As we broiled beneath a relentless sun in the Chihuahuan Desert, next to countless rows of improbably green cotton plants, I expected Ramon Tirres to tell me that water is his most precious resource. In the valley south of El Paso, Tirres grows cotton and pecans, and for the past 23 years, he’s farmed in the midst of historic drought. But as the wiry 71-year-old toed the dirt next to one of the canals that waters his fields, Tirres told me he’s facing a more pressing shortage: “The big issue we’re having now is finding workers,” Tirres said. “God almighty, is it hard.”Three years ago, Tirres began working to get an H-2A employment visa for a Mexican farmhand, one of a small pool of workers…