Water rationing begins Sunday, but city residents won't discover how much water they will be allotted for another two weeks.

Beginning May 15, customers will receive notices telling them how much water they can use, the San Francisco Water Department said Friday.The rationing program, adopted Thursday by the city's Public Utilities Commission, is aimed at reducing consumption by 25 percent compared with 1987.

Among other things, the new rules prohibit washing sidewalks, operating decorative fountains and serving water to restaurant patrons except on request.

The rationing program will depend on public cooperation, and the only penalties will be increased rates for excessive use and possible installation of flow restrictors for big wasters, said James Cooney, general manager at the water department.