24-Month Study Model (24MS)

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Overview

The 24-Month Study Model (24MS) is one of three river system models developed and maintained by Reclamation to support operational decision-making and planning for the Colorado River Basin. All three models are implemented in the RiverWare modeling platform and share some key features. The Mid-term Probabilistic Operations Model (MTOM) was developed directly from 24MS so the two models have very similar structures and operations. 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.

24MS is used for short-term (1-2 years) projections for operational decision-making, and is typically 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 24MS run is packaged and distributed as the 24-Month Study Reports.

Relevance

The 24MS projections 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 avidly follow the 24-Month Study Reports, as do many Upper Basin stakeholders as well.

Model Structure and Operation

Figure 1. While this shows the Mid-term Probabilistic Operations Model (MTOM) in object view, MTOM's model structure and objects are very similar to those of the 24-Month Study Model (24MS), including the 12 reservoirs in the bullet list at left, which are shown with the purple triangles. (Source: Reclamation)

Basically, 24MS takes a projected inflow sequence 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 total of 16 inflow points are used to represent both inflows to those reservoirs and intervening flows between reservoirs.

The inflow sequences 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 (1) the month when 24MS is run, and (2) the future month that is projected.

Upper Basin water demands are not explicitly modeled in 24MS (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 24MS. Lower Basin water demands are modeled based on monthly schedules provided by the water users.

The output from 24MS 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

As suggested above, 24MS is run 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, four times a year (January, April, August, October), two additional 24MS runs 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 (as happened in January 2021), both of these additional 24MS runs are done monthly instead of four times per year to provide basin stakeholders with additional guidance to prepare for drought impacts.

Data and Tools

24-Month Study Reports

The output of the primary monthly run of the 24-Month Study Model (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 24MS are performed either 4 times per year, or monthly, depending on projected reservoir conditions. These runs, based on probable minimum and probable maximum inflows, bracket the uncertainty in the primary run.

Probable Min/Probable Max runs


Additional Resources

State of the Science Report

Chapter 3 of the State of the Science report, Primary Planning Tools, describes the three primary Reclamation system models in detail.