Researchers: Drought-Busting Spring Rains May Slow in Future

In early 2015, the Colorado River Basin was facing an unprecedented water shortage. Across the Mountain West, crops were withering, reservoir levels were falling, and state and local governments were imposing historic curtailments.

Come that spring, however, a nearly threefold increase in the basin’s average precipitation sent the river system from severe shortage to extreme flooding in a matter of weeks. Water managers rejoiced at the “Miracle May.”

Drought-busting “miracles” have been fodder for newspaper headlines for more than a century — popping up every decade or so when remarkably rainy springs bring an end to dismally dry winters. But these springtime deluges may become less frequent and less substantial under climate warming.

“There’s that old saying, ‘miracles never cease,’ but of course, these were always rare events, which is why they were called 'miracles' to begin with,” said Utah State University climate scientist Binod Pokharel, the lead author of a new paper on unforeseen, extreme springtime precipitation capable of abruptly ending prolonged droughts. “In the future, it seems these events might become fewer and farther between.”

To come to that conclusion, the researchers had to come up with a definition for something that is, by its nature, very hard to define.

“I’ve spent a good part of my life quantifying different sorts of clouds, which are obviously very nebulous things,” Pokharel said. “But miracles? That’s really hard."

The authors noted that a lot of folk wisdom has gone into identifying these sorts of events.

"You might say, ‘Well, the crops were dying and then there was a lot of rain, and that felt like a miracle.’ And of course, you’d be right," said study co-author Matthew LaPlante, an associate professor of journalism at Utah State and climate researcher whose related and recently published work focused on rare and unexpected extreme snowpacks in the Colorado River Basin. "But if we want to test whether there could be more or fewer of these extreme dry-to-wet transitions in the future, there need to be specific criteria.”

To develop those parameters, the study employed a co-production approach, bringing together scientists with water managers from around the West, several of whom went on to become co-authors of the study published in the Journal of the American Water Resources Association. Together, the scientists and local experts defined a “miracle” winter-to-spring transition as one in which four consecutive anomalously dry months are followed by at least three consecutive anomalously wet months. Those conditions were met in about 10 percent of all years since 1960.

Using multiple climate projection models, the researchers found the “miracle” events would be less likely under continued warming, but none of the scenarios suggested that the Colorado River Basin wouldn’t still benefit from the occasional drought-ending deluge through the end of the century.

James Eklund, a co-author of the paper and water policy adviser to state and local governments across the West, said these findings can help avoid “missed opportunities for water storage, groundwater recharge and maintenance of ecological flows."

In many cases around the globe, climate scientists have identified oceanic and atmospheric arrangements of temperature and pressure that often precede specific weather patterns by many months, allowing for stronger meteorological predictions.

The researchers in this study did not identify ways to better predict individual “miracle” springs. But Jake Serago, a co-author and water resource engineer for the Utah Department of Natural Resources, said great benefits can be derived from an improved assessment of the past and potential future periodicity of extreme dry-to-wet transitions.

"Understanding the likelihood of such 'miracle' events is important for the hydrologic prediction and water planning we do," Serago said.

Kripa Akila Jagannathan, a co-author and climate adaptation researcher at Lawrence Berkeley National Laboratory who specializes in co-production projects, said it was gratifying to see climate science made relevant and actionable for decision-makers.

"The insights and experiences shared by the water managers were instrumental in shaping our study, right from generating the research question,” Jagannathan said. “The iterative process helped to ensure that these findings resonate with real-world water management challenges."