Recent findings indicate that California’s Central Valley, often referred to as America’s breadbasket, derives nearly half of its groundwater from the Sierra Nevada mountains. This is crucial for a region where, in many areas, irrigation is heavily dependent on groundwater.
Recent findings indicate that California’s Central Valley, often referred to as America’s breadbasket, receives almost half of its groundwater supply from the Sierra Nevada mountains. This is crucial for a region where, in many areas, irrigation is heavily dependent on groundwater.
While it’s easy to observe the changes in above-ground reservoirs due to rainfall and snowfall, the aquifers—natural water sources located deep underground—remain out of sight, often hundreds of feet below the surface. “They resemble large bathtubs filled with water and sediment,” explained UC Riverside’s associate professor of groundwater hydrology, Hoori Ajami.
Research has long established that the Sierra Nevadas play a vital role in supplying water to the Central Valley aquifer, but this new study led by UC Riverside is the first to accurately measure the impact of the mountains on groundwater. Published in the journal Water Resources Research, the research represents that the contribution from the Sierras can be as high as 53% in the southern Central Valley.
It’s not only the snowmelt and streams from rainfall that feed the aquifer below; there are also aquifers within the Sierra mountains that seem to contribute to the Central Valley aquifer.
“This research demonstrates a significant connection between the mountains and the valley,” stated Sandra Armengol, the lead author of the study and a former postdoctoral researcher at UCR.
Given that groundwater is often hidden, measuring it can be challenging. The research team employed a comprehensive strategy to determine the origins of the water in the Central Valley aquifer.
Part of their approach utilized well water samples from both the southern Central Valley and the mountains, collected by the U.S. Geological Survey. The UCR researchers then analyzed the chemical compositions—such as levels of calcium, sulfates, sodium, and both radioactive and stable isotopes—to ascertain the water’s sources and age.
Carbon-14 and tritium are the chemicals used to determine the age of the water. Carbon-14 is also utilized in archaeology to date artifacts that are tens of thousands of years old, while tritium helps identify water that is less than 50 years old.
The UCR team discovered that the groundwater from the Sierra flowing into the Central Valley shows a wide range of ages. Some samples were as young as 4 years old, while others were about 102 years old, and certain samples dated back over 40,000 years. This variability indicates that the water cannot simply be replenished once consumed.
“It takes a significant amount of time for precipitation to percolate through sediment layers to recharge aquifers. If we maintain current pumping levels, our aquifers in the Central Valley will face considerable depletion,” Ajami warned.
The groundwater shortage, exacerbated by the droughts from 2012 to 2016, led to excessive groundwater extraction, resulting in land subsidence that has affected infrastructure like dams and canals which deliver surface water to the valley.
In response, former Governor Jerry Brown enacted the Sustainable Groundwater Management Act in 2014, mandating the creation of sustainability plans for overdrawn basins to mitigate issues such as groundwater depletion and land sinking. Nonetheless, additional actions are necessary to effectively manage the groundwater supply.
“To determine a sustainable pumping rate for an aquifer, I need to understand both the inflow of water and the outflow of groundwater. Just like managing a bank account, your expenditures must align with your income,” Ajami explained.
This study contributes to efforts aimed at assessing that inflow, or recharge, supported by a $630,000 National Science Foundation CAREER award won by Ajami in 2020 for research in hydrology, education, and outreach. Further studies are planned to assess the Sierra Nevada recharge in greater detail.
“For aquifers facing over-extraction, like the Central Valley, more precise studies to determine recharge rates are essential,” noted Armengol. “However, we are optimistic this work will facilitate better management of this invaluable resource, which is crucial for food production.”
