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NASA satellites reveal groundwater levels beneath the surface

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Four scientists wearing lab coats, masks and hair nets, work on the trapezoid-shaped GRACE-Follow On satellites in a laboratory.

Scientists work on the GRACE-Follow On Mission satellites in a laboratory in November 2017. (Mathias Pikelj/NASA)

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Keeping tabs on groundwater is more important than ever. Most of the western United States is suffering the most extreme drought in 12 centuries. And climate change only promises to make things worse.

Aquifers — porous underground rock or earth containing groundwater — can provide lifelines to farmers and cities. But pumping them dry causes the land around them to sag, a process known as subsidence. It can create sinkholes, destroy infrastructure and potentially collapse the underground water storage formation.

But groundwater is hard to monitor — it’s sometimes located thousands of feet under the surface. That’s where satellites come in.

Kyra Kim is a postdoctoral fellow at NASA’s Jet Propulsion Laboratory, where researchers have created a model to track groundwater levels with two satellite systems. One uses fluctuations in gravitational pull to measure the mass of water underground, and the other uses radar to map the topography of the surface.

The following is an edited transcript of host Meghan McCarty Carino’s conversation with Kim.

Kyra Kim: We were able to synthesize the two missions together to understand what is the relationship between groundwater depletion and land subsidence. So using this technology, we’re able to track groundwater at global scale. That wasn’t, you know, very easy before with just traditional field methods.

Kyra Kim (Courtesy Hannah Dekker)

Meghan McCarty Carino: And how does this add to our understanding of groundwater depletion?

Kim: Traditionally, I think, even myself, as a scientist, I had focused a lot more on the long-term changes. And so that actually makes it really hard for us to understand some of the seasonal changes that we might be seeing with water table rise. But we only looked at some of the monthly changes that were happening. And so we can then start to understand these water table and land changes at scales that are maybe more relevant to management and for, you know, seasonal changes.

McCarty Carino: Right, so you’re able to approximate a little bit more closely real-time estimates of the groundwater, compared to these much more long-term horizons.

Kim: Exactly.

McCarty Carino: And why is it important to track groundwater?

Kim: Unlike surface water — for example, rivers and lakes — which we can see with the naked eye, groundwater is underneath our feet. And obviously, that makes it really hard for us to understand how much of it is there and how much we’re using. And so I like to say that it’s easy to think of it like a savings account. Especially in areas like California, where we have a lot of agricultural activity, water demand is very, very high and in big metropolitan areas as well. And when we have surface water, it’s easy to use that water. But then when we have conditions like drought, or surface water is just not readily available, groundwater is something that we rely on like a savings account. If we can’t track how much there is as easily, then it becomes kind of [like] you’re using money from your savings account, but you don’t quite know how much of it there is.

Artist rendering of spacecraft associated with the NASA/German Research Centre for Geosciences Gravity Recovery and Climate Experiment Follow-On mission, also known as GRACE-FO, which can monitor groundwater storage. (Image courtesy NASA/JPL-Caltech)

Related links: More insight from Meghan McCarty Carino

We’ve got the full NASA-level explanation of the model Kyra Kim worked on and the satellites it draws data from.

One of the systems is known as GRACE, for Gravity Recovery and Climate Experiment. It’s made of two satellites that chase each other — as Kim evocatively described it — like Tom and Jerry, the cartoon cat and mouse. One satellite chases the other exactly 137 miles behind, but slight variations in the Earth’s gravitational pull, from features like groundwater or ice caps, cause that distance to fluctuate. This gives a sense of the mass of those features and how they change over time.

For instance, GRACE has been used to track the melting of ice sheets in Greenland and Antarctica. Because they exert less gravitational pull as they melt, scientists concluded Greenland had lost about 5,000 gigatons of ice over 15 years. NASA says it’s enough to cover the entire state of Texas in an ice sheet 26 feet high.

When it comes to measuring groundwater, having better real-time data could be useful in California. The state is just starting to manage groundwater use to keep the supply sustainable.

During times of drought, like now, about 80% of irrigation water for the state’s enormous agriculture business comes from underground aquifers, which landowners could previously tap with pretty much no limits.

As for that megadrought I mentioned that the West has been experiencing, scientists say it’s the worst in at least 1,200 years. That’s as far back as the tree rings they studied go. Researchers told The New York Times that the higher temperatures that come with climate change increase evaporation and will likely keep the drought going.

Before this, the longest droughts on record were about 30 years. We’re now at 22.

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