FABENS, Tex. — On maps, the mighty Rio Grande meanders 1,900 miles, from southern Colorado’s San Juan Mountains to the Gulf of Mexico. But on the ground, farms and cities drink all but a trickle before it reaches the canal that irrigates Bobby Skov’s farm outside El Paso, hundreds of miles from the gulf.

Now, shriveled by the historic drought that has consumed California and most of the Southwest, that trickle has become a moist breath.

“It’s been progressively worse” since the early 2000s, Mr. Skov said during a pickup-truck tour of his spread last week, but he said his farm would muddle through — if the trend did not continue. “The jury’s out on that,” he said.

Drought’s grip on California grabs all the headlines. But from Texas to Arizona to Colorado, the entire West is under siege by changing weather patterns that have shrunk snowpacks, raised temperatures, spurred evaporation and reduced reservoirs to record lows.