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.

The program is getting renewed attention this week as the Senate works to complete a $955 billion, five-year farm bill. It voted Thursday 75 to 22 to hold a final vote on the bill early next week.

Two Western Democrats, Representative Earl Blumenauer of Oregon and Senator Tom Udall of New Mexico, have introduced legislation that would ensure that water saved by taxpayer-financed irrigation systems would stay in underground water tables or streams and not be used by farmers to expand their growing operations.

Mr. Udall had planned to offer his bill as part of the farm legislation, but it never came up for a vote. Mr. Blumenauer will introduce his legislation when the House begins working on its version of the bill this month.

“Farmers and the American public recognize that conservation of our resources is good for our crops, our land and our nation’s future,” Mr. Udall said while introducing the bill last month. “America’s farmers have a long history of innovation and adaption of new technologies and practices. Our legislation would encourage those who seek to implement new practices that increase quality production through sound management of our precious resources.”

A study by researchers at the University of California, Davis, this year concluded that Kansas farmers who received payments under the conservation subsidy were using some of their water savings to expand irrigation or grow thirstier crops, not to reduce consumption.

Another study by researchers at New Mexico State University in 2008, which studied an area running from Colorado to New Mexico, came to the same conclusion.

“Policies aimed at reducing water applications can actually increase water depletions,” the researchers said.

According to data from the Environmental Working Group, a Washington research group, the government has provided about $4.2 billion in conservation subsidy payments to landowners since 1997. About $1 billion has been used to help agricultural producers increase the efficiency of irrigation.

Five states — Arizona, California, Colorado, Texas and Utah — account for nearly half of the program’s spending on irrigation equipment, data from the Environmental Working Group shows.

“This is a critical issue because groundwater is important for a number of reasons, and not just agriculture,” said Craig Cox, a senior vice president of the Environmental Working Group. The incentive program “has done a lot of good in helping farmers practice better conservation, but its needs some changes.”

Mr. Cox said environmental organizations do not want lawmakers to cut financing for the water conservation subsidy. They want the program changed so farmers receiving payments would be restricted from increasing their water use.

According to the United States Geological Survey, while the population has nearly doubled over the last 50 years, water consumption has tripled. Farm irrigation accounts for 80 percent of the water use nationwide, according to the Agriculture Department. Western states, where water resources have been diminishing for years, make up some of the largest users of water through irrigation.

“Given that we just had the worst drought in the last 50 years, lawmakers need to really look at this program and how it’s having the opposite effect of what was intended,” Mr. Cox said. “Buying better equipment does not save water. Irrigation is the poster child for why we need reform.”