FRESNO, Calif. — Up and down the San Joaquin Valley, vast fields that once grew cotton lie fallow, remnants of a boom and bust fueled by a worldwide demand for premium T-shirts and linens.

Farmers here have fallowed acres of Pima cotton by the thousands, threatening the region’s unlikely reign as the world’s biggest producer of the specialty cotton, also called Supima.

Environmentalists say that farmers should never have bet so heavily on a thirsty cash crop in this dry swath of central California — particularly a crop used for luxury clothing, as opposed to food.

As recently as 2011, American farmers planted a near-record 306,000 acres of Pima cotton, almost all of that in the San Joaquin Valley, consuming an estimated 249 billion gallons of water. That’s enough to meet the average yearly water needs of about 1.9 million households. But now, with reservoirs nearly dry, farmers in California’s hardest-hit districts have no surface water to irrigate their crops. At the same time, cotton prices have slumped, hurt by a global glut. Farmers may harvest as little as 100,000 acres of Pima cotton in California this year, according to the latest forecasts.