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SLO County needs a lot more rain, officials say

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SLO County's recent rainstorm had locals digging out their rain boots and led to greener hills, but it wasn't enough to alleviate the local drought.

According to SLO County Water Resources Division Manager Courtney Howard, we're going to need more rain to get back to where we should be.

"There was some rise [in reservoir levels], however, they're still below where they were this time last year," Howard said. "So we haven't fully recovered, we're going to definitely need more rain, and more consistent rain."

SAVE WATER SLO County's recent rainfall helped increase reservoir levels, but they're still below average. - PHOTO COURTESY OF SLO COUNTY WATER
  • Photo Courtesy Of Slo County Water
  • SAVE WATER SLO County's recent rainfall helped increase reservoir levels, but they're still below average.

Kate Ballantyne, deputy director of SLO County Public Works, said it's still early in the rainfall season, and December's storms were a good start. But big rainfall doesn't always correlate with a large rise in reservoir levels.

"That's primarily because when the soil is dry and it rains, the moisture locks in and it doesn't necessarily run off into the reservoirs," Ballantyne said.

The Lopez Dam, which supplies water to the Five Cities area, receives an average annual rainfall of 21 inches, but has only gotten 11.21 cumulatively this water year, according to Dec. 27 data from the Water Resources Advisory Committee. More than 8 of those inches came during December rainfall, but the reservoir is still only at 53 percent of its total average.

"Where we are today after all that rain is about where the lake level was in mid-October of 2021," Ballentyne said of Lopez Lake.

Rainfall consistency is the key here, both for reservoirs to be filled and groundwater to be replenished.

"It's really important how the rain falls," Howard said. "Even if we get a certain amount of quantity to get up to average [rainfall], if it's intermittent it doesn't saturate and get into our reservoirs. It evaporates."

The amount of rain retained also depends on the watershed conditions. Howard said graded and rocky terrain, such as that found on Lake Nacimiento's watershed, is better because it encourages water to flow into the reservoir and not dry up.

Ballentyne said Lopez Lake has historically needed about 9 or 10 inches of rain before water starts to run off into the lake.

"Each watershed definitely performs differently," Ballantyne said. "If it continues to rain without a hot, dry spell in between, we expect to see more runoff into [Lopez] Lake. But if we do have a hot spell we kind of backtrack and it's like you're starting over."

While urban areas rely more on reservoirs like Lopez and Nacimiento, most of the county's rural areas rely on groundwater, which is also impacted by rainfall.

"It's a similar concept [to reservoirs], where we need the ground to saturate for it to actually get down into the groundwater basin," Howard said.

If a rain event is too isolated, the ground may dry up faster, allowing less of the water to percolate down into the basin.

"It has stayed cool, so that's helpful. It's not drying up as fast," Howard said of the weather since the last rainfall in late December. "If it stays saturated by the time the next rain comes, then it will get down to the basin sooner, rather than having to resaturate again."

Ballantyne said the best kind of rain is constant and steady, which allows the county to get the amount of rain it needs without encountering flooding or damage.

"It was great that we got rain and we're feeling hopeful, but definitely cautious looking at the dry January that's forecasted," she said. "We're really hoping for more rain because that's what we need to move out of the drought."

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