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Study: Drought-breaking rains more rare, erratic in US West

by Mary Sewell

BILLINGS, Mont. — Rainstorms grew more erratic, and droughts much longer across most of the U.S. West over the past half-century as climate change warmed the planet, according to a comprehensive government study released Tuesday that concludes the situation is worsening. The most dramatic changes were recorded in the desert Southwest. The average dry period between rainstorms grew from about 30 days in the 1970s to 45 days between storms now, said Joel Biederman, a research hydrologist with the U.S. Department of Agriculture Southwest Watershed Research Center in Tucson, Arizona.

The consequences of the intense dry periods that pummeled areas of the West in recent years were severe — more intense and dangerous wildfires, parched croplands, and not enough vegetation to support livestock and wildlife. And the problem appears to be accelerating, with rainstorms becoming increasingly unpredictable and more areas showing longer intervals between storms since the turn of the century compared to prior decades, the study concludes.

The study comes with almost two-thirds of the contiguous U.S. beset by abnormally dry conditions. Warm temperatures forecast for the next several months could make it the worst spring drought in nearly a decade, affecting roughly 74 million people across the U.S., the National Oceanic and Atmospheric Administration said.

Water use cutbacks, damaged wheat crops, more fires, and lower reservoirs in California and the Southwest are possible, weather service and agriculture officials have warned. Climate scientists call what’s happening in the West a continuation of a “megadrought” that started in 1999.

While previous research documented a decline in total rainfall for much of the West, the work by Biederman and colleagues put more focus on when that rain occurs. That has significant implications for how much water is available for agriculture and plants such as grasses with shallow roots and need a steadier supply of moisture than large trees.

“Once the growing season starts, the total amount of rainfall is important. But if it comes in just a few large storms, with really long dry periods in between, that can have really detrimental consequences,” study co-author Biederman said in an interview. The total amount of rain in a year doesn’t matter to plants — especially if rains come mostly in heavy bursts with a significant run-off — but consistent moisture is what keeps them alive, said UCLA meteorologist Daniel Swain, who writes a weather blog about the West and was not part of the study.

The new findings were published in the journal Geophysical Research Letters. Researchers led by University of Arizona climate scientist Fangyue Zhang compiled daily readings going back to 1976 from 337 weather stations across the western U.S. and analyzed rainfall and drought data to identify the changing patterns. Other parts of the region that saw longer and more variable droughts included the southwest Rocky Mountains, the Colorado Plateau, and the Central Plains.

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