In january, it rained a lot in Southern California. The usual street intersections flooded. Water tumbled down the Los Angeles River. And houses in areas ravaged by fires last fall seemed in danger of sliding off their hilltop perches.
It was chaotic, as always -- but desperately needed. The wet weather came after the driest year on record in the L.A. Basin -- less than 3.5 inches of rain. Coupled with below-average rainfall in 2006, lack of rain in 2007 had fed fears of a drought. Do last month's downpours mean we can stop worrying now?
There is no simple, single definition of drought. In any region, there are periods of below-normal precipitation. These dry periods become a drought when demand for water exceeds supply. In this sense, we may be in a permanent drought throughout the Western United States.
Wet and dry extremes are a natural part of California's climate. Since 1900, the state has experienced eight multiyear dry periods. Major droughts occurred in 1929-1934, 1976-77 and 1987-1992. Researchers have identified more extreme dry periods going back centuries.
So far, 2008 has been a wet year. At the end of January, rainfall in downtown Los Angeles totaled just over 12 inches, well above the seasonal average of 8 inches. The Sierra Nevada snowpack, the major source of fresh water for Southern California, is currently 18% above average for this time of year. And meteorologists forecast more rain and snow.
But we need a lot of rain to make up for last year's shortfall. Precipitation throughout California in 2007 was only 65% of normal. It was between 15% and 30% of normal in Southern California. The Sierra snowpack was a meager 25% of normal. While reservoirs in Southern California are relatively full, they supply only a small fraction of the region's water. Reservoirs in the rest of the state, by contrast, are low compared to the average for this time of year. And groundwater basins throughout California remain overtapped. Just as a single dry year does not necessarily constitute a drought, a single wet year may not end one.
But several new factors are making it harder for water managers and planners to say if we're in -- or out -- of a drought. One of the most important is climate change caused by global warming. In its most recent assessment, the Intergovernmental Panel on Climate Change, a scientific body that studies the effect of human activity on the climate, noted that droughts have become more common. It forecast that droughts will become more frequent and intense as atmospheric greenhouse gas concentrations continue to rise. And as temperatures warm, crops, lawns and swimming pools will likely require more water.
