The twenty‐first century Colorado River hot drought and implications for the future

Abstract

Between 2000 and 2014, annual Colorado River flows averaged 19% below the 1906–1999 average, the worst 15‐year drought on record. At least one‐sixth to one‐half (average at one‐third) of this loss is due to unprecedented temperatures (0.9°C above the 1906–1999 average), confirming model‐based analysis that continued warming will likely further reduce flows. Whereas it is virtually certain that warming will continue with additional emissions of greenhouse gases to the atmosphere, there has been no observed trend toward greater precipitation in the Colorado Basin, nor are climate models in agreement that there should be a trend. Moreover, there is a significant risk of decadal and multidecadal drought in the coming century, indicating that any increase in mean precipitation will likely be offset during periods of prolonged drought. Recently published estimates of Colorado River flow sensitivity to temperature combined with a large number of recent climate model‐based temperature projections indicate that continued business‐as‐usual warming will drive temperature‐induced declines in river flow, conservatively −20% by midcentury and −35% by end‐century, with support for losses exceeding −30% at midcentury and −55% at end‐century. Precipitation increases may moderate these declines somewhat, but to date no such increases are evident and there is no model agreement on future precipitation changes. These results, combined with the increasing likelihood of prolonged drought in the river basin, suggest that future climate change impacts on the Colorado River flows will be much more serious than currently assumed, especially if substantial reductions in greenhouse gas emissions do not occur.

Greater Temperature and Precipitation Extremes Intensify Western U.S. Droughts, Wildfire Severity, and Sierra Nevada Tree Mortality

“…5b, c). The most severe instances were seen in the Sierra Nevada and Wasatch Range, highlighting the impact of sustained severe drought in watersheds (Cayan & Dettinger 2001, Udall & Overpeck 2017 (Figs. 3, 5b).…”

Evidence that Recent Warming is Reducing Upper Colorado River Flows

“…Woodhouse et al (2016) and Udall and Overpeck (2017) are the first studies to show a negative effect of recent warming on UCRB streamflow in the instrumental record. Although climate model projections suggest a decrease in UCRB streamflow in part because of increases in temperature (Christensen et al 2004;Christensen and Lettenmaier 2007;Hoerling and Eischeid 2007;McCabe and Wolock 2007;Vano et al 2014;Ficklin et al 2013;Kopytkovskiy et al 2015;Udall and Overpeck 2017), there has been only sparse research that has qualitatively or quantitatively documented a negative effect of increases in temperature on UCRB streamflow in the instrumental record. Woodhouse et al (2016) and Udall and Overpeck (2017) are the only studies to date that have addressed this issue, and they suggest that the effects of projected warming are becoming detectable.…”

“…Bureau of Reclamation 2012;Udall and Overpeck 2017). A number of studies have examined the effects of projected future warming on streamflow in the Colorado River basin (Christensen et al 2004;Christensen and Lettenmaier 2007;Hoerling and Eischeid 2007;McCabe and Wolock 2007;Vano et al 2014;Ficklin et al 2013;Kopytkovskiy et al 2015;Foster et al 2016;Udall and Overpeck 2017).…”

“…In another recent study, Udall and Overpeck (2017) examined the effect of temperatures on UCRB streamflow during the instrumental period and based on future projections from climate models. Udall and Overpeck point out that between 2000 and 2014, annual Colorado River streamflow averaged 19% below the 1906-99 mean, the worst 15-yr drought on record.…”

Cascading impacts of climate change on southwestern US cropland agriculture

“…Recent declines are driven by rising temperatures which reduce snow water equivalent, even when there are increases in precipitation (Mote et al 2005). On average, 2000-2014 Colorado River annual flows were 19% below the long-term 1906-1999 mean (Udall and Overpeck 2017). Although precipitation changes explain much of the interannual variability in streamflow, temperature is an important driving variable impacting Upper Colorado streamflow; temperature alone explained a mean streamflow departure of − 7% over the last 30 years (McCabe et al 2017).…”