prologue

Study GCMs (runs) Colorado R Runoff by mid 21st century

Christensen et al. 2004 1 (3) -18%

Milly 2005 12 (24) -10 to -20%

Hoerling and Eischeid 2006 18 (42) -45%

Christensen and Lettenmaier 2007

11 (22) -6% (-40 to 18%)

Seagar et al. 2007 19 (49) -16% (-8 to -25%)

Information from Table 5-1 in Western Water Assessment (WWA) report for Colorado Water Conservation Board “Colorado Climate Change: A Synthesis to Support Water Resource Management and Adaptation.” Oct 2008 (available online at: http://wwa.colorado.edu/publications/reports/WWA_ClimateChangeColoradoReport_2008.pdf)

Prologue

Dennis LettenmaierJulie Vano

Brad Udall

Jonathon OverpeckHolly HartmannKiyomi Morino

Dan Cayan Hugo HidalgoTapash Das

Greg McCabe (USGS)Robin Webb (NOAA) Marty Hoerling (NOAA)Levi Brekke (Reclamation)Kevin Werner (NWS RFC)+

*RISA: Regional Integrated Science and Assessments

Understanding Uncertainties in Future Colorado River Streamflow

JAN 2014

JA Vano and Collaborators

Dennis Lettenmeier, (UW, CIG, CIRC)Brad Udall (WWA)Dan Cayan, Tapash Das,

Hugo Hidalgo (CAP)Jonathon Overpeck, Holly Hartmann,

Kiyomi Morino (CLIMAS)Robin Webb, Marty Hoerling (NOAA)Greg McCabe (USGS)Levi Brekke (Reclamation)Kevin Werner (NWS RFC)

Hydrology model

RCM

GCM

Stream-flow

CLIMATE

LAND

SURFACE

MGMT

IMPACT

Adapted from Vano et al 2014: Figure 1

Hydrology model

RCM

GCM

Stream-flow

boundaryconditions

P-E, R

Stat. down

P-E, R

flow routing

CLIMATE

LAND

SURFACE

MGMT

IMPACT

Adapted from Vano et al 2014: Figure 1

1. Seager et al. 20072. Seager et al 20133. Milly et al. 2005

Hydrology model

RCM

GCM

Stream-flow

boundaryconditions

P-E, R

Stat. down

P-E, R

flow routing

CLIMATE

LAND

SURFACE

MGMT

IMPACT

Adapted from Vano et al 2014: Figure 1

1. Seager et al. 20072. Seager et al 20133. Milly et al. 20054. Christensen et al. 20045. Christensen and Lettenmaier 20076. Cayan et al 20107. USBR 2011

Hydrology model

RCM

GCM

Stream-flow

boundaryconditions

P-E, R

Stat. down

P-E, R

flow routing

CLIMATE

LAND

SURFACE

MGMT

IMPACT

Adapted from Vano et al 2014: Figure 1

1. Seager et al. 20072. Seager et al 20133. Milly et al. 20054. Christensen et al. 20045. Christensen and Lettenmaier 20076. Cayan et al 20107. USBR 20118. Gao et al 20119. Rasmussen et al 201110.Gao et al 2012

1.GCM, Emission Scenario & Period of Analysis

2.Spatial scale3.Land Surface models4.Statistical Downscaling

GCM, Emission Scenario & Period of Analysis

Hydrology model

RCM

GCM

Stream-flow

CLIM

ATE

LAND

SURF

ACE

MGMT

IMPA

CT

Uncertainty #1:

Different GCMs, A1B scenario

-19%

-13%

Adapted from Vano et al 2014: Figure 3

-19%

-13%

-24%

Different GCMs, A1B scenario

Same GCMs, Different Emissions Scenarios

-8%

-13%

-15%

-8%

-15%

-17%

-10%

Same GCMs, Different Emissions Scenarios & Different Periods of Analysis

1. Model subset size and composition will impact projections of future streamflow.

2. Emission Scenario and Period of Analysis may matter for some model subsets.

Lesson #1:

Spatial scale.

Hydrology model

RCM

GCM

Stream- flow

CLIM

ATE

LAND

SURF

ACE

MGMT

IMPA

CT

Uncertainty #2:

Basin-wide, PRCP amounts are roughly equivalent in summer and winter

BUT

winter PRCP is much greater in headwaters and more efficiently produces runoff.

