Lake Tahoe loses half-million acre-feet a year to evaporation
The 2015 State of the Lake report was funded by the California Tahoe Conservancy, IVGID Waste Not Program, League to Save Lake Tahoe, Parasol Tahoe Community Foundation, Tahoe Area Sierra Club, Tahoe Fund, Tahoe Lakefront Owners Association, Tahoe Regional Planning Agency, Tahoe Water Suppliers Association and the Toiyabe Chapter of the Sierra Club, among other supporters.
TERC partners with other research institutions and with resource agencies in the Tahoe Basin to address the current challenges facing Lake Tahoe.
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The amount of water lost by Lake Tahoe to evaporation could fill Walker Lake in two years, according to the annual State of the Lake report.
Evaporation caused the largest loss of water from Lake Tahoe in 2014. By the end of the year, 52 inches of water had evaporated from the lake.
Geoffrey Schladow, director of the UC Davis Tahoe Environmental Research Center and author of the annual report, explained an inch of water across Lake Tahoe’s expanse is equal to 3.5 billion gallons of water.
That means 182 billion gallons of water simply vaporized from the Lake in 2014, according to Schladow’s math. That translates to 558,536.7 acre feet, or enough to fill Hawthorne’s Walker Lake, which has a capacity of a million acre feet, in two years.
“One of the things we’re fortunate here at Tahoe that makes it a great place to study evaporation is in the summer, there’s usually almost no rain, and what is coming in at the streams is miniscule, and what goes out from the Truckee River is gauged — or in the case of this year, it’s zero,” Schladow said. “The only water gain or loss — or the great majority of it — is evaporation, so the change in water level in those late months is nearly all due to evaporation.”
Among findings this year, the report suggests the Lake’s renowned clarity and blueness are mutually exclusive, and at times of the year when clarity increases, blueness actually decreases and vice versa.
While clarity and the Lake’s blueness may not be tied the way it was once assumed, those disparate elements and the increasing rate of evaporation all have something in common, Schladow said, the lake is getting warmer.
Tahoe’s water temperatures, are the highest they’ve ever been, Schladow said.
In 2014, annual average surface temperatures reached an all-time high of 53 degrees Fahrenheit, and winter experienced that season’s warmest surface water temperature in recorded history.
Meanwhile, the number of days the Lake experienced its coldest temperatures totaled only 29 days. Compared with every prior recorded year, that represents the fewest days of freezing temperatures ever on the Lake.
“The Lake has been getting warmer for the last few years, and because of the drought, it is getting drier,” Schladow said.
While it may seem confusing these elements tie into one another.
The Lake’s clarity is controlled by sediment. Blueness is controlled by algal concentration, according to the report, which in turn is driven by the level of nutrients available to the algae.
Think of one of those weeble-wobble toys from your childhood, Schladow explained. Their heaviness at the bottom ensures whatever is happening on top is going to move back and forth, but never falls over.
Tahoe’s water is acting in a similar way, he said. The heavier, colder water at the bottom isn’t moving, while its lighter, warmer water continues to be blown around by surface winds, speeding up the evaporation rate and reducing the Lake’s margins.
The resulting effect leaves little mixing of the water, a natural process that’s impacting production of a vital nutrient to the Lake’s aquatic life: nitrate.
In 2014, Lake Tahoe didn’t mix to its full depth for the third consecutive year, due to warmer water and record-high levels of stability, according to the report.
This lack of deep mixing led to the highest nitrate-nitrogen levels on record, 20 micrograms per liter.
Algae historically goes through a cycle in which it feeds on the nitrate, dies, and decomposes as it sinks to the bottom of the Lake and becomes nitrogen, which in turn helps create more nitrate. But that didn’t happen in 2014.
As the Lake level dropped below its natural rim, the lack of mixing and an overproduction of nitrate persisted. The resulting increase in algal growth has manifested itself all over the Lake, which in turn has helped increase the Lake’s blueness.
“This does not mean that clarity should be dismissed,” said Shohei Watanabe, a postdoctoral researcher. “Rather, it shows that algae concentrations and nutrient input should be managed more closely to truly keep Tahoe blue and clear.”
Watanabe led the blueness study in collaboration with NASA-Jet Propulsion Laboratory and Laval University. Watanabe is also responsible for creating a “Blueness Index,” which quantifies the lake’s color by using data produced by submerged research buoys.
“I think the blueness is something we haven’t noticed in years past because we haven’t had the opportunity and the people to study that particular issue,” Schladow said. “The amount of evaporation surprised me. I’ve seen other estimates made by agencies that put it at a half or a third of that number.”