The time between rainfalls has become longer in many parts of the western United States in the last 50 years, and the rains have occurred more erratically in that time, researchers say. Photo by Joel Biederman/Agricultural Research Service
April 6 (UPI) -- Over the last 50 years, precipitation patterns across the American West have become more and more erratic.
According to a new survey of weather records in Western states -- published Tuesday in the journal Geophysical Research Letters -- temperatures are rising, droughts are getting longer and annual rainfall totals are declining.
Records showed yearly rainfall totals across the West have declined by an average of four inches during the last 50 years.
Meanwhile, the longest dry spell in a given year increased to an average of 32 days in 2020 from an average of 20 days in 1970.
"The greatest changes in drought length have taken place in the desert Southwest," senior co-author Joel Biederman said in a press release.
"The average dry period between storms in the 1970s was about 30 days; now that has grown to 45 days," said Biederman, a research hydrologist with the ARS Southwest Watershed Research Center in Tucson, Ariz.
Though rainfall totals have declined over the last half-century, precipitation patterns have become more variable in recent decades. Western states have experienced longer, more intense droughts, as well as bigger storms.
"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.
For the study, researchers analyzed actual rainfall data from 337 weather stations situated throughout the western United States.
The data showed that in some places -- including Washington, Oregon and Idaho, as well as parts of Montana, Wyoming and the Dakotas -- drought intervals are getting shorter, not longer, and rainfall totals have increased slightly.
Previously, climate models have predicted rising temperatures would push the mid-latitude jet stream northward, carrying more moisture from the Pacific Ocean to parts of the Western United States.
"We were surprised to find widespread changes in precipitation have already occurred across large regions of the West," said study co-senior author William K. Smith, an assistant professor of natural resources at the University of Arizona.
"For regions such as the desert Southwest, where changes clearly indicate a trend toward longer, more erratic droughts, research is urgently needed to help mitigate detrimental impacts on ecosystem carbon uptake, forage availability, wildfire activity, and water availability for people," Smith said.