5-Year Probabilistic Projections

A minor error was found in the originally published August 2021 5-year probabilistic projections (published on September 22, 2021). The error resulted in Lake Powell and Lake Mead initial conditions of approximately 0.5 ft lower than intended. The impact of this error had an insignificant effect on these 5-year probabilistic projections. The results below were updated to fix the error on October 13, 2021.

Overview

Five-year projections of future conditions, currently through 2026, in the Colorado River system are typically updated every January, April, and August, while probabilistic results for the 2-year period are updated every month. The “mid- to long-term projections” are generated using a combination of Colorado River Mid-term Modeling System (CRMMS) for the current year’s projections along with the Colorado River Simulation System (CRSS) for projections in year 2 and beyond.

The most recent 5-year projections of future Colorado River system conditions were produced in August of 2021 using the following assumptions:

  • Initial Conditions: CRSS was initialized in January 2022 with the August 2021 Most Probable 24-Month Study projected end-of-year reservoir conditions.
  • Hydrology: Index sequential method applied to the 1988-2019 historical record, i.e., Stress Test hydrology; a total of 32 future projections (i.e., traces) in the Stress Test hydrology (1 set of initial conditions from the August 2021 24-Month Study x 32 hydrologic inflow sequences).
  • Water Demand: Upper Basin demands per the 2016 UCRC depletion demand schedules; Lower Basin demands developed in coordination with the Lower Basin States and Mexico.
  • Policy: 2007 Interim Guidelines, Lower Basin Drought Contingency Plan, and Minute 323 are modeled reflecting Colorado River policies.

Additional details are available in 5-Year Projections Modeling Approach. All modeling assumptions and projections are subject to varying degrees of uncertainty. Please refer to this discussion of uncertainty for more information.

There was an important change to the modeling assumptions for the Upper Basin Drought Response Operations between the June 2021 and August 2021 projections. The June 2021 CRSS included assumptions to approximate additional releases from Flaming Gorge, Navajo, and/or the Aspinall Unit based on the Agreement for Drought Response Operations at the Initial Units of the Colorado River Storage Project Act (UB DRO). These assumptions were removed from the August 2021 CRSS (beginning in 2022) while these operational plans are under development, and in order to provide a clearer depiction of future risks. A comparison of how this change affects system conditions is provided in the table below, while this presentation provides a more detailed analysis of the effects.

Projections

5-year probabilistic results presented in the tables below are reported as the percentage of projected Lake Powell and Lake Mead operations that fall below critically low elevations or are within each operational tier in the next five years.

Chance of Reaching Critically Low Reservoir Elevations
CRSS Projections - June 2021 (with UB DRO) vs. August 2021 (without UB DRO)
Run 2022 2023 2024 2025 2026
Lake Powell
less than
3,525 feet
June 2021 79% 30% 25% 30% 34%
August 2021 88% 53% 41% 44% 41%
Difference +8% +24% +15% +14% +6%
Lake Powell
less than
3,490 feet
June 2021 0% 5% 17% 16% 22%
August 2021 3% 34% 25% 28% 34%
Difference +3% +29% +8% +12% +13%
Lake Mead
less than
1,025 feet
June 2021 0% 17% 44% 58% 63%
August 2021 0% 25% 44% 59% 62%
Difference 0% +8% -1% +1% 0%
Lake Mead
less than
1,000 feet
June 2021 0% 0% 9% 21% 23%
August 2021 0% 0% 12% 19% 22%
Difference 0% 0% +3% -2% -2%
*All results computed as the chance of falling below the threshold in any month in the calendar (water) year for Lake Mead (Lake Powell). Projections assume 1988-2019 resampled natural flows (Stress Test Hydrology). In contrast to the June 2021 projections, August 2021 results do not assume Upper Basin Drought Response Operations beyond 2021.
Chance of Reservoir Operating Conditions
August 2021 CRSS Projections
Upper Basin - Lake Powell - Operating 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
Lower Basin - Lake Mead - Operating 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 Year 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 ft) 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 ft) 0 16 62 34 34
DCP Contribution / Mexico’s Water Savings (Mead < 1,050 and > 1,045 ft) 0 12 3 3 12
DCP Contribution / Mexico’s Water Savings (Mead <= 1,045 and > 1,040 ft) 0 3 12 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 12 9
DCP Contribution / Mexico’s Water Savings (Mead <= 1,030 and >=/> 1,025 ft) 0 0 12 3 12
Shortage / Reduction - 3rd Level (Mead < 1,025 ft) 0 0 6 41 41
DCP Contribution / Mexico’s Water Savings (Mead </<= 1,025 ft) 0 0 6 41 41
*All results computed as the percent of traces with a system condition. Projections assume 1988-2019 resampled natural flows (Stress Test Hydrology). In contrast to the June 2021 projections, these results do not assume Upper Basin Drought Response Operations beyond 2021. Percentages shown in this table may not be representative of the full range of future possibilities that could occur with different modeling assumptions.

Download Tables

Archived Tables

Previous tables (starting in January 2012) are also available. Select the date (a new window will open):

The following two figures show a combination of historical and projected reservoir elevations for Lake Powell and Lake Mead, respectively.

Projections of Lake Powell (top) and Lake Mead (bottom) end-of-December reservoir elevations. The colored region, or cloud, for the hydrology scenario represents the minimum, 10th percentile, 90th percentile, and maximum of the projected reservoir elevations. Solid lines represent historical elevations (black), and median projected elevations for the scenario (yellow). Horizontal gray lines represent important elevations for operations.


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Last updated: 2021-10-13