Heavy Rainfall Does Little to Replenish Ground Water : Drought: Clay soil layers, evaporation and thirsty roots allow only a fraction to soak into basins supplying the county.


Despite the wettest March on record in Ventura County, experts predict that important ground-water basins will absorb only a fraction of the rain that fell.

Soil conditions, including impenetrable clay layers that lie below the surface of much of the populated area of the county, dramatically limit the amount of water that can seep into the basins below ground.

The clay layers have teamed up with evaporation and thirsty plant roots to allow only about 10% of the rainfall to soak into shallow basins underground, experts say.

“It’s like a five-gallon water bottle sitting out there in the rain with only the small mouth open to the sky,” said Luke Hall, hydrologist for Ventura County Flood Control.

Far less water, about 1% of total rainfall, eventually makes its way down through hundreds of feet of soil to the deepest and largest basins that provide much of the ground water used in the county.


“As a result, the water we pump from those aquifers is not being replenished,” Hall said, referring to the deepest basins.

In October, at the beginning of the current rain year, the county had a 29-inch rainfall deficit. The deficit had reached about 40 inches before recent rains began Feb. 26, but has now been reduced to about 28 inches.

“Basically, we bought ourselves a year,” said Hall, who also teaches geology at Ventura Community College. “But it was not enough to end the drought.”

Nevertheless, the 12 inches of rain that fell over the past month on the Oxnard Plain, combined with runoff from mountains and rivers, will raise ground-water levels in the shallow basins by several feet over the next two to four weeks, said James T. Gross, a ground-water geologist and resource manager with the United Water Conservation District.

Many of those basins had reached record low levels before storms began on Feb. 26.

Ground-water basins supply two-thirds of the county’s water during normal years. In 1989, cities, farmers and residents pumped nearly 280,000 acre-feet of water from the ground. Farmers used about 85% of that. An acre-foot of water supplies two California families for a year, or covers one acre of land with water one foot deep, experts say.

The county’s dependence on ground water is expected to increase drastically if restrictions on imported water continue.

Already, many farmers whose imported water supplies have been cut by 90% are drilling wells or activating old wells to make up the deficit.

Hydrologists estimate that ground-water basins in the county contain up to 5 million acre-feet of water. But much of that water is too high in mineral content for drinking or crop irrigation.

When the usable basins are overpumped, as they have been on the Oxnard Plain, the remaining water can become contaminated, they say. In the shallow Oxnard Aquifer and parts of the deep Fox Canyon Aquifer, overpumping has allowed seawater to contaminate corners of the freshwater supplies.

And overpumping in inland basins can draw in poor-quality water from the sides and bottoms to the central parts of the pools.

“We suspect that is occurring now in the Santa Clara River Valley,” Gross said. Studies are under way to determine the extent of any contamination.

United and the county’s Fox Canyon Ground Water Management Agency are working to replenish their aquifers and limit pumping.

United operates the Freeman diversion dam on the Santa Clara River, which diverts water into settling basins in the Oxnard Forebay, an area of sandy soil that water can easily penetrate. Along the 20-square-mile forebay, water filters through the soil and into Oxnard Plain aquifers.

That is the only portion of the Oxnard Plain where water can soak into the basins, said LaVern Hoffman, a Ventura County ground-water hydrologist. The rest of the plain has clay layers underground that prevent rainwater from seeping in, he said. Instead, it pools above the clay and eventually runs into the ocean.

The same kind of clay layer prevents water from seeping into much of the Pleasant Valley and Los Posas basins that stretch from the Oxnard Plain northeast toward Moorpark, Hoffman said. Simi Valley has some sandy soil that allows water to soak into underground basins, but the water there is too high in mineral content to use for drinking water, he said.

In the Ojai Valley and the Ventura River Valley, sandy soil allows water to percolate quickly through the ground. During rainy times, the three basins recover quickly, but two of them are also heavily pumped by water districts and farmers, Hoffman said.

The sandy bottom of the Santa Clara River also accepts water readily. The easy replenishment, along with treated sewer water that is discharged upstream in Newhall, has helped keep basins healthy in Fillmore and Piru through the drought, Hoffman said.

But the Santa Paula basin benefits less from the Newhall water because it is farther downstream. And that basin is heavily pumped by farmers and the cities of Santa Paula and Ventura, which reduced it to a record low level before the rains, Hoffman said.

“The rainfall will go a long way toward getting us back to where we want to be,” Gross said. “But we’ve had nearly five years of below-average rainfall. It was not enough and can’t sink in fast enough to make up for the dry years.”


Much of the southern part of Ventura County is covered with an impenetrable clay layer that prevents water from seeping into undergound water basins. When rainwater hits the clay layer four or five feet underground, it pools up and runs off into the ocean through underground streams.

But about 25% of the ground has sandy alluvium soil that allows the water to seep into the ground-water basin.

Once the water sinks into the sandy ground, about 90% of the water remains in the upper-level aquifers, such as the shallow Oxnard or Mugu aquifers. Only 10% of the water eventually seeps down into the deep Fox or Grimes canyon aquifers, and that process takes decades.

Source: Ventura County Public Works Department