Colorado River Basin 

April 2021 5-year Projected 

Future Conditions

Alan Butler

Glen Canyon Dam Adaptive Management Program Technical Work Group Meeting

June 16, 2021
Colorado River Mid-term Modeling System (CRMMS)

24-Month Study Mode CRSS

(Manual Mode)

Ensemble Mode

(Rule-based Mode)

Primary Use

AOP tier determinations and 

projections of current

conditions

Risk-based operational planning 

and analysis 

Long-term planning, comparison 

alternatives

of 

Simulated Reservoir

Operations Operations input manually Rule-driven operations

Probabilistic or 

Deterministic

Deterministic – single 

hydrologic trace

Deterministic OR 

Probabilistic 35 (or more) 

hydrologic traces

Probabilistic – 100+ traces

Time Horizon (years) 1 - 2 1 - 5 1 - 50

Upper Basin Inflow Unregulated forecast, 1 trace Unregulated ESP forecast, 35 traces Natural flow; historical, paleo, 

climate change hydrology

or 

Upper Basin Demands Implicit, in unregulated inflow forecast Explicit, 2016 UCRC assumptions

Lower Basin Demands Official approved or operational Developed with LB users

Reclamation Operational Modeling

Model Comparison

2
Approach for Official April 2021 Probabilistic Modeling

1. Use the April Colorado River Mid-term Modeling System 

(CRMMS)

– Ensemble mode using 35 future inflow sequences to project 2021 operations

– 2021 hydrology per the Colorado Basin River Forecast Center’s April official 

forecast consisting of 35 future inflow sequences

2. Initialize CRSS with all 35 CRMMS MTOM mode projections 

of December 2021 reservoir conditions, 2022 Lake Powell 

operating tier, and 2022 Lake Mead operating condition

3. Use CRSS to project 2022-2026 conditions using 114 

hydrologic inflow sequences from the observed natural flow 

record (1906-2019), i.e., “Full Hydrology”, and 32 hydrologic 

inflow sequences from the observed natural flow record 

(1988-2019), i.e., “Stress Test Hydrology”

– Full Hydrology: 35 initial conditions x 114 hydrologic inflow 

sequences = 3,990 total simulations in CRSS

– Stress Test Hydrology: 35 initial conditions x 32 hydrologic 

inflow sequences = 1,120 total simulations in CRSS

4. Compute probabilities across all future traces

Lees Ferry Natural Flow
Important Caveats

• April 2021 projections use the April 2 forecast from the Colorado River 

Basin Forecast Center

• 2021 forecast has declined since the April forecast

• Most probable April – July inflow declined by 1.4 maf between April 2 and June 3. 

• Projections are sensitive to initial conditions that are based on the 2021 

forecast
April 2021

Full Hydrology vs. Stress Test Hydrology
Upper Basin – Lake Powell

Percent of Traces with Event or System Condition 

Results from April 2021 CRMMS MTOM Mode/CRSS using the Full Hydrology and Stress Test Hydrology (values in percent)

Event or System Condition 2021 2022 2023 2024 2025 2021 2022 2023 2024 2025

Equalization Tier (Powell ≥ Equalization [EQ] Elevation) 0 0 6 12 17 0 0 0 3 7

Equalization – annual release > 8.23 maf 0 0 6 12 17 0 0 0 3 7

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

Upper Elevation Balancing Tier (Powell < EQ Elevation and ≥ 3,575 ft) 100 3 36 49 50 100 3 31 41 39

Upper Elevation Balancing – annual release > 8.23 maf 0 2 35 45 44 0 2 30 39 36

Upper Elevation Balancing – annual release = 8.23 maf 100 <1 1 4 5 100 <1 <1 2 3

Upper Elevation Balancing – annual release < 8.23 maf 0 0 0 <1 0 0 0 0 <1 0

Mid-Elevation Release Tier (Powell < 3,575 and ≥ 3,525 ft) 0 91 51 31 23 0 91 65 45 35

Mid-Elevation Release – annual release = 8.23 maf 0 0 0 <1 2 0 0 0 0 5

Mid-Elevation Release – annual release = 7.48 maf 0 91 51 30 21 0 91 65 45 30

Lower Elevation Balancing Tier (Powell < 3,525 ft) 0 6 7 8 10 0 6 4 11 18

Below Minimum Power Pool (Powell < 3,490 ft) 0 0 1 4 6 0 0 <1 9 12

Notes: 

