From a useful op-ed in today's Los Angeles Times:
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.
Was chatting with a scientist friend about this topic a couple of months ago. Though not a hydrologist, he was sure there was a good scientific definition of drought. And he's right: there are several scientific measures of drought, which NOAA blends (like scotch) in various drought index formulations.
But the fact remains: because drought is in part a measure of demand, as well as supply, it's surprisingly difficult to say when in a thirsty region like Southern California when a drought begins...or ends.
