Is time running out to save Lake Tahoe?

UC DAVIS (US) — By the year 2100, average snowpack in the Lake Tahoe basin is expected to decline by as much as 60 percent and prolonged droughts will become more common.

Based on climate change, scientists say floods will increase and a new threat  to the lake’s unique ecology will emerge in the second half of the century.

“Public dollars are funding restoration programs in the Tahoe region, which is a special place for millions of people,” says Geoff Schladow, director of the Tahoe Environmental Research Center at the University of California, Davis.

“For these programs to succeed, resource managers need to know what to expect in the coming decades.

“Will we have more or less snow, rain, and runoff? Will the erosion controls and stormwater basins we are devising now still be useful in 30 or 50 years? What impact might climate change have for Lake Tahoe’s water quality and aquatic ecology?”

Previously researchers have drawn on 100 years of data to describe changes in temperature and precipitation that have already occurred in the Tahoe region.

The new report combines those findings with sophisticated computer models to produce detailed local projections out to the year 2100.

The scientists considered two possible future carbon emission scenarios—one “business as usual”—in which population growth and national and international policies affecting global climate change remain unchanged—and the other “optimistic,” assuming slower growth and aggressive climate action.

“While there is always some uncertainty when projecting this far into the future, the results appear reasonable,” says John Reuter, associate director of the center.

“They provide environmental managers and scientists with our first detailed glimpse of the potential impact of climate change on precipitation, runoff, water quality, and plant and animal resources in Lake Tahoe.”

This glimpse gives scientists and resource managers a better chance of taking strategic steps to minimize potential impacts of climate change, including:

  • Precipitation: No strong increase in the amount of annual precipitation was predicted, but the area will see a continuing shift from snowfall to rain, and earlier snowmelt and runoff. By the end of the century, precipitation in some years could be all rain and no snow. The peak snowmelt in the Upper Truckee River will occur four to six weeks earlier by the end of the century.
  • Water flow: Floods will be larger and more frequent in the middle of the century. Peak water flows in the Upper Truckee River, the largest river flowing into Lake Tahoe, will more than double, with the future “100-year flood” resembling today’s “1,000-year flood,” threatening roads and bridges.
  • Water quality: Even though more stormwater runoff is forecast, today’s best management practices will be able to accommodate the bulk of the increase. The simulations showed that predicted growth in runoff will cause only a 10-percent decline in the performance of urban stormwater control devices configured to current requirements.
  • Droughts: Droughts will be more severe, especially toward the end of the century and on the east side of the basin.
  • Truckee River water supply: There are likely to be longer periods when the lake falls below its natural rim and water stops flowing into the Truckee River toward the end of the century.  In the last 110 years, the lake has fallen beneath its natural rim on only 20 occasions, and only for a few months or years at a time, but these periods could be as long as 10 to 20 years, eliminating a large part of the downstream water supply for Reno, Pyramid Lake and agriculture.
  • Lake turnover (vertical mixing): By the second half of the 21st century, there are likely to be decades-long periods when the lake stops mixing to the bottom. Historically, this complete mixing or turnover has occurred once every four years, on average. Turnover carries oxygen from the surface to the bottom of the lake. Without oxygen, a large part of the lake will be inhospitable to trout and other game fish.
  • Lake clarity: Prolonged absence of mixing could start a physical-chemical process resulting in the release of large amounts of phosphorus from the lake’s bottom sediments. If this phosphorus reaches the surface, it will feed algae. Algae are one factor contributing to reduced water clarity. In addition, when algae sink to the bottom and decompose, oxygen is taken from the deep water, thus creating a downward spiral in water quality and clarity.

“The Total Maximum Daily Load (TMDL) program, which prioritizes fine-particle removal to improve clarity, may be the most important local action to be taken to help mitigate against climate change,” says Schladow.

Phosphorus, the limiting nutrient at Lake Tahoe, will be reduced in direct proportion to the removal of fine particles, and this will help reduce the oxygen demand of the lake and increase its resilience to change.

“Lake Tahoe is very close to at least two tipping points. First, a decrease in deep mixing could trigger abrupt changes in water quality and clarity. Second, our modeling results show that a decades-long shut-off of lake outflow by the end of the century is likely, cutting off a large part of the water supply for Reno and other downstream users,” says researcher Robert Coats, a consulting hydrologist.

“When we add the expected terrestrial impacts of increased drought—especially tree deaths and wildfire—it is clear that by 2100, the Tahoe Basin will be different from the one we know today.”

The study was funded by the U.S. Forest Service and the U.S. Bureau of Land Management, through the Southern Nevada Public Lands Management Act.

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