Water levels behind California’s most important dams dropped in December, a month when they almost always rise--a new and crucial sign that the state’s most severe drought since the 1930s is intensifying its painful grasp, state water officials said Wednesday.
Water stored in reservoirs is the key to California’s survival through the dry summer months typical of the state’s climate. Most precipitation that occurs in winter runs off into rivers in spring, yet the peak demand for water on farms and in cities doesn’t come until the heat of summer. Until the dams transformed the California landscape, most runoff escaped to the ocean before summer.
December is usually the month when dams that were drained during the summer refill. Typically, 16% of the entire rainy season’s precipitation occurs in December in the northern mountains, the source of most water for the reservoirs relied upon by California cities and farms.
But this December was a washout for rain and snow in the north, spoiling hopes that this winter would deliver normal precipitation and end the 5-year-old drought. Statewide precipitation was 24% of normal for December.
“We lost December and that’s hard to recover from,” Doug Priest, manager of the state’s emergency drought center, said Wednesday. “We’re now looking at a 1 in 10 chance of having a normal year.”
As of Jan. 1, the 155 key dams monitored by state officials held just 54% of their usual New Year’s water supply, Priest said. On Dec. 1 the reservoirs held 57% of their average for that time of year. Last summer ended with the dams at 60% of normal.
Water stored behind the dams totals 12.04 million acre-feet, less than half the 26 million acre-feet in reservoirs in September, 1986, at the start of the drought. An acre-foot, the volume of water that would cover an acre to a depth of a foot, is about the amount an urban family of four uses in a year.
Since 1986, the reserve held behind dams for just such a drought has allowed most of the state to escape severe water rationing. Now the reserve is all but gone, state water officials say, and a heavy winter was needed to avert mandatory rationing in most cities and a cutback of up to 65% for farmers.
January, normally the wettest month in California, so far promises to be no better than December, despite the rain Wednesday in Southern California. In the northern Sierra Nevada, the crucial watershed for the dams that supply the State Water Project, less than 15% of the usual January precipitation has fallen.
Clear skies over Lake Tahoe most of the day Wednesday were evidence that unusual weather patterns are cheating the Sierra Nevada of snow. A storm blew in from the Pacific earlier in the week, seemingly headed here, but its moisture peeled off to pelt Southern California and the Pacific Northwest. Most of the Sierra Nevada has just a thin covering of snow, while Lake Tahoe itself sits a foot below its natural rim.
Snow surveys conducted last week to assess the summer water picture found that the American River basin between Lake Tahoe and Sacramento was covered with only 30% of its usual Jan. 1 snowpack, the Sierra Nevada south of here with about 20%. Together, those two watersheds supply most of the water for farms in the San Joaquin Valley and cities in the San Francisco Bay Area.
Lake Oroville, which collects Feather River runoff for the State Water Project, had a little better snowpack at 33% of normal, but the reservoir itself was lower than ever for this time of year.
Storms in December had promised to lay down more snow, but they reached the mountains with less moisture than expected. They also gave up their precipitation in a pattern exactly reversed from the usual--more rain and snow fell at lower elevations than higher in the Sierra Nevada. Hydrologists prefer the precipitation to fall at higher elevations so it will remain frozen until the spring runoff.
Los Angeles, with an independent water supply in the Owens Valley and eastern Sierra, faces a somewhat different water picture than most California cities. But a snow survey taken last week found the snowpack that Los Angeles depends upon in equally bad shape.
Atop Mammoth Mountain, the ski area where the city Department of Water and Power takes quick measures of snow conditions, analysts found that a typical column of snow would yield 4.7 inches of water. The deeper and wetter the snow, the more water it contains. Last year in early January the snow on Mammoth held 7 inches of water; normal is 19.8 inches.
“We’re off to a very, very slow start,” said Dennis Williams, a DWP executive.