California’s five-year drought taxed the state’s water supplies like never before, especially its groundwater. Many areas of the state saw huge drops in aquifer water levels, with resulting surface subsidence and even damage to infrastructure such as roads and canals.
As a result, water agencies and scientists began looking for ways to monitor groundwater more closely. One that emerged uses sensors mounted on Earth-orbiting satellites.
It’s difficult to imagine that satellites can detect changes in groundwater. The fact that they can indicates just how much groundwater consumption affects the planet: Losses in groundwater can be detected from changes in the Earth’s gravitational force and by precisely measuring land surface elevation.
The latter technique has been perfected by Estelle Chaussard, a professor of geology and geophysics at the State University of New York at Buffalo. In a new study, Chaussard and colleagues explain how they used radar measurements of the Earth’s surface provided by Italian satellites to monitor groundwater in California’s Silicon Valley.
In particular, Chaussard’s team used a technique called InSAR (Interferometric Synthetic Aperture Radar) to measure tiny vertical movements of the Earth’s surface caused by changes in underground water levels. They looked at deformation, changes in the land caused by the falling water levels in the aquifer, and worked closely with the Santa Clara Valley Water District, which provided data from numerous drinking water wells. The district serves 1.9 million people in Santa Clara County by delivering water on a wholesale basis to numerous local governments and private water companies.
Water Deeply recently interviewed Chaussard to learn more about this new method of tracking groundwater.
Water Deeply: Is it the first time this kind of satellite data has been used to measure groundwater?
Estelle Chaussard: The first time it was used to look at groundwater was in 2014, when I looked at Silicon Valley to try to reconstruct the history of deformation of the basin.
The new thing about what we did in this [latest] work was to use a new satellite technique that’s really the best we can get right now. The big difference is, we can look every day at deformation of this area. And that’s very important when we look at a system that is undergoing a drought and a recovery period. We can really pinpoint when the system is undergoing changes. We’re really looking at improvements of techniques in terms of how we can monitor these resources from space.
Water Deeply: What is the source of the satellite data you used?
Chaussard: The data comes from a constellation of four satellites operated by the Italian space agency. When I did the research in 2014 – the very first work – we had access solely to InSAR satellite data that provided only one image of the deformation, at most, every 35 days. The work we just published is very different. We have data up to every day, so that was a major improvement. It’s because of the better satellite constellation – the design of the orbits of how these four newer satellites are oriented.
NASA has never had an InSAR satellite. But the first NASA InSAR satellite, a project with the Indian space agency, should be launched in 2021. Up to now, everything has been coming from Europe.
Water Deeply: Why did you choose to examine Silicon Valley?
Chaussard: At the time, I was living in Berkeley and I was visiting nearby, so we had the interest in the area that was local at the time, and that’s where we had the contact with the water district, which was very important. In some places, it’s really hard to get access to well water data. A good relationship with the Santa Clara Valley Water District made us more interested in working in this area.
Water Deeply: What is the accuracy of the ground deformations that you’re measuring?
Chaussard: We can measure changes in elevation on the order of a few millimeters. We can detect really small changes in elevation. It’s a radar system, it’s not a laser system. We’re using changes in phase of the radar signal over time to measure changes in elevation.
Water Deeply: How do you get water volume measurements by monitoring land elevation?
Chaussard: We need data from a few wells to be able to understand the properties of the aquifer, how the elevation changes relative to changes in water. We have a few [pieces of] well data from the Santa Clara water district, and we basically create a map of how the wells relate to elevation changes. That’s why we need a close relationship with the water district.
The Santa Clara Valley was kind of an ideal case. We have a lot of wells that are closely monitored. So we can compare what the InSAR predicts with what the real well changes are from the actual well data.
Water Deeply: How much water did the Silicon Valley aquifer lose?
Chaussard: We mapped, throughout the basin, the elevation changes and the corresponding changes in water level. We found the maximum changes in elevation associated with the drought was about 5–6cm [2–2.5in] of subsidence. And the water in the wells went down by about 30–35m [100–115ft]. And that was in the center of the basin, so in the area south of Sunnyvale. As you go from the center of the basin toward the sides, you have less and less elevation change, because the wells pumping water are in the center of the basin.
Overall, the Santa Clara Valley lost water measured at 0.09 square km [0.035 square mile] during the drought. Normal variation is 0.02 square km [0.008 square mile]. So the water loss increased by about 4.5 times during the drought.
Water Deeply: How much did the aquifer recover?
Chaussard: What we found that was extremely interesting was that the recovery started before the precipitation started. What that really meant was that the conservation efforts of the water district really worked. We started seeing this recovery in late 2014, while precipitation didn’t really start until early 2016. We had a few months where we started seeing elevations going back up because the water district [took] a lot of measures to reduce water usage.
The second thing we found was that, by early 2017, the water in wells was back to pre-drought levels. That is really important because it means there was no permanent change in the aquifer. There was no damage to the aquifer – it remained healthy.
Water Deeply: How was this research important to the water district?
Chaussard: We were able to learn that their measures to restrict water usage really worked. They were able to get [groundwater] recovery started in advance of the precipitation. Their water conservation efforts really had an impact on the aquifer. It was meaningful to them to see that water went back to pre-drought levels, and they did not damage the aquifer. They were able to keep the porosity as they had it before the drought. The Santa Clara Valley Water District did a very good job in this sense, compared to the Central Valley of California, where there has been a lot of pumping and there is permanent damage to the aquifer.
Water Deeply: The Santa Clara Valley Water District also relied on imported water, didn’t it?
Chaussard: Yes, the water district actually imports water from the reservoirs that are basically on the Diablo Range mountains to the east (via the State Water Project). What they did is they used aqueducts to bring this water into the aquifer. In the case of drought, when there’s a lot of pumping and not much water in the form of precipitation, they can use this imported water to restore what normally would be received by surface water.
Water Deeply: How does your work compare to the satellite estimation of California groundwater conducted by NASA’s Jay Famiglietti?
Chaussard: Jay is working with a completely different form of satellite technology, the GRACE satellite. What the GRACE satellite is measuring is changes in the gravity field over time. What we are measuring with InSAR is changes in elevation – basically ground deformation. It’s two different things.
The downside of GRACE is the maximum resolution we can get is a 200 square km [77 square mile] area. The aquifer we have in Silicon Valley is basically smaller than one pixel in the GRACE data. The deformation that we have from our InSAR technique is more of a scale that can be used by a water district.
Water Deeply: Is there a need for more satellites collecting InSAR data?
Chaussard: There is a need for satellites that have basically global coverage, and the more frequent data we can get the better we can understand what deformation means on the ground. So there is always a need for more global coverage and more free data.
Some of these satellites don’t provide free data. For example, the Italian space agency: We need to have an agreement to get the data free of charge.
Estimating a cost is pretty hard. A lot of the work with InSAR is trying to find access to the data that doesn’t neces
sarily require purchasing the data. For this work, we collaborated with the Italian space agency – that allowed us to have the data for free. But in the future it would be a case-by-case basis figuring out the cost.