OAKLAND -- The East Bay Municipal Utility District's board of directors declared a water shortage Tuesday and asked its 1.3 million customers in Alameda and Contra Costa counties to voluntarily reduce their water use until further notice.

The conservation plan calls for residential customers to only irrigate three days a week, never on consecutive days, and only at night or early morning before dawn.

The water agency said customers also should check for indoor leaks and other ways to reduce unnecessary water usage.

EBMUD said large irrigators are being asked to reduce their water consumption by 25 percent and other non-residential customers are being asked to upgrade plumbing and irrigation systems, check for leaks or waste and to cut back on discretionary water use.

The water district said it's urging greater conservation because the winter of 2007 has been one of the driest in its 84-year history, yielding less snow and rain than necessary to fill its water supply reservoirs next fall.

EBMUD says there's less than half of the normal runoff this year, so it projects that it will begin the next rain season on Oct. 1 with its reservoirs only about 80 percent full.

District officials say that if the winter of 2008 also is dry, it could lead to further dwindling of water supplies and create a drought scenario that would require mandatory rationing.

EBMUD said it anticipates a water supply shortfall of 15,000 acre feet by Oct. 1.

Its conservation plan is aimed at recovering 10,000 acre feet of water.