Long-Term Hydrologic Projections for 

the Colorado River Basin and Methods 

for Considering Uncertainty

James Prairie, PhD, Upper Colorado Basin Region

GCD Adaptive Management Plan

October 13, 2021
State of the System (Water Years 1999-2021)1,2

Water Year 2022 Forecast

Sept Min Prob: 4.74 maf (44%)

Sept Most Prob: 8.20 maf (76%)

Sept Max Prob: 16.00 maf (148%)
Current 

Drought in a 

Historical 

Context

Figure 12 Sequence Average plot from Salehabadi, et al (2020) “The Future Hydrology of the Colorado River Basin” https://qcnr.usu.edu/coloradoriver/files/WhitePaper4.pdf

*based on August 2021 provisional natural flow calculations for 2020 & 2021 https://www.usbr.gov/lc/region/g4000/NaturalFlow/provisional.html

* * *
5-Year Probabilistic 

Projections and 

Beyond
Precipitation Variability

“... the average annual precipitation over the past 20 years (2000–2019) does not stand out relative to periods of the same length earlier in the observed 

record.” Ch.2 p.75 SoS Report

Figure 2.6: Upper Basin 

water-year precipitation 

compared with Colorado 

River at Lees Ferry wateryear natural streamflow, 

1906–2019. The correlation 

between the two timeseries is 0.77 (R2 = 0.61) 

over the entire record, with 

higher correlations over 

more recent periods. (Data: 

precipitation, NOAA NCEI; 

streamflow, Reclamation)

Figure taken from Colorado River Basin Climate and Hydrology—State of the Science Report: https://wwa.colorado.edu/publications/reports/CRBreport/ColoRiver_StateOfScience_WWA_2020_Chapter_2.pdf
Increasing Temperature Trend

Annually-averaged temperature for the Colorado River Basin, 1895–2018, shown as departures from a 1970–1999 average. The gray line is a 10-year running average plotted on 

the 6th year. A 40-year linear trend (dashed yellow line) shows 2.4°F of warming from 1979–2018. Figure taken from forthcoming Colorado River Basin Climate and Hydrology—

State of the Science Report: https://www.usbr.gov/lc/region/programs/research-reports-etc/Final_CRB_SoS_Project_Overview.pdf
Lees Ferry Observed Natural Flow Record (1906-2021)

2

5

8

11

14

17

20

23

26

1906 1913 1920 1927 1934 1941 1948 1955 1962 1969 1976 1983 1990 1997 2004 2011 2018

Annual Flow (maf)

Year

Observed 1906-1987 Stress Test 1988-2019 Provisional 2020-2021 5-Year Avg 5-Year Avg

Avg Annual Flow 1906-2019: 14.7 maf Avg Annual Flow 1988-2019: 13.3 maf
Lake Powell and Lake Mead End-of-December Elevation

August 2021 CRSS Projections with 1988 – 2019 Resampled Hydrology

Now available at https://www.usbr.gov/lc/region/g4000/riverops/crss-5year-projections.html
Distributions of Alternative Hydrology Scenarios

Colorado River natural Flow at Lees Ferry Gaging Station
0%

2%

4%

6%

8%

10%

12%

14%

202120222023202420252026

Percent of Traces

Year

 2007 Projections (1905-2005 Hydrology, 1999 Demands; 2007 Interim Guidelines

 CMIP3 Hydrology; 2016 Demands; Current Policies Continue

 CMIP5 Hydrology; 2016 Demands; Current Policies Continue

 Full Hydrology; 2016 Demands; Current Policies Continue

 Stress Test Hydrology; 2016 Demands; Current Policies Continue

Scenario

* Not official Projections, 

based on August 2020 CRSS modeling with Lake Powell initial elevation of 3,592 feet. Lake Powell’s 9/21/21 elevation is 3,547 feet 

** CMIP3 and CMIP5 ensembles span 3 different emissions futures and were downscaled using Bias Correction Spatial Downscaling (BCSD)

**

**

Shifting Risk*

Risk of Lake Powell dropping below 3,490 feet in any month
0%

5%

10%

15%

20%

25%

30%

35%

40%

202120222023202420252026202720282029203020312032203320342035203620372038203920402041204220432044204520462047204820492050

Percent of Traces

Year

 2007 Projections (1905-2005 Hydrology; 1999 Demands; 2007 Interim Guidelines

 CMIP3 Hydrology; 2016 Demands; Current Policies Continue

 CMIP5 Hydrology; 2016 Demands; Current Policies Continue

 Full Hydrology; 2016 Demands; Current Policies Continue

 Stress Test Hydrology; 2016 Demands; Current Policies Continue

Scenario

Current Policies Expire

* Not official Projections, 

based on August 2020 CRSS modeling with Lake Powell initial elevation of 3,592 feet. Lake Powell’s 9/21/21 elevation is 3,547 feet 

** CMIP3 and CMIP5 ensembles span 3 different emissions futures and were downscaled using Bias Correction Spatial Downscaling (BCSD)

**

**

Shifting Risk*

Risk of Lake Powell dropping below 3,490 feet in any month
Concerns with Planning under Deep Uncertainty

• Deep uncertainty occurs when probabilities of any given set of future conditions cannot be estimated with confidence 

• Translation: it is impossible to determine the most appropriate planning 

assumptions

• Choices of hydrologic ensemble and other assumptions about the 

future are likely to be controversial

• Statistics-based analysis may be unreliable as the sole basis for understanding system or planning for future

• Risk = percent of traces; completely dependent on the composition of the chosen ensemble of traces

• “acceptable” level of risk, risk reduction, etc. are common planning metrics 

but the implications of the underlying calculations are not well understood by stakeholders
Decision Making under Deep Uncertainty

• Decision Making under Deep Uncertainty (DMDU) methods 

incorporate concepts and techniques that help address the challenges 

of planning under climate change

• Shift away from statistics-based risk analysis

• Focus on robustness- performance is good enough in a wide range of 

futures

• Fundamental concepts

• Wide range of futures, all equally likely

• Vulnerability analysis based on observable conditions

• Adaptation based on observable signposts as conditions evolve (and 

uncertainty decreases)
James Prairie: jprairie@usbr.gov