Last year was, for Southern California water management, perfectly wet. By that I mean a good snowpack in the Sierra Nevada and the Colorado River Basin.

I’m stealing a wonderful phrase here from a new paper by the University of Arizona’s Connie Woodhouse (the full paper’s behind a paywall): “A Long View of Southern California Water Supply: Perfect Droughts Revisited.”

The impact of drought on water resources in arid and semiarid regions can be buffered by water supplies from different source regions. Simultaneous drought in all major source regions — or perfect drought — poses the most serious challenge to water management. We examine perfect droughts relevant to Southern California (SoCal) water resources with instrumental records and tree?ring reconstructions for the Sacramento and Colorado Rivers, and SoCal.

Woodhouse and colleagues found five “perfect droughts” in the 20th century, which they concluded was not unusual when they used tree rings to extend the record back in time. But….

Although the causes of perfect droughts are not clear, given the long?term natural variability along with projected changes in climate, it is reasonable to expect more frequent and longer perfect droughts in the future.