Alan F. HamletDennis P. Lettenmaier

Center for Science in the Earth System Climate Impacts Group

and Department of Civil and Environmental EngineeringUniversity of Washington

December, 2004

Quantifying Hydrologic Changes in Pacific Northwest Watersheds

Chehalis River 1999(~2.5 ft above flood stage)

Changing PNW Floods Risks

Summary of Past Research

In the 20th century, risks of moderate floods have been non-stationary in time and are strongly linked to the variability of PDO and ENSO

Risks of extreme flooding events are not necessarily linked to PDO and ENSO in the same way as are moderate events (sample size problems)

Challenges in Evaluating Potential Changes in Flood Risks

Severe floods by definition occur infrequently Problems:

Sample size is very smallStreamflow measurement errors (models may be better) Regulation effects are significant in almost all large watershedsFitting of probability distributions is imprecise (and inappropriate?)

Changes in climate or land use that affect flooding can be gradual, have varying spatial extent, and affect different areas at different times. Problems:

Flood risks are probably not stationary in timeDetection using conventional statistical procedures is problematic (need

models and larger samples)Ability to identify cause and effect is frequently difficult

(e.g. climate or land use?)

Conclusions:•Need hydrologic models•Need long, homogeneous records of temperature and precipitation

Use of a Hydrologic Model with Long Precipitation and Temperature Records

VICHydrology Model

Meteorological Records from 1915-2003•De-trended Temperatures

•Observed Precipitation Variability

Variability of Runoff In Different

River Basin Typesfor A Consistent

“Early” and “Late” 20th CenturyTemperature

Regime

• Have increasing temperatures in transient snow basins systematically increased flood risks in early winter as climate change scenarios suggest they should?

• Have changes in temperature and precipitation in snowmelt dominant basins increased or reduced spring flood risks? What is the relative role of winter temperature (loss of snow) and spring precipitation in these changes?

• Has the variability of floods in snow melt dominant and transient snow basins been systematically altered by the loss of snowpack due to a more direct coupling of precipitation and runoff production?

• How have flood risks changed in rain dominant basins? What aspects of evolving precipitation variability are most important in these basins and what is the role of climate and topography?

• What role have observed changes in ENSO frequency and decadal variability associated with the PDO and PNA played in observed changes in flood risk? Have the influence of these climate indices remained constant over time? What are the implications for projecting flood risks forward in time?

Research Questions:

Effects of Climate Change on Pacific Northwest Watersheds:

Decision Support for Pacific Northwest Watershed Planning Units

Hydrologic Data for Watershed Planning Units

Watershed planning units frequently have limited financial resources and may have no access to hydrologic models or even observed hydrologic data in some cases.

Sensitivity to regional warming will inform decisions on whether to invest these limited resources in obtaining better information about the impacts of climate change.

Macro-scale hydrologic simulation models can provide a great deal of information about the potential impacts of climate change on particular watersheds in the Pacific Northwest which can help inform these decisions.

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Quantifying Changes in Irrigation RequirementsChanges in Average July PotET over the Southern Plain Region in Idaho Current Climate vs. MPI2040 scenario (+ 4° C)

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