Valley Public Radio - Live Audio

Subsidence Could Cause $1-Billion In Damage

Jan 13, 2016

California’s prolonged drought is once again causing the valley in sink. Groundwater pumping to keep water flowing and plants growing is resulting in the valley floor to settling and sinking in what is known as subsidence. As the water is pulled out the ground underneath fills the space and settles. In some places, the land is subsiding as much as a foot a year.

A historic image tracking previous subsidence in the valley
Credit USGS

  Hydrologist Jim Borcher says the Valley has experienced sinking before, but now it is back.

"We have a 2,700-square mile area where we have had more than a foot of subsidence in the Tulare Basin. And further north in the central San Joaquin Valley about 1,200-square miles with a foot of subsidence or more near El Nido. That is between Madera and Los Banos. We had a lot of damage over a period decades. And we seem to forget about these problems when we are not using the ground water system and ground water levels recover. And then are shocked to find out it is back again.”

Borcher has run the numbers and says infrastructure damage could cost as much as $1 billion in damage.

"As the water level rises in the canals, because of subsidence pooling, it can over top the canal embankment. It can erode the non-concrete parts of the embankment. And the water level has to increase to higher and higher heights to get the flows that are needed downstream. Some of the canals may not run the right direction anymore and we may have to install pumping facilities. Subsidence damages wells. When the land compacts and settles it tugs on the well casing it can collapse telescopically into itself,”

Borcher also says that roads can be damaged, cities are more prone to flooding, and canal linings can fracture. And the process is also a cycle. As more water is pumped out deeper wells need to be drilled further accelerating subsidence.

And even if the El Nino brings heavy rain to the valley, the subsidence effect cannot be reversed. Borcher says the underground water is ‘fossil water’ that took millennia to build up.

“Just because we get a few years of rain and recharge we are not going to refill the aquifer to the level it was before the subsidence. And you are never again going to get the water back into the clays. Water in the clays is called water of compaction. When it comes out it can’t go back in. It is like an inheritance. A box of gold that’s buried in the back yard. If you take some out you can’t put any back in there. And if you do take some out you better really need it because it is a one-time resource,”

Borcher, however, is hopeful for the state’s new groundwater management plan. While the plan will likely take decades to develop, he says it is the first time California has gotten serious about how to sustainably manage the irreplaceable asset.