It may come as a surprise that as of 2015, most of the water taken out of US ground- and surface-water sources was withdrawn by the electricity sector. Irrigation is a close second, and public supply is a distant third.

In 2015, thermal power generation—anything that burns fuel to create gas or steam that pushes a turbine—used 133 billion gallons of water per day. That water is mostly for cooling the equipment, but some of it is also used for emissions reduction and other processes essential to operating a power plant.

Those gallons are mostly freshwater, but some near-coast power generators do use saline or brackish water to operate. Much of the water is returned to the ecosystem, but some of it is also lost in evaporation. The water that is returned can often be thermally polluted, that is, it's warmer than what's ideal for the local ecosystem.

But the electricity sector is getting more water-conscious. According to the Energy Information Administration (EIA), the amount of water used per kilowatt-hour (kWh) generated across the country has been falling since 2014. That year, the electricity sector used a staggering 15.1 gallons of water per kWh it produced. By 2017, it only used 13 gallons per kWh.

Thank renewable energy and the plummeting price of natural gas. Wind and solar power, which use no water for operation, have become much more prevalent in the last four years, and combined cycle gas turbines—which combine a gas turbine with a steam turbine—use much less water than a traditional coal or gas plant.

Coal and nuclear power plants, both thermal systems that are very water-intense, have been retired at significant rates throughout the US since 2014. New coal and nuclear haven't been added in a significant way since 2014, either.

EIA points out that thermal power plants in the Eastern US use much more water than thermal power plants in the West, because they're built for their surroundings. Generally, eastern plants can take advantage of large rivers and lakes, so they tend to be "once-through cooling" plants, where lots of water is withdrawn from the surrounding ecosystem, but a lot of water is also put back into it. "Because water is scarcer in some western regions, many power plants in western states are fitted with recirculation, dry cooling, or hybrid cooling systems, which require less water for cooling than once-through cooling systems," EIA writes.

EIA compares Maryland to Utah and Arizona. Both states generate more than 85 percent of their electricity from thermoelectric power plants, but in 2017, Maryland's electricity sector withdrew 47 gallons of water per kWh it produced, while plants in Utah and Arizona withdrew less than half a gallon of water per kWh. Those who live in dry western states (currently being made drier by climate change) may have heard the phrase "whiskey is for drinking; water is for fighting." When water is as scarce as it is, investing in expensive cooling technologies that minimize water withdrawal is a reasonable measure.

Still, EIA says that water use by thermoelectric plants remains significant despite the changes of the past several years. "In 2017, the total volume of water withdrawn by thermoelectric power plants in the United States was more than twice the amount that flows over the Niagara Falls each year."