Projected future hydrology

Overview

Traditionally, long-range water planning in the Colorado River Basin and elsewhere has operated under the assumption (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 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.

The same primary tools--global climate models (GCMs)--that are used to diagnose ongoing climate changes and project future climate conditions, are also used to project future hydrology, typically in conjunction with basin-scale hydrologic models. The typical approach uses output from multiple GCMs--specific future changes in temperature and precipitation from each model--and then translates those climate changes into runoff changes using a 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 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 has and will further increase evapotranspiration (ET) and reduce basin runoff, by about 4-9% per degree F of warming. Precipitation 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. However, a relatively large long-term increase in precipitation (+5-10%) would be necessarily to counteract the effects of warming temperature on runoff, and across the GCMs, projections with large increases in precipitation are less prevalent than projections with other precipitation outcomes that result in net declines in basin runoff over the next several decades.

Using 32 GCM generated as part of CMIP5, downscaled with LOCA techniques and simulated using VIC, results in future projected streamflow at Lees Ferry which <can fill in details according to which figure we use - I like the State of the Science, but if we want to diversify, we could, thoughts?>

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Relevance

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.

Additional resources

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).