Since about 2020, North America has been in a drought that has caused water shortages, threatened crop yields, and killed wildlife. After a wet winter this year, drought conditions have improved. But that doesn’t mean that water supply woes are over.
Even after rain returns to dried-out areas, the impact of precipitation drought (also known as meteorological drought) persists in rivers for months or even years. In new research published in the Journal of Hydrology, scientists reported that the lag time between the return of regular rain and the recovery of a river to its normal conditions can be years long. Climate change is worsening drought severity, which further puts rivers at risk of prolonged recovery, the authors write.
“This is really important for understanding how meteorological drought impacts reverberate through the water cycle,” said Olivia Miller, a hydrologist at the Utah Water Science Center who was not involved in the new research.
Total precipitation is a more observable indicator of drought, but other types of drought exist. Baseflow drought, for example, refers to the depletion of groundwater that feeds streams, lowering a stream’s baseflow. Baseflow drought affects the availability of water for drinking, agriculture, and ecosystems.
Streamflow drought, which refers to abnormally dry stream conditions, has worsened in the United States since the 1950s. These droughts are now more severe and longer lasting. Emerging research shows that the same is true for baseflow droughts.
“It takes time for our groundwater reservoirs to recover because it takes time for water to infiltrate,” said Hoori Ajami, a study coauthor and groundwater hydrologist at the University of California, Riverside. “You may have precipitation back to normal, but not baseflow.”
In the new study, researchers analyzed data on precipitation, streamflow, baseflow, and more from 358 river catchments in the United States between 1982 and 2012. They found that the lag time between the end of a precipitation drought and the end of a baseflow drought was as long as nearly 3.5 years, with an average lag time of 2.8 months.
Parts of the West Coast, South, and Great Plains showed longer recovery times, which the authors attributed to rising temperatures over time. The researchers also found that baseflow droughts became more severe and prolonged over the decades analyzed because of increasing temperatures.
Water Management on a Drying Continent
River recovery lag times are increasing as climate change makes Earth’s atmosphere thirstier, said Jeffrey Mount, a geomorphologist and senior fellow at the Public Policy Institute of California. A drier, hotter atmosphere sucks up precipitation that falls onto a catchment before that water can replenish the groundwater supply.
Increasingly volatile weather, spurred by climate change, may also affect river baseflow recovery, Mount said. Instead of regular periods of rain spread consistently throughout the years, many parts of the United States are now seeing periods of heavy precipitation followed by intense dry periods. When all that water falls at once, the ground quickly saturates, and more water flows away as runoff rather than entering the groundwater and replenishing baseflow.
Baseflow recovery is a “very big deal” in the West, Mount said, because many parts of the region don’t get much rain in the summer months and rely on river baseflows to maintain water supply.
“One key message we want to send is that people must be careful about managing the water they have,” said hydrologist Sanghyun Lee, the lead author of the study, in a statement. “Because watershed boundaries often cross state or international lines, preserving precious water resources will require more cooperation.” Lee was formerly a postdoctoral scholar in Ajami’s lab and is now a postdoctoral fellow at the U.S. Department of Agriculture.
Some policies governing major waterways in the United States are struggling to keep up with increased demand for water and lower streamflows, making accurate water forecasts crucial. This year, a state auditor found that water managers in California had “significantly overestimated” the state’s water supply during the recent drought, a problem Mount said occurred in part because of a limited understanding of baseflow recovery times.
Continuing to track those lag times will help policymakers and water managers maintain sustainable water usage in the face of drought, Miller said.
—Grace van Deelen (@GVD__), Staff Writer