Colorado River Mid-term Modeling System (CRMMS)

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The Colorado River Mid-term Modeling System (CRMMS) is one of two river system models developed and maintained by Reclamation to support operational decision-making and planning for the Colorado River Basin. Both models are implemented in the RiverWare modeling platform and share some key features. RiverWare is an object-oriented modeling platform in which the objects may be inflow points, river reaches, diversions, reservoirs, canals, pipelines, and other water-resource features.

CRMMS was created in 2021 from the integration of two existing models: the 24-Month Study Model (24MS) and the Mid-term Probabilistic Operations Model (MTOM)--itself an offshoot of 24MS.

CRMMS is run in two modes, which respectively map onto the functions of the former 24MS and MTOM models:

  • 24-Month Study Mode is used primarily for short-term (1-2 years) projections for operational decision-making, and is run in a deterministic mode, meaning that a single inflow scenario is used to generate a single output trace for analysis. The model is run every month to provide operational updates as inflow hydrology forecasts and demand projections evolve. The output of the primary run is packaged and distributed as the 24-Month Study Reports.
  • Ensemble Mode is used primarily for mid-term (1-5 years) system projections for risk-based operational planning and analysis. CRMMS is also run in Ensemble Mode to create 2-year probabilistic projections that supplement the deterministic projections used for the 24-Month Study Reports.


The CRMMS deterministic projections, in 24-Month Study Mode, of Lake Powell and Lake Mead levels for the coming water year determine the tier for the Annual Operating Plan and thus also the annual release volume from Lake Powell, and any reductions in delivery to the from Lake Mead to the Lower Basin states and Mexico. This fact alone means that Lower Basin stakeholders closely follow the 24-Month Study Reports, as do many Upper Basin stakeholders as well.

The envelope of the CRMMS Probabilistic Mode projections of Lake Powell and Lake Mead levels over the 5-year mid-term period provides guidance to Reclamation and basin stakeholders regarding the likelihood of undesirable system outcomes over that period (e.g., Lower Basin delivery reductions, reservoir levels falling below the power pool). This information can help water managers and users prepare for those outcomes both collectively, and individually, in accordance with their own risk tolerances.

Model structure and operation

Figure 1. While this shows the former Mid-term Probabilistic Operations Model (MTOM) in object view, the current CRMMS model structure and objects are essentially identical, including the 12 reservoirs in the bullet list at left, which are shown with the purple triangles. (Source: Reclamation)

CRMMS takes a monthly inflow sequence (details below) and runs it through prescribed operational rules at 12 Reclamation reservoirs to project the monthly conditions at those reservoirs over the next 24 months. A set of inflow points (16 in 24-Month Study Mode; 19 in Ensemble Mode) are used to represent both inflows to those reservoirs and intervening flows between reservoirs.

Upper Basin water demands are not explicitly modeled in CRMMS (with three exceptions); instead, the inflow sequences from CBRFC have the expected Upper Basin diversions and consumptive use already subtracted from the flows that would have occurred under natural conditions. The exceptions are the Gunnison Tunnel, Azotea Tunnel, and Navajo Indian Irrigation Project, which are represented as separate diversions in CRMMS. Lower Basin water demands are modeled based on monthly schedules provided by the water users.

The output from CRMMS consists of monthly projected elevations and storage, releases downstream, and hydropower generation (if applicable) for the following 12 reservoirs:

  • Fontanelle
  • Flaming Gorge
  • Taylor Park
  • Blue Mesa
  • Morrow Point
  • Crystal
  • Vallecito
  • Navajo
  • Lake Powell
  • Lake Mead
  • Lake Mohave
  • Lake Havasu

The specific inflows to CRMMS and details of processing of CRMMS output depends on the mode and what analyses the model is supporting.

To support the 24-Month Study Reports (monthly):

The inflow sequences for 24-Month Study Mode are provided by the NOAA Colorado Basin River Forecast Center (CBRFC) and are based on a blend of CBRFC's seasonal streamflow forecasts and average historical inflows, depending on the month when CRMMS is run, and the future month that is being projected.

