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

Figure 1. The Colorado River Simulation System (CRSS) model schematic for the Upper Green River basin, showing Fontanelle Reservoir and Flaming Gorge Reservoir (purple triangles), 10 inflow points (gage "houses"), and several dozen diversion points. Compare with the similar figure for the 24-Month Study Model (24MS) (Source: Wheeler et al., 2019)

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. There are 12 reservoirs; 9 in the Upper Basin and 3 in the Lower Basin:

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

The only differences from 24MS and MTOM are that Vallecito Reservoir in Colorado is not modeled separately in CRSS, while CRSS includes Starvation Reservoir in Utah, which is not separately modeled in 24 MS and MTOM.

CRSS has 29 inflow points (Figure 2), compared to 16 points in 24MS and 19 points in MTOM. More important than the number of the points is the nature of the inflows themselves. In 24MS and MTOM the inflows are forecasts of partly naturalized streamflows, which means that Upper Basin water demands are already "baked" into the inflows, while Lower Basin demands are represented by fixed schedules. In CRSS, however, the inflows are sequences of fully naturalized streamflows, which allows a much broader set of water demands in both the Upper and Lower Basins to be specified, and adjusted, in CRSS independently of the inflow scenarios. The CRSS inflow sequences are developed within Reclamation using one or more sources of hydrologic information: the historical observed hydrology, paleohydrology, and climate-model-projected future hydrology.

Figure 2. Map of the Colorado River Basin showing the 29 inflow points for CRSS. (Source: Lukas and Payton, 2020)

In total, CRSS represents 115 delivery points across the basin. In the Upper Basin, these delivery points typically represent aggregations of many water users. While there is flexibility to adjust diversions and depletions at each point, in practice the Upper Basin demands in CRSS have been set according to depletion schedules provided by the Upper Colorado River Commission (UCRC). In the Lower Basin, each mainstream water user is individually represented, with their water use set according to the current policy or alternate policy being analyzed (e.g., 2007 Interim Guidelines, 2019 Drought Contingency Plans).

The output from CRSS consists of monthly projected elevations and storage, releases downstream, and hydropower generation (if applicable) for the 12 modeled reservoirs, as well as consumptive uses. Given the length of the runs (usually 20-50 years), these outputs are usually summarized in time-series plots, from which results for specific time-slices are extracted (e.g., % of runs in 2050 that show Lake Mead below 1025').

Data and Tools

Colorado River System 5-Year Projected Future Conditions

The MTOM/CRSS projections of future conditions of the Colorado River system are updated by Reclamation at least twice annually (January 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.

Colorado River System Projected Future Conditions - Alternative Future Hydrology Scenarios

This Reclamation page shows the same results of the latest MTOM/CRSS projections as the Colorado River System 5-Year Projected Future Conditions page, but in time-series form, and with an explanation of the development of the alternative future hydrology scenarios, i.e., the Stress Test hydrology.

Studies featuring long-term CRSS projections

Reclamation does not produce regular operational CRSS projections at long-term (>5-year) timescales equivalent to the twice-per-year MTOM/CRSS 5-year projections. Instead, CRSS is run at longer timescales as needed for specific studies and policy analyses. Below are links to the studies (and report volumes therein) that feature CRSS results, in reverse chronological order:

Drought Contingency Planning (2019)

Ten Tribes Tribal Water Study (2018)

Minute 323 (2017)

Minute 319 (2012)

Basin Study (2012)

Interim Guidelines EIS (2007)

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.

Water Resource Modeling of the Colorado River: Present and Future Strategies

This 2019 white paper provides a detailed overview of CRSS and its operation, and then evaluates its utility in analyzing alternative management paradigms for the Colorado River. The report was authored by Kevin Wheeler, David Rosenberg, and Jack Schmidt, and produced by the Center for Colorado River Studies at Utah State University.

MTOM/CRSS General Modeling Information

This Reclamation page summarizes the key components of the MTOM/CRSS runs: the models' setups and assumptions, the inflow scenarios, and the reservoir operations rules.

New and Notable Research (2020-present)

[<URL FOR PAPER>]

Summary