Storms may fill low local lakes

Fill ’er up.

That’s what area water managers are hoping will happen to reservoirs in San Luis Obispo County as a series of storms moves through the Central Coast over the next two weeks.

Three years of drought have depleted reservoirs — a vital drinking water source — prompting some local agencies to plan for water shortage emergencies. Santa Margarita, Lopez and Whale Rock reservoirs are all about half full, while Nacimiento Lake is at only 13 percent of its capacity.

But help is on the way. The storm door is wide open, forecasters say.

As many as five nearly continuous storms are expected to roll across the county next week, dumping as much as 10 inches of rain on coastal valleys and as much as 20 inches on higher elevations. That much rain is causing emergency officials to worry about localized flooding.

Another three wet storms are expected the week after. If all these storms materialize as expected, rainfall totals could jump to 170 percent of normal, said Rob Johnson, chief of water resources planning for the Monterey County Water Agency, which owns Nacimiento.

“This looks like a great start,” he said. “The way the jet stream is setting up, the storms will all have access to the coast.”

There are many examples in recent decades of reservoirs being dangerously low, only to fill up after a couple of months of plentiful rain, said Gary Henderson, San Luis Obispo’s water division manager. This is particularly true during periodic El Niño weather events.

El Niño weather patterns usually result in increased rainfall in California. The 1997-98 El Niño event was the strongest on record.

Local forecaster John Lindsey said the last time storms set up in this pattern was February of 1998. That month, more than 15 inches of rain were recorded at Cal Poly.

Lindsey said a moderate El Niño is occurring now — the phenomenon in which the Pacific near Peru and Ecuador warms more than normal. That change affects how the upper-atmosphere wind known as the jet stream flows over the West Coast.

Right now, the jet stream is sinking down from Canada, and that will help usher the storms forecast for next week, Lindsey said.

How lakes will fill

Each reservoir fills up at a different rate depending on the size of its watershed — the area of land that drains into the lake. So it is impossible to make blanket predictions about reservoir levels, water managers say.

At opposite extremes are Santa Margarita and Whale Rock reservoirs. Santa Margarita holds half as much water as Whale Rock, but its watershed is five times larger, so it fills much faster, Henderson said.

“If we get the rain they are calling for, there’s a good chance Santa Margarita will fill up and start spilling,” he said. “But I don’t expect Whale Rock to fill up all the way.”

Those lakes are the key drinking water sources for San Luis Obispo.

Similarly, Monterey County officials are doubtful that Nacimiento will fill up all the way. The same is true for Lake San Antonio in southern Monterey County, which is at 35 percent of capacity.

But, even if these reservoirs do not fill, their levels stand to rise dramatically. Plentiful rainfall also recharges underground aquifers, another important source of domestic water for many communities in the county.

State impact

Experts agree the drought must end in the next 12 weeks if California’s depleted reservoirs are to recover in 2010. The question is: Will the rain and snow fall in the right place to replenish the state’s largest reservoirs?

El Niño often brings more snow to the Southern Sierra than the northern part of the range. But the state’s largest reservoirs — Shasta and Oroville — are in the north.

The two reservoirs need 110 percent of average runoff this year if they are to provide the normal supply of water for millions of Southern Californians, millions of San Joaquin Valley farmland acres and a faltering ecosystem in the Sacramento-San Joaquin Delta.

About half the rainfall season is already over, and the snowpack — a rough predictor of later runoff — is only about 75 percent of average in the Northern Sierra.

It’s not just how much water, but where it falls, that will matter.

About 75 percent of the state’s water supply originates in Northern California, which is why Shasta and Oroville are linchpins for water-scarce Southern California cities, Valley farms and the ecosystem. Shasta has only 64 percent of the water it usually holds this time of year, and Oroville 45 percent.

This story includes staff and wire reports.