LOS ANGELES (AP) — Old-school cooling systems on the roofs of larger Los Angeles buildings may be wasting billions of gallons of water each year.

The Los Angeles Times reported Saturday that some of these “swamp coolers” are so inefficient they can use as much water as all the bathrooms, drinking fountains and kitchens in the buildings below.

As California’s drought persists, officials are looking to supplement savings already won when property owners ripped out lawns or installed water-miserly appliances.

Los Angeles Mayor Eric Garcetti last month encouraged building owners to upgrade cooling towers, with a hope of saving 100 million gallons annually.

That’s still a drop in the bucket.

Across all Los Angeles County, an area much larger than the city, cooling towers waste more than 2.5 billion gallons each year, estimated David Hodgins of the Los Angeles Better Building Campaign, which promotes conservation.

That would be enough to serve 15,000 households.

It’s also a guess, because there is little data about the number of tower and their condition, Hodgins said.

Cooling towers as big as trucks dot many high-rise or other large commercial buildings because they’re remove large amounts of heat better than compressors familiar in homes. Pipes carry in heated water, which trickles through a membrane and a giant fan pulls air through it. The process removes heat by evaporation. The cooled water then is recirculated to the building.

Systems that are old, as many are, are inefficient.

Evaporation raises the mineral content of the remaining water, which becomes corrosive enough that it has to be flushed.

Only about 10-20 percent of buildings have optimally efficient cooling systems, Hodgins said. And only about 300 are known to have been upgraded.

“There’s a lot to do,” he said.