Unraveling the Mystery of Groundwater Recharge in California’s San Joaquin Valley
Nestled between the towering Sierra Nevada mountains and the sprawling San Joaquin Valley, a significant scientific investigation is underway. This is not a classic detective story featuring a human suspect, but rather an exploration of a natural process: the journey of snowmelt as it infiltrates underground to replenish the critically depleted groundwater reserves in the region.
The investigator in this scenario is not a traditional detective but a NASA aircraft equipped with cutting-edge radar technology. This advanced machinery, sensitive enough to detect ground movements as thin as a nickel, is being utilized to potentially solve one of the most pressing water management challenges in the American West—preventing the depletion of groundwater supplies.
Erin Urquhart, the program manager for NASA’s Earth Action Water Resources, emphasizes the importance of this technological advancement. “NASA’s technology could revolutionize our precision in tracking where snowmelt is recharging groundwater,” she explains. “This information is crucial for farmers, water managers, and policymakers who are striving to make informed decisions to safeguard water supplies for both agriculture and communities.”
In late February, a NASA aircraft equipped with the Uninhabited Aerial Vehicle Synthetic Aperture Radar (UAVSAR) embarked on the first of six planned flights for the year. These flights cover a 25-mile stretch of the Tulare Basin in the San Joaquin Valley, an area that experts believe holds the key to maintaining water supplies for one of the United States’ most productive agricultural regions.
The groundwater in the San Joaquin Valley is largely replenished by the melting snow from the Sierra Nevada. However, as Stanford University geophysicist and professor Rosemary Knight points out, “For generations, we’ve managed water in California without truly understanding where that meltwater infiltrates the ground to replenish the groundwater.”
This process is largely invisible, as the moisture seeps through rock and sediment, disappearing beneath orchards and fields. As the water flows downhill, it follows a predictable pattern, filtering into rivers and streams, with some of it eventually soaking into the ground at the valley’s edge or as the waterways fan out across the valley. This movement of water underground can create slight pressure, resulting in an upward push of the surface. Although these movements are imperceptible to the human eye, NASA’s sophisticated radar technology can detect them.
Yunling Lou, who leads the UAVSAR program at NASA’s Jet Propulsion Laboratory in Southern California, elaborates, “Synthetic aperture radar doesn’t directly detect water. It measures changes in surface elevation—less than a centimeter—that indicate where water is present.”
These surface bulges form what Knight describes as an “InSAR recharge signature.” By monitoring how these bulges migrate from the mountains into the valley, the team hopes to identify where groundwater replenishment occurs and quantify the amount of water naturally recharging the system.
Previous satellite-based InSAR (Interferometric Synthetic Aperture Radar) research has demonstrated that land in the San Joaquin Valley experiences uplift and subsidence with the seasons, as groundwater is replenished by Sierra snowmelt. However, satellite radars could not uniquely identify the specific paths of recharge. By combining satellite data with images of underground sediments obtained through an airborne electromagnetic system, Knight’s team successfully mapped major hidden subsurface water pathways responsible for aquifer recharge.
NASA’s airborne UAVSAR system promises to provide even more detailed data, potentially allowing researchers to gain a clearer view of where and how quickly water is reabsorbing into the ground and recharging depleted aquifers.
California’s Central Valley, which includes the San Joaquin Valley, is a vital agricultural hub, producing over a third of the nation’s vegetables and two-thirds of its fruits and nuts. The region relies heavily on groundwater, especially during drought years. Consequently, water managers sometimes impose restrictions on groundwater pumping when aquifer levels drop. This has led some farmers to drill increasingly deeper wells, escalating costs and further depleting reserves.
Aaron Fukuda, general manager of the Tulare Irrigation District, underscores the importance of understanding recharge patterns. “Knowing where recharge is happening is essential for effective water management,” he says. “During dry years, we can direct flood releases to areas that recharge efficiently, avoiding places where water would merely evaporate or take too long to soak in. In wetter years, like 2023, it’s even more crucial—we need to move water into the ground swiftly to prevent flooding and maximize absorption.”
NASA’s ongoing efforts to monitor and manage Earth’s water resources utilize a variety of advanced technologies that complement one another, each offering unique insights into groundwater management challenges. The upcoming NISAR (NASA-ISRO Synthetic Aperture Radar) mission, a collaboration between NASA and the Indian Space Research Organisation (ISRO) scheduled for launch in the coming months, will provide global-scale radar data to track land and ice surface changes, including groundwater movement signatures, every 12 days.
Additionally, the GRACE satellites, operated by the German Aerospace Center, German Research Centre for Geosciences, and NASA, have revolutionized global groundwater monitoring by detecting minute variations in Earth’s gravity, offering a broad view of monthly water storage changes across vast regions. The Gravity Recovery and Climate Experiment and Follow-On (GRACE and GRACE-FO) missions have revealed significant declines in aquifers, including those in California’s Central Valley. Nonetheless, their coarser resolution necessitates complementary tools that can precisely identify recharge hotspots.
Together, these technologies form a powerful suite of tools that bridge the gap between regional-scale monitoring and localized water management. NASA’s Western Water Applications Office (WWAO) plays a crucial role in ensuring that this wealth of data is accessible to water managers and others, offering platforms like the Visualization of In-situ and Remotely-Sensed Groundwater Observation (VIRGO) dashboard to facilitate informed decision-making.
Stephanie Granger, WWAO’s director at NASA’s Jet Propulsion Laboratory, emphasizes the value of these airborne campaigns. “Airborne campaigns like this one in the San Joaquin test how our technology can deliver tangible benefits to American communities,” she says. “We collaborate with local water managers to evaluate tools that have the potential to strengthen water supplies across the Western United States.”
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