The U.S. electric power sector is putting a strain on the nation's water resources, a Union of Concerned Scientists report warns.

The report -- "Water-Smart Power: Strengthening the U.S. Electricity System in a Warming World" -- released this week as much of the nation grappled with a heat wave, says more than 40 percent of U.S. freshwater withdrawals are used for power plant cooling.

"But during drought or hot weather, when plants cannot get enough cooling water, for example, they often must cut back or completely shut down their generators as happened repeatedly in 2012 at plants around the country," the report said.

The U.S. Department of Energy released a report last week on the effects of climate change on the energy sector, citing instances of energy disruption due to climate conditions.

"Last August, Dominion Resources' Millstone Nuclear Power Station in Connecticut shut down one reactor for two weeks because the temperature of the intake cooling water, withdrawn from the Long Island Sound, was too high and exceeded technical specifications of the reactor," the government report said.

The UCS report notes the nation's power sector is currently on a path that would primarily replace coal with natural gas, currently projected to supply 60 percent of the country's power by 2050.

Although the shift from coal to natural gas would decrease water use, those declines in water withdrawals won't occur mostly until after 2030 -- "a 20-year delay that leaves the power industry unnecessarily vulnerable to drought and exacerbates competition with other water users," the UCS report says.

"Our electricity system clearly isn't able to effectively meet our needs as we battle climate change and face a future of expanding electricity demand and increasing water strain," said Erika Spanger-Siegfried, co-author of the report and a senior analyst in the UCS Climate & Energy Program, in a statement.

If the nation's power sector were instead to follow a path that includes strong investments in renewable and energy efficiency, the UCS report says, water withdrawals could drop as much as 97 percent from current levels by 2050.

That scenario would also cut carbon emissions 90 percent from current levels.

"Making low-carbon, water-smart choices is a high-stakes effort. The choices we make in the near term to define the power sector of this century will affect water resources, our climate and long-term hydrology, and the power sector's long-term resilience," said Peter Frumhoff, UCS director of science and policy.