1 Modeled operations include the 2007 Interim Guidelines, Upper Basin Drought Response Operations, Lower Basin Drought Contingency Plan, and Minute 323, including the Binational Water Scarcity Contingency Plan. 2 Reservoir initial conditions on March 31, 2021 were simulated using the April 2021 MTOM based on the CBRFC unregulated inflow forecast ensemble dated April 2, 2021. 3 Each of the 35 initial conditions from MTOM were coupled with 114 hydrologic inflow sequences from the Full Hydrology that resamples the observed natural flow record from 1906-2019 for a total of 3,990 

traces analyzed and with 32 hydrologic inflow sequences from the Stress Test Hydrology that resamples the observed natural flow record from 1988-2019 for a total of 1,120 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.
Event or System Condition 2021 2022 2023 2024 2025 2021 2022 2023 2024 2025

Lower Basin – Lake Mead

Percent of Traces with Event or System Condition 

Results from April 2021 CRMMS MTOM Mode/CRSS using the Full Hydrology and Stress Test Hydrology (values in percent)

Surplus Condition – any amount (Mead ≥ 1,145 ft) 0 0 0 1 4 0 0 0 0 0

Surplus – Flood Control 0 0 0 0 <1 0 0 0 0 0

Normal or ICS Surplus Condition (Mead < 1,145 and > 1,075 ft) 100 3 6 17 19 100 3 8 9 6

Recovery of DCP ICS / Mexico’s Water Savings (Mead >/≥ 1,110 ft) 0 0 0 4 9 0 0 0 0 <1

DCP Contribution / Mexico’s Water Savings (Mead ≤ 1,090 and > 1,075 ft) 100 3 5 11 10 100 3 7 9 3

Shortage Condition – any amount (Mead ≤ 1,075 ft) 0 97 94 82 77 0 97 92 91 94

Shortage / Reduction – 1st level (Mead ≤ 1,075 and ≥ 1,050) 0 97 81 37 34 0 97 71 31 33

DCP Contribution / Mexico’s Water Savings (Mead ≤ 1,075 and > 1,050 ft) 0 97 81 37 34 0 97 71 31 33

Shortage / Reduction – 2nd level (Mead < 1,050 and ≥ 1,025) 0 0 13 44 32 0 0 21 60 36

DCP Contribution / Mexico’s Water Savings (Mead ≤ 1,050 and > 1,045 ft) 0 0 11 9 6 0 0 17 6 7

DCP Contribution / Mexico’s Water Savings (Mead ≤ 1,045 and > 1,040 ft) 0 0 2 9 6 0 0 4 11 6

DCP Contribution / Mexico’s Water Savings (Mead ≤ 1,040 and > 1,035 ft) 0 0 <1 11 8 0 0 0 16 6

DCP Contribution / Mexico’s Water Savings (Mead ≤ 1,035 and > 1,030 ft) 0 0 0 10 7 0 0 0 17 6

DCP Contribution / Mexico’s Water Savings (Mead ≤ 1,030 and ≥/> 1,025 ft) 0 0 0 5 6 0 0 0 9 10

Shortage / Reduction – 3rd level (Mead < 1,025) 0 0 0 1 11 0 0 0 <1 25

DCP Contribution / Mexico’s Water Savings (Mead </≤ 1,025 ft) 0 0 0 1 11 0 0 0 <1 25

Notes: 

1 Modeled operations include the 2007 Interim Guidelines, Upper Basin Drought Response Operations, Lower Basin Drought Contingency Plan, and Minute 323, including the Binational Water Scarcity Contingency Plan. 2 Reservoir initial conditions on March 31, 2021 were simulated using the April 2021 MTOM based on the CBRFC unregulated inflow forecast ensemble dated April 2, 2021. 3 Each of the 35 initial conditions from MTOM were coupled with 114 hydrologic inflow sequences from the Full Hydrology that resamples the observed natural flow record from 1906-2019 for a total of 3,990 

traces analyzed and with 32 hydrologic inflow sequences from the Stress Test Hydrology that resamples the observed natural flow record from 1988-2019 for a total of 1,120 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.
Questions

Alan Butler – rabutler@usbr.gov

5-year projections available at:

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

https://www.usbr.gov/lc/region/g4000/riverops/crss-alt-hydrology.html