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Colorado River Hydrology, Aridification and System Imbalances

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Water Fluency for Journalists – Session 2

August 12, 2021

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Brad Udall

Senior Scientist/Scholar

Colorado Water Center

Colorado State University

Bradley.Udall@colostate.edu

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• Talk Overview
• Basin Overview
• Hydrology Since 1964
• Unregulated Inflows into Powell
• Combined Powell + Mead Contents
• 5 Key Studies since 2016
• Runoff Efficiency
• Temperature Sensitivity
• Megadrought
• Aridification vs. Drought
• Solutions
• Process and Solutions for New 2026 Agreement
• Alternative Management Options Study

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Colorado River�

= Major Diversion

• 7 States, 2 Nations, 29 Tribes
• 8% of area of the Lower 48
• Annual “Natural” Flow ~14.75 MAF

= Hudson River

• Worst drought in gaged record started 2000 ~12.4 MAF/yr

= ~18% decline annually

• 40 M People
• All of the Major Cities in SW US
• 4.5m Irrigated Acres
• Fully Allocated in 1922
• Complex Use Agreements
• Withdrawals equaled Supplies ~2000
• New Projects still contemplated
• No longer reaches the ocean

“Unregulated Inflows”

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• Actual Inflows into Powell with effects of Upstream Reservoir Operations Removed

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• If you added Upper Basin Demands to these flows you’d have ~ “Natural Flows”

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• Tie Directly to Compact Section III D Non-Depletion Provision

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• Can not release from Powell more than Inflow over time

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Jan 2000: �Powell+Mead 90% Full, 47 MAF

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April 2022:�Powell+Mead less than 30% Full, 15 MAF

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Loss of 32 MAF or 1.4 MAF/Year

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Colorado River:�Non-Stationarity �Evident

• Warming Everywhere
• Record Setting Drought
• Temperature Induced Losses
• Snow Loss
• Earlier Runoff
• Less Productive Runoff
• Dust

• Temperature can be a major flow driver (normally we just think about precipitation)
• Since 1988 flows have been less than expected given winter precipitation
• Warm temperatures exacerbated modest precipitation deficits in the Millennium Drought

Geophysical Research Letters, 2016

• Precipitation declines only partially explain flow loss
• ~ 66% of the loss

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• Temperature increases explain the remainder
• ~ 33% of the loss

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• Why?
• More Evaporation
• Thirstier Atmosphere

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• Temperature-Induced Losses
• Now = ~6% - 10%
• 2050 = ~20%
• 2100 = ~35%

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- 2017

• Hydrology Model-based Study using Historical Data
• Run model with and without temperature change

4 Key Basins (Green + Blue) produce ~55% of all runoff

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Findings

• ~50% of Decline due to Higher Temperatures
• More Evaporation of all kinds

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• ~50% of Decline due to Changing Precip Patterns
• Precipitation shift to less productive basins

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In both Central Plains and Southwest, Multi-decadal* Drought Risk exceeds 80% in 21^st Century

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Other studies have shown 21^st Century megadroughts can even occur with increases in precipitation

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* Defined as 35 years or more

Percent Chance of Multi-Decadal

Drought Risk,

Southwest US using 3 metrics

Science Advances, 2015

Emerging Megadrought

2000—2018 2^nd Driest 19-year period since 800 AD

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Caused by Natural Variability aided by anthropogenic drying

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About 50% due to humans

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Without anthropogenic drying, would be a moderate drought

Science, April 2020

• Attempt to ‘reconcile’ the wide range of CR Temperature Sensitivities
• Answer: -9.3 %/°C !
• Mid-century flow loss (only Temps)
• -14% to -26% RCP4.5
• -19% to -31% RCP8.5
• Mid-century flow loss (both Temps & Precip)
• +5% to -24%
• +3% to -40%
• Key Finding: As shiny, reflective snow declines, absorbed radiation goes up (2/3 of the cause)

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Aridification – not a drought

• Declining Snowpack and earlier runoff
• Higher Temperatures – 1.25C
• Drying Soil
• Thirsty Atmosphere (holds more moisture)
• Moving storm tracks (less certain, but a worry)
• Shorter Winter/Longer Fall
• Megadrought

• Tenuous Supply – Demand Balance now
• A gradual and incremental approach won’t work
• Upper Colorado River Commission Demands too high
• Combined Powell and Mead Storage should be the management metric
• Changes in Reservoir Operations will not solve the supply-demand imbalance
• Have control over demands but not over hydrology
• Consumptive water uses must be matched to available supplies
• Requires Upper Basin limitations and substantially larger Lower Basin reductions than are currently envisaged
• Without demand reductions, high probability of reservoirs falling to 15 maf or less

Key Findings

The Next Agreement…

• IG + DCP Agreements in force thru 2026
• Negotiations underway
• Very Difficult Problems
• Lower Basin Overuse
• Upper Basin “Delivery Obligation” vs “Non-Depletion Obligation”
• Upper Basin Desire to Increase Demands
• Tribal Needs and Equity
• Declining Flows
• Many Parties
• 7 states, 30 Tribes, Mexico, Federal Government
• Solutions not at all clear
• But good working relationships
• Important Role for Tribes

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DCP Signing Ceremony at Hoover Dam

Prudent Planning and “Reasonable Worst-Case Future”

• What scenarios should we be modeling for?
• Water-supply policy is ultimately a political / policy decision, informed by science
• We suggest: “Reasonable Worst-Case Future”
• Definition: Future that is both politically possible to plan for, and climatologically possible, without being on the extreme tail of any distribution
• What science should inform that decision?
• Known Science
• Past 21 years of flows, precipitation and temperature
• Temperature impacts on flow
• Future temperature projections
• All evidence points to declining flows
• Unknown Science – mostly precipitation
• Low confidence in projected increases
• Might save the day, but is it prudent to count on this?
• Precipitation might also decrease – see Hoerling et al, 2019
• Ultimately a Policy Decision of What is Prudent and Possible to Plan For
• Balancing of politically possible and climatologically problematic
• Prudence dictates modeling using flows less than last 21 years, but how much less?

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Fleck – Udall Science Editorial on Colorado River Risk

May 28, 2021

• Hydrology Model Study over 4 Big Western River Basins
• Warming applied by single month/season

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• CRB most sensitive to annual warming: -16% flow loss with 3C warming (implies ~5%/°C loss)

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• Summer Warming most important.
• Affects flow that summer and following summer via soil moisture deficits.

• 20% Flow Decline over last century
• 50% of that due to climate change (i.e. 10% flow loss)
• Climate models show 1.2°C warming and 3% precip decline

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• Precipitation Elasticity of ~ 2
• Temperature Sensitivity of ~ -2.5% to -7% /°C

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• Warming is 1/3 of the decline (~3 % of flow)
• Precipitation Loss is 2/3 of decline (~7 % of flow)

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• What’s New:
• 1. Attribution of 1981- 2010 precipitation decline to climate change
• 2. Lower Temperature Sensitivity

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Hoerling, Barsugli, Livneh, Eischeid, Quan, Badger, 2019

With Climate Change

Without Climate Change

Precipitation

Runoff

Temperature

Climate Model Results 1981-2010

Sophisticated Multi-model Multi-Ensemble GCM Effort with and without added greenhouse gasses

Record Setting Temperatures and Precipitation June-August 2020