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Wednesday, March 27, 2024 | Back issues
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Scientists develop new method to monitor underground water loss in California’s Central Valley

The new model uses data from two different satellites to measure the level of water underneath the ground.

(CN) — A team of California scientists have developed a new method for monitoring underground water levels on a month-to-month basis.

Eight researchers from NASA’s Jet Propulsion Laboratory in Pasadena and the Berkeley Lab spent the last few years studying the Tulare Basin in California's Central Valley, an incredibly productive area of farmland that grows roughly 40% of the fruits, vegetables, and nuts in the United States. That productivity is only possible by extensive groundwater pumping, since the region only receives five to 10 inches of rainfall every year.

"After decades of pumping, underground water resources are dwindling," JPL said in a press release. "Wells in the Tulare Basin now must be drilled as much as 3,500 feet deep to find adequate water."

Up until now, there has been no good way to measure just how badly the underground aquifers are depleted.

"Groundwater is so hard to measure," said Kyra Kim, a post-doctorate researcher at JPL specializing in hydrology. "You can’t just measure the level. It’s impossible to see with your naked eye. It’s like not knowing how much money is in our savings account."

Team leader Donald Vasco, a hydrogeophysicist at the Berkeley Lab, agrees. "We don’t know basic things like how water flows from the Sierra mountains. We know how it flows to the ocean, but some of it goes underground. We don’t know how the deeper aquifers are being replenished. People are drilling deeper and deeper. We don’t know how fast they’re being replenished. We know they’re overdrawing them — they’re using more than is coming back in. We don’t know how sustainable things like that are."

Often, when the level of groundwater sinks the land above it sinks as well — a process called subsidence. Scientists already knew the Tulare Basin was sinking drastically, about a foot every year. But that sinking isn't continuous — the ground rises and falls throughout the year. And those fluctuations don't always line up with the weather. For example, the water table rises after heavy rainfall. But sometimes, after it rains, the ground in the Tulare Basin will sink.

The researchers combined data on water loss from the Gravity Recovery and Climate Experiment, or GRACE, which measures month-to-month changes in gravity in different parts of the earth, with data from the European Space Agencies Sentinel-1 satellite, which measures movement on the earth's surface, to create a new model for how underground water is depleted and regenerated.

"It’s pretty phenomenal technology," said Vasco.

The model, published recently in Scientific Reports, assumes two different layers of ground, each of which behave differently. The top layer is permeable soil — sand and dirt, with a layer of water mixed in, called the water table. Below that is a layer of hard clay. And below that lies the aquifer, an underground reservoir of water. Researchers realized the soil in the top layer compresses quickly after being pumped of its water, whereas the clay in the middle compacts more slowly.

"The method sorts out how much underground water loss comes from aquifers confined in clay, which can be drained so dry that they will not recover, and how much comes from soil that’s not confined in an aquifer, which can be replenished by a few years of normal rains," JPL said in a statement.

Whereas depletion of water inside the soft soil is nothing to worry about, draining too much water from a clay aquifer can damage it, hurting its ability to store water in the future.

"Some of that change is permanent," said Kim.

But it's not all bad news. The new model can be something of a reassurance, at least in the short term.

"We did see a rise in the groundwater table that was correlated to the snow in the Sierra," said Kim. "If it snows, you see that in the groundwater, although the ground might be sinking in response."

A map map showing changes in the mass of water, both above ground and underground, in California from 2003 to 2013, as measured by NASA’s GRACE satellite. The darkest red indicates the greatest water loss. The Central Valley is outlined in yellow. (NASA/JPL)
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Categories / Environment, Technology

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