Projected future hydrology
Traditionally, long-range water planning in the Colorado River Basin and elsewhere has operated under the assumption of stationarity, that the future would closely resemble the past, at least in a statistical sense (average and variability). Thus, Reclamation and other water agencies relied on resampling the historical gaged hydrology (see Streamflow) to construct scenarios of future hydrology. Starting in the 1970s, and especially after 2000, much longer records of streamflow obtained from Paleohydrology have provided an expanded perspective on past hydrologic variability beyond that available in the gaged hydrology.
But there is now clear evidence that the hydrology of the Colorado River Basin is already systematically changing, and that the future hydrology will be substantially different than the past, as a result of anthropogenic climate change. While the gaged hydrology and paleohydrology can still provide important information about hydrologic variability to inform long-range planning, they cannot capture the likely future systematic changes in hydrology in the basin.
To capture these systematic changes, researchers deploy the same global climate models (GCMs) used to project future climate conditions, usually in conjunction with basin-scale hydrologic models. The typical approach takes output from multiple GCMs, i.e., specific future changes in temperature and precipitation from each model, and then translates those climate changes into basin runoff changes using a separate hydrologic model. Before being run through the hydrologic model, the relatively coarse-grained GCM output needs to be downscaled (e.g., from a 150-km grid to a 12-km grid) to match the spatial resolution of the finer-grained hydrologic model. (GCMs can and do model the entire hydrologic cycle, but their coarse resolution, and resulting poor representation of complex topography, limits the reliability of the GCM's raw hydrologic output, e.g., soil moisture, snowpack, runoff, etc.)
As noted in Projected future climate, all GCM projections of future climate regardless of emissions scenario indicate further increases in temperatures of the basin, and this warming by itself will increase evapotranspiration (ET) and reduce basin runoff, by an estimated 4-9% per degree F of warming. Precipitation, however, is the most important driver of runoff on a year-to-year basis, and the GCMs are in much less agreement regarding the direction of future precipitation change for the basin. In some projections of future basin hydrology, there is a sufficiently large increase in annual precipitation to overcome the effect of warming temperatures, and so runoff increases. But in most (65-90%) projections of future basin hydrology driven by GCMs, across many different studies, the impact of warming combined with the precipitation outcome leads to net declines in basin runoff over the next several decades.
Figure 1 shows an ensemble of 32 simulations of changes in future Colorado River flow at Lees Ferry (i.e., Upper Basin runoff), from 32 GCM projections run under the moderate RCP4.5 emissions scenario, downscaled with the LOCA method and then run through the VIC hydrologic model. By the mid-21st century, streamflows have declined in more than two-thirds (21) of these projections, even though precipitation increases over the 21st century in most of the 32 projections. Figure 2 shows only the 16 simulations with smaller precipitation changes (up or down) over the 21st century. This reduced ensemble more clearly shows the impact of future warming on streamflow, and deemphasizes the potential for large future precipitation changes, since there is much less confidence in specific precipitation outcomes than in the overall warming.
Long-range planning for Colorado River basin water supply and other resources needs to account for likely future changes in climate and hydrology caused by anthropogenic climate change, most critically future declines in annual runoff. Accordingly, recent basinwide planning activities (e.g., 2012 Colorado River Basin Study, 2018 CRB Ten Tribes Partnership Tribal Water Study) and broader-scale Reclamation assessments (2021 SECURE Water Act reports) have analyzed scenarios of future hydrology derived from projections from global climate models (GCMs) with additional hydrologic modeling.
Data and tools
While many different datasets of projected future hydrology for the Colorado River Basin have been produced for specific studies and reports over the past decade, relatively few of them are accessible to non-expert users, either to visualize in a web portal or to download; these are listed below. Users looking for graphics and additional details of projected future hydrology for the basin may first want to look at the reports listed under Additional resources farther down on this page, and also the individual studies referenced in those reports.
A User Guide to Climate Change Portals provides descriptions of several web tools for visualizing projected future hydrology based on CMIP5 model projections, including the three listed below.
The Climate Toolbox is a diverse set of interactive tools developed by researchers at U. Cal-Merced and partners. All projected climate data are from the MACA-downscaled CMIP5 dataset. The projections of hydrologic variables in Climate Toolbox are based on a reduced ensemble (10 vs. 20) of CMIP5-MACA climate projections (temperature and precipitation) that have been run through the VIC hydrologic model. Note that with 10 members, this ensemble of hydrologic projections may not reflect the full range of potential climate (and hydrology) futures shown across all CMIP5 projections.
The Future Streamflows tool allows users to generate monthly hydrographs of modeled historical and future streamflows (in cubic feet/second) for over 140 gages in basins across the western U.S. and Canada, including 15 gages within the Colorado River Basin. This is especially useful for visualizing the changes in streamflow timing associated with future climate change.
The Future Boxplots allows users to generate boxplots of modeled historical and future area-averaged projected runoff (in inches) for user-selected points, rectangular areas, counties, and HUC8 watersheds.
The National Climate Change Viewer (NCCV) allows users to generate zoomable change maps, time series, monthly climographs, and scatterplots for multiple climate and hydrology variables, including area-averaged runoff (in inches or cm). The viewer's analysis areas are river basins/watersheds (HUC2, HUC4, and HUC8). All projected climate data are from the MACA-downscaled CMIP5 dataset (all 20 models); the hydrology variables such as runoff are derived from a simple water-balance model driven with the projected temperature and precipitation.
This portal allows downloading of the CMIP5-LOCA-VIC projected hydrology dataset, from which Figures 1 and 2 are derived, in netCDF or ASCII (text) format. (There are no visualization tools on the site.) The tab "Projections: Complete Archive" allows users to download the entire dataset for all of CONUS; most users will want to select the "Projections: Subset Request" tab and step through the tabs to select the time period, area of interest, dataset and models, and data format.
State of the Science Report
Chapter 11 of the State of the Science report details the projected future hydrology for the basin, in section 11.7, following section 11.6, which covers the projected future climate (temperature and precipitation changes). Chapter 11 also described the methods and data used in these projections: Global climate models (GCMs; 11.2); CMIP (11.3); Emissions scenarios (11.4); Downscaling (11.5). Chapter 6 describes in detail the hydrologic models such as VIC that are used to translate the climate model output into hydrologic changes.
Reclamation develops these reports every 5 years under the SECURE Water Act to analyze projected risks to water supplies in the West using the best available science and highlight collaborative efforts to mitigate those risks.
The West-Wide Assessment report provides detailed estimates of changes in temperature, precipitation, snowpack, and streamflow across the West using the CMIP5-LOCA-VIC projections, as well as additional climate-risk analyses. The projected future climate and hydrology of the Colorado River Basin is addressed in section 3.2.