New Paper Assesses Groundwater Quantity and Water Table Levels in the U.S.

     I recently posted about a series of studies by researchers at UC Santa Barbara regarding water table levels around the world, the problem of groundwater depletion, and possible solutions. Another recent study maps water table levels in the U.S. In this study, researchers at Princeton University and the University of Arizona took data from about 800,000 wells and applied a machine-learning model to estimate the depth of the water table nationwide. They estimated aquifer freshwater levels down to 1300 feet, much deeper than most groundwater wells.

     One of the problem areas in the U.S. where groundwater is depleting fastest is the Central Valley area, which is our country’s agricultural powerhouse. It is noted that in some areas, like the desert Southwest, the groundwater is considered “fossil water,” and if depleted, it will take years to be recharged without severe interventions. There, when groundwater depletes, it often dries up connections to surface water, such as wetlands and small tributaries.

     According to an article in the LA Times:

“The total quantity of water underground is still immense. The scientists found nationwide there is roughly 250 billion acre-feet, or 13 times the volume of the Great Lakes.”

“Data compiled by lead author Yueling Ma show the Colorado River watershed has about as much groundwater as the volume of the Great Lakes, while California has about 70% of that.”

     The map below shows that the uncertainty level, as exemplified by the inter-quartile range (IQR), is generally lower in the eastern U.S. and higher in the western U.S. 

     The database for California showed 6000 water wells that have dried up since 2013. However, only 13 ran dry in 2025, so the rate has likely slowed.

     The new study utilized a machine learning approach. It emphasizes the need for higher local spatial resolution in order to get a better estimate of groundwater quantities. Therefore, the study utilized a high-resolution (approximately 30 m) approach to calculate groundwater storage. Other studies often utilize a much lower 1 sq. km resolution. 

     One known finding confirmed by the study is that groundwater has large spatial variability, which poses challenges for management. They show that wow resolution estimates systematically underestimate accessible groundwater. This is why higher spatial resolution is important to get accurate estimates. The first graphic below shows the mismatch between the spatial scale resolutions of common remote sensing methods and modeling. The second graphic below shows the loss of detail when the spatial resolution is too low.

     According to the paper:

“Groundwater systems are not a uniform reservoir and have significant spatial variability across multiple scales. Water table depth is often portrayed as a subdued replica of topography, however, groundwater recharging at higher elevations can travel great distances laterally underground to topographic lows, also known as groundwater convergence. Groundwater can maintain shallow water table depth in areas of local convergence during dry conditions, the same way that baseflow supports streamflow. These groundwater-land surface connections are of great importance to both watershed dynamics and ecosystem function often helping to sustain vegetation through drought.”

     The authors note that machine learning sidesteps computational barriers to bridge scales.

“We combine our water table depth product (Fig. 2) with spatially variable estimates of porosity from previous literature to calculate total groundwater storage (see SI S1.3 and Table S1). We estimate a total groundwater storage of 306,500 km3, with an uncertainty range of 291, 850 km3 to 316, 720 km3. This estimate includes all groundwater above a depth of 392 m (this depth is the deepest depth for which we have reliable porosity data, and roughly the limit of active circulation in the hydrologic cycle.”

     The authors conclude that their higher-resolution estimates can help with local -scale groundwater management strategies:

“Despite the groundwater challenges we currently face and the anticipation of future challenges, large-scale estimates of the quantity of groundwater can be connected to the local-scale water table depth to provide additional information critical in water management. The accessibility of groundwater under agricultural regions demonstrates the uncertainty in the national water supply used to sustain food production.”

 

    

References:

 

How much water do we have? Scientists map the water beneath our feet. Ian James. Boiling Point Newsletter. Los Angeles Times. April 9, 2026. How much water lies underground? Scientists finally have an answer - Los Angeles Times

High resolution US water table depth estimates reveal quantity of accessible groundwater. Yueling Ma, Laura E. Condon, Julian Koch, Andrew Bennett, Amy Defnet, Danielle Tijerina-Kreuzer, Peter Melchior & Reed M. Maxwell. Communications Earth & Environment volume 7, Article number: 45 (January 2026). High resolution US water table depth estimates reveal quantity of accessible groundwater | Communications Earth & Environment