When Will Lake Mead Go Dry?
Exposed turbine intakes and the "bathtub ring" at Lake Mead. Photo: Craig Miller
You can see a slide show of the retreating waters at Lake Mead and Hoover Dam and listen to my radio feature from The California Report. Also, The American Experience will rerun its documentary on Hoover Dam, Monday night on most PBS stations.
The Las Vegas Sun has a digital clock on its website, counting down to a theoretical doomsday when the city's principal source of water would go dry. Wagering on that question may not have found its way into the sports books on the Strip–but it did become a lively pastime among engineers and hydrologists, when a report emerged from San Diego's Scripps Institution, with a dire forecast. The paper, by climate physicist Tim Barnett, put the odds at 50-50 that Lake Mead, the giant reservoir behind Hoover Dam, would reach "dead pool" by 2017. That's the point at which the dam shuts down and neither hydroelectric power nor water emerges from it.
The Barnett study "definitely raised eyebrows throughout the basin," admits Terry Fulp, deputy director of the U.S. Bureau of Reclamation's Lower Colorado Region, which operates Hoover Dam and Lake Mead. As it turns out, Barnett was a bit pessimistic. Subsequent work by him and others revealed that he overestimated the evaporation rate at Lake Mead, and omitted inflows below a certain point on the river.
The bottom line, according to Balaji Rajagopalan at the University of Colorado: Doomsday is not quite that near at hand. But that doesn't mean it's not on the horizon. "After 2027, the demand increase outpaces the supply decrease," Rajagopalan told me in a recent interview. "And that’s why much of the risk explodes from 2027 to 2057."
All of these studies are couched in probabilities, much in the same way that the Corps of Engineers talks about a "100-year" flood. Rajagopalan says: "Even in our study, we have a 50% risk [of dead pool], but that occurs in 2057. And that makes a big difference in terms of water managers, what they can do."
One of those managers is Pat Mulroy, who directs the Southern Nevada Water Authority. Her constituents rely on Lake Mead for 90% of their water, so she says she's not inclined to wait around for a consensus. "I mean, during the entire period of the ‘90s when we were bickering with our friends in the lower basin over surpluses, there was zero probability that the drought that we’re currently in was going to happen," Mulroy told me. "I’ve lost confidence in probabilities."
The Bureau's Fulp says the Colorado system leans heavily on the huge water storage capacity of Lake Mead and its sister reservoir upstream, Lake Powell. "We’ve known for decades that this system is highly variable and that’s why so much storage was built." When filled to capacity (which it was, more or less, 10 years ago), Lake Mead alone can hold enough to put an area the size of Pennsylvania under a foot of water. But a 10-year drought has left Mead at just over 40% of capacity (so think of flooding something more the size of Costa Rica). Just as current evidence and climate models both point toward lessening flows on the Colorado, many parts of the southwest still see relatively high population growth.
Scientists continue to run their statistical models aimed at handicapping the Colorado's demise as a dependable bringer of water. But as Fulp sums it up, "It’s really a debate about when. It’s not really 'if."
I regret an error of my own that appeared in the radio feature. I misstated the number of people in southern Nevada who are dependent on water from the Colorado. The correct number is about two million.
Yosemite's Fiery Future
Photo: Tim Walton
California’s Yosemite National Park has been scarred by several big fires in recent years—the latest contained less than two months ago. But new research affirms that this crown jewel among national parks is likely to have even more fire in its future.
In late August, when fire crews attacked the Big Meadow Fire in Yosemite, it was hard to blame nature for the 74-hundred acres lost. That was a "prescribed burn" that got out of hand (or "escaped," as the official report puts it). But nine out of ten wildland fires in the Sierra start with a lightning strike. Newly published work suggests that as California’s climate changes, the combination of warmer temperatures, less snow and more lightning strikes could mean 20% more fires by mid-century.
USGS research forester emeritus Jan van Wagtendonk co-authored the study with James Lutz at the University of Washington. He says they studied 20 years of Yosemite fire data to identify a trend. The mechanism starts with the oft-cited warming scenario, causing more rain and less snow at upper elevations.
"What happens in the mountains is that, as snow recedes in the spring the moisture in the fuels follows," says van Wagtendonk. "The fuel starts drying out earlier and we extend the fire season by having more days available for fires to burn."
But there’s another wildcard in the deck: lightning. Separate studies suggest that higher concentrations of atmospheric carbon dioxide will set the stage for more lightning strikes.
