Colorado River Water Availability Assessment Under Climate Variability

Colorado River Water Availability Assessment Under Climate Variability

Annie Yarberry1, Balaji Rajagopalan2,3 and James Prairie4

1. Humboldt State University, Arcata, CA2. University of Colorado, Boulder, CO

3. CIRES, University of Colorado, Boulder, CO4. USBR, University of Colorado, Boulder, CO

AGU Fall 2010

Background• 60 MAF reservoir storage

• 4 times annual flow• 50 MAF in Lake Powell and Mead

• Increasing Demand• Decreasing Streamflows• Compacts/agreements made in the

wettest part of early 20th century • Under stress in recent decades

• Water supply risk and sustainability

19141919

19241929

19341939

19441949

19541959

19641969

19741979

19841989

19941999

20040

2000000400000060000008000000

100000001200000014000000160000001800000020000000

Total Colorado River Use 9-year moving average.

NF Lees Ferry 9-year moving average

Calendar Year

Annu

al F

low

(MAF

)

Background…

2000 ~ 2008•Declining lakes Mead and Powell •5 years of 10 maf/yr•66% of average flows•Worst drought in historic record•How bad can it go??••Climate change portends 0 ~ 25% reduction in the coming 4-5 decades

0

5000000

10000000

15000000

20000000

25000000

30000000

Year

Volu

me

(MAF

)

120 Foot drop13 maf lostCurrent: ~48%, 12 maf

Lake Mead Volume in Millions of Acre-Feet 1935-2008

Climate Change – Back to the Future?

• Long dry epochs are very common• 20th century unusually wet• Climate change studies indicate a

consensus of • 0 ~ 30% decline in mean flows in 4-5

decades

Climate Change – Water supply Risk• What is the risk to water supply under climate change – can

management mitigate? (Rajagopalan et al., 2009) • Basin-wide simple water balance model

• Entire storage as a ‘bath tub’• Stochastic streamflow ensembles from

observed+paleo+climate change projections (Prairie et al., 2008)

• Water supply risk (i.e., risk of drying) is small (< 5%) in the near term ~2026, for any climate variability (good news)• Risk increases dramatically by approximately 7 times in

the three decades thereafter (bad news)• Smart operating policies and demand growth strategies

need to be instilled

This Study – Research Question• What is the probability distribution of optimal “yield” from

the given storage capacity in the basin and ensemble of streamflow sequences?• Can be a complementary tool for stakeholders to make

risk-based planning and development decisions.

Methodology – Constrained Optimization(Linear)Y = Yield (MAF)Spillt= Overflow (MAF)Qt = Paleo-reconstructed inflow (MAF/yr)K = Reservoir capacity (MAF)St-1 = Previous year storage (MAF)St = Current storage (MAF)

Minimum Storage is specifiedSystem storage = 60MAF•Average storage is computed for the optimal yield Yopt, as the average of:

Methodology…• 10,000 50-year ensembles of streamflow sequences

each generated using observed and paleo flows (Prairie et al., 2008) – Natural Variability

• 10 and 20 percent linear flow reductions applied to incorporate climate change projections (Rajagopalan et al., 2009) – Climate Change Projections

• For each ensemble:• optimal yield• average storage• standard deviation of storage

• For the three scenarios - natural variability, 10, and 20% flow reductions – we explored five reservoir conditions• Initially full• Minimum storage at:• Zero MAF; and 15%, 30%, and 40% of capacity

ResultsDensity

12 14 16 18 20

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5 10 15 20 25

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Natural Variability – Minimum Storage of Zero

Optimal Yield (MAF/yr)

Average Storage (MAF)

Storage Standard Deviation (MAF)

Den

sity

12 14 16 18 20

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sity

10 20 30 40 50

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sity

5 10 15 20

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20% Streamflow Reduction – Minimum Storage of Zero

