FRESNO, Calif. (AP) _ The San Joaquin Valley's farm economy suffered a $545 million blow last year because of the drought and ''will slowly be crushed'' unless irrigation water returns to normal, a study said.

Nine thousand jobs were lost and 253,000 acres were left fallow because there wasn't enough water for all the fruits, nuts, vegetables and cotton normally grown in the valley, said the study by a private consulting firm.

''If these shortages continue, the agricultural industry, which provides 50 percent of the valley's jobs, will slowly be crushed,'' Sacramento-based Northwest Economic Associates said.

''Economists are already beginning to see the symptoms of an economic death spiral in which the number of farmers remaining in business decreases as water supplies dry up.''

Lost farm revenue totaled $281.5 million in 1991, the report said. That represents 2.5 percent of the $11.2 billion gross crop value for the eight- county region in 1990, the last year for which state statistics have been compiled.

Lost sales by businesses that supply valley farmers added $264.3 million in farm-related revenue losses, said the survey, released Thursday by California Farm-Water Coalition, a group of water districts and agribusiness companies working toward long-range solutions to the state's water needs.

''People driving on Interstate 5 may think farmers aren't hurting because they see so many crops,'' said Duane Paul, vice president of the firm that made the survey. ''But what they don't see are the jobs that were lost and the suffering of all businesses in the valley communities.''

The drought, in its sixth year, has cut federal water allocations sharply in many parts of the valley, forcing farmers to arrange water transfers from other areas, pump irrigation water from underground sources or switch to less thirsty crops.

The Sierra Nevada snow pack, which supplies most of the state's water, remained about two-thirds of normal despite storms in February.