Water experts have long had a saying: "If you can't measure it, you can't manage it."

But they also have long known that determining how much water is diverted from rivers and how much is pumped from wells is an inexact and expensive science.

Now, a method developed by the Idaho Department of Water Resources and the University of Idaho that uses surface-temperature readings from government satellites is letting officials measure how much water is lost from a piece of land through evaporation and through transpiration, the release of water vapor by plants.

Water resource managers in Idaho and other states see the method as the best way to measure water consumption to help settle growing regional disputes over water supplies without lawsuits. More than 90 percent of the water used in Idaho's 3.4 million acres of irrigated agricultural land is consumed by evaporation and transpiration.

The data already have been used to help Idaho planners evaluate the impact of population growth, protect salmon and steelhead habit and settle questions in the conflict between groundwater irrigators and surface-water irrigators in the Magic Valley.

Previously, officials had to look at well-pumping records and electricity use toestimate each irrigation district's usage. Now they can measure water consumption field by field with greater precision and less cost.

"This tool would allow the state of Wyoming or Colorado to independently verify what's going on in California," said Tony Willardson, executive director of the Western States Water Council. "It probably wouldn't be safe for someone in a Colorado Department of Natural Resources truck to drive around in California to see how much water they're using."

The method could help farmers cope as Idaho's climate warms.

"The water conflicts that are brewing are intense," said James Levitt, director of the Program on Conservation Innovation at the Harvard Forest, Harvard University.

"If you don't have water you can't farm. Climate change is actually happening now. This will allow government and farmers to adapt. Not every farmer in Idaho subscribes to global warming as a proven theory. But they want to know where their water is."

Willardson said the method can allow irrigation districts or other entities to conserve water and save the surplus for drier times. The same principle applies to farmers who can "bank" their rights to consume water and lease or sell those rights to other users.

The data are also crucial to government programs that buy back water rights - basically paying farmers to let their land dry up so the water can flow into streams where steelhead and salmon spawn.

The data helped settle a century-long fight between Colorado and Kansas over water in the Arkansas River.

In Oregon, data helped conserve water in Klamath Basin salmon habitats by helping scientists work with ranchers to withhold irrigation from certain pastures.

In California, the method eased fears that water transfers to Los Angeles and San Diego would increase the salinity of Imperial Valley farmland.

In Texas, the method revealed that invasive saltcedar trees were using less water than expected, indicating a costly eradication was likely not necessary.

University of Idaho water resources engineer Rick Allen developed the method at the Kimberly Research and Extension Center near Twin Falls.

"Our method treats everyone the same, and that has been a big factor in its acceptance," Allen said.

Trying to track who uses water and where has been expensive and difficult. Most monitoring now is done at the county level by monitoring flow in irrigation canals or by monitoring electrical use at individual wells.

Drawing on a Dutch model that employed satellite imagery, Allen refined it using pioneering work by now-retired James L. Wright of the U.S. Department of Agriculture's Agricultural Research Service at Kimberly.

Idaho Department of Water Resources satellite imagery experts Tony Morse and Bill Kramber at Boise saw the potential in Allen's work and helped refine it for public use.

Recently the method's future has been in jeopardy because NASA was not planning to include the $100 million thermal infrared sensor needed to record surface temperature in the next Landsat satellite, scheduled to launch in 2012.

The currently orbiting Landsat 5 and 7 were launched in 1984 and 1999 and were designed to last only three to five years.

After much pressure from Western politicians, it appears NASA will include the sensor in Landsat 8. A final decision is expected by the end of the year, according to Jim Irons, a project scientist for the Landsat Data Continuity Mission at the Goddard Space Flight Center in Maryland.