HASKELL COUNTY, Kan. — Forty-nine years ago, Ashley Yost’s grandfather sank a well deep into a half-mile square of rich Kansas farmland. He struck an artery of water so prodigious that he could pump 1,600 gallons to the surface every minute.

Last year, Mr. Yost was coaxing just 300 gallons from the earth, and pumping up sand in order to do it. By harvest time, the grit had robbed him of $20,000 worth of pumps and any hope of returning to the bumper harvests of years past.

“That’s prime land,” he said not long ago, gesturing from his pickup at the stubby remains of last year’s crop. “I’ve raised 294 bushels of corn an acre there before, with water and the Lord’s help.” Now, he said, “it’s over.”

The land, known as Section 35, sits atop the High Plains Aquifer, a waterlogged jumble of sand, clay and gravel that begins beneath Wyoming and South Dakota and stretches clear to the Texas Panhandle. The aquifer’s northern reaches still hold enough water in many places to last hundreds of years. But as one heads south, it is increasingly tapped out, drained by ever more intensive farming and, lately, by drought.