Hydrologic Implications of 20th Century Warming and Climate Variability in the Western U.S.

Alan F. HamletProf. Dennis P. Lettenmaier (Chair)

Phd Final Exam May, 2006

Committee:Dennis P. Lettenmaier (chair)Deirdre Meldrum (GSR)Stephen BurgesDaniel CayanRichard PalmerNathan Mantua

CIG:Philip Mote Edward MilesAdrienne Karpov

Hydro Group:Andy WoodTed BohnKostas AndreadisJenny Adam

Acknowledgements:

Family and Friends:Carys KresnyRhys HamletAnya KresnyBill Kennedy

Western Water Assessment:Martyn Clark

Background andIntroduction

Natural Climate Influence Human Climate Influence

All Climate Influences

Natural AND human influences explain the observations of global warming best.

A history of the PDOwarm

coolwarm

A history of ENSO

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000

Pacific Decadal Oscillation El Niño Southern Oscillation

DJF Temp (°C) NDJFM Precip (mm)

PNW

CA CRB

GB

Cool Season Climate of the Western U.S.

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may jun jul

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sep

Are

a A

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ate

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(de

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in m

m)

precipitation

swe

runoff+baseflow

soil storage

evapotranspiration

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in m

m)

precipitation

swe

runoff+baseflow

soil storage

evapotranspiration

20th Century Climate

2040s Scenario(+ 2.25 C + 4% Pcp)

Seasonal Water BalanceNaches River

More runoff in winter and early spring, less in summer

At almost every USHCN station, winters warmed

+ signs: warming but not statistically significant

Climate change experiments have suggested that in temperature sensitive areas of the West, we should already be able to see the effects of global warming in the historic snow and streamflow records.

Using models we should be able to more fully analyze these changes, as well as other hydrologic effects which are not typically measured.

Why Do We Need Model Simulations of the Historic Record?

•Longer Record (Avoids problems with decadal variability from 1950 forwards)

•Spatial Coverage (high and low elevations not in the observations), river basin scale impacts.

•Temporal Resolution (daily time step)

•Full suite of hydrologic variables and consistency amongst these variables

•Explicit sensitivity analysis for effects of temperature and precipitation

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(sm

oo

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)

PNW

CA

CRB

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PDO PNW Trend

CRB Trend

Cool Season Precipitation Anomalies Compared to the PDO

-0.845-0.264-0.438-0.053

(Regional to PDO Correlation R2 )

Snow Model

Schematic of VIC Hydrologic Model and Energy Balance Snow Model

PNW

CACRB

GB

•How have variations in temperature and precipitation from the early 20th Century on (1916-2003) affected trends in hydrologic variables such as snowpack, volume and timing of runoff and baseflow, seasonal evaporation and soil moisture, and flood risk in the western U.S.?

•Is a consistent global warming signal apparent over the western U.S. in this period, and is it possible to make a clear distinction between “natural” variations such as decadal precipitation variability and more systematic effects associated with global warming signals? Are temperature and precipitation different in this regard?

•What role do regional climatic regimes and topographic variations play in defining the role of temperature and precipitation variability on hydrologic variations? What areas of the western U.S. are most sensitive to changes in temperature or precipitation changes and why?

Overview of Research Questions:

•Do the hydroclimatic variations observed in the western U.S. over the 20th century corroborate simulations of climatic changes produced by global climate model scenarios? For instance, is a hypothesis of wetter conditions in the western U.S. due to an intensified global hydrologic cycle born out in the observations? If so, how have these changes affected hydrologic variability?

•How do flood risks vary in time and how can these risks be characterized and predicted in the context of interannual and interdecadal climate variability and longer-term variations associated with global warming?

Research Questions (cont.):

Research Topics

1.Methods for producing long meteorological driving data sets

2.Effects of observed climate variability on snowpack trends

3.Effects of observed climate variability on trends in runoff, soil moisture, and evaporation

4.Evaluating changing flood risks in the context of climate variability and global warming

1) Met Data Processing

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ea

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ow

(c

fs)

unadjusted

observed

Problems with Temporal Inconsistencies in Meteorological Records(S. F. Flathead River at Hungry Horse Dam, MT)