The study assumes a 17% reduction in snowpack by 2050—under the relatively modest B1 warming scenario, drawn from IPCC models. The results are in line with other climate studies that imply not just more fires, but more intense fires as the climate warms. It's a trend, says van Wagtendonk, that has already started:
"We were able to trace, through satellite imagery, the change that we’ve seen in the severity of those fires just over the past 20 years, so it’s been obvious to us from those data that whatever temperature trends are occurring today are already having an effect on increased severity."
"We see more of the same," said the forester, "and a continued increase in both size, number and severity of fires."
Yosemite fires from 1984-2005. The black triangles indicate fires sparked by lightning. Image: International Journal of Wildland Fire.
Scott Stephens, an associate professor of fire science at UC Berkeley, says lightning is changing the landscape in more ways than one.
Stephens recently told KQED's Central Valley Bureau Chief, Sasha Khokha: "In Yosemite and Sequoia-Kings Canyon, they manage quite a few lighting fires in the wilderness area, away from people, and they allow these things to burn for months and months and months to try to allow that lighting fire to begin to shape the landscape again like it did 100 or 200 yrs ago. Those types of events probably increased the resiliency of the forest to deal with climate change and other impacts."
The article is published in the current issue (10/27) of the International Journal of Wildland Fire.
KQED's Central Valley Bureau Chief, Sasha Khokha, contributed to this post, as well as to the radio report.
Seeding Clouds for Hydropower
PG&E cloud seeders located near Burney Falls, CA. Photo: PG&E
Christina Aanestad's radio feature for Climate Watch airs Monday morning on The California Report.
Wringing Hydropower Out of the Clouds
By Christina Aanestad
When cloud seeding began in the 1950’s there were no laws governing weather modification. According to Maurice Roos, Chief Hydrologist with the state Department of Water Resources (DWR), it wasn’t until the late 1970’s when a storm in a seeded area near Los Angeles flooded, that regulations governing weather modification were included in the state’s water code. In the West, “Most of the states have legislation that governs the conduct of weather modification activity,” says Brant Foote, director of the Research Applications Lab at the National Center for Atmospheric Research (NCAR) in Boulder, Colorado.
Government oversight has changed over the years. Today in California, state regulations have slackened. “As for the State’s role, it is mainly informational. There are no permits or licenses,” said Roos. According to Roos, all cloud-seeding projects required permits until the law was reformed. “The old law required licenses and permits but it was repealed in the 1980’s. There was a general move toward deregulation in the government–mainly to reduce costs.” Today, according to Roos, Sponsors of cloud-seeding projects must notify DWR and county governments of the project, “This can be a letter or, for DWR, an e-mail notice,” he said. “They also have to publish a Notice of Intention in the county or counties affected by their proposed operations.”
Most of what this reporter learned was from Roos’ institutional memory, and going directly to sponsors of cloud-seeding operations–about 15 intermittent projects around the state. Data on cloud seeding at the state level is scattered, according to Roos. “We used to have an annual report that was published. Last time I tried to find it, it was in an archived box and nobody knew where it was,” said Roos, who added that budget cuts and deregulation mostly gutted the oversight program.
Despite lax oversight, the State of California wants to use weather modification as part of its 2009 Water Plan, which states:
“Cloud seeding has advantages over many other strategies for providing water. A project can be developed and implemented relatively quickly…it could offset some of the loss in snow pack expected from global warming.”
According to the plan, some regulation remains: weather modification sponsors need to comply with the California Environmental Quality Act [CEQA]. But not all seeding has to comply with environmental regulations. PG&E contends that an environmental impact report is not required for its Pit-McCloud River project because it is privately funded, with equipment on private lands,” said Roos.
That has locals groups near Mount Shasta concerned with PG&E’s proposed project in the Pit and McCloud River watersheds. “It’s a clear unequal treatment between public agencies and private entities,” said Angelina Cook with the Climate Council and the Mount Shasta Community Rights Project. “Private corporations require more government oversight and regulation to ensure accountability for the their practices.” But compiling all cloud-seeding data in California into one reference source today would be “a labor of love,” says Roos. “There’s no funds for it,” he said.
Cook says she is working on a cloud-seeding ban in Mount Shasta City, which may include a chemical trespass for silver iodide, the common chemical used in cloud seeding. “If silver iodide is found in the area, PG&E would be liable,” said Cook.
But Roos, who says cloud seeding is mostly benign, asks where one would draw the line. “There’s all kinds of influences on the air like people driving their cars, diesel trucks running around,” said Roos. Just as California has increased its regulations on air emissions in the state, some like Cook would like to see tougher regulations for weather modification as well.
Meanwhile, the state’s 2009 water plan also urges more research and development into cloud-seeding. Research could include cloud seeding’s impact on global climate change, and it’s effectiveness. The plan also identifies areas that could provide optimal results from cloud seeding, mostly in Northern California, along the Sacramento, Trinity and Russian Rivers.
Cloud Seeding Projects in California
View Cloud Seeding Projects in California in a larger map
To find references to cloud-seeding in the state’s water plan, look under Volume 3, then for "Precipitation Enhancement."
Parsing the White House Climate Report
At least one researcher cited in the 196-page climate impacts report issued this week by the Obama administration is not impressed with the final product. Roger Pielke of the University of Colorado's Center for Science & Technology Research has written a blog post critical of the report and in particular, the way in which his work was interpreted. If you'd rather not plow through the entire post, John Tierney has an overview of Pielke's critique on his blog for the New York Times.
2004 California heat wave, from the Aqua satellite. Image: NASA
The report was arguably the first to break down both observed and projected effects of climate change into coherent regional summaries. For the purposes of the report, California was considered part of the Southwest region, which included states as far east as Colorado and New Mexico.
Not surprisingly, many of the points raised in the Southwest section (beginning on p. 129) have to do with water supply. Most have been reported or discussed in our Climate Watch coverage, either here or in our radio reports. Selected "highlights" include:
- Past climate records based on changes in Colorado River flows indicate that drought is a frequent feature of the Southwest, with some of the longest documented “megadroughts” on Earth.
- The prospect of future droughts becoming more severe as a result of global warming is a significant concern, especially because the Southwest continues to lead the nation in population growth.
- Human-induced climate change appears to be well underway in the Southwest. Recent warming is among the most rapid in the nation, significantly more than the global average in some areas.
- Projections suggest continued strong warming, with much larger increases under higher emissions scenarios compared to lower emissions scenarios. Projected summertime temperature increases are greater than the annual average increases in some parts of the region, and are likely to be exacerbated locally by expanding urban
heat island effects.- Water supplies in some areas of the Southwest are already becoming limited, and this trend toward scarcity is likely to be a harbinger of future water shortages. Groundwater pumping is lowering water tables, while rising temperatures reduce river flows in vital rivers including the Colorado.
- Projected temperature increases, combined with river-flow reductions, will increase the risk of water conflicts between sectors, states, and even nations.
- Increasing temperature, drought, wildfire, and invasive species will accelerate transformation of the landscape.
- Under higher emissions scenarios, high-elevation forests in California, for example, are projected to decline by 60 to 90 percent before the end of the century.
- In California, two-thirds of the more than 5,500 native plant species are projected to experience range reductions up to 80 percent before the end of this century under projected warming.
- Projected changes in the timing and amount of river flow, particularly in winter and spring, is estimated to more than double the risk of Delta flooding events by mid-century, and result in an eight-fold increase before the end of the century.
- A steady reduction in winter chilling could have serious economic impacts on fruit and nut production in the region. California’s losses due to future climate change are estimated between zero and 40 percent for wine and table grapes, almonds, oranges, walnuts, and avocados, varying significantly by location.
By the way, Pielke's critique does not directly address anything in this list, though his work does involve weather-related disasters, which would include floods. Asked by a commentator on his blog if he thinks the entire report should be dismissed based on the flawed interpretation of his research, Pielke replied: "I wouldn't think so and would certainly hope not. At the same time the section which covers my research does not give me a lot of confidence in the process that led to the report."
More Adventures "Paddling to the Sea"
Here are some more journal entries from participants in the Paddle to the Sea project, to raise awareness of river and water supply issues. For most, it's a relay event and they're taking a leg or two of the voyage from the upper Tuolumne to the San Francisco Bay. But two paddlers have pledged to go all the way. Here are some of their observations.
Facing the froth--and mortality--on the Tuolumne. Photo by Jayne Johnson.
Emilio Martinez: whitewater section, Tuolumne River
Once the green terror subsided and I knew I might not drown, rafting the "T" (river rat lingo) was an absolute blast! Our blue rafts, manned by a crew of four apiece, plus an experienced guide at the helm, put in about ten miles upstream from where the Clavey River and the Tuolumne River meet in a thunderous embrace. And I am not ashamed to tell you that seeing the river run at 7000+ cubic feet per second was exhilarating to the point of terror. Whether due to climate change or pure luck, the Tuolumne was in fine form that weekend: racing down its bed like some aquatic serpentine creature from another dimension, wildly snaking its way through ancient bedrock as it bucked and thrashed, arching into the wind, pummeling its way through narrow confines of rock and tree as only a force of nature unleashed can.
For two days, thirty of us clung to the River, loving it and fearing it, crawling out of our rafts each day, tired and hungry, but somehow spiritually cleansed by the only worldly thing I've ever known that I can call sacred.
Unlike my rafting cohorts–who likely must travel some distance to the "T" to pay it homage–as denizens of Modesto's "Airport District," we are blessed with the Tuolumne as a constant benevolent feature of our neighborhood (emphasis on hood), which squats on the River's banks, roughly one square mile of humanity and squalor broiling in the San Joaquin Valley, and whose inhabitants are more known for our deficits than our merits, for our God-fearing ways rather than earth-loving hearts.
My first encounters with the "Tuolumne" were a mix of horror (fear of drowning), awe (Pentecostal baptisms), joy (childhood treasure hunts at the Modesto landfill - now filled up and landscaped into something called the Tuolumne River Regional Park), and food (Mexican fiestas, where beer, barbeque, and corridos mixed nicely).
And, as befits my low-income neighborhood: I am still largely ignorant of the ecology of this gorgeous river.
So, speaking for myself, my trip down the Tuolumne River is something like an awakening to nature in the Garden of Eden, for there is precious little vocabulary in the noisy, backward streets of the Airport District for the marvels of nature that abound on the banks, canyons, and hillside forests that flank the River: though we children of the Airport grow up touching the River almost daily, we are taught almost nothing about its nature or how to address it.
Shame.
Still, I figure (hope) that my vocabulary for its flora and fauna and geology will grow as I flow downstream towards the San Francisco Bay; that I'll be able to distinguish an oak from alder some day, egret from swallow, poison ivy from blackberry vine, and that my eyes will see "salmon" and not just vague swimming things.
I exaggerate, but not by too much.
Emilio Martinez grew up in the Central Valley and currently lives in Modesto. The rivers are uncharted territory for him and have inspired his poetic side.
Segerstrom and Martinez on the river. Photo by Jayne Johnson.
Owen Segerstrom: whitewater section, Tuolumne River:
It is a privilege to be on the "T" (Tuolumne River) during a peak snow melt flow like the one we experienced over the weekend. My cousin Tom, son of Sierra Mac founder and TRT (Tuolumne River Trust) board member Marty McDonnell, said it was the second highest flow he had ever seen (out of an estimated thirty-five trips).
Our guide, an east coaster working his first season for Sierra Mac, opined that the T would be the national gold standard in whitewater if such flows were a regular occurrence. We took a nasty swim on a 4+ rapid called "Frame Crusher," a humbling reminder of the river's power. The excitement of the water conditions was rivaled only by the magnificence of the river canyon itself. As if to remind us not to stay away for too long, a bald eagle swooped above us on an up-canyon trajectory less than a mile before the take-out.
Owen Segerstrom grew up in Sonora, near the upper reaches of the Tuolumne River, and spent his childhood and teenage years exploring the Tuolumne and Clavey swimming holes. He’s also done quite a bit of rafting on the Tuolumne.
Paddling to the Sea
Jessie Raeder is Bay Area Organizer for the Tuolumne River Trust. More than 200 paddlers are expected to take to the river between now and June 7, for this river awareness relay. Raeder offers this dispatch from the starting line, near the headwaters in Yosemite National Park.
By Jessie Raeder
Paddle to the Sea got off to a roaring start over the weekend. Melting snow caused by an early heat wave in the Sierras had the river pumping at much higher flows than rafters typically face. The Tuolumne is usually rated Class 4–but goes to Class 5 when flows get over 4000 cubic feet per second. This weekend saw the river flowing at 7000 cfs! For a while it looked like we might have to cancel all the whitewater trips due to safety concerns, but in the end one trip did go on and needless to say it was an epic journey for our Paddle-to-the-Sea team.
A paddler takes on the Clavey. Photo: Patrick Koepele, Tuolumne River Trust
Meanwhile, an international team of 12 hotshot kayakers ran the Clavey River, a tributary of the Tuolumne and one of only three remaining free-flowing rivers in the Sierra. The Clavey is a class 5+ river that's rarely run and only by experts.
Paddle to the Sea is a three-week festival to foster stewardship of the Tuolumne River. Hundreds are joining in this epic journey from the Sierra to the Sea. Kayakers and rafters will begin on the upper stretches of the Clavey and Tuolumne Rivers, travel through the Central Valley where canoers take the lead, pass through the confluence of the Tuolumne and San Joaquin Rivers, and sea kayakers will finish the trip in San Francisco Bay.
A changing and increasingly unpredictable water supply will be the first way that most people in California experience climate change affecting their own lives. Here in the Bay Area, tap water for 2.5 million people comes from the Tuolumne River, whose headwaters start with melting snow from the Lyell Glacier (picture attached). That glacier is on the retreat, and more frequent droughts are expected throughout the Sierra.
Paddle to the Sea is meant to demonstrate that issues that affect one section of the river ripple up and down the watershed. Bay Area water users share this resource with farmers in Modesto, anglers in Yosemite, the commercial salmon industry on the Pacific, and a host of native fish, plant, and wildlife species, many of which are endangered.
As population and demand for water continues to grow, California will be faced with many questions about how we use water, and where it will come from. The Tuolumne River Trust is working to ensure that we turn to water efficiency and water recycling as much as possible–alternatives which are far more sustainable and renewable than continuing to take additional water from the Tuolumne River, which has been the solution most turned to in the past.
Pika One Step Closer to ESA Listing
American pika. Photo by Chris Ray.
UPDATE: Federal fish & wildlife authorities have decided to proceed with a full review of the American pika, for potential listing under the Endangered Species Act. The US Fish & Wildlife Service will formally publish its decision this week, including this summary:
"We, the U.S. Fish and Wildlife Service, announce a 90-day finding on a petition to list the American pika (Ochotona princeps) as threatened or endangered under the Endangered Species Act of 1973, as amended. We find that the petition presents substantial scientific or commercial information indicating that listing of the American pika may be warranted. Therefore, with the publication of this notice, we are initiating a status review of the species, and we will issue a 12-month finding to determine if the petitioned action is warranted. To ensure that the status review is comprehensive, we are soliciting scientific and commercial data regarding this species. We will make a determination on critical habitat for this species if, and when, we initiate a listing action."
The Center for Biological Diversity (CBD) first petitioned for listing in 2007, and then followed with a lawsuit a year later, when federal authorities shelved the request.
The significance of this week's decision, according to a CBD news release, is that "the pika will become the first mammal considered for protection under the Act due to global warming in the continental United States outside of Alaska."
Last month a San Francisco court ruled that state wildlife officials wrongly denied the CBD's petition for listing under California's ESA. So it looks like the little critter will get a fresh review at both the state and federal levels.
Pivotal Week for Pika Protection
American pika. Photo by Chris Ray.
Note that an update to this story was posted on May 6.
The hamster-sized, high-elevation haymaker known as the American Pika has had its "day" in court–and then some. Now it may be making inroads toward listing as a threatened species, while questions persist over whether that would be premature.
Friday was the deadline for officials at the U.S. Fish & Wildlife Service to decide whether to further consider the pika for listing under the federal Endangered Species Act.
The San Francisco-based Center for Biological Diversity (CBD) has been pursuing listing for the pika under both the state and federal Endangered Species Acts. On April 16, a Superior Court judge in San Francisco ruled that the California Fish and Game Commission applied too stringent a standard, when it voted last year to reject the CBD's petition to list the pika under the California law. The CBD says it expects the court to formally order the state to go back and take a second look at whether the critter deserves protection.
Meanwhile federal wildlife officials had until May 1 to decide whether to formally review the pika's plight and consider listing it under federal law. A response is expected to be published in the Federal Register this week.
Complicating the case is an apparent difference between the fate of pika populations in the Great Basin, where field research clearly shows pika colonies in trouble, and colonies in the Sierra Nevada range, which may be faring better.
Pika thrive only at high elevations, in the rocky conditions known as talus. Their band of tolerance for temperature is very narrow, so some biologists see them as an indicator species for global warming. Temperatures that humans may consider merely balmy, can be fatal for pika.
Chris Ray, an ecologist at the University of Colorado, has studied pika in the mountain ranges of the Great Basin. She's identified and ranked several stress factors that pose threats to the animals, including habitat shrinkage and exposure to both heat and cold.
Ray, who presented her latest research at the USGS-sponsored Pacific Climate Workshop last month, is cautious about endorsing an ESA listing just yet, saying: "I do not think there are data indicating that the species as a whole is in danger of extinction, however the loss of isolated populations from the Great Basin has me concerned."
"I think it's very reasonable to consider potentially listing some sub-populations of pika." Ray says that in order to do that, a case would have to be made that there are genetically distinct sub-species of pika. In its petition, CBD claims that five sub-species have been identified in California. But scientists at UC Berkeley and the U.S. Forest Service who have done field research in the Sierra, have said it's less clear that those colonies are in trouble.
CBD staff biologist Shaye Wolf says a 1995 study found "evidence for four genetic units across the pika range, roughly grouped as Sierra Nevada, Cascades, Southern Rockies, and Northern Rockies. However, better genetic analyses using more sensitive genetic markers (like microsatellites) are necessary to understand pika population structure."
Wolf says that for its ESA petition, the CBD drew on a 1981 study that used population distribution to break out 36 "subspecies" of pika.
Sierra Snow Season Ends with a Whimper
Surveyor Frank Gehrke takes one last poke at the season's shrinking snow pack. Photo by Craig Miller.
When veteran snow surveyor Frank Gehrke stuck his aluminum tube into the thinning snow course at Phillips Station this morning, the "Mount Rose" snow gauge stopped dead a few inches in. He then withdrew a sad little cylinder of corn snow, about the size of a flashlight battery. There was barely a foot of snow left at this measuring station, 6,900 feet above sea level. Water content: 35% of normal. Yikes. Of course, that's just one station among many surveyed on a monthly basis to help handicap the coming summer's water supply.
The numbers are in for the season's last snow survey: On average throughout the Sierra Nevada, water content clocks in at 66% of normal for this date. Last year at this time it was 72% of normal. The southern third of the Sierra came in at 61%.
So even with that hope-lifting late-season burst of precipitation that started in mid-February, we ended up even worse than last year–at least in terms of the snowpack. Some local reservoirs filled up nicely with the soggy spring. The trouble, says Gehrke, is that "The big ones didn't."
Some municipal water districts have vacillated on rationing plans for the summer. But the big state and federal systems that supply irrigation water to farms have largely stuck with their drastic cuts in allocations this year.
The bottom line, according to state water director Lester Snow: “When combined with extremely dry years in 2007 and 2008, low storage in the state’s major reservoirs, restrictions on Delta pumping, a growing population and prediction of increasingly unpredictable weather patterns due to climate change, it is clear the problems facing California will persist beyond this year and this drought.”
Official drought proclamations have been a source of some controversy since the rain finally began falling in February. The Department of Water Resources has produced a statewide water plan and put it up for comment until June.
Early Runoff More than Theory
This post has been modified based on clarifications by the study's lead author, which are outlined in her comment, below.
A recent study seems to confirm what many have already surmised: The spring melt from the Sierra snowpack is happening sooner.
To get a handle on the timing of mountain runoff, a team led by Iris Stewart of Santa Clara University pulled together data from 52 stream gauges up and down California. For her study, Stewart says she chose only water courses unaffected by dams and diversions, with at least 20 years of continuous data.
Stewart's data shows that over the 60 years spanning 1948-2008; 80% of the gauges show the "stream pulse" that accompanies peak runoff, coming consistently sooner in the season–an average of about 10 days sooner, though at least one location had shifted up by more than a month. In fact, combining all of the metrics in the study, Stewart says only one gauge showed a later trend.
The trend seems remarkably consistent. Stewart says that despite a warming trend over the past ten years, she has not seen any acceleration of the trend within that period.
Stewart cautions that there's more work to do on this and was reluctant to draw broad inferences from the study. Runoff in a particular stream is affected by many factors, including the elevation, slope, aspect (which direction it's facing), vegetation cover and soil composition. Stewart says further study of these variables will better help identify the most vulnerable streams. But the latest results seem consistent with an earlier study in which Stewart found "earlier runoff on a continental scale."
Scientists are concerned that as average temperatures rise, California's mountains will see more rain, less snow–and what snow there is will melt off sooner. Reservoirs can only retain so much runoff at once, so if more of the "frozen reservoir" dissipates earlier in the season, farms and cities stand to be caught short of water before the rains return.
Stewart, an assistant professor at SCU's Environmental Studies Institute, presented her findings this morning to researchers at the Pacific Climate Workshop (known as PACLIM, the conference does not have a website), a semi-annual gathering of climate scientists doing front-line research around North America. The conference in Pacific Grove is organized by the USGS office in Menlo Park.
Over the course of four days, about 60 researchers will hear findings on the climatic implications for fire, fog, glaciers, the ocean and wildlife, among other topics.
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