Colorado River Basin 

August 2021 5-year 

Projected Future Conditions

Alan Butler

Glen Canyon Dam Adaptive Management Program Technical Work Group Meeting

October 12, 2021
Presentation Overview

• August 2021 5-year

Probabilistic Projections

• Website Updates

2
Reclamation Operational Modeling

Model Comparison

3
Approach to 5-Year Probabilistic Projections

• Future hydrology produced by Reclamation by resampling a period of the natural 

flow record ... “Stress Test Hydrology”

• Future inflow scenarios are generated using 1988 – 2019 data ... 32 years = 32 scenarios

• In contrast to our prior approach, will not include the “Full Hydrology” (1906-2019) scenario

• Produced using Reclamation’s Colorado River Simulation System (CRSS) 

• Official “long-term” planning model- used to produce official projections, policy development 

and planning studies 

• Upper Basin water demand develop in coordination with the Upper Basin States and the UCRC 

• Lower Basin water demand develop in coordination with the Lower Basin States and Mexico

• Operations consistent with the 2007 Interim Guidelines, Minute 323 and the Lower Basin DCP

• In contrast to the June results, August results will not assume UB Drought Response Operations 

beyond 2021

4
Lake Powell and Lake Mead End-of-December Elevation

Corrected August 2021 CRSS Projections with 1988-2019 Resampled Hydrology

Lake Powell Lake Mead
Comparison of Corrected August 2021 Without UB DRO and June 

2021 Projections with UB DRO

Chance of Reaching Critical Reservoir Elevations, Stress Test Hydrology

All results computed as the chance of falling below the threshold in any month in the calendar (water) year for 

Lake Mead (Lake Powell).
Upper Basin – Lake Powell

Percent of Traces with Event or System Condition 

Results from Corrected August 2021 CRSS without Upper Basin Drought Response 

Operations (values in percent)

Event or System Condition 2022 2023 2024 2025 2026

Equalization Tier (Powell ≥ Equalization [EQ] Elevation) 0 0 0 6 6

Equalization – annual release > 8.23 maf 0 0 0 6 6

Equalization – annual release = 8.23 maf 0 0 0 0 0

Upper Elevation Balancing Tier (Powell < EQ Elevation and ≥ 3,575 ft) 0 19 25 28 25

Upper Elevation Balancing – annual release > 8.23 maf 0 19 25 28 22

Upper Elevation Balancing – annual release = 8.23 maf 0 0 0 0 3

Upper Elevation Balancing – annual release < 8.23 maf 0 0 0 0 0

Mid-Elevation Release Tier (Powell < 3,575 and ≥ 3,525 ft) 100 34 44 31 34

Mid-Elevation Release – annual release = 8.23 maf 0 0 0 6 9

Mid-Elevation Release – annual release = 7.48 maf 100 34 44 25 25

Lower Elevation Balancing Tier (Powell < 3,525 ft) 0 47 31 34 34

Notes: 

1 Modeled operations include the 2007 Interim Guidelines, Lower Basin Drought Contingency Plan, and Minute 323, including the Binational Water Scarcity Contingency Plan. 2 Reservoir initial conditions on December 31, 2021 were simulated using the August 2021 Most Probable 24 Month Study. 3 Stress Test Hydrology uses 32 hydrologic inflow sequences that resamples the observed natural flow record from 1988-2019 for 32 traces analyzed.. 4 Percentages shown in this table may not be representative of the full range of future possibilities that could occur with different modeling assumptions. 5 Percentages shown may not sum to 100% due to rounding to the nearestpercent.
Lower Basin – Lake Mead

Percent of Traces with Event or System Condition 

Results from Corrected August 2021 CRSS without Upper Basin Drought Response Operations (values in percent)

Event or System Condition 2022 2023 2024 2025 2026

Surplus Condition – any amount (Mead ≥ 1,145 ft) 0 0 0 0 0

Surplus – Flood Control 0 0 0 0 0

Normal or ICS Surplus Condition (Mead < 1,145 and > 1,075 ft) 0 6 3 0 9

Recovery of DCP ICS / Mexico’s Water Savings (Mead >/≥ 1,110 ft) 0 0 0 0 0

DCP Contribution / Mexico’s Water Savings (Mead ≤ 1,090 and > 1,075 ft) 0 6 3 0 3

Shortage Condition – any amount (Mead ≤ 1,075 ft) 100 94 97 100 91

Shortage / Reduction – 1st level (Mead ≤ 1,075 and ≥ 1,050) 100 78 28 25 16

DCP Contribution / Mexico’s Water Savings (Mead ≤ 1,075 and > 1,050 ft) 100 78 28 25 16

Shortage / Reduction – 2nd level (Mead < 1,050 and ≥ 1,025) 0 16 63 34 34

DCP Contribution / Mexico’s Water Savings (Mead ≤ 1,050 and > 1,045 ft) 0 13 3 3 13

DCP Contribution / Mexico’s Water Savings (Mead ≤ 1,045 and > 1,040 ft) 0 3 13 9 0

DCP Contribution / Mexico’s Water Savings (Mead ≤ 1,040 and > 1,035 ft) 0 0 9 6 0

DCP Contribution / Mexico’s Water Savings (Mead ≤ 1,035 and > 1,030 ft) 0 0 25 13 9

DCP Contribution / Mexico’s Water Savings (Mead ≤ 1,030 and ≥/> 1,025 ft) 0 0 13 3 13

Shortage / Reduction – 3rd level (Mead < 1,025) 0 0 6 41 41

DCP Contribution / Mexico’s Water Savings (Mead </≤ 1,025 ft) 0 0 6 41 41

Notes: 

1 Modeled operations include the 2007 Interim Guidelines, Lower Basin Drought Contingency Plan, and Minute 323, including the Binational Water Scarcity Contingency Plan. 2 Reservoir initial conditions on December 31, 2021 were simulated using the August 2021 Most Probable 24 Month Study. 3 Stress Test Hydrology uses 32 hydrologic inflow sequences that resamples the observed natural flow record from 1988-2019 for 32 traces analyzed.. 4 Percentages shown in this table may not be representative of the full range of future possibilities that could occur with different modeling assumptions. 5 Percentages shown may not sum to 100% due to rounding to the nearestpercent.
Website Updates

9

https://www.usbr.gov/lc/region/g4000/riverops/coriver-projections.html
CRMMS visualization tool demo

10

https://www.usbr.gov/lc/region/g4000/riverops/cr

mms-2year-projections.html
Questions

Alan Butler – rabutler@usbr.gov

5-year projections available at:

https://www.usbr.gov/lc/region/g4000/riverops/crss-5yearprojections.html