Preprocessing Regridding

Lapse Temperatures

Correction to RemoveTemporal

Inhomogeneities

HCN/HCCDMonthly Data

Topographic Correction forPrecipitation

Coop Daily Data

PRISM MonthlyPrecipitation

Maps

Schematic Diagram for Data Processing of VIC Meteorological Driving Data

Preprocessing Regridding

Lapse Temperatures

Correction to RemoveTemporal

Inhomogeneities

HCN/HCCDMonthly Data

Topographic Correction forPrecipitation

Coop Daily Data

PRISM MonthlyPrecipitation

Maps

Preprocessing Regridding

Lapse Temperatures

Correction to RemoveTemporal

Inhomogeneities

HCN/HCCDMonthly Data

Topographic Correction forPrecipitation

Coop Daily Data

PRISM MonthlyPrecipitation

Maps

Schematic Diagram for Data Processing of VIC Meteorological Driving Data

Result:Daily Precipitation, Tmax, Tmin

1915-2003

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adjusted

unadjusted

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unadjusted

observed

Root square error

Comparison of adjusted vs. unadjusted VIC simulations(S. F. Flathead River at Hungry Horse Dam, MT)

Simulated vs Observed

Evaluation of Streamflow Simulations of the Colorado River at Lee’s Ferry, AZ

Trends in Temperature and Precipitation in the Western

U.S.

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(sm

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Global

TMAX

Regionally Averaged Cool Season Temperature Anomalies

0.740.630.760.62

(Regional to Global Correlation R2 )

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CRB

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Global

Regionally Averaged Cool Season Temperature Anomalies

TMIN

0.840.870.940.73

(Regional to Global Correlation R2 )

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Regionally Averaged Cool Season Precipitation Anomalies

PRECIP

Rel. Trend %/yr Trend (°C/yr) Trend (°C/yr)

Precipitation Tmax Tmin

DJF

Avg

Tem

pera

ture

DJF

Avg

Tem

pera

ture

Rel. Trend %/yr Trend (°C/yr) Trend (°C/yr)

1916-2003

1947-2003

Trends in Cool Season (Oct-Mar) Precipitation and Temperature

Rel. Trend %/yr Trend (°C/yr) Trend (°C/yr)

Precipitation Tmax Tmin

DJF

Avg

Tem

pera

ture

DJF

Avg

Tem

pera

ture

Rel. Trend %/yr Trend (°C/yr) Trend (°C/yr)

1916-2003

1947-2003

Trends in Warm Season (Apr-Sept) Precipitation and Temperature

2) Effects of Temperature and Precipitation Variability on Snowpack Trends in the

Western U.S.

Met Data1915-2003

VIC SWELinear Trend

Analysis

Overview of Simulation and Analysis

•1916-2003 •1924-1976 (warm to cool PDO)•1947-1997 (cool to warm PDO)•1924-1946 with 1977-1997 (warm to warm PDO)

Linear Trends:

Experiments:•Base—combined effects of temp and precip trends•Static Precip—effects of temperature trends only•Static Temp—effects of precipitation trends only

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

Trends in April 1 SWE 1950-1997

1950-1997 relative trends in April 1 SWE vs DJF temperature

ObsVIC

ObsVIC

ObsVIC

ObsVIC

Trend %/yr

DJF

avg

T (

C)

Trend %/yr

Overall Trends in April 1 SWE from 1947-2003

Trend %/yr

DJF

avg

T (

C)

Trend %/yr

Temperature Related Trends in April 1 SWE from 1947-2003

Trend %/yr

DJF

avg

T (

C)

Trend %/yr

Precipitation Related Trends in April 1 SWE from 1947-2003

1916-2003

Trend %/yr

DJF

avg

T (

C)

1925-1946with1977-2003

Trend %/yr

DJF

avg

T (

C)

DJF

avg

T (

C)

1947-2003

Decadal Variability Doesn’t Explain the Temperature Related Effects to Snowpack

b) Max Accum. c) 90 % Melt a) 10 % Accum.

DJF

Tem

p (C

)

Change in Date

Change in Date

DJF

Tem

p (C

)

Change in Date

Change in Date

DJF

Tem

p (C

)

Change in Date

Change in Date

Trends in the Date of Snow Accumulation and Melt

1916-2003

Effects ofTemperatureonly

Effects ofPrecipitationonly

Effects ofTemperatureandPrecipitation D

JF T

emp

(C)

Change in Date

DJF

Tem

p (C

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Change in Date

Change in Date

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Change in Date

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Change in DateD

JF T

emp

(C)

b) Max Accum. c) 90 % Melt a) 10 % Accum.1916-2003

3) Trends in Seasonal Runoff, Evaporation, and

Soil Moisture

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): 1136-1155

March June

Relative Trend (% per year)

Trends in simulated fraction of annual runoff in each month from 1947-2003 (cells > 50 mm of SWE on April 1)

Trend %/yrD

JF T

emp

(°C

)Trend %/yr

DJF

Tem

p (°

C)

Trends in March Runoff Trends in June Runoff

Trends in Soil Moisture

April 1

July 1

Trends in Simulated Soil Moisture from 1947-2003

Trend %/yr

DJF

Tem

p (°

C)

DJF

Tem

p (°

C)

Trend %/yr

Trend %/yr

DJF

Tem

p (°

C)

