Even as California struggles with surface flooding, the state is going dry underground, triggering sinking in parts of the great San Joaquin Valley, according to a new NASA report released by the Department of Water Resources.
The most comprehensive study yet of the problem reveals the startling pace and extent of the damage: NASA satellites found the ground subsiding up to 20 inches in a seven-mile area near the Fresno town of Tranquillity, because the state’s subterranean water supply was drained to record lows by farms and towns coping with the recent drought.
Previous imagery revealed the ground subsiding almost everywhere in two main subsidence “bowls”: one, between Modesto and Tulare, and the second between Huron and Kettleman City. New images show that these two bowls — covering hundreds of square miles — grew wider and deeper between spring 2015 and fall 2016.
Even worse, the sinking is threatening the stability of the California Aqueduct. The report shows that subsidence caused by groundwater pumping near Avenal in Kings County has caused the California Aqueduct to drop more than 2 feet.
As a result of the sinking, the aqueduct at this stretch can carry a flow that’s 20 percent less than it was design to carry. Water project operators must reduce flows to avoid overtopping the concrete banks of the aqueduct in those sections that have sunk.
The NASA analysis also found subsidence of up to 22 inches along the Delta-Mendota Canal, a major artery of the Central Valley Project, operated by the U.S. Bureau of Reclamation. The CVP supplies water to about three million acres of farmland and more than two million Californians.
The state’s Department of Water Resources recently completed a land survey along the aqueduct and found about 70 miles in Fresno, Kings and Kern counties sank more than 1.25 feet in two years.
“The rates of San Joaquin Valley subsidence documented since 2014 by NASA are troubling and unsustainable,” said DWR Director William Croyle. “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.”
NASA’s satellites can map surface deformations of a fraction of an inch over large areas — making it possible to monitor groundwater and subsidence as never before.
This is old water, not easily replaced by this winter’s torrents. If everyone stopped pumping immediately, it will take at least 50 years for the Central Valley’s aquifers to naturally refill, as rain and snowmelt from the mountains slowly seep underground, according to the U.S. Geologic Survey.
And rains cannot expand the underground deep layers of clay, sand and gravel that store the state’s great freshwater aquifers. When pumped too dry, they collapse, stacking like pancakes.
But this winter’s rain could help slow the trend by boosting flow from reservoirs, so farmers are less reliant on aquifers to support their crops. The Central Valley, home to the world’s largest swath of ultra-fertile Class 1 soil, is the backbone of California’s $36.9 billion a year, high-tech agricultural industry.
Also helpful is the historic Sustainable Groundwater Management Act, enacted by Gov. Jerry Brown in September 2014, which requires local governments to form sustainable groundwater agencies that will regulate pumping and recharge to better manage groundwater supplies.
