Potential effects of climate change on the Columbia River Basin: Hydrology and

water resources

Dennis P. LettenmaierDepartment of Civil and Environmental Engineering

and Climate Impacts Group

University of Washington

UW School of Communications/Knight Foundation 

Seminar for JournalistsJune 28 2002

JISAO Climate Impacts GroupJoint Institute for the Study of Atmosphere and Oceans

A regional perspective on a global problem

What is known? On a global basis, temperature has been increasing,

appears to be well out of the range of natural variability

Picture is less clear for hydrology Modeling is the basis for future projections

Models essentially all agree on direction of temperature change and regional coherence, less so on precipitation

Some consistency in direction of model predictions for precipitation change over land at intermediate to high latitudes

Source: IPCC 2001

Source: IPCC 2001

What is a climate model?

• Abstraction of the true climate (“average weather”) system, based on the laws of physics

• Typically a set of equations, solved numerically, over a (global) grid, for “climate time scales” (decades to centuries)

• In practice, so-called General Circulation Models (GCMs) are close cousins of numerical weather prediction models

Source: NRC 1975

Source: IPCC 2001

Source: IPCC 2001

Source: IPCC 2001

Interpreting the hydrologic implications

• Scale mismatch – the critical roadblock• Interpreting hydrologic consequences

requires a river basin perspective, GCM scale of degrees lat-long doesn’t resolve any but the very largest rivers

• Topographic effects on precipitation and temperature at poorly resolved by global GCMs

Topography as a constraint (or why the situation is not hopeless)• In the PNW (and most of the West) streamflow originates

predominantly as winter snowfall (> 70% westwide)• Topography exerts a strong control on partitioning of

precipitation into rain vs snow (~ 6oC/1000 m mean lapse rate), and amount of precipitation (orographic enhancement)

• Hence hydrologic implications of warming can be extracted from large scale information about warming, as can some information about spatial distribution of precipitation (changes will be most important over mountainous source areas, especially in interior of west where lowlands tend to be arid or semi-arid)

Climate Scenarios

Transient GCM Simulations for Increasing CO2 and Aerosols

AdjustmentsTo ObservedMeteorology

Delta Precip,Temp

HydrologicModel (VIC)

Natural Streamflow

ReservoirModel (ColSim)

DamReleases,Regulated

Streamflow

PerformanceMeasures

Reliability of System Objectives

ColSimReservoir

Model

VICHydrology Model

Changes in Mean Temperature and

Precipitation from GCMs

Overview of ColSim Reservoir Model

Physical Systemof Damsand Reservoirs

Reservoir Operating Policies

Reservoir StorageRegulated StreamflowFlood ControlEnergy ProductionIrrigation ConsumptionStreamflow Augmentation

0100000200000300000400000500000600000700000800000900000

1971

1972

1973

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1980

Flow

(cfs

)

Streamflow Time Series

VIC Simulations of April 1 Average Snow Cover ExtentMPI ECHAM4 Scenarios

Base Case ~2025 ~2045

Snow Accumulation and Ablation

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Changes to Mean Hydrographs Columbia Basin 2045

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Water Year

Flo

w (

cfs

)

HC 2045 TempChange Only

HC 2045 PrecipChange Only

Base

The Dalles

Libby

Lower Granite

McNary

Grand Coulee

Snake R.

Mica

Corra LinnPriest Rapids

Chief Joseph

Ice Harbor

Columbia Falls

Hungry Horse

Oxbow

1

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FirmEnergy

Non-FirmEnergy

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SnakeIrrigation

LakeRooseveltRecreation

System Objective

Rel

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) Base Case

HC 2025

HC 2045

HC 2095

MPI 2025

MPI 2045

ColSim Reliability of System Objectives

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irm E

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irm E

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Gra

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Flo

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Current Climate

ECHAM4 2040's

HadCM2 2040's

HadCM3 2040's

PCM 2040's

VIC/ColSim 2040’s

DALLES

0

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dec

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apr

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augA

vera

ge

Flo

w (

cfs)

Base

ECHAM4

HadCM2

HadCM3

PCM

-$140

-$120

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$0

Current 3.3 4.3 6 8.3 33.3

Chan

ge in

Ann

ual P

ower

Rev

enue

(milli

ons)

$0.0

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Aver

age A

nnua

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age (

millio

n)

Mean power revenue change

Mean annual f lood damage