In response to an expected dry summer and uncertainty about future water supplies, San Diego County Water Authority officials are asking residents to reduce their water consumption by about 10 percent.

Officials with the county agency will officially announce today what they are calling the "20-gallon challenge," calling on every resident to reduce by 20 gallons the amount of water they use.

Most people use about 180 gallons a day, an authority spokesman said.

The water agency serves as the wholesale water supplier for 24 member water agencies in the San Diego region.

The water authority appeal follows calls from other water agencies in the state for their customers to reduce their water consumption as the region faces the prospect of future water shortages, an authority spokesman said Wednesday.

The Colorado River provides 60 percent of the region's water supply and is in the throes of an eight-year drought. In addition, the past two years have been two of the driest on record in Southern California and for the Sierra Nevada snowpack, which supplies 40 percent of the region's water needs.

"If we are faced with another dry year or multiple dry years then we are going to have to save as much water in storage as possible," water authority spokesman John Liarakos said Wednesday. "We don't know what is going to happen."

Metropolitan, which supplies much of San Diego County's water, has about 2.5 million acre feet of water stored as a hedge against drought, Liarakos said. But it's expected that the agency will have to begin drawing down on that supply this summer, he added.

An acre-foot is enough water to supply the water needs of two families of four for one year.

Agency officials say that people could save as much as 4.5 gallons of water by running the dishwasher only when it's full, another two gallons by turning off the water while brushing one's teeth and 15 to 50 gallons by washing only full loads of laundry.

So far, no local agency in the state has instituted mandatory conservation of water supplies, Liarakos said.

"But that could happen if there is not enough water to go around," he said. "Think of this as saving for a not rainy day."

The San Diego area is not the only part of the state where water officials are concerned about conserving. Earlier this month Long Beach officials issued an urgent call for residents to cut back on water use, and San Francisco area residents are already conserving.

Also earlier this month, Southern California's main water supplier, the Los Angeles-based Metropolitan Water District voted to approve $6.3 million for a public relations campaign to convince people to save water.

- Contact staff writer William Finn Bennett at (760) 740-5426 or wbennett@nctimes.com.