Sinking ground near El Nido could impact flood fighting

Estimated subsidence in the San Joaquin Valley from 1949 to 2005.
Estimated subsidence in the San Joaquin Valley from 1949 to 2005. Department of Water Resources

Land in the El Nido area sank an additional 16 inches from spring 2015 to fall 2016, a report from NASA shows, and the subsidence may affect flood-control measures after the first wet year in five years.

The report released earlier this month found that a subsidence bowl about 25 miles in diameter centered on El Nido continued to sink near the East Side Bypass canal from May 2015 to September 2016. The bypass carries floodwater off the San Joaquin River in Fresno.

That could complicate flood-fighting efforts later this spring, when snowmelt occurs and the water flows down the San Joaquin River system through small channels, said Jeanine Jones, interstate resources manager with the state Department of Water Resources.

Subsidence is caused by groundwater pumping and has been a problem in the San Joaquin Valley for decades. It’s been particularly bad in the past five years of drought as growers’ surface water supply dwindled and they became more dependent on groundwater.

The state Department of Water Resources is working with local water districts to analyze whether surface deformation may interfere with flood-fighting efforts, particularly as a heavy snowpack melts this spring.

“The rates of San Joaquin Valley subsidence documented since 2014 by NASA are troubling and unsustainable,” said William Croyle, DWR director, in a statement. “Subsidence has long plagued certain regions of California. But the current rates jeopardize infrastructure serving millions of people. Groundwater pumping now puts at risk the very system that brings water to the San Joaquin Valley. The situation is untenable.”

The NASA report depends on satellite radar mapping. The most recent report found record rates of subsidence in El Nido and the Tulare County area of Corcoran. The report also found subsidence intensified near Tranquillity in Fresno County.

Despite the wet year, much of the damage from subsidence is irreversible, Jones said.

“A shallow aquifer that comes in contact with surface water will recharge much more quickly than a deep aquifer,” Jones said. “But pumping often is occurring in the deep aquifers, and that’s causing the subsidence. Once it’s happened, it’s happened.”

The wet weather will, however, provide more surface water to growers, who will in turn pump less water, she said, slowing the rate of subsidence.

For the coming irrigation season, the Merced Irrigation District’s board of directors approved unlimited water deliveries for half the price of last year.

MID also is using the stormwater in recharge basins, with one of them in El Nido. The recharge basin in the El Nido area spans approximately 18 acres. Starting in January, the basin helped replenish about 18 acre-feet of groundwater per day, with flows being diverted from storm runoff in Mariposa Creek, MID reported.

Brianna Calix: 209-385-2477