Trend %/yr

DJF

Tem

p (°

C)

Trends in April 1 SM Trends in July 1 SM

Trends in the Dates of 50% WY runoff, 80% max soil moisture recharge, and 50%

WY ET

BR

FTR

FPR

DJF

Tem

p (°

C)

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Tem

p (°

C)

DJF

Tem

p (°

C)

BR

FTR

FPR

BR

FTR

FPR

Trend days/50 yr

Effects ofTemp alone

Effects ofPrecip alone

Cumulative Trends in the Date of Hydrologic Events

(1947-2003)

50% WY Runoff 80% Max SM 50% WY ET

Trends in the “Runoff Ratio”(runoff/precipitation)

Trend Oct-Mar PCP

Tre

nd R

unof

f R

atio

Effects of Cool Season Precipitation Trends on Trends in the Runoff Ratio

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Temperature Related Downward Trends in Annual Streamflow at The Dalles Compared with the Effects of Precipitation Variability

Black trace = constant precipMagenta trace = with precip variability

4) Evaluating Systematic Changes in Flood Risks

Avg WY Date of Flooding VIC

Avg

WY

Dat

e o

f F

loo

din

g O

BS

Ln

(X

100

/ X

mea

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O

BS

Ln (X100 / Xmean) VIC

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

Zp

X1

00 G

EV

flo

od

/mea

n f

loo

dRed = VICBlue = OBS

5-yr

20-yr

10-yr

50-yr

100-yr

Tem

pera

ture

Historic temperature trend

in each calendar month

1915 2003

Detrended Temperature Driving Data for Flood Risk Experiments

“Pivot 2003” Data Set

“Pivot 1915” Data Set

y = 0.0326x - 9.1654

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observed tmin

detrended tmin

Linear (observed tmin)

Linear (detrended tmin)

Trends in January TMIN for a VIC cell in the Cascades

+ 2.8° C

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

X20 2003 / X20 1915

DJF

Avg

Tem

p (

C)

DJF

Avg

Tem

p (

C)

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

X20 2003 / X20 1915 X20 2003 / X20 1915

DJF

Avg

Tem

p (

C)

DJF

Avg

Tem

p (

C)

DJF

Avg

Tem

p (

C)

X100 2003 / X100 1915 X100 2003 / X100 1915X100 2003 / X100 1915

X100 2003 / X100 1915 X100 2003 / X100 1915X100 2003 / X100 1915

DJF

Avg

Tem

p (

C)

DJF

Avg

Tem

p (

C)

DJF

Avg

Tem

p (

C)

X100 nPDO / X100 2003 X100 cPDO / X100 2003X100 wPDO / X100 2003

X100 nPDO / X100 2003 X100 cPDO / X100 2003X100 wPDO / X100 2003

DJF

Avg

Tem

p (

C)

DJF

Avg

Tem

p (

C)

DJF

Avg

Tem

p (

C)

X100 nENSO / X100 2003 X100 cENSO / X100 2003X100 wENSO / X100 2003

X100 nENSO / X100 2003 X100 cENSO / X100 2003X100 wENSO / X100 2003

X100 cENSO / X100 2003

Effects of Cool ENSO on Flood Risks in Larger Basins

DJF

Avg

Tem

p (

C)

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

X20 ’73-’03 / X20 ’16-’03

X20 ’73-’03 / X20 ’16-’03

Summary of Temperature Related Effects

•Large-scale changes in the seasonal dynamics of snow accumulation and melt have occurred in the West in the 20th century as a result of increasing temperatures.

•Temperature-related effects are, in general, organized spatially according to mid-winter temperature regimes.

•Hydrologic changes include earlier and reduced peak snowpack, more runoff in March, less runoff in June, and corresponding increases in simulated spring soil moisture and decreases in summer soil moisture.

•Flood risks appear to be declining overall due to warming, but the model 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 warmer areas).

•Based on scenarios, we expect that the intensity and rates of change of temperature-related effects will increase as global warming progresses in the 21st century.

Summary of Precipitation Related Effects

•Consistent changes in cool-season precipitation volumes are not apparent in the West. Warm season precipitation, however, seems to be increasing over most of the West.

•Changes in cool season precipitation variability are apparent since 1975, but the cause is not yet clear, and it is not possible to say whether these changes are related to global warming, should be considered systematic in nature or not, etc.

•Unlike temperature-related effects, precipitation-related hydrologic effects are frequently distributed geographically. (e.g. ENSO variations via storm track effects, or large scale changes in precip. variability affecting the entire West).

•Although this study has highlighted some important differences between temperature and precipitation changes and their relation to global warming, a number of important questions remain about how best to represent future precipitation variability and uncertainty in global warming scenarios.