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Decades of climate data reveal longer droughts, erratic rainfall in Southwest

With climate change, scientists expect the Southwest U.S. to become hotter and drier over the next century. And according to research published in the journal Geophysical Research Letters on Tuesday, that trend has been well underway for the last half-century.

Researchers from the U.S. Department of Agriculture’s Agricultural Research Service and the University of Arizona analyzed daily meteorological observations recorded from 1976 to 2019. Compiled by the Global Historical Climatology Network, the data spanned 337 long-term weather stations distributed across the American West.

According to the analysis, temperatures in the Southwest U.S. rose an average 0.36 degrees Fahrenheit per decade over the last 45 years. For every 10 years since 1976, dry spells grew by 0.6 days, with longer droughts appearing more often since 2000.

“The greatest changes in drought length have taken place in the desert Southwest. The average dry period between storms in the 1970s was about 30 days; now that has grown to 45 days,” explained Joel Biederman, a research hydrologist with the ARS Southwest Watershed Research Center in Tucson, Arizona in a statement.

Not only is the region getting hotter and drier, the American Southwest is also experiencing longer dry intervals in between ever varying precipitation.

Impacts vary among the Desert Southwest, the Southwest Rockies/Colorado Plateau, and the Central Plains, with southern latitudes experiencing longer dry spells than northern latitudes. Meanwhile, annual rainfall increased throughout Washington state, Oregon and Idaho as well as the northern plains of Montana and Wyoming.

Biederman said “these changes support what models have predicted as a consequence of climate change: a northward shift in the mid-latitude jet stream, which brings moisture from the Pacific Ocean to the western United States.”

Longer droughts are likely to shorten grazing opportunities for cattle and put regions at risk for longer wildfire seasons. Longer dry spells additionally influence ecosystem carbon cycles and may limit water availability for mankind.

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“Consistency of rainfall, or the lack of it, is often more important than the total amount of rain when it comes to forage continuing to grow for livestock and wildlife, for dryland farmers to produce crops, and for the mitigation of wildfire risks,” Biederman said.

The paper concludes with a call for future research to focus on water availability.

“Future research aimed at better understanding and projecting the impacts of water resource changes across these various aspects of socioecological drought is urgently needed to mitigate future dryland degradation and loss of the essential ecosystem services they provide,” the researchers warn. “Especially in light of evidence of an emerging North American megadrought.”

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Joel Biederman/Agricultural Research Service

The time between rainfalls has become longer and the rains occurred more erratically in the western U.S. during the last 50 years.