For millennia, communities throughout North America have adapted to the ebb and flow of waterways. Water infrastructure provides reservoirs for times of drought and flood control for instances of deluge.
Drought is a way of life in some parts of the United States, said Jeffrey Mount, a geomorphologist and senior fellow at the Public Policy Institute of California. “What you worry about is whether you’re picking up a trend.” Long-term shifts in streamflow could signal a fundamental change in climate that scientists believe the country’s infrastructure is not designed to endure.
Unfortunately, such a trend is emerging. In the first comprehensive picture of streamflow in the United States, scientists reported that streams in the South and West have gotten drier in the past 70 years. Though unsurprising to many, the result is worrisome. The finding was published in the journal Water Resources Research.
What Is a Drought, Anyway?
“There are a lot of flavors of drought,” said Adam Ward, a hydrologist at Oregon State University who was not involved in the study. Abnormally dry stream conditions, a collective phenomenon termed streamflow drought, tend to follow prolonged meteorological drought, which is brought about by a lack of precipitation; the length of the lag depends on factors such as the size of the basin.
Previous anecdotal evidence and empirical data from some locations have shown that streamflow droughts have gotten more severe in recent decades.
Stakeholders define streamflow drought differently, often considering daily, seasonal, or long-term average flows. However, a barge moving through the Mississippi River doesn’t care what time of year it is, just whether there’s enough water flowing, said John Hammond, a hydrologist at the U.S. Geological Survey (USGS) and lead author of the new streamflow study.
In the new study, researchers looked at measurements of streamflow between 1921 and 2020, gleaned from USGS gauges in 555 watersheds throughout the continental United States. To reveal any long-term changes arising solely from shifts in climate, the group focused on streams without dams or other management systems. The researchers evaluated the history of drought on these streams, considering several thresholds of fixed long-term average and variable seasonal average flows.
The data showed that in the South and West, streamflow droughts got longer between 1951 and 2020, regardless of threshold. Worse yet, droughts in these regions are becoming more intense. “[Recent] droughts have caused there to be a lot more missing water from the system,” Hammond said. Conversely, streamflow droughts generally got shorter and less intense in the East and North. These trends track with increasing aridity in the West, signaling that as the climate continues to dry out, streamflow drought will get worse.
The study’s scale “helps us see that the future of New Hampshire and the future of Sacramento can be very different,” Ward said. “It helps us understand why the EPA and Army Corps [of Engineers] have regions instead of one-size-fits-all policies.”
The results are useful for policymakers because they confirm there is a long-term trend in drought, said Mount, who was not involved in the study. “The larger question is, Can you use this to help you develop policy that adapts to these changing conditions?”
Day of Reckoning
The study highlighted that understanding big-picture climate trends is critical in developing sound water policy. “The implications of changing streamflow are going to cascade through some of our regulations in a way that we don’t necessarily appreciate,” Ward said.
The changing climate is forcing the United States to evaluate the scope and efficacy of the Clean Water Act, for instance. Legal battles (including an upcoming Supreme Court decision) have been fought over the act for 50 years, with some policymakers questioning “what does it mean to count as a waterway?” Ward said.
“When you get more frequent droughts, waterways go from flowing all the time to flowing sometimes,” Ward explained, a situation leading to a gray area regarding what features should be protected by a law written to address rivers—in other words, what counts as a waterway. Ultimately, Ward said, “none of our regulations are set up for changing streamflows.”
“I think we’re here on a day of reckoning on drought policy,” Mount said. Many of the country’s major water infrastructure projects and policies date back a century, when the climate and streamflow were very different.
“All of that needs a rethink,” Mount said.
—Jennifer Schmidt (@DrJenGEO), Science Writer