Outside Stephanie Woolstenhulme’s office window, the first snowfall of the season has dusted the streets. She looks delighted, and not just because she’s a skier. Her community needs all the precipitation it can get to replenish the springs and aquifers that water its roughly 1,600 residents. “Water is gold here,” she says.

Ever since European settlers crossed the Rockies, access to water has defined the development of the American West. Water irrigates farms, hydrates households, powers machinery. But a prolonged drought that began in 2000 has become the Southwest’s driest 22-year period in 12 centuries, according to analyses of tree-ring records.