SAN DIEGO -- Residents and businesses that use water from the city of San Diego will have to reduce usage by 20 percent if regional water agencies order drastic cutbacks, Mayor Jerry Sanders said Friday.

Sanders issued his water conservation recommendations at a news conference where he said he wants a plan ready to go by July 1 in case the County Water Authority and Metropolitan Water District call for the cuts.

"We have created the plan because the state water agency and our water wholesalers have informed us that water supply conditions are tenuous and they are likely to reduce the city's allocation by mid-year," Sanders said. "If that happens, the city must have its own allocation plan in place with methodology that can reduce overall use as fairly and equitably as possible."

The plan is based on individual water use histories going back three years, so there will be no benefit to using a lot of water now in order to make future cutbacks easier, said Alex Ruiz, the assistant director of the city's Water Department.

Water Department customers will receive a four-month allocation of water with their bills, and future bills will show whether their usage is on target, Ruiz said.

Penalties will be enforced after two billing periods, he said. The Water Department will charge either $2.42 or $4.85 for every 748 gallons of water over the customer's target. Water officials regard 748 gallons as one unit.

Ruiz said fines will be steeper for repeat offenders.

California has been plagued by drought conditions for several years and rainfall is below average for most of the state this year. And, the Sierra snowpack is drier than usual.

Recent Northern California rains and a storm expected over the weekend have slightly lessened the odds mandatory water cutbacks will be necessary.

Even if the 20 percent reduction is not ordered this year, the mayor said San Diegans need to make long-term changes in their water usage.

"If you've yet to start your conservation efforts, the time to start is now," said Sanders, who noted that he is an avid weekend gardener who has already begun making water sacrifices.

About 21 percent of the city's water customers have already reduced consumption to the point where they will receive a "Super Saver" credit, Sanders said.

"We will recognize past conservation efforts and won't hold past conservation against you," Sanders said.

The credit exempts single-family homeowners who use less than 4,488 gallons per month from having to make cutbacks

Conversely, homeowners found to regularly use more than 14,960 gallons monthly will have to reduce consumption by more than 20 percent, the mayor said.

If the cutbacks required by the state and water wholesalers is less than 20 percent, the mayor's plan will be adjusted.

Under the plan, customers will be able to appeal their allocations.