1585

Figure from Vano et al., BAMS, January 2014

0 100 200 300 400 500 600 700 800 900 1000Runoff (mm/year)

0 100 200 300 400 500 600 700 800 900 1000Runoff (mm/year)

1/8˚ 1/2˚ 1˚ 2˚

Grid spacing (degrees)

Annu

al A

vera

ge R

unof

f abo

veLe

es F

erry

(mm

/yr)

1/8˚ 1/2˚ 1˚ 2˚

Adapted from Vano et al 2014: Figure 4

Grid spacing (degrees)

Annu

al A

vera

ge R

unof

f abo

veLe

es F

erry

(mm

/yr)

Adapted from Vano et al 2014: Figure 4

Sensitivity: The % change in runoff for an imposed

increase in T.

Coarser spatial resolutions tend to be more sensitive to change from both warming and precipitation reduction.

Lesson #2:

Land surface representation.

Hydrology model

RCM

GCM

Stream-flow

CLIM

ATE

LAND

SURF

ACE

MGMT

IMPA

CT

Uncertainty #3:

• Grid-based simulations of land-surface processes using principles of energy and water balance

• Daily timesteps with some sub-daily processes

• Forcing data: PRCP, T, specific humidity, wind speed, air pressure, and surface incident shortwave and longwave radiation

* elasticity is a ratio of: the percent change in annual model

runoff to the percent change in annual

precipitation.

elas

ticity

* (r

unoff

:prc

p)

Adapted from Vano et al 2014: Figure 5

sens

itivi

ty*

(%∆

per d

eg C

)

* sensitivity is the percent change in runoff for an imposed increase in T.

Adapted from Vano et al 2014: Figure 5

Hydrology models show:

1. substantial differences in sensitivities to T increases;

2. similar responses to precipitation change;

3. differences in P elasticity and T sensitivity are generally smaller in headwater regions.

Lesson #3:

Statistical downscaling.

Hydrology model

RCM

GCM

Stream-flow

CLIM

ATE

LAND

SURF

ACE

MGMT

IMPA

CT

Uncertainty #4:

200 km 1 month10-20 km sub-daily

inmcm.a2giss.a2

hadcm3.a2ipsl.a2

pcm.a2mri.a2

csiro.a2mpi.a2gfdl.a2

miroc.a2cnrm.a2

-20% 0% 20% 40% 60% 80% 100% 120% 140%

15%

12%

9%

15%

0%

5%

1%

5%

5%

1%

-5% differencedelta methodBCSD

percent of historical flows

CNRMMIROCGFDLMPICSIROMRIPCMIPSLHADCM3GISSINMCM

Glob

al C

limat

e M

odel

s (A

2 em

issio

ns sc

enar

io)

+6%

The choice of downscaling method can affect the magnitude of the climate signal leading to differences in long-term projected runoff.

Lesson #4:

We identified four major reasons for discrepancies in Colorado River projections:

1. GCMs and emissions scenarios;2. Spatial scale or the ability of models to

simulate the disproportionate contributions to Colorado River discharge of the relatively small, high elevation runoff source areas;

3. Sensitivities of land surface hydrology models to P and T changes;

4. Methods used to statistically downscale GCM scenarios.

Re-cap

Implications for Decision makers

From past AR4 studies, we can say with high likelihood that in the Colorado:• Temperatures (T) will rise in the Colorado over the

coming decades• Precipitation (P) less certain, but will likely

decline on annual basis

From our analysis• Warmer T (ignoring P) will reduce runoff production

(our estimates -6.5 3.5% per C at Lees Ferry)• Change in P results in streamflow response of 2 to

3 times (5% decline in P results in 10-15% decline in streamflow)

• Coarse spatial resolution of models does not resolve high elevation hydrologic processes that dominate Colorado River basin runoff production

The diversity of approaches to projecting future streamflow in the scientific literature requires further analysis to make apples-to-apples comparisons in order to better understand sources of uncertainty.

Parting words…

Thank you!Research funded by NOAA through its RISA (Regional Integrated Sciences & Assessments) Project and its National Integrated Drought Information System

The Uncertainty Prayer*Grant us…The ability to reduce the uncertainties we can;The willingness to work with the uncertainties we cannot;And the scientific knowledge to know the difference. -

*http://www.wucaonline.org/assets/pdf/actions_whitepaper_120909.pdf

• 14 co-authors, from academia and federal agencies, many authors of divergent papers

• Reached agreement on four key sources of future uncertainties AND important certainties for decision makers in the Colorado River basin

• Documented for the larger science and management communities how to approach seemingly disparate results, particularly timely with new results being released with the 5th Assessment Report (AR5)

Summary

Where research evolving (not ranking of research priorities):• New climate change projections• Increased spatial resolution of climate models• Improved land surface simulations• New paleoclimate reconstruction and model

evaluation• Improved observational records• Strengthening connection with management

community

Future research directions

As climate science evolves, understandings will improve. However, there will be no single magic bullet. Therefore, a continued effort to communicate and incorporate uncertainty is needed.