You probably know your Social Security number, your driver’s license number and perhaps the latest wrinkle in mattress marketing, your sleep number.

But do you know your drought number?

The latter represents the amount of water you are allowed to use per day. If you don’t know it, you probably should. Not knowing could cost you money. As California’s severe drought moves into a fourth year, state and local water agencies are working on something called “allocation-based rate structures,” a kind of precursor to water rationing that’s all the rage in Sacramento and in some areas such as Santa Cruz, Irvine and Santa Monica.

Here’s how it works: Your local water company, special district or city assigns you and your household a number in gallons — a daily water allocation. Usually, one number applies to maximum indoor water use, i.e. showers, kitchen and bathroom faucets, dishwashers, clothes washers, etc., and an extra allocation is assigned for outdoor use such as lawn irrigation.

Using census records, aerial photography and satellite imagery, an agency can determine a property’s efficient water usage.

At the Irvine Ranch Water District, number of residents, amount of landscaping and even medical needs are factored into a household’s water allocation or water budget.

“We want you to stay within that budget. That way we know you are using water in an efficient way,” according to an instructional video on the Orange County water agency’s website.

While some call it a more equal way to meter out mandatory water conservation, others call it social engineering. Some say the idea simply will not work.

In July, the State Water Resources Control Board passed stage one emergency regulations, giving powers to all local water agencies to fine $500 per violation.

“We were concerned with the lack of alarm we were hearing,” said Felicia Marcus, chair of the State Water Resources Control Board. “Our reservoirs are low. Half of the state’s storage is gone. It’s a frightening situation.”

Beginning sometime in October, the state water board will have collected sufficient data from local water agencies to report how much water per person per day residents of the state are using. Already, Marcus said Los Angeles residents are down to 89 gallons per person per day, from more than 100 earlier in the year. Sacramento water use has dropped as well, but it is still way higher per capita than Los Angeles, she said.

After the new numbers are crunched, the state board could order the local agencies to implement stronger water-use regulations, such as banning all watering of lawns and all decorative fountains, she said.

Right now, Marcus is recommending that water agencies, particularly retail ones, switch to water-budgeted allocations. But it is unlikely the state board will assign every resident a drought number. That would be left up to local agencies and cities.

She said at the very least, water agencies and cities should move toward “incentivizing water use through different kinds of rate structures” but declined to get more specific.

“Rates send a powerful signal,” she said. “It is one thing to ask for voluntary action … but that can only go so far. Rates should be designed so you send a financial signal (to customers)” and to enable them to know their water use and compare that with their neighbors.

“This winter, we will be looking at this information on water production, specifically gallons per capita per day. That will give us a better story of what is going on and people can compare themselves with other communities. We will then consider more regulations,” Marcus said.

Santa Monica model

Some local agencies are implementing a drought number model. In Santa Monica, the City Council passed a first-reading in August of an ordinance that would apply an indoor water allocation of 68 gallons per-person-per-capita for every single-family home with four people, said Gilbert Borboa, water resources manager for the city of Santa Monica.

“A customer uses beyond that allocation, then it is possible some penalties might apply,” he said. Today, residents are using about 88 gallons per person per day in Santa Monica, he said.

The City Council will vote on the water-allocation plan next month. Santa Monica will work on allocations for bigger households, apartments and condominiums and for commercial establishments such as hotels, he said. Indoor use is “essential” for health and safety, while the ordinance labels outdoor use as “non-essential.”

“Watering plants and lawns make them all look green and pretty, but that is not essential. Essential is for sustaining life (indoor use),” Borboa said.

The per person allocation is gaining momentum as are other water conservation ideas.

“I believe it is coming. There are agencies already doing it,” said Mike Touhey, a member of the Upper San Gabriel Valley Municipal Water District which serves nearly 1 million residents.

Eastern Municipal Water District, which covers communities in the Inland Empire from Riverside to Hemet, has enacted a Stage 2 drought plan. Each single-family household with three residents gets 60 gallons per person per day. An outdoor allocation is provided based on whether a house has a pool or turf or both. Any household going over the total allocation will be charged an “excessive rate,” according to the plan.

Matt Lyons, director of planning and conservation for the city of Long Beach, said the water allocation method is deeply flawed. Lyons said calculating an accurate allocation for a household or a hotel can’t be done because aerial images don’t tell the whole story such as elevation, or what lies under a canopy of trees — factors that affect water use.

“No. We have not embraced that at all,” he said. “You can’t do it with any degree of accuracy or without being intrusive.”

Instead, Long Beach has seen water conservation successes since 2009 through extensive outreach programs, including paying homeowners $3.50 per square foot to remove thirsty lawns and replace them with drought-tolerant plants.

So far, 1,400 residents have eliminated their lawns and on average, each uses 22 percent less water, he said. The city has seen a drop in water use of 10,000 acre-feet from 2007 to 2009, he said. “Behavior change is driving this water savings,” he said.

Making water hogs pay a top-tier rate is another trend gaining popularity among water agencies.

For example, Irvine charges a “wasteful” rate of $12.60 per hundred cubic feet, well above the $1.34 base rate.

Pasadena City Council member Margaret McAustin said Pasadena Water and Power, like many cities including Los Angeles, bill customers using tiered water rates. Customers pay more when they use more. But this doesn’t stop residents who can afford higher bills from wasting water, she said.

“We are talking about putting in a new tier for people who use a lot of water,” she said.

Jonathan Parfrey, executive director of Climate Resolve and a former commissioner at the Los Angeles Department of Water and Power, likes the water-allocation system.

“This is a terrific idea. People need to live within their water means. That is what a water budget offers,” he said. He’d like to see cities add more, steeper tiers, charge for excessive water use and re-direct the revenues into conservation and water recycling.

Lyons, of Long Beach, cautioned against charging too much for water because it could be a violation of Proposition 218, which says water agencies can only charge for the cost of water service. If a municipal agency charges more for water, it can’t make a profit, so it will have to charge someone else less. This leads to inequities, he explained.

He characterized water budgeting as a passing fad. “In water conservation, people are always looking for or thinking they found the magic bullet,” he said. “We’d rather change the water culture.”