Colorado River Simulation System (CRSS)

Overview

The Colorado River Simulation System (CRSS) 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. 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.

CRSS is used for long-term (5-50 years) projections for planning studies, criteria development, and risk analyses, and is run in a probabilistic mode, meaning that a large set (30-1000+) of inflow scenarios, representing plausible hydrologic futures, are used to generate an ensemble of output traces for analysis. Currently, CRSS is run twice per year to support the MTOM-CRSS 5-year projections, with additional runs performed as needed to support specific studies and analyses.

The latest version of CRSS reflects over 50 years of Reclamation computer simulations of the Colorado River system for long-term planning, dating back to 1969. The CRSS moniker was first attached to a FORTRAN model in the early 1980s, which was then implemented in RiverWare in 1996.

Relevance

Multi-decadal CRSS projections of Lake Powell and Lake Mead levels and other system outcomes under different supply, demand, and policy assumptions have been fundamental to many important basin studies and policy documents over the past few decades, including the 2007 Interim Guidelines EIS, the 2012 Basin Study, Minutes 319 (2012) and 323 (2017), the 2018 Ten Tribes Tribal Water Study, and the 2019 Drought Contingency Plans. CRSS is also key to the official 5-year system projections (MTOM-CRSS) performed twice per year.

Model Structure and Operation

CRSS has both greater complexity, especially in representing water uses, and greater flexibility than the 24-Month Study Model (24MS) and the Mid-term Probabilistic Operations Model (MTOM). Those two models allow for different supply (inflow) assumptions, but have fixed demand assumptions and reservoir operating rules. CRSS allows for different supply, demand, and reservoir-operation assumptions to be specified, thus making it useful for policy analysis.

The representation of Reclamation's reservoir system in CRSS is very similar to 24MS and MTOM.

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.

Research Directions

New and Notable Research (2020-present)

[<URL FOR PAPER>]

Summary