Increasing influence of air temperature on upper Colorado River streamflow

Abstract

This empirical study examines the influence of precipitation, temperature, and antecedent soil moisture on upper Colorado River basin (UCRB) water year streamflow over the past century. While cool season precipitation explains most of the variability in annual flows, temperature appears to be highly influential under certain conditions, with the role of antecedent fall soil moisture less clear. In both wet and dry years, when flow is substantially different than expected given precipitation, these factors can modulate the dominant precipitation influence on streamflow. Different combinations of temperature, precipitation, and soil moisture can result in flow deficits of similar magnitude, but recent droughts have been amplified by warmer temperatures that exacerbate the effects of relatively modest precipitation deficits. Since 1988, a marked increase in the frequency of warm years with lower flows than expected, given precipitation, suggests continued warming temperatures will be an increasingly important influence in reducing future UCRB water supplies.

Evidence that Recent Warming is Reducing Upper Colorado River Flows

“…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. Given these previous studies and continued warming in the UCRB (Udall and Overpeck 2017), two critical questions need to be answered, namely, 1) are the effects of temperature on UCRB flow now substantial enough to be discernable amid natural UCRB flow variability, and 2) can these effects be quantified at both annual and seasonal time scales?…”

“…They attribute at least one-third of the decrease in UCRB streamflow during this period to increased temperature. 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.…”

“…Results indicated that cool-season precipitation explains most of the variability in water-year streamflow; however, spring/summer temperature and, to a lesser extent, antecedent fall soil moisture appear to have substantial effects on UCRB streamflow under certain conditions. For example, the effects of recent droughts on UCRB streamflow have been amplified by increased temperatures, which exacerbated the effects of relatively modest precipitation deficits (Woodhouse et al 2016). …”

“…Recently, Woodhouse et al (2016) examined the influence of precipitation, temperature, and antecedent soil moisture on UCRB water year (October through September) streamflow over the past 100 years. Results indicated that cool-season precipitation explains most of the variability in water-year streamflow; however, spring/summer temperature and, to a lesser extent, antecedent fall soil moisture appear to have substantial effects on UCRB streamflow under certain conditions.…”

Future shift of the relative roles of precipitation and temperature in controlling annual runoff in the conterminous United States

“…Thus, sensitivity of water budget to climate change may be discrepant across time and space. Although the possible underestimation of the influence of temperature in altering regional water resources has been discussed recently (Sospedra-Alfonso et al, 2015;Woodhouse et al, 2016), a comprehensive evaluation under different climate backgrounds and land-cover compositions is still lacking.…”