Stanford study shows where – and why – recharge is uplifting sunken ground

New research reveals why some rivers in the San Joaquin Valley are causing the ground to uplift when others aren’t. The answer lies beneath the ground’s surface. 

A new study from scientists at Stanford University combines satellite data with airborne electromagnetic (AEM) flight data to see exactly what’s happening with recharged water from the Sierra Nevadas. 

The satellite process, called interferometric synthetic aperture radar (InSAR,) bounces signals onto the ground which can read over time where ground has uplifted due to groundwater recharge. The data, from the wet year of 2017, shows water traveling through the valley underground uplifting the surface as it moves. 

But other areas didn’t see the same effect. The study points out two sites where there are natural waterways, one near Fresno and one near Visalia. The Fresno site didn’t see any uplift while the Visalia site did.  

“You can’t figure out what it means until you can slice below the ground surface,” said Rosemary Knight, a George L. Harrington Professor of Earth Sciences at Stanford University and one of the authors of the study. 

That’s where the AEM comes in. AEM data shows what the subsurface is made of. The researchers flew AEM surveys over some of the areas of uplift that were found through the InSAR data. The AEM data reveals types of strata up to 300 meters below the surface. 

They found that only areas with significant clay saw uplift. If the ground is made up of too much coarse material, like sand and gravel, the water sinks in fast but simply moves through it and doesn’t uplift. 

It’s an answer Knight and other researchers suspected but needed to combine the practices to confirm. 

Overpumping groundwater in the valley has caused compaction in the lower aquifer which results in subsidence, land sinking. Some areas of the valley have subsided so badly, it has damaged vital infrastructure. A 33-mile section of the Friant-Kern Canal, which runs from Fresno south to Arvin, had sunk because of overpumping. That “sag” had  limited the canal’s carrying capacity by 60%. 

The research is exciting and has potential real world impacts, said Knight. 

“Over time, we know that the climate is likely to change, the timing of the snow melt is likely to change, there could be less snow in the Sierras,” said Knight. “It’s really important that we understand when and where these recharge processes are occurring so that we can do whatever we can do to protect these recharge sites.”

The data could be used to “zone for recharge,” said Knight. And it could be important to use this mapping to avoid development in areas where it could impact prime recharge spots, she added. 

This is the focus of another study Knight and her colleagues are submitting in less than a month. For that study, they analyzed AEM data across the entire valley to determine where recharge facilities should be constructed and where on-farm recharge would be best. 

“The more we understand how nature works, the more we can ourselves work to ensure that the decisions we make about land use are not interfering with natural processes that are maintaining our groundwater systems,” said Knight. 

Groundwater managers are looking forward to how the research could direct water management in the future. 

“It is exciting to see Stanford again pushing the envelope on geophysics, groundwater hydrology, and understanding of local sustainable groundwater management activities,” wrote Aaron Fukuda, general manager at Tulare Irrigation District and interim general manager of Mid-Kaweah groundwater sustainability agency, in an email. “As Stanford is finding, groundwater recharge, subsidence, and uplift are all new frontiers for some aspects. We continue to watch and learn from what science is telling us.” 

“We just have to start tapping into what we can do with these data to better understand management of our groundwater systems,” said Knight.