Graph by Jonathan Thompson using data from the California Energy Almanac
Credit: Graph by Jonathan Thompson using data from the California Energy Almanac

The water-energy nexus spans the world of electricity generation and water movement, particularly in Western states. It takes water to produce steam for coal, natural gas and nuclear power plants, and they usually need water to cool them down. Huge amounts of electricity are needed to pump water across the desert; the Southern Nevada Water Authority is Nevada’s biggest user of electricity, and the Central Arizona Project relies heavily on the Navajo Generating Station to keep water moving through the canals.

Surely the most obvious link between water and energy, and between climate and electricity generation, though, is found at the West’s numerous hydroelectric generation stations, and here in California—deep in a nasty drought—we’re feeling that link in a painful way.

The relationship is pretty simple: More water in a reservoir or river equals more potential for generating electricity by releasing that water to turn turbines. All of California’s reservoirs are far below the average levels for this time of year. New Melones reservoir is sitting at just 17 percent of capacity; Shasta reservoir, one of the state’s biggest hydroelectric power plants, is only 49 percent full. Meanwhile, Lake Mead and Lake Powell, backed up behind the Southwest’s two biggest hydropower plants, are at critically low levels. This is impacting electricity generation, without a doubt, and doing so during the hottest months of summer, when the grid is already stressed.

We wanted to get a more concrete sense of exactly how the drought is affecting hydropower generation in California and beyond, so we dug into the data and crunched some of the numbers. The map below in the “Media” field gives a rundown on the biggest hydropower generators in the Southwest, including all of California’s plants with a generating capacity of 200 megawatts or more, along with Glen Canyon Dam, Hoover Dam and Davis Dam on the Colorado River. Click on the icons, and you’ll see a graph of power generation from 2009 through 2014.

Here are the highlights of the number-crunch.

In California, drought has taken a direct hit on hydropower generation, as is quite evident in the map below, and the graph above. In 2014, California’s collective hydropower plants kicked out about 17 million megawatt hours of juice. Not bad, except that it was only about one-third of what they produced in 2011, an unusually wet year, and about half of an average year in pre-drought times.

The average home uses about 11,000 kilowatt hours of power—or 11 megawatt hours—each year. In other words, something like 2.8 million fewer homes were powered by hydroelectricity in 2014 as compared to 2011 in California. And things have only gotten worse since.

The good news is that burgeoning wind and solar in the state have stepped in and taken up some of the slack. The bad news is that more natural gas power has also been needed to fill the deficit.

Drought diminishes hydropower production from Colorado River dams as well, though, as directly as in California. Hoover Dam is a huge hydroelectric power plant, with a nameplate capacity as large as some of the biggest coal-fired power plants, and its “fuel” supply is running out as Lake Mead reaches historically low levels. So it’s no wonder that folks are concerned about its status as a power producer. So far, though, drought hasn’t had as severe an impact on hydropower production here or in Glen Canyon and Davis Dams, above and below Hoover, respectively. That’s because the operators of these dams can’t withhold a bunch of turbine-turning water just because their reservoirs are running on empty. In fact, Hoover dam’s turbines produced 33,000 more megawatt hours of power during the first quarter of this year, even as it approached record low levels, than during the first quarter of 2014. Yet dry times still do impact hydropower production, because the lower the reservoir, the less force the water has to turn the turbines. Lake Mead’s “dead pool”—the level at which it could no longer turn the turbines—was once 1,050 feet, about 25 feet below what it is now (and a level that is not in the cards at least for a few more years). But new wide-head turbines have been installed over the last decade, which lowers dead pool to about 950 feet, thus extending Hoover’s hydropower life for a while.

Hydroelectricity is an especially valuable form of power. Hydro is a good source of baseload power, meaning it can put a steady stream of juice into the grid around the clock. But it’s also valuable in that grid operators can crank it up or down relatively quickly, meaning it can be used to balance out variable power sources like wind and solar, or it can step in to account for a sudden surge of power demand, due to everyone turning on their air conditioners in the hottest time of the day, coupled with a drop in solar generation as the sun dips toward the horizon. It’s in this capacity that Hoover and Glen Canyon are huge assets to the southwestern grid.

If drought-induced hydropower loss is going to happen anywhere, though California’s a good place for it. If any other state lost tens of millions of megawatthours of power from one source, they’d likely replace it with coal-fired power. But California is fast on its way to completely phasing out coal, and it’s also under the most robust renewable portfolio standards, so it can only get so dirty when looking for a stand-in for hydro.

Jonathan Thompson is a senior editor of High Country News, where this story first appeared