Thursday, April 2, 2015

California’s Deepening Drought: Ten Things to Know

California’s four-year struggle with drought entered an ominous new phase on Wednesday, as the California Department of Water Resources conducted its annual April 1 survey of the crucial Sierra snowpack. Runoff from snowmelt across the Sierra Nevada provides about 30% of the state’s average water supply. Nobody expected good numbers to emerge from the April 1 survey, given the obvious lack of snow across the Sierra, but the report was still a shocker. As the agency put it in its headline, without any need to exaggerate, “Sierra Nevada Snowpack is Virtually Gone.”

Figure 1. A study in high contrast: the Sierra snow survey being conducted at the Phillips course on March 28, 2013 (top) and April 1, 2015, with California governor Jerry Brown addressing reporters (bottom). Image credit: California Department of Water Resources.

As of Wednesday, the snowpack held just 1.4 inches of water--only about 5% of its usual water content for the date, compared to a previous record low of 25% in both 1977 and 2014. At the Phillips snow course (elevation 6800 feet), the ground was completely bare for the first time in 75 years of early-April measurement. In a typical year, that course would be covered by more than five feet of snow. California’s wet season is rapidly drawing to a close, so there’s little hope of any major recovery in the snowpack, which is dwindling fast at a time when it ought to be peaking (see Figure 2). 

In a press conference linked to the survey results, California governor Jerry Brown announced the state’s first-ever mandatory water restrictions, including a 25% statewide cut in municipal water use compared to 2013 usage. California is the nation’s most populous state and the world’s eighth-largest economy, so the drought and its implications will be major news for many weeks and months to come. What are some of the key take-away points for now?

•  This winter has not been the driest in California history. Some 40% of California’s precipitation comes in the form of powerful “atmospheric river” storm systems, separated by long dry spells. This year the divide was especially sharp, with a sequence of storms in December and another in February providing the great bulk of this winter’s rain and snow. Across most of the state, these storms were bountiful enough to push precipitation totals for the water year to date (October 1 - March 31) somewhat higher than the values seen over the same period in 2013-14, though still below average. The totals through March 31 were roughly 70 - 90% of average across northern California, 50 - 80% in central CA, and 40 - 60% in the south. San Francisco’s wet December (11.70”) obscures the fact that downtown SF followed up with its driest January-to-March in 156 years of record keeping. As noted by Bay Area consulting meteorologist Jan Null, only 1.59” fell, compared to the previous record of 2.31” set in 2013. (The next driest Jan-Mar was 3.20” in 1851.)

Figure 2. The already-pathetic snowpack in the Sierra Nevada has been dropping just at the time it would be peaking in a typical year. Image credit: California Department of Water Resources.

• This winter has been the warmest in California weather history, by far. On a statewide level, what’s truly historic hasn't been the lack of precipitation but the extremely mild temperatures. According to NOAA’s National Climatic Data Center, the average temperature for California during meteorological winter (Dec - Feb) of 49.5°F was far above the previous winter record of 47.2°F, set in 1980-81. Most recently, towns and cities across the state shattered records for their warmest March, including Sacramento as well as many reporting stations in the Los Angeles and San Diego areas. 

Figure 3. Temperatures averaged well above normal across most of the western U.S. in March. Image credit:Western Regional Climate Center.

• The “snow drought” is truly off the charts. This winter’s extreme mildness meant that much of the usual snowfall over the Sierra Nevada either arrived as rain or melted quickly. Never in its recorded history has the Sierra ended up with so little snow by winter’s end. Most of the ski areas in the Lake Tahoe region were closed by mid-March, weeks ahead of schedule. Some high-elevation rivers and streams will struggle to avoid running completely dry this year.

• Reservoirs are fuller than you might expect--but not for long. Thanks in large part to the unusually sizable fraction of rain vs. snow in the Sierra, reservoirs benefited from generous winter runoff, although most are still well below average (see Figure 4). According to Jan Null, the state’s reservoirs as a whole are now about 5% closer to full capacity than they were on June 30 of last year. Alas, the current numbers should drop more quickly than usual this spring, since there will be very little snow melt to replenish the reservoirs.

Figure 4. Reservoirs across California (blue levels) were running well below capacity (top of bars) and historical averages (red lines). Blue values below each reservoir show the percent of capacity; red values show the percent of long-term average. Image credit: Western Regional Climate Center.

