Evaporating and warmer Lake Tahoe has more clarity

While Lake Tahoe’s iconic blueness and clarity often garner all the attention during annual State of the Lake reports, it was the lake’s increasing rate of evaporation in 2014 that most surprised the report’s author this year.

While the lake’s water level continues to drop due to a number of factors, it’s actually helping improve clarity, which is the best it’s been in more than a decade, improving from 70.2 feet in 2013 to 77.8 feet in 2014, according to the report released Thursday morning.

However, 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 1 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.

“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.”

KEEPING TAHOE BLUE?

Schladow spoke about the 2015 State of the Lake during a special presentation Thursday evening in Incline Village.

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 lake’s increasing rate of evaporation all have something in common, Schladow said — the lake is getting warmer.

Tahoe’s water temperatures, which have been recorded since 1910, 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.

HIGHEST NITRATE-NITROGEN LEVELS EVER

While it may seem confusing these elements tie into one another, the logic isn’t that far-fetched, Schladow said.

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 lake’s 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.

“Nitrate is one of the nutrients to come in and feed the algae in the lake,” Schladow said.

‘BLUE AND CLEAR’

Like the circle of life, 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. And the cycle starts all over again.

But that didn’t happen in 2014, due to the lake’s record-setting levels of stability, according to the report.

As the lake level dropped below its natural rim due to evaporation and the drought, the lack of mixing and an overproduction of nitrate persisted through 2014. 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 at UC Davis TERC. “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.”