WASHINGTON — Millions of dollars in farm subsidies for irrigation equipment aimed at water conservation have led to more water use, not less, threatening vulnerable aquifers and streams.

From Wyoming to the Texas Panhandle, water tables have fallen 150 feet in some areas — ranging from 15 percent to 75 percent — since the 1950s, scientists say, because the subsidies give farmers the incentive to irrigate more acres of land. Other areas, including several Midwestern states, have also been affected.

The Environmental Quality Incentives Program, first authorized in the 1996 farm bill, was supposed to help farmers buy more efficient irrigation equipment — sprinklers and pipelines — to save water.

But the new irrigation systems have not helped conserve water supplies, studies show. And researchers believe that the new equipment may be speeding up the depletion of groundwater supplies, which are crucial to agriculture and as a source of drinking water.