Groundwater loss prompts more California land sinking

Kyle Murray and Rowena Lohman examined the southern portion of California's Central Valley. They found that in spite of heavy rains in early 2017, groundwater extraction for agriculture and human use leads to a continual sinking of land, as seen from satellites.

Despite higher-than-normal amounts of rain in early 2017, the large agricultural and metropolitan communities that rely on groundwater in central California experienced only a short respite from an ongoing drought.

When the rain stopped, drought conditions returned and the ground has continued to sink, by up to a half-meter annually, according to a new Cornell study in Science Advances.

“With the heavy storms in early 2017, Californians were hopeful that the drought was over,” said Kyle Murray, a Cornell doctoral candidate in the field of geophysics. “There was a pause in land subsidence over a large area, and even uplift of the land in some areas. But by early summer the subsidence continued at a similar rate we observed during the drought.”

Murray and Rowena Lohman, Cornell associate professor in earth and atmospheric sciences, examined satellite imagery of the San Joaquin Valley. This region, like many others in the western United States, faces ongoing groundwater extraction, happening faster than it can be replenished. Their research, “Short-lived Pause in Central California Subsidence After Heavy Winter Precipitation of 2017,” was published Aug. 29 in Science Advances.

About 80 percent of groundwater usage in California is agricultural. In the farming region of the Tulare Basin in central California, growers have been extracting groundwater for more than a century, said the researchers. Winter rains in the valley and snowmelt from the surrounding mountains replenish the groundwater annually to some extent, but drought has parched the valley since 2011.

Between 1962 and 2011, previous studies had found that the average volume of groundwater depletion each year was at least a half cubic-mile. Using satellite-based measurements between 2012 and 2016, depletion of the Tulare Basin groundwater volume was estimated at 10 miles cubed, which is comparable to five times the water volume of Cayuga Lake (in the New York’s Finger Lakes region) in that period of time.

Fresno and Visalia border the Tulare Basin to the north, with Bakersfield to the south. About 250 agricultural products grow there with an estimated value of $17 billion annually, according to the U.S. Geological Survey. The valley holds about 75 percent of the California’s irrigated agricultural land and supplies 8 percent of the United States’ agricultural output.

“The California water problem has been examined from different angles in the past decades. This is one piece of that,” said Lohman. “In this research, we observed 50 centimeters of subsidence during the drought, and then heavy rains and snowfall over several months in the spring occurred and subsidence slowed down. Six months after the rain – in summer, 2017 – we observed that subsidence rates are again approaching their previous high levels. It was business as usual.”

As an engineering problem, subsidence damages infrastructure, causes roads to crack and give rise to sinkholes – expensive problems to fix, said Lohman. “One of the places where it really matters in California is the aqueduct system that brings water to the region. They’re engineered very carefully to have the correct slope to carry a given amount of water,” she said. “Now, one of the major aqueducts in that area is bowed and can't deliver as much water. It’s been a huge engineering nightmare.”

Groundwater – as an agricultural and municipal resource – is incredibly important to communities in central California and elsewhere. Said Lohman: “The subsidence we see is a sign of how much the groundwater is being depleted. Eventually, the water quality and cost of extracting it could get to the point where it is effectively no longer available.”

Funding for this research was provided by NASA.

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