Sierra snowpack levels to taper, says research; could mean spring, winter flooding
Rescuers rafted to people’s homes to pluck them to safety. Bridges washed out. Mud caked several residents’ floors. North Tahoe bore the brunt of an estimated $35 million in damage countywide.
That 14-inch onslaught of rain that rang in 1997 wasn’t the kind of precipitation North Tahoe and Truckee are used to around New Year’s.
But in decades to come, global warming could accelerate snowmelt enough to cause similar winter and spring flooding, according to preliminary projections to be presented next week by researchers from the U.S. Geological Survey and the University of California, San Diego’s Scripps Institution of Oceanography.
Scientists Mike Dettinger and Noah Knowles spent the past year researching how a climate increase of 2.5 degrees Celsius over the next 60 years would affect Sierra Nevada snowpack levels.
All other factors being the same – precipitation levels, greenhouse gas emissions, etc. – the impact of a temperature change as slight as 2.5 degrees Celsius could be dramatic.
Preliminary results suggest the temperature change would cause snow to melt rather than build up, reducing snowpack and thereby changing the seasonal cycle water managers rely on to meet the needs of millions of Californians and Nevadans.
“In a large part of the Sierra, you end up with a considerable reduction – down to 10 percent of today’s levels in some places, virtually removing the snow,” Dettinger said.
Mid-elevations of 5,000 feet and below would sustain the most dramatic effects of global warming, but higher elevations could still be affected, he said.
While the pair’s research encompasses the entire Sierra Nevada, Dettinger also has applied the same scientific models to specific sites within the Sierra, including the Carson River and Merced River valleys south of the Tahoe Basin.
Although both rivers end at an elevation of about 5,000 feet, their loss of snowpack likely won’t be quite as dramatic as sites on the western slope.
He projects the maximum average snowpack levels to decrease by 25 percent at the Carson River and as much as 50 percent on the Merced River.
“It’s still significant,” Dettinger said. “It doesn’t amount to a removal of snow, like it does in other places, but it still changes the amount considerably.”
Even though he and Knowles stress that these results are preliminary and require further research, they said the consequences could be far-reaching enough to warrant discussions on water management policies. Indeed, they plan to present their findings at a state Department of Water Resources meeting next week in Los Angeles, where a panel of advisors will continue their work to draft the state’s next water plan, which is scheduled for release in 2003.
They’ll have much to talk about.
Lower snowpack, less spring snowmelt
“If you don’t have snowpack accumulating as much, you have higher flood peaks during the rainy season,” Knowles said. “Conversely, since you do have more water running straight off, there will be less water recharging the reservoirs. That has a whole range of potential impacts.”
Current water management practices, including those at Lake Tahoe and area reservoirs, are based on typical cycle of winter snow accumulation followed by steady spring snowmelt.
A change in that cycle could render those management policies ineffective.
“Because that runoff happens during winter, when people aren’t so much wanting the water to come down, there are risks of there being a flood,” Dettinger said. “Under current management situations, if the water isn’t there in snowpack by April or March, then it might as well not be there. If it comes in January or February, it’s not of use to you.”
Water managers such as Federal Water Master Garry Stone are bound by strict rules regarding lake levels, streamflows and release rates from lakes and reservoirs to rivers and streams.
In the case of the 1997 flood, Stone followed those rules and maintained Lake Tahoe at a level that wouldn’t exceed the federally mandated elevation of 6,229.1 feet, as projected under that season’s snowpack levels.
The warm winter storm disrupted the balance of the system, Stone said.
This year’s dry winter presents the flip side to that coin.
Snowpack was just 40 percent of normal in the Tahoe Basin, the spring runoff fell well short of normal, creating a different set of management challenges.
“We basically do our planning based on end-of-the-month snow totals during winter time,” he said. “We have to operate on the same basis we’ve been operating under for 100 years – day-to-day and week-to-week and month-to-month. We have to follow the rules that are established.”
Impact on wildlife, plantlife
Aside from water management, a change in precipitation patterns could impact wildlife and plantlife, depending on the severity, said Tahoe Research Group researcher Bob Richards.
“With less storage of snow, it could change things,” Richards said. “If the climate becomes drier, the vegetation in the basin could change to more arid types of flora. We could see more sagebrush and less trees that love water. If the streams are flowing less of the year, you could have a reduction or even a disappearance of some of the plants and animals that utilize those stream courses.
“As our famous forefather said, everything is hitched to everything else. Get one change here and it can ripple through the entire ecosystem.”
Even so, more research is needed on global warming and its impacts, he said.
As such, Knowles and Dettinger will continue their research in an attempt to bring forth more reliable data.
“Next is to try to get harder numbers and something that provides more than just a talking point for managers,” Knowles said.
In the meantime, these preliminary data can launch discussions about the potential consequences of global warming, a concept that is often debated without real understanding, he said.
“The overreaching message of this work is that even though there’s a lot of uncertainty involved in the problem of global warming and the impacts, this kind of work really helps make it clear that what may seem like an abstract problem with no relevance to our lives really can have significant impacts on people’s everyday lives – if not for them, then for their children.”
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