Much of the West is experiencing drought conditions this year, and a new study has found that dry periods between rainstorms are actually becoming longer as the decades go on.
Research conducted by the USDA’s Agricultural Research Service (ARS) and the University of Arizona in the Western U.S. found that rain has been falling in fewer and sometimes larger storms, with longer periods of dryness in between. The total amount of yearly rainfall has decreased an average of 0.4 inches over the last 50 years, while the longest dry period in each year increased from 20 to 32 days.
“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,” said co-senior author Joel Biederman, a research hydrologist with the ARS Southwest Watershed Research Center in Tucson, AZ.
The report emphasized the length of dry periods can often have a larger impact on socioecological systems than the total amount of rainfall.
“While the total amount of precipitation is important for these systems, key aspects of precipitation timing, such as the length of dry periods between precipitation events, can strongly influence ecosystem services including ecosystem carbon uptake, productivity for grazing and forage, wildfire frequency, and intensity, and water availability for societal use,” the report read.
Although greater portions of the West have been showing longer drought intervals since 2000 compared to previous years, there are a few notable exceptions to the patterns. Washington, Oregon and Idaho, and the Northern Plains region of Montana, Wyoming, and the most western parts of North and South Dakota experienced some increases in total annual rainfall and decreases in drought intervals. Biederman attributed this to a northern shift in the mid-latitude jet stream, which brings moisture from the Pacific Ocean to the West.
“We were surprised to find widespread changes in precipitation have already occurred across large regions of the West,” said co-senior author William K. Smith, assistant professor at University of Arizona.
“For regions such as the desert Southwest, where changes clearly indicate a trend towards 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.”
Current drought conditions
As of May 13, the U.S. Drought Monitor shows much of the West in some stage of drought. Approximately 84 percent of the region is in drought and 47 percent is in extreme or exceptional drought. Drought has recently worsened in California, Oregon, Washington, Montana, and Utah.
California has experienced very poor snowpack conditions and is experiencing some of its driest water years on record. In response, Gov. Gavin Newsom (D) has extended drought emergency declarations to 41 counties.
Arizona ranchers have been reported as hauling water for cattle for the past month because dirt stock tanks are completely dry. The drought has also caused juniper trees to die off in parts of central and northern Arizona.
Northwestern Oregon has expanded into areas of severe and moderate drought as streamflow and soil moisture levels continue to degrade. In southwestern Montana, moderate drought has expanded due to below-normal precipitation during the past three months, low streamflows and reductions in irrigation allotments.
According to the National Oceanic and Atmospheric Administration’s National Centers for Environmental Information, California and Nevada had their sixth driest April on record and their third driest October through April period. Idaho, Oregon and Washington experienced their third driest April on record. The Four Corners states experienced their driest 12- and 24-month periods on record for the region. — Anna Miller, WLJ managing editor
