Declining runoff in the American Southwest as temperatures increase

In the American Southwest in recent years major river basins such as the Rio Grande have exhibited droughts and reduced runoff efficiency (the proportion of precipitation that ends up in the river). In a recent study tree ring-derived data from outside of the Upper Rio Grande basin were used to reconstruct estimates of precipitation within the Upper Rio Grande basin stretching back to 1571. This was combined with with a streamflow reconstruction within the basin over the same period to enable researchers to reconstruct runoff ratios over the past 445 years. They found that runoff ratios depended primarily on precipitation, but also on temperature. High temperatures exacerbate low runoff rations by 2.5–3 times. During those times when there was reduced snowpack in the mountains, warm temperatures reduced the runoff ratio more than when the snowpack was deep. The reconstruction also revealed that the decline in runoff ratio since 1986 is unprecedented in the historical record. The 1980s were an unusually wet period for the Upper Rio Grande, while the 2000s and 2010s have been unusually dry. Since the 1980s temperatures in the Upper Rio Grande have increased by nearly 2°F. The decline in runoff ratio between 1986 and 2015 is unprecedented in the last 445 years. Assessing recent declines in Upper Rio Grande runoff efficiency from a paleoclimate perspective, Flavio Lehner et al., Geophysical Research Letters, 11 May 2017, DOI: 10.1002/2017GL073253