16 35 17

173516

Results

12 14 16 18 20

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PDF

10 20 30 40 50

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5 10 15 20 25

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PDF

All Scenarios – Minimum Storage at Zero

Optimal Yield (MAF/yr)

Average Storage (MAF)

Storage Standard Deviation (MAF)

Natural Variability10% Flow Reduction20% Flow Reduction

16 35 17

Results..Density

10 12 14 16 18

0.00

0.05

0.10

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0.25

0.30

Density

35 40 45 50 550.00

0.02

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Density

4 6 8 10 12 14

0.00

0.05

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Natural Variability - Minimum Storage at 40% Capacity

Optimal Yield (MAF/yr)

Average Storage (MAF)

Storage Standard Deviation (MAF)

Density

10 12 14 16 18

0.0

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Density

35 40 45 50 55

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4 6 8 10 12 14

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20% Streamflow Reduction - Minimum Storage at 40% Capacity

16

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35

35

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Results..

10 12 14 16 18

0.0

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PDF

30 35 40 45 50 55

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4 6 8 10 12 14

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PDF

All Scenarios - Minimum Storage at 40% Capacity

Optimal Yield (MAF/yr)

Average Storage (MAF)

Storage Standard Deviation (MAF)

Natural Variability10% Flow Reduction20% Flow Reduction

16 35

Results..

ConditionYield (MAF/yr)

12.7 13.5 14.4 16Reliability (%)

Minimum storage of zeroNatural Variability 99 99 97 6320% flow reduction 99 94 70 9

Minimum storage at 40% capacityNatural Variability 92 83 65 1920% flow reduction 82 57 25 1

Results..

Summary and Conclusion• A simple system-wide water balance model to assess

‘Optimal Yield’ for a given storage and streamflow sequence was developed for the Colorado River Basin• Natural variability, 10% and 20% reduction in mean

flows due to climate change were considered• Reliability of current consumption ~ 12.7MaF • ~99% when the system is let to go dry for any

flow scenario• Drops to ~82% when minimum storage is set to

24MaF and for 20% reduction due to climate change scenario

Summary and Conclusion• Reliability of planned demand of ~ 13.5MaF Drops

to ~57% when minimum storage is set to 24MaF and for 20% reduction due to climate change scenario

• Higher demands have progressively less reliability

• The PDFs of ‘optimal yields’ provide stakeholders with estimates of risks for various scenarios

• If specific sub-system ‘yields’ and their risks are desired – full system model (e.g., CRSS) needs to be run

Acknowledgements

• NSF-REU Program at University of Colorado at Boulder, Summer, 2010

Thank You!

Questions?

References

Rajagopalan, B., K. Nowak, J. Prairie, M. Hoerling, B. Harding, J. Barsugli, A. Ray and B. Udall; Water supply risk on the Colorado River: Can management mitigate?, Water Resources Research, 45, W08201, 2009.

Christensen, N., A. Wood, N. Voisin, D. Lettenmaier, and R. Palmer; The effects of climate change on the hydrology and water resources of the colorado river basin, Climatic Change, 62, (1-3), 337-63.

U.S. Department of Interior, U.S. Bureau of Reclamation; Colorado River interim guidelines for lower basin shortages and the coordinated operations for lake powell and lake mead, Final EIS, 2007.

Climate Change Studies

Early Studies – Scenarios, About 1980Stockton and Boggess, 1979 Revelle and Waggoner, 1983*

Mid Studies, First Global Climate Model Use, 1990sNash and Gleick, 1991, 1993McCabe and Wolock, 1999 (NAST)IPCC, 2001

More Recent Studies, Since 2004Milly et al.,2005, “Global Patterns of trends in runoff”Christensen and Lettenmaier, 2004, 2006Hoerling and Eischeid, 2006, “Past Peak Water?”Seager et al, 2007, “Imminent Transition to more arid climate state..”IPCC, 2007 (Regional Assessments)Barnett and Pierce, 2008, “When will Lake Mead Go Dry?”

National Research Council Colorado River Report, 2007