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 long relied on resampling the historical gaged hydrology to construct scenarios of future hydrology. Starting the 1970s, and especially after 2000, much longer records of streamflow obtained from Paleohydrology have provided an expanded perspective on past--and thus potential future--hydrologic variability.

But there is mounting 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. The 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, pioneered in the 1990s, 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 hydrologic models of varying complexity. 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 actually model the entire hydrologic cycle, but because of their coarse resolution and resulting poor representation of complex topography like in the Colorado River headwaters, the hydrologic output of GCMs--soil moisture, snowpack, runoff, etc.--is less reliable than the output of dedicated, higher-resolution hydrologic models that are driven with the climate output of GCMs.)

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. The GCMs are in much less agreement regarding the direction of future precipitation change; however, a relatively large increase in precipitation (+5-10%) would be necessarily to counteract the effects of warming temperature on runoff, and such scenarios are much less prevalent than climate scenarios that result in declines in basin runoff.

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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Figure 1. CAPTION. SOURCE.)

Relevance

A better understanding of the future is critical to water planning and management in the Colorado River basin. Climate change-informed hydrology has already increased awareness that business as usual is problematic and is being used to consider what is needed to prepare for a warmer, drier future, as evidenced in SECURE Water reports and elsewhere.

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).