Hotter conditions have played a much greater role in reducing flow during the ongoing Millennium Drought than in a mid-20th century drought
By Sarah Stanley
The Colorado River flows through seven U.S. states and northern Mexico, before discharging into the Gulf of California. Along the way, it provides drinking water to millions of people and irrigates thousands of square kilometers of cropland. However, although annual precipitation in the region increased by about 1% in the past century, the volume of water flowing down the river has dropped by over 15%.
New research by Xiao et al. examines the causes behind this 100-year decline in natural flow, teasing out the relative contributions of rising temperatures and changes in precipitation. This work builds on a 2017 paper showing that rising temperatures played a significant role in reduced flows during the Millennium Drought between 2000 and 2014.
Rising temperatures can lower flow by increasing the amount of water lost to evaporation from soil and surface water, boosting the amount of water used by plants, lengthening the growing season, and shrinking snowpacks that contribute to flow via meltwater.
To investigate the impact of rising temperatures on Colorado River flow over the past century, the authors of the new paper employed the Variable Infiltration Capacity (VIC) hydrologic model. The VIC model enabled them to simulate 100 years of flow at different locations throughout the vast network of tributaries and subbasins that make up the Colorado River system and to tease out the effects of long-term changes in precipitation and temperature throughout the entire Colorado River.
The researchers found that rising temperatures are responsible for 53% of the long-term decline in the river’s flow, with changing precipitation patterns and other factors accounting for the rest. The sizable effects of rising temperatures are largely due to increased evaporation and water uptake by plants, as well as by sublimation of snowpacks.
Additional simulations with the VIC model showed that warming drove 54% of the decline in flow seen during the Millennium Drought, which began in 2000 (and is ongoing). Flows also declined because precipitation fell on less productive (i.e., more arid) subbasins rather than on highly productive subbasins near the Continental Divide. This contrasts strongly with an earlier (1950s–1960s) drought of similar severity, which was caused almost entirely by below-normal precipitation over most of the basin.
The authors note that the situation is complex, given different long-term trends and drought response across the basin, as well as seasonal differences in temperature and precipitation. Still, the new findings support an argument from the 2017 research that as global warming progresses, the relative contribution of rising temperatures to decreased Colorado River flow will increase.