ABSTRACT:

Of concern to Colorado River management, as operating guidelines post-2026 are being considered, is whether water resource recovery from low flows during 2000–2020 is possible. Here we analyze new simulations from the sixth generation of the Coupled Model Intercomparison Project (CMIP6) to determine plausible climate impacts on Colorado River flows for 2026–2050 when revised guidelines would operate. Constrained by empirical estimates of Lee Ferry gauge (through which over 85% of the river flow passes) flow sensitivity to meteorological variability, effects of CMIP6 projected precipitation and temperature changes are integrated to project Lee Ferry flow. The critical importance of precipitation, hitherto largely discounted, is emphasized. Model projections indicate increased precipitation in the Upper Colorado River basin due to climate change, which alone acts to increase river flows 5%–7% (relative to a 2000–2020 climatology). Depending on the river’s temperature sensitivity, this wet signal compensates some, if not all, of the depleting effects from basin warming. Considerable precipitation variability is demonstrated, driving a much greater range of plausible Colorado River flow changes for 2026–2050 than previously surmised from treatment of temperature impacts alone: precipitation-induced Lee Ferry flow changes of -25% to +40% contrast with a -30% to -5% range from expected warming effects only. Consequently, extreme low and high flows are more likely. Lee Ferry flow projections, conditioned on an initial drought state in 2000–2020, analogous to observations, reveal substantial recovery odds for water resources, albeit with elevated risks of even further flow declines than recent .