Water leaders say that we’ll have plenty of water in the future. They base their confidence on a model that says droughts will last no longer than three years. But here’s a problem. We’re now in the fourth year of the current drought.

Two weeks ago, we wrote that people shouldn’t worry about running out of water in the long term even with lots of new development projected in the region. County and city water officials said they’ve planned for all those new homes and businesses.

But the length of the current drought shows that sometimes those plans can go wrong. Some other assumptions water officials are counting on: New water from the Colorado River will flow uninterrupted and that water customers will continue using less per person than they have in the past. These assumptions face their own questions.

None of this is to say that our water assumptions are bad and we actually are at risk of running dry. San Diego currently has a healthy supply of water, especially compared with other regions of the state that are dipping into reserves. Local water officials say they’re constantly evaluating all their projections to ensure that the spigot will keep working.

That starts with the projection that droughts will only last three years. Dana Friehauf, the top water resource specialist at the County Water Authority, said the agency might try to plan for longer droughts in the future.

“We don’t know if this is going to continue along and that’s why it’s so important for people to conserve,” she said. Last week, the agency announced new conservation measures.

Even during this drought, the county expects to have 99 percent of the water it will need over the next year. Because the state has ordered municipal water customers to cut their consumption by an average of 25 percent, the county will even have water on hand that customers will not be allowed to use. That water will be stored in San Vicente Reservoir for future years, something that will come in handy if the drought continues.

One assumption for the future is that per person water consumption in San Diego will decrease by 20 percent this decade. That reduction is mandated by a 2008 state law. Currently, the county as a whole is on track to meet those goals, the water authority said.

A lot of the reason the region can accommodate new growth now is thanks to drastic reductions in per-person water consumption that already happened. Last year, the region used less drinking water than it did in 1990, even though there were a third more people living here, according to the authority.

Part of this reduction was driven by more efficient homes. According to homebuilding industry figures, new three-bedroom, single-family homes use about half as much water as homes built in 1980.

“You want to get people out of older, wasteful houses into newer, more efficient ones,” said Michael McSweeney, a spokesman for the Building Industry Association of San Diego.

There are other circumstances that could throw a wrench in long-term water plans and cause significant water shortages.

A major problem could come from a dispute between the Imperial Irrigation District in Imperial County and the state of California over the Salton Sea. The dispute could unravel a major agreement that also allows San Diego to get water from the Colorado River. Without that water, San Diego would be in trouble.

And the drought — what happens if this drought lasts five, six, or even 10 years?

If the drought continues, officials across the county would likely scramble to find other sources of water and force consumers to make even deeper cuts. Some of the city’s long-term plans, for instance, rely on recycled drinking water from a system it has yet to construct.

There are some rules on the books that would allow officials across the county to demand residents to cut back. One of them is to halt new development. That is a concept that some areas are toying with right now, although that is because of the state-ordered cuts and not an actual shortage of water.

The county is ready to open a new desalination plant. Future plants would provide a source of new water as well. Though the water that comes from them is expensive compared with other sources, it is considered dependable because it doesn’t rely on rains and rivers.

“If things continue to worsen, that would be an option to look at, along with a lot of other options,” Friehauf said.

The city is now largely dependent on the County Water Authority for water. Without new water supplies or demand reductions, the city anticipates shortfalls.

There are a variety of interlocking long-term plans. The city has several projections that rely on the County Water Authority’s. The County Water Authority’s projections rely in part projections by the Metropolitan Water District, which supplies water to agencies across Southern Californian. And Metropolitan’s projections rely, in part, on projections from the state Department of Water Resources.

A major earthquake, for instance, could cut San Diego off from its major water sources, though it has water in storage for such an emergency. Further up the state, a major earthquake in Northern California could collapse levees in the Sacramento-San Joaquin Delta that keep salt water away from vital fresh water supplies. That, in turn, would disrupt delivery of Northern California water to Southern California and cause shortages across this half of the state.