When it rains, it pours — and that has San Francisco water officials looking into charging property owners a new “storm-water fee” to help with the upkeep of the city’s aging sewer system.

The first target will be the owners of vacant lots.

“It’s a tax on rain,” fumed Jason Sanders, who was just notified that effective July 1, he will be assessed $31.46 a month for runoff on his vacant lot on Ashbury Street.

According to the Public Utilities Commission, every inch of rainfall that falls on the 47-square-mile city puts about 430 million gallons into the storm-water runoff system.

And it all needs to be treated before it flows into the bay.

Right now, owners of vacant lots are exempt from having to pay the wastewater fees that are assessed to other properties. Utilities commission officials say they want to make all property owners pay to keep up the city’s wastewater and sewage treatment system.

So the PUC has notified 500 lot owners who don’t have either sewer service or an active water account that the agency is considering levying a flat rate of $31.46 a month, or $377 annually, per vacant parcel to pay for treating runoff.

The rain fee would increase by $3.70 a month, or $44.40 a year, by 2022.

The bill represents “the average amount an existing customer pays for storm-water treatment costs,” according to the notice from PUC General Manager Harlan Kelly Jr.

But for Sanders, who says all the rain that falls on his property soaks into the ground, it’s “just another way of ripping off customers.”

The $188,760 the city expects to get from the empty-lot owners would be just a drop in the bucket compared with the $732 million that the utilities agency collected in 2017 from city sewage and water bills.

And though vacant lots are first in line, officials said the new storm-water fee could eventually spill over to all households and commercial properties citywide.

“I’m not saying we are going to do that, but that discussion is happening now,” said PUC spokesman Tyler Gamble.

As for how that affects the commission’s fairness argument, Gamble would say only that this is the beginning of a process for overhauling the billing system — with rates adjusted up or down to reflect a customer’s separate sewer and storm-water costs.

A state bill signed into law by Gov. Jerry Brown in October opened the gate for other cities across California to start adding storm-water costs to tax bills without going to the voters.

Seattle, Portland and Washington, D.C., already charge their customers a storm-water drainage fee. In Seattle, the fee generated nearly $121 million in 2016, a city report shows.

San Francisco’s water and wastewater bills went up an average of nearly 39 percent in the past five years. Storm-water fee or not, the PUC is already in the midst of planning for another round of similar increases over the next five years.

Unhappy property owners will have a chance to sound off when the PUC meets April 10 to formally adopt the new storm-water charges.

Mental challenge: The city’s new lockdown mental health facility at St. Mary’s Medical Center was opened to a mix of public praise for the idea and private prayers that Sacramento lawmakers will change conservatorship laws to make it easier to commit homeless people with psychiatric problems to long-term treatment.

Barbara Garcia, director of the city Department of Public Health, said what’s needed for the $5 million-a-year San Francisco Healing Center to be effective is for judges to be given more latitude to commit people with a long history of mental health issues and nuisance-law violations. At present, judges make such decisions only on the basis of how homeless people appear after a three-day hold, when they are often sobered up and no longer a danger to themselves.

“We’re talking about the city’s 40 to 50 most chronic cases,” said Mayor Mark Farrell as he toured the floor. “We call them frequent fliers — those who are repeatedly going through our hospital system and our emergency rooms, using our ambulances, using our Fire Department and Police Department.”

Frustration over the revolving door at the mental wards like those at St. Mary’s and San Francisco General prompted state Sen. Scott Wiener to introduce SB1045.

“The bill will allow judges to take into account how many times the person had a 72-hour psychiatric hold in the past year or that they had been taken to the emergency ward 30 times,” Wiener said.

Still, Wiener knows he’s up against 50 years of liberal policies toward the mentally ill and concerns about depriving them of their civil rights.

“We overly depend on locking people up already,” said Jennifer Friedenbach, executive director of the Coalition on Homelessness. “We should be building up our community mental health system so that people don’t get to that point of needing to be locked up.”

“We need both,” said Farrell, who added, “We need to have a bottom line of decency and decorum on our streets, and this is part of that solution.”