (C)
X / X
X / X
X / X

20. X100 wPDO / X100 2003 X100 nPDO / X100 2003 X100 cPDO / X100 2003
Fig 7 Warm, neutral, cool PDO 100-year flood
DJF Avg Temp (C)
DJF Avg Temp (C)
DJF Avg Temp (C)
X100 wPDO / X
X100 nPDO / X
X100 cPDO / X

21. X100 wENSO / X100 2003 X100 nENSO / X100 2003 X100 cENSO / X100 2003
Fig 8 Warm, neutral, cool ENSO 100-year flood
DJF Avg Temp (C)
DJF Avg Temp (C)
DJF Avg Temp (C)
X100 wENSO / X
X100 nENSO / X
X100 cENSO / X

22. Effects of Cool ENSO on Flood Risks in Larger Basins
X100 cENSO / X

23. Regionally Averaged Cool Season Precipitation Anomalies

24. 20-year Flood for “ ” Compared to “ ” for a Constant Late 20th Century Temperature Regime
DJF Avg Temp (C)
X20 ’73-’03 / X20 ’16-’03
X20 ’73-’03 / X20 ’16-’03

25. Summary and Conclusions
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?