Determining the amount of water across any landscape