Critical Effects of Precipitation on Future Colorado River Flow

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. We constrain projected flows for Lee Ferry, the gauge through which 85% of the river flow passes, using its estimated sensitivity to meteorological variability together with CMIP6 projected precipitation and temperature changes. The critical importance of precipitation, especially its natural variability, is emphasized. Model projections indicate increased precipitation in the Upper Colorado River basin due to climate change, which alone increases 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 internal decadal precipitation variability (~5% of the climatological mean) is demonstrated, driving a greater range of plausible Colorado River flow changes for 2026–2050 than previously surmised from treatment of temperature impacts alone: the overall precipitation-induced Lee Ferry flow changes span −25% to +40% contrasting 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 initial drought states akin to 2000–2020, reveal substantial recovery odds for water resources, albeit with elevated risks of even further flow declines than in recent decades.