Laura Harper farms the 34 acres of pecan trees in the Mesilla Valley of southern New Mexico that her late mother, Sally, established as one of the region’s first organic orchards. She has depended on the Rio Grande to irrigate. But decades of drought have made the river an unreliable source of water. For much of the year, says Harper, 45, the river is “cut off” — dammed up at a reservoir called Elephant Butte to manage its flows.

One day in March, the river was almost entirely dry near Harper’s orchard. One day in September, it was running full, swelled by the release of water from the reservoir upstream. The levels ebb and flow each year; what has been steady is the persistence of overall drought.

In a warming climate, Harper worries that this valley may not be able to sustain the thirsty orchards that produce a healthy and lucrative pecan crop. She and her fellow farmers have been pumping more water from underground.