Scientists map fastest pathways for replenishing Central Valley groundwater

Depleted groundwater threatens communities, agriculture, and ecosystems in California’s Central Valley, which produces much of the nation’s fruit, vegetables, and nuts. But the same acres where farmers have long cultivated thirsty crops might be critical for refilling aquifers, Stanford scientists have found. 

In a paper published April 17 in Earth and Space Science, the researchers used electromagnetic geophysical data to identify areas across the Central Valley where water released on the surface could rapidly flow into aquifers to “recharge” groundwater. 

“We were hoping to see a relatively big portion of agricultural land that’s suitable for recharge, and that’s what we’re seeing,” said lead study author Seogi Kang, who worked on the research as a postdoctoral scholar in geophysics in the Stanford Doerr School of Sustainability and is now an assistant professor at the University of Manitoba.

Tapped groundwater

Water held in sediments below the surface is an important resource for drinking water and irrigation, especially in dry years, and supports freshwater-dependent ecosystems. During droughts, groundwater provides up to 70% of water supplies in the Central Valley. But right now, water is pumped out faster than it is replenished, threatening supplies and causing issues like land subsidence due to the loss of water pressure and resulting compaction. To help restore balance, water agencies are looking for where they can recharge groundwater using excess surface water available in wet years. 

The trick is finding where this extra water will seep into aquifers rather than pond on the surface. Some land sits atop porous sand and gravel from old streambed sediments that allow water to move steadily down to the water table. But other parts of the Central Valley feature dense clay layers that prevent water from seeping into deep aquifers, leading it to evaporate from the surface. 

If farmland stays saturated too long, that can also pose a risk of crop diseases or destabilize the roots of orchard trees. “Surface water is very valuable,” said senior author Rosemary Knight, the George L. Harrington Professor in the Doerr School of Sustainability. “You don’t want to put it somewhere where it’s not going to provide a benefit in terms of recharge.”

Finding flow paths

To survey the Central Valley’s recharge potential, the team analyzed a trove of electromagnetic data collected by a helicopter-hoisted sensor that crisscrossed the region with a total of 20,000 kilometers of flight paths. The dangling sensor creates a magnetic field that extends below the ground, which allows it to detect subtle differences in how easily electrical current flows through materials up to 300 meters below the surface.

By comparing this data to logs from drilled wells, the team determined a relationship between the ease of electrical current flow and sediment type. Electrical current moves readily across clay layers and is blocked in areas dominated by sand and gravel, a pattern that Knight had used to interpret the same type of data in the Central Valley in previous research. 

Using equations that predict how the drilling logs and electromagnetic data relate to sediment type, the team built a web application called “fastpath,” which groundwater agencies, consultants, and land owners can use to identify the fastest pathways for water through the sands and gravels in an area. The application was developed with funding from Stanford’s Sustainability Accelerator. 

In response to requests from water managers who tried out the app, the team then used the fastpath software to create maps assessing recharge suitability across the entire Central Valley. The maps include a few metrics for assessing groundwater recharge potential. One is the average proportion of sand and gravel between the surface and water table. But since even thin layers of clay-rich material can impede water or cause it to move laterally, they also included other metrics such as the length of the porous “fastpaths” from the surface to groundwater. 

In total, the team found that up to 13 million acres of the Central Valley may be suitable for recharging groundwater. The largest portion of this area occurs on agricultural land, with most corresponding to orchards, field crops, and vineyards.

The researchers have made their data publicly accessible online for anyone to use in groundwater recharge planning. Users can choose between maps showing sand and gravel percentage, “fastpath” length, and the distance to a clay barrier or the water table. This flexibility is important because a “suitable” site for infiltration is subjective; if a crop could be ruined by possible ponding, then it may be better to choose a site with rapid infiltration. In other places, slower flow might be acceptable.

Next, Knight intends to continue building on the electromagnetic geophysical data to solve other groundwater problems, such as determining the best places to inject water for counteracting ground subsidence or to promote healthy ecosystems. “How do we take advantage of all these data that are now available, to go from sensors to solutions,” she said. “That’s what I’m all about.”

Rosemary Knight is also a senior fellow at the Stanford Woods Institute for the Environment. Meredith Goebel, Stanford physical science research scientist, is also a co-author. 

The research was funded by the Sustainability Accelerator in the Stanford Doerr School of Sustainability, the Gordon and Betty Moore Foundation, and the United States Department of Agriculture.