A growing population coupled with diminishing freshwater supplies should force major changes in the ways the world's farmers tend their crops in coming decades, a recent study recommends.

Because agriculture uses about 70 percent of the globe's fresh water every year, farming should be the focus of intense conservation efforts, Cornell University professor David Pimentel said.

"We in the U.S. waste a lot of water in contrast to other people," said Pimentel, whose study appeared in the journal BioScience. "Agriculture is going to have to give up water as the population grows.

"States like California, Colorado, Texas and Nebraska are going to have to make some major changes."

The study said farmers should use water-conserving irrigation methods combined with water and soil conservation practices to minimize run-off.

It also suggests governments eliminate water subsidies to farmers to encourage more efficient use, work to reduce water pollution and protect forests and wetlands.

In parts of Arizona, water from major aquifers is being withdrawn more than 10 times faster than it can be recharged by rainfall. In California, agriculture accounts for about 3 percent of the state's economic production but consumes

85 percent of the fresh water.

The United Nations estimates the world population will rise to 9.4 billion by 2050 from about 6.3 billion now. Increasing demand for water already is causing problems.

Pimentel cites the Ogallala aquifer, under parts of Nebraska, South Dakota, Colorado, Kansas, Oklahoma, New Mexico and Texas, that supplies water to a fifth of all irrigated land in the country.

The underground water source has dropped 33 percent since 1950 — half the volume of Lake Erie, said Sandra Postel, director of the Global Water Policy Project in Amherst, Mass.

Don Adams, executive director of Nebraskans First, a statewide coalition of groundwater irrigators, acknowledged the effects of drought but said Nebraska should not be lumped with other states.

While the Ogallala aquifer might face declining water levels elsewhere, he said, its levels recently increased under Nebraska.

And more farms here are employing best management practices, with more irrigators switching to center pivot irrigation and away from flood irrigation, he said.

But the problems persist worldwide, from the Chenaran plain in northeastern Iran to Guanajuato, Mexico. Of particular concern is Asia, home to 60 percent of the world's population but only 30 percent of its fresh water. Postel says water efficiency will have to double to meet future needs.

"The pace of the problem is proceeding faster than the pace of the solution," Postel said.

By 2050, "water will to be the most critical resource issue we face in the entire world," said Bob Stallman, president of the American Farm Bureau Federation and a Texas-based rice farmer. "Frankly, I think wars will be fought over water. There are already border disputes in some parts of the world between countries over water."