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Executive Summary for January 4th

Water lost to evaporation in major reservoirs is at last getting serious attention from researchers, who are beginning to measure the losses precisely and trying to estimate future losses from climate change. Meanwhile, other experts are trying to capture water from fog.

Published on Jan. 4, 2016 Read time Approx. 4 minutes

Reservoir Evaporation, Ignored No More

Leave a dish of water out in the sun, and what happens? Slowly, you’ll lose all that water.

The same happens with water-storage reservoirs all over the world. But evaporative losses at these critical storage facilities has gone ignored for a long time. Well, no longer.

“We can no longer afford to lose this amount of water. Once it is lost it is gone,” said Bob Grossman, a retired scientist from the University of Colorado at Boulder Department of Atmospheric and Oceanic Sciences, who helped organize a recent conference on reservoir evaporation. “The neglect of evaporative loss as the cost of doing business in a water-abundant world will likely cut into the bottom line as scarcity looms.”

No one is sure just how much potable water is lost to evaporation. But, of course, it depends to some extent on local geography and climate. There’s been very little research on the subject. One study estimates about 500 billion gallons of water evaporate annually from storage on the Colorado River system. This represents roughly 10 percent of the total natural flow of the Colorado River Basin — about five to 10 times the amount of Denver’s annual water use.

Several new efforts should begin to provide some answers. Justin Huntington, a hydrologist at the Desert Research Institute in Reno, is deploying sophisticated evaporation sensors at numerous reservoirs in California, including Folsom Lake in the Sacramento area.

“It does no good just to know what evaporation is off Folsom Reservoir,” Huntington told Capital Public Radio. “We need to know what evaporation is off all the reservoirs off the west slope of the Sierra and country wide. A 10 percent error in evaporation off this reservoir is worth millions and millions of dollars in terms of water to irrigators.”

The U.S. Bureau of Reclamation published a study early in 2015 that attempted to estimate how climate change would affect evaporation rates at several large reservoirs throughout the West. The study found that evaporation rates are likely to increase, although the rate varies considerably.

Shasta Reservoir, the largest in California, was part of that study. The results estimated evaporation at Shasta could increase from 7.6 to 14.7 percent by 2080.

The big question, obviously, is what to do about this. At present, there aren’t many answers. The large size of water storage reservoirs makes most solutions impractical. They can’t simply be sheeted in plastic or mesh like a swimming pool cover.

Thin films of organic compounds are one solution being studied. Another trick is “shade balls,” which got a lot of media attention when 96 million of them were spilled into a small Los Angeles reservoir earlier this year. In reality, these were not a sensible solution economically, and may also pose ecological concerns.

Another option is decidedly old-school: Shift water storage underground. Which is just one reason we’re hearing more talk these days about investing in groundwater storage and aquifer recharge, and less talk about building new reservoirs.

“One thing we do know is that you can only reduce evaporation and not eliminate it unless you store it underground,” said Katja Friedrich, a University of Colorado, Boulder, associate professor of physics and precipitation research. “But that has its own set of problems. Our intention is to help water managers reduce evaporation for current and future reservoirs.”

Snagging Water From the Air

Ever stood in a California coastal redwood forest on a foggy day? Then you know what happens: You get wet.

Even if it’s not raining, fog means a wet day within a redwood forest. The trees are so good at capturing the water in that fog that they produce a constant gentle rain as water drips off the branches.

Now scientists are trying to figure out how to capture water from the fog in a controlled manner.

Dan Fernandez, a professor of science and environmental policy at California State University, Monterey Bay, has developed and deployed dozens of “fogcatchers” throughout the region, testing their effectiveness with different materials and in different coastal areas. The devices look like giant window screens held aloft, designed to capture fog moisture the way redwood trees do.

“I was excited about the idea of being able to collect water from fog. It’s so unique and novel and interesting,” Fernandez tells Bay Nature Magazine. “My thinking was, maybe this could be a way to address some of the water concerns that we’re having in California.”

On a good day, a fog collector can extract up to 9 gallons of potable water. More typical days yield 1 or 2 gallons.

That’s a fraction of the 100 gallons of water per day consumed by the average American. You’d need a fog fence much larger than a house to fill the needs of an average household. So, although they are relatively cheap at $100-$200 a piece, fogcatchers may never be as scalable as, say, rooftop solar.

Still, the idea of using fogcatchers to supplement water supplies is appealing. For example, a single fogcatcher could easily irrigate a small container garden, or provide enough water to flush one toilet for a day.

Top image: In this photo taken Sept. 17, 2014, houseboats are docked at Lake Shasta’s Bay Bridge resort near Redding, Calif. After three years of drought the water level at the lake has dramatically receded. One contributor is simple evaporation, which recent studies estimate will increase with climate change. (Rich Pedroncelli, Associated Press)

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