Even though it’s rained more than normal across California, most of San Diego’s reservoirs are still not full. About 41 percent of the storage space in these artificial lakes remains empty.

Most of this is just the way things are in San Diego. Here, most of the region’s drinking water comes from the Colorado River and the melted snow of Northern California. Only about 5 percent of urban San Diego’s water comes from local rainfall.

Every time it rains big, people wonder what’s happening to keep it from wasting away into the ocean or whether their favorite lake is going to be full of water for fishing and boating. That’s all especially true this year. But even though it has rained more than normal, San Diego is not like other parts of the state, where many major dams are 80 percent full or more.

For one thing, the water that falls is the cheapest source available, so it’s the first sent to customers.

Now, San Diego’s biggest reservoir, San Vicente, is less than a tenth of the size of the state’s largest reservoir, Shasta Lake. San Vicente is about 77 percent full; Shasta is 96 percent full. This is what it means to live in a region as arid as San Diego.

There are 54 dams in San Diego County regulated by the state. Only about a dozen of them are plumbed to hold the imported water on which the region depends, and most are relatively small.

That leaves the rest dependent on rain and snow that rarely falls. This year, there’s been more precipitation than usual across the county – 126 percent more than normal in downtown San Diego, 148 percent more in Ramona, 115 percent more in Oceanside. In Northern California, by contrast, the Sierra Mountains have nearly 200 percent more snow than normal – and the Sierras get far more rain and snow to begin with.

The Otay Reservoir spilled for the first time in several years. But that is the exception rather than the rule in San Diego.

Part of that is climate and soil – some rains were not heavy enough, so the water was absorbed by the ground. Part of it is that water officials are taking water out of reservoirs as quickly as they can so they can serve customers free water from the sky rather than expensive water imported from hundreds of miles away.

At least one dam, the El Capitan Reservoir near Alpine, is not allowed to be filled all the way because of concerns about stability and safety. The city of San Diego is currently paying for an engineer to review El Capitan and eight other dams it operates. City officials say they are not sure if there is a serious problem at El Capitan, but that the restriction has not yet affected the dam’s operations or reduced its usefulness, in large part because there hasn’t been enough water to fill it all the way to the top.

The lowest of the region’s major reservoirs, Lake Morena, off Interstate 8 near Campo, is 91 percent empty. The city’s water department, which operates the lake and dam there, has for several years been taking out as much water as it can. At one point, the lake held only about 2 percent of its capacity.

That city began doing that back when Jerry Sanders was mayor and wanted to avoid raising water rates, said water department spokesman Brent Eidson. By taking free rainwater out of the reservoir, the city avoided having to buy imported water, which costs several hundred dollars per household.

There’s no precise figure yet for how much money ratepayers will save because of this year’s rains, but the city is expected to release a figure at the end of the budget year.

When the city buys imported water from the Colorado River or Northern California, it buys the water through the San Diego County Water Authority. The Water Authority, in turn, gets most of its water from the Metropolitan Water District of Southern California, which delivers the vast majority of the region’s water.

But if the city can use the water that falls as rain, it doesn’t have to pay for the expensive imported water. On some days, the city has been able to get 30 million gallons of free water out of the El Capitan reservoir, which is roughly 20 percent of the water the city needs for a day. The city only has to pay to treat the water so it is safe to drink, a relatively minimal expense.

Across the region, that free water goes fast.

“You’re foolish not to use it as quickly as you can, because it may evaporate or saturate,” said Jim Fisher, the Water Authority’s director of operations and maintenance.

At the city’s reservoirs, about 5 to 7 percent evaporates in an average year, so it doesn’t pay to keep it sitting around for another dry spell.

Still, both the city and the county have policies to keep enough water on hand in case of dry spells and emergencies. One of the biggest fears is that an earthquake will cut San Diego off from Metropolitan’s system, leaving the county to fend for itself until pipelines can be repaired. The Water Authority stores enough water in the county to be entirely cut off for two months.

The Water Authority recently wrapped up a $1.5 billion series of projects to prepare the region for an emergency. The projects include new pipelines but also more space to hold water. As part of that, the Water Authority built the new Olivenhain Dam near Escondido and raised the San Vicente Dam near Lakeside, which allowed more water to be stored behind it.

Ry Rivard was formerly a reporter for Voice of San Diego. He wrote about water and power.

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