CRMMS is run in 24-Month Study Mode monthly with a single inflow sequence, representing the most probable inflows. The most probable run in August--specifically, its projected elevations of Powell and Mead on January 1st--is used to set the tier for the Annual Operating Plan. The release from Lake Powell may be subsequently adjusted based on the results of the April run (projected elevations on September 30th).

Additionally, at least four times a year (January, April, August, October), two additional runs in 24-Month Study Mode are done, using probable minimum and probable maximum inflow sequences provided by CBRFC. These two runs, which represent 90% and 10% exceedance probabilities, help bracket the uncertainty in the main most probable run. Under the terms of the 2019 Drought Contingency Plans, whenever a probable minimum run shows Powell going below 3525' at any time in the next 24 months, both of these additional runs are done monthly, instead of four times per year, to provide basin stakeholders with additional guidance to prepare for drought impacts. Accordingly, these monthly runs have been done starting in January 2021 and continuing today (March 2022).

To support 5-year CRMMS mid-term projections and risk analyses (three times per year; January, April, August):

Each sequence begins with the basin moisture conditions as of the CRMMS run month and then is driven by the historical temperatures and precipitation for one of 30 periods sampled from the 1991-2020 observational record. This method is essentially the same as that used by CBRFC for their ensemble streamflow prediction (ESP) forecasts, made between December and July for seasonal April-July streamflows, and for the same reasons: the ensemble represents the uncertainty in the weather over the coming months, which will have a strong influence on the flows until the end of the year.

Reclamation then computes and reports the percent of these traces in which particular event or system condition occurs. These percentages represent the actual future risk only if all of the modeling assumptions are correct, so it is more reasonable to treat the numbers as conditional risk estimates, where the conditions are the key modeling assumptions.

Data and tools

24-Month Study Reports

The output of the primary monthly run of CRMMS in 24-Month Study mode (using most probable inflows) is released around the 15th of each month. These results are formatted by Reclamation as a standardized set of data tables totaling 17 pages. The Upper Colorado and Lower Colorado regional offices package this set of data tables differently; Upper Colorado adds a 9-page narrative summary for the Upper Colorado reservoirs, while Lower Colorado adds a single cover page with very a brief summary. Note that both report "flavors" contain the results for both Upper Basin and Lower Basin reservoirs.

24-Month Study Reports - as distributed by Upper Colorado Region

24-Month Study Reports - as distributed by Lower Colorado Region

Supplemental 24-Month Study runs

As noted above, two supplemental runs of CRMMS in 24-Month Study mode are performed either 4 times per year, or monthly (as is true as of March 2022), depending on the projected reservoir conditions. These runs, based on minimum probable and maximum probable inflows, bracket most of the uncertainty in future conditions.

Probable Min/Probable Max runs

CRMMS Visualization Tool

This visualization tool shows the results of the latest month's runs of CRMMS in both 24-Month Study mode and Ensemble mode, for all 12 reservoirs represented in CRMMS. From the homepage, select the latest month, then the reservoir of interest, then "Dashboard" to see modeled monthly inflow, outflow, elevation, and storage, going out 24 months from the latest month.

Colorado River System 5-Year Probabilistic Projections

The CRMMS 5-year probabilistic projections of future conditions of the Colorado River system are typically updated by Reclamation in January, April, and August. The linked page shows the results of the most recent projections in tabular form, along with a description of the modeling approach and assumptions.

Additional resources

CRMMS General Modeling Information

Under the two "CRMMS" tabs, this Reclamation page provides information about CRMSS as run in both 24-Month Study mode and Ensemble mode: model setup and assumptions, the inflow scenarios, and the reservoir operating rules. The tabs "Overview", "Hydrology", "Policy", "Uncertainty", and "Modeling Assumptions" contain information germane to both CRMMS and CRSS.

State of the Science Report

Chapter 3 of the State of the Science report, Primary Planning Tools, describes the Reclamation system models in detail, although it uses the old nomenclature of "24-Month Study Model" and "MTOM" in lieu of CRMMS.