• High-elevation fires could be ferocious this summer. California’s lower-elevation grassland and shrubland fires tend to be at their worst when a wet year fosters growth that dries out over the following year. It’s a somewhat different picture in the high-elevation forests of the Sierra, where unusually dry, warm conditions in springtime are the main factor setting the stage for summer wildfire. The lack of snowpack and the persistent record heat suggest that a very serious wildfire risk could emerge at higher altitudes this summer. In its spring/summer outlook issued on Wednesday, the National Interagency Fire Center projected above-average wildland fire risk by June and July across western Idaho, southwest Arizona, and most of California, Oregon, and Washington. “The current drought is very likely going to increase the number and severity of fires in the mountains, but will likely reduce fire activity in the foothills due to reduced grass growth,” USGS research scientist and Sierra fire expert Jon Keeley told me in an email. 

• Agriculture may face a year of reckoning. Some 80% of California’s developed water supply is used by agriculture, and the recent drought has prompted many growers to pump from groundwater at increasing rates. The state has lagged in forcing public disclosure of groundwater use, so it’s hard to assess how quickly the aquifers are being depleted, but any natural recharge will take many years, and the cost of pumping goes up significantly as the water table goes down. The state’s Mediterranean climate, with its wet, mild winters and dry, warm summers, is ideal for growing a huge array of crops, but some are far more water-thirsty than others, and irrigation techniques (such as flooding an entire field vs. using drip lines) make a huge difference in how much water is needed. This year may force large segments of California agriculture to confront hard choices.

•  Other parts of the Southwest are also contending with serious drought. Unlike California, the Southwest has been in near-continuous drought since 2000, with only modest spells of relief. Nevada’s Lake Mead, which provides water to Las Vegas as well as Los Angeles, continues to be far below capacity, and projections are that the lake level will hit another record low this spring, following a record low last summer. With snowpack across the upper Colorado River basin again less than average, Arizona’s Lake Powell (Figure 5, below) is also expected to remain close to its record lows. The decline in both lakes is prompting serious long-term concern about water and power availability throughout the region.

Figure 5. “Bathtub ring” water lines are visible on March 30, 2015, on a section of Lake Powell formerly under water near Big Water, Utah. Lake Powell is currently at 45 percent of capacity and is at risk of seeing its surface elevation fall below 1,075 feet above sea level by September, which would be the lowest level on record. The Colorado River Basin supplies water to 40 million people in seven western states. Image credit: Justin Sullivan/Getty Images.

• Even a strong El Niño may not save the day. For more than a year, Californians have been teased by the off-and-on signals that a major El Niño event could bring a wet year to the state. Computer models are now increasingly emphatic that El Niño conditions will intensify to at least moderate levels by summer, and climatology would favor any such event growing during the fall and winter. But while strong El Niño events do raise the odds of a wet winter substantially, a drier-than-average year is still possible, and weak-to-moderate El Niño events have a much less consistent influence.

•  There’s really no telling how long the drought will last. Paleoclimatology tells us that megadroughts lasting one to several decades--even centuries--have struck California over the last 1200 years. The factors that produce such long-lived drought have yet to be nailed down, although some research has pointed to persistent La Niña-like conditions in the tropical Pacific. Putting wishful thinking aside, policymakers and citizens must grapple with the chance that the current drought could continue for years more. What’s more, the effects of any precipitation downturn are increasingly likely to be exacerbated by above-average temperatures. As we’ll explore in an upcoming post, unusual heat is becoming the norm when California experiences long dry spells, and this raises major concerns about the impacts of future drought in a warming climate.

•  Any winter could still bring catastrophic floods to California. Paradoxical and perverse as it may seem, a warming planet tends to boost both ends of the hydrologic spectrum. Warmer air temperatures enhance evaporation from land, making dry soils even drier, while also increasing evaporation from the oceans, adding moisture to the air and increasing the ability of a given storm to dump rain or snow. When it does rain in California, it can pour--as evidenced by the devastating floods of 1861-62, which inundated much of the Central Valley and put downtown Sacramento under ten feet of water. A similar event today could produce more than $300 billion in economic impacts, according to a 2011 USGS study. New research is digging into the latest round of coordinated climate modeling (CMIP5) and what it reveals about possible changes in California hydrology. Such work is still ongoing, but one study led by Michael Warner (University of Washington), published last February in the Journal of Hydrometeorology, suggests that average winter precipitation along the North American west coast could increase by 11% to 18% by the end of the 21st century. The bump-up appears to be almost completely due to richer atmospheric moisture rather than increased winds. The study indicates that the most extreme days (as measured by vertically integrated water vapor) could see anywhere from 15% to 39% more precipitation. Thus, in a state renowned for extremes of rain and snow, there are signs that the future may hold even more “extreme extremes.”