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SAN DIEGO — Nearly 54,600 San Diegans live in areas that could be subject to flooding during El Nino-fueled rainstorms over the next several months, according to a report released Tuesday by the National University System Institute for Policy Research.

The affected residents live in mostly known flood plains that appear in U.S. Geological Survey data used by insurance companies.

The NUSIPR report said 54,560 residents — or 1.75 percent of San Diego County residents — live in 100-year flood zones. Report author Vince Vasquez based the figure on geographic information systems and the 2010 census.

The geographically largest vulnerable areas are the Tijuana River Valley and the San Diego neighborhood of Palm City below the South San Diego Bay National Wildlife Refuge, the report said.

Other areas that could be prone to flooding are alongside watersheds of the numerous rivers running from the mountains west toward the Pacific Ocean.

In total, around 21,700 residential units and almost 4,800 businesses are in potential flood zones, or around 4.8 percent of all businesses in the county, Vasquez said.

“The weather conditions this winter are unknown, but there are clear repercussions for the regional economy,” said Vasquez, NUSIPR senior policy analyst. “Given the history of El Nino disasters, storm preparedness by local governments, proprietors and home owners is essential this season.”

Heavy rains could also cut attendance at area theme parks and the San Diego Zoo, though storms might also prompt San Diegans to not travel elsewhere and opt for local attractions — mitigating any detrimental impact.

Vasquez said the outlook was also mixed for the county’s agriculture, valued at $1.85 billion in 2013.

Research shows that El Nino in other regions lowered crop yields and increased pests, disease and rot. On the other hand, more rain and a longer, warmer summer has also been beneficial for some crops, he said.

According to Vasquez, a 1998 report from the California Coastal Commission on that winter’s El Nino identified $700,000 in storm-related damage to the Oceanside Harbor, and $1.6 million in damages in Del Mar.

The same report noted widespread seacliff erosion, damage to the Pacific/Mission Beach boardwalk, and frequent closures of the Highway 101 section through Cardiff-by-the-Sea.