Colorado River System Projected Future Conditions - Alternative Future Hydrology Scenarios

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    Colorado River System Projected Future Conditions - Alternative Future Hydrology Scenarios

Control Display Icon  Overview

There has always been significant uncertainty associated with projecting future hydrology in the Colorado River Basin, as clearly illustrated by the high variability of inflow from year to year across more than a century over the historical observed record (1906 through present). Decades of Colorado River Basin research along with recent hydrologic conditions suggest that this uncertainty is likely to increase.

Because hydrologic uncertainty has a greater impact on projections of system conditions than any other modeling inputs, it is critical to explore its scope and implications. In recognition of this need, Reclamation’s Upper and Lower Colorado Regions launched a multi-faceted research and development program to enable the use of alternative methods for projecting future hydrology in Colorado River planning studies. This program, now called the Colorado River Basin Research-to-Operations (CRB-R20) Program, continues to fund research into Basin climate and hydrology along with other topics.

Methods developed to explore hydrologic uncertainty include resampling the historical record (either from the observed record or a derived record using a “proxy” such as tree-ring data), generating future inflow data that preserve key statistics of this historical record while adding a random component, and using hydrologic models to simulate runoff based on temperature and precipitation projections from general circulation models. Each of these methods was used to develop future hydrology scenarios during the Colorado River Basin Water Supply and Demand Study in its Water Supply Assessment. Reclamation is also actively exploring new methods through its CRB-R2O Program.

This page compares projections of reservoir elevations and system conditions for multiple alternative future hydrology scenarios.

Control Display Icon  Alternative Future Hydrology Scenarios

Comparing projections of the Colorado River system using different future hydrology scenarios, while keeping other modeling assumptions the same, illustrates how sensitive the system is to the assumed future hydrology and highlights how risk may change with different future hydrology assumptions.

The method used to generate future inflows in the current projections includes resampling the historical natural flow record (1906-2018) using the Index Sequential Method (ISM), referred to here as “Full” hydrology. One alternative to the Full hydrology scenario applies ISM to a shortened period of the natural flow record, 1988-2018, and is referred to as “Stress Test” hydrology. The Stress Test hydrology scenario removes the earlier portion of the natural flow record and focuses on the recent (approximately 30 years) hydrology, which has a 11% drier average flow than the Full hydrology. Use of the Stress Test scenario is supported by multiple research studies that identified a shifting temperature trend in the Colorado River Basin in the late 1980s that affected runoff efficiency and resulted in lower average flows for the same amount of precipitation (McCabe et al. 2017, Udall and Overpeck 2017, Woodhouse et al. 2016).

The historical Colorado River natural flow at the Lees Ferry gaging station in Arizona, is shown in the next figure with the Stress Test hydrology highlighted.

Colorado River Natural Flow at Lees Ferry Gaging Station, Arizona

Control Display Icon  Future Projections

The figures below compare the Colorado River system conditions projected using two different scenarios for future hydrology while keeping the other modeling assumptions the same. Results are presented as a range of future elevations and in probabilistic terms that quantify the chance of occurrences of particular events in each scenario. It is important to note that the two hydrology scenarios explored here may not be representative of the full range of future possibilities that could occur.

Lake Powell and Lake Mead Elevations

The next two figures show a combination of historical and projected reservoir elevations for Lake Powell and Lake Mead, respectively. The colored region, or cloud, for each alternative hydrology scenario represents the 10th to 90th percentile range of the projected reservoir elevations. Solid lines represent historical elevations (black), and median projected elevations for each alternative hydrology scenario (yellow, green). Dashed gray lines represent important elevations for operations, and vertical lines mark the adoption of the 2007 Interim Guidelines and 2019 Drought Contingency Plans.