BILLINGS, Mont. (AP) — 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 sweeping government study released Tuesday that concludes the situation is worsening.

The most dramatic changes were recorded in the desert Southwest, where 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 cons...