About 37 percent of the contiguous United States was in at least a moderate drought as of April 7. Droughts appear to be intensifying over much of the West and Southwest as a result of global warming. Over the past decade, droughts in some regions have rivaled the epic dry spells of the 1930s and 1950s.

The relationship between the climate and droughts is complicated. Parts of the country are becoming wetter: East of the Mississippi, rainfall has been rising. But global warming also appears to be causing moisture to evaporate faster in places that were already dry. Researchers believe drought conditions in these places are likely to intensify in coming years.

There has been little relief for some places since the summer of 2012. At the recent peak last May, about 40 percent of the country was abnormally dry or in at least a moderate drought.