WASHINGTON — Like the old song, ``Love and marriage, love and marriage . . . you can't have one without the other,'' so it goes with energy and water.

It takes a lot of water to produce energy. It takes a lot of energy to provide water. The two are inextricably linked, and claims on each are rising.

``The water supply is as critical as oil,'' said Charles Groat, a geologist and expert on the problem at the University of Texas in Austin.

In return, ``water use requires a tremendous amount of energy,'' said Peter Gleick, the president of the Pacific Institute for Studies in Development, Environment and Security in Oakland, Calif.

As the United States tries to lower its dependence on foreign oil by producing more energy from domestic sources such as ethanol, however, it's running low on fresh water.

Water is needed for mining coal, drilling for oil, refining gasoline, generating and distributing electricity, and disposing waste, Gleick said.

``The largest use of water is to cool power plants,'' he said at a panel of experts on ``The Global Nexus of Energy and Water'' in Boston last month.

According to Vince Tidwell, a water-management expert at the Sandia National Laboratory in Albuquerque, N.M., more than 40 percent of the water that's withdrawn from rivers, lakes and wells is used for energy. The rest goes mainly for irrigation.

Most of the water used for energy is returned to its source, but by then it's often heated or polluted and of lesser value.

As a result, ``increased use of brackish or degraded water may be required in some areas,'' the Energy Department warned Congress in a report last year.

Conversely, vast amounts of energy are needed to pump, transport, treat and distribute water.

For example, the California State Water Project, which pumps water over the Tehachapi mountains to the Los Angeles Basin, is ``the largest single use of energy in California,'' Gleick said.

Heating water to wash dishes or clothes or to take a shower is a greedy consumer of energy.

``Running a hot water faucet for five minutes is the equivalent of burning a 60-watt light bulb for 14 hours,'' Gleick said. ``Maybe the best way to save energy is to save hot water.''

Most historic battles over water have come from the demands of agriculture for scarce supplies in arid regions. But the energy sector's needs are beginning to affect water policy and vice versa.

Gleick cited these examples: The Tennessee Valley Authority had to reduce the output from a nuclear power plant to avoid overheating the Tennessee River. London rejected a proposed water-desalinization plant because it would use too much energy. Amsterdam had to build wind turbines to generate energy before it could build a desalinization plant in the Netherlands.

One difficulty is that there's no high-level authority to coordinate energy and water usage. At least 20 federal agencies, along with a multitude of state and local governments, have a hand in matter.

``No one is in charge,'' said Groat, a former director of the U.S. Geological Survey in Washington. ``Energy planners assume we will have enough water. Water planners assume will have enough energy.''

The problem is going to get worse, according to Michael Webber, a mechanical engineer at the Center for International Energy and Environmental Policy, a policy-research group of scientists and engineers at the University of Texas in Austin.

``Future fuels are likely to be very water-intensive,'' he said. ``They all require a lot of water.''

For example, driving one mile on ethanol consumes 600 gallons of water to irrigate the corn from which it's made, Webber said in an e-mail. Even plug-in hybrids, which are touted as the most efficient way to power electric cars, need to withdraw 10 gallons of water for every mile traveled, he said.

``Instead of miles per gallon of gasoline, we're switching to gallons of water per mile,'' he said.

Unfortunately, water supplies are shrinking even as energy demands increase.

``Climate concerns and declines in groundwater levels suggest that less fresh water, not more, may be available in the future,'' according to ``Energy Demands on Water Resources,'' an Energy Department report published last year.

``Available surface water supplies have not increased in 20 years, and groundwater tables and supplies are dropping at an alarming rate,'' the report says. ``Some regions have seen groundwater levels drop as much as 300 to 900 feet over the past 50 years.''

``If we're switching from foreign oil to domestic water, we've got to make sure we've got it,'' Webber said.

ON THE WEB

The Energy Department' report "Energy Demands on Water Resources."