Unless the 1991 rainy season is wetter than last winter, water supplied to California farmers for irrigation could be reduced by half to two-thirds, state and federal officials say. Urban water users who rely on state reservoirs could suffer cuts of 13 percent if the drought runs into a fifth year, officials said. Federal deliveries to municipal customers could drop 25 percent to 50 percent. In an annual drought-outlook conference titled ``What if ... 1991 is Dry?'' state, federal and local officials said Tuesday that another critically dry winter could be catastrophic for many sectors of California's economy and the environment. ``I hope we don't have to experience the `what if' situation,'' said Chet Bowling, drought coordinator for the federal Central Valley Project, the largest water supplier in California. A draft report released Tuesday by the state Department of Water Resources said a fifth drought year would leave California as bad off as it was in 1977, the driest year on record. This year, the State Water Project and the Central Valley Project cut farm deliveries by 50 percent, and the CVP cut municipal supplies by 25 percent to 50 percent, depending on each city's contract. It was only the second time in the history of both projects that deliveries were disrupted - the first time was 1977. Agriculture suffered an estimated $480 million in drought-related losses in 1990. If the drought continues, the State Water Project estimates reductions to agriculture of 63 percent, while the federal project cuts would be about 50 percent. The state report said San Francisco and some other Bay area cities could be forced to cut water use by half. Some cities in southern California already are curtailing some water uses. Most of California's available water - 82 percent, according to the U.S. Geological Survey - is used for irrigating crops. California leads the nation in water use, at 30.3 billion gallons a day. Agriculture accounts for more than $16 billion of the state's $300-billion- plus annual economy. While the drought that began in 1986 is less severe than 1976-77, the state report said it poses an unprecedented threat to fish and wildlife. ``California's population of 30 million people and their needs for water and space has combined with the drought to create what is truly a natural disaster,'' the report said. Although water authorities acknowledge that it is too early to forecast what kind of winter California will have - most of the state's rainfall is recorded in December, January and February - the report said there were some disturbing historical trends. ``Looking back in history, dry years and wet years seem to cluster slightly more than would be expected by chance. But no one so far has been able to develop a predictive model for California for next year from these irregular patterns.'' Storage in California's major reservoirs as of Oct. 1 was about 60 percent of average.