Tucson and the desert Southwest are enduring one of their driest spells on record.

 Rebecca Sasnett / Arizona Daily Star

Drought conditions in the West, particularly the desert Southwest, have intensified over the past 45 years, with less precipitation and longer and more frequent dry spells between storms, a new study says.

The Southwestern deserts that include Tucson were slammed the hardest by far, the study found.

The Margo Fire burned through salt cedar in the San Pedro River burned 12 structures in Dudleyville north of Tucson on April 8, 2021. State Route 77 was closed south of the town. Video by Rick Wiley / Arizona Daily Star

Annual precipitation in our region dropped by eight times more than in the West as a whole from 1976 to 2019. The total decline of 3.2 inches in the Southwest compared to 0.4 inches regionwide.

The study also found a significant increase in the West in both the average and longest numbers of dry days in between storms, and increased annual fluctuations in the size of individual precipitation events and in the lengths of dry periods. The Southwest also experienced increased annual variability in total annual precipitation.

As covered in this study, the desert Southwest ranges from the Southern California desert on the west, across Southern Arizona’s Sonoran Desert, and into New Mexico and West Texas’ Chihuahuan Desert.

Also, the average longest annual period of dry days between storms rose 50% in the Southwest in that period. While the percentage increase in dry days was the same as that in the entire West, the Southwest’s total increase was much bigger: 17 days to 48 total, compared to a 12-day increase to 32 total across the West.

Overall, the Southwest endured a “critical combination” of reduced total precipitation, increasing year-to-year rainfall variability and longer dry spells between storms, said University of Arizona assistant professor William Smith, one of the study’s six co-authors.

The study found particularly sharp contrasts in drought-related trends between the Southwest and the Pacific Northwest. Not only did rainfall increase since 1976 in the Northwest, the typical number of dry days between storms also decreased. Portions of Montana, Wyoming and the Dakotas also had improving rainfall conditions, the study found.

The study’s bottom line for our region is that worsening drought conditions carry potential consequences for everyone — people and animals who drink water, crops that need irrigating, grasslands and cacti that need moisture, and wildlife and cattle that need grass and other desert foods.

It’s been well known for quite some time that the West is in a prolonged drought that has intensified since 2000. But this study stretches the drought’s duration as far back as the middle 1970s. But weather stations checked by researchers have shown these trends were more common since 2000, Smith said.

The findings come as Tucson and the desert Southwest endure one of their driest spells on record — a period not even covered by this study.

In Tucson, 2020 was the driest calendar year on record. The periods April 2020 through March 2021 and March 2020 through February 2021 were the driest and second driest 12-month periods on record, National Weather Service records show. Four of the 10 driest 12-month periods on record have occurred since January 2020, the weather service says.

The outlook also continues to be poor for the region’s single biggest drinking water supply.

Colorado River Basin snowpack currently ranges from 55% to 85% of historical norms across the seven-state basin. April through July river runoff into Lake Powell is likely to be only 45% of its 30-year average, says the federal Colorado Basin River Forecast Center.

The new study’s researchers are now extending their work to cover 2020 conditions, said Smith, adding that their report highlights that such extreme years are becoming more common.

“Years like 2020 — the potential to have them more often in the future is increasing,” he said.

Fallout from continued drought could include shifts in vegetation communities, more widespread mortality in grasslands and in other vegetation, and more intense fires, said Smith. The drying also means less forage for cattle, potential declines in crop yields and less water available for human consumption, he said.

“In general, we’re definitely indicating that water is becoming less available across the region,” said Smith, who is with the UA’s School of Natural Resources and the Environment.

A UA news release said the study’s findings portend “ominous trends for the desert Southwest, including Southern California, Arizona and New Mexico, regions that already experience relatively high mean temperatures and lower annual rainfall or snowfall.

“For these regions, substantial multidecade evidence demonstrates droughts are becoming longer and more frequent,” the news release said.

The study didn’t address why the Southwest is drying out more than the rest of the West. Asked for an explanation, author Smith pointed to a 2020 study by UCLA’s Park Williams that concluded this region is in an “emerging megadrought.”

That study attributed such conditions to natural variability in climate patterns, aggravated by human-caused warming, Smith noted.

Williams reported that human-caused trends in temperature, relative humidity and precipitation were estimated by 31 climate models to have accounted for 47% of the drought’s severity from 2000 to 2018.

The warming weather pushed “an otherwise moderate drought onto a trajectory comparable to the worst megadroughts since 800” A.D., concluded Williams, an associate professor at UCLA’s Geography Department.

The new drought study was published Tuesday, April 6, in the journal Geophysical Research Letters, run by the American Geophysical Union. The union describes itself as “an international, nonprofit, scientific association whose mission is to promote discovery in Earth and space science for the benefit of humanity.”

Besides Smith, co-authors include lead author Fangyue Zhang, a postdoctoral researcher at UA’s natural resources school; Dong Yan, a third UA researcher; and researchers from the U.S. Geological Survey, the U.S. Agriculture Department and the University of Iowa.

The new study examined precipitation and temperature records at 337 weather stations across the West.

While previous research documented a decline in total rainfall for much of the West, this study 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 that have 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 Joel Biederman of the Agriculture Department in Tucson told The Associated Press.

The total amount of rain in a year doesn’t matter to plants — especially if rains come mostly in heavy bursts with large runoff — but consistent moisture keeps them alive, said UCLA meteorologist Daniel Swain. He writes a weather blog about the West and was not part of the study.

Specifically, the study found:

  • Maximum daily temperatures across the West rose an average of 0.36 degrees Fahrenheit per decade.
  • Longer droughts became more common. The average number of dry days in the entire West rose 0.6 of a day per decade. The longest dry interval between storms rose 2.4 days per decade.
  • In the Southwest, year-to-year variability of the number of wet days increased at nearly 60 weather stations. Such variability decreased at 39 weather stations, mostly in the Northwest.
  • Also in the Southwest, year-to-year precipitation variability rose by 1.8% per decade. The longest dry spell rose by 4.7 days per decade.

Become a #ThisIsTucson member! Your contribution helps our team bring you stories that keep you connected to the community. Become a member today.

This article includes information from The Associated Press. Contact reporter Tony Davis at tdavis@tucson.com or 349-0350.