Tax reform under Proposition 13 prevents California cities and counties from collecting an estimated $13 billion a year, according to a UC Davis study. The shortfall is created because homes, businesses and vacant lots are assessed at about half the actual market value. (The estimate uses a 1 percent property tax rate, taxing at actual market value.) The finding is from a study by Steven Sheffrin, director of the Center for State and Local Taxation and professor of economics at UC Davis, who with two colleagues conducted the research with state funds. It's doubtful Prop. 13, upheld last year by the U.S. Supreme Court, ever will be substantially changed for homeowners. But the researchers do advocate a "split-roll" tax system to raise more revenue and allow more flexibility for cities and counties. For example, if assessments for all non-homeowner property -- commercial, industrial, rental and vacation property -- were brought to current market value and taxed at a 1 percent rate, an estimated $8.6 billion of additional property tax would have been raised in 1992. For a copy of the study summary, media should contact the California Policy Seminar at (510) 642-5514. For other information, media should contact Sheffrin at (916) 752-1576. Tip by Lisa Crumrine Klionsky, News Service, 752-9841.