"When push comes to shove, water could become the real throttle on renewable energy," Michael E. Webber, an assistant professor at the University of Texas Austin who studies the relationship between energy and water, told the Times.

Water efficiency is especially problematic with solar thermal farms, which use mirrors arranged in long troughs. "Trough technology has been more financeable, but now trough presents a separate risk -- water," said Nathaniel Bullard, a solar analyst with New Energy Finance, a London research firm. Photovoltaic power plants, while typically more costly and less efficient than solar thermal farms, consume less water, mainly to wash the solar panels.

Solar developer BrightSource Energy is hoping to benefit from the water consumption issue with a technology that places mirrors on towers, producing a higher-temperature steam than from a trough system. It then uses a dry cooling method, which does not adversely affect power output.

In the meantime, water consumption is a contentious issue with state regulators for a number of big solar projects. Already, solar developers in California have had to resort to technologies that are less water intensive because local officials have refused to make large quantities of water available for their projects.

So far California has 35 large-scale solar projects planned that could generate 12,000 megawatts of electricity, each project with different water needs. In the southern part of the state, BrightSource Energy's dry-cooled Ivanpah project would need some 25 million gallons of water a year, mostly to wash mirrors. But in California's Mojave Desert, 705 million gallons would be swallowed up by the wet-cooled Abengoa Solar project, less than half the size of the Ivanpah project.

In Nye County, Nevada, requests for water from renewable energy developers "far exceed the amount of available water," said Joni Eastley, chairwoman of the county commission.

German developer Solar Millennium announced plans to build two large solar farms in the state's arid Amargosa Valley. Hard-hit from the recession, the hundreds of jobs the project would create was welcome news to the Nevada community. But now the people are divided ever since the company disclosed that the project would require 1.3 billion gallons of water a year -- about 20 percent of the valley's available water. While some are worried about the impact of the solar farms on the environment, others are hoping to earn money selling their water rights.

As the population increases and renewable energy development continues, water scarcity will become a problem throughout the United States, not just in the arid western states, predicts Daniel M. Kammen, director of the Renewable and Appropriate Energy Laboratory at the University of California, Berkeley.

"When we start getting 20 percent, 30 percent or 40 percent of our power from renewables, water will be a key issue," Kammen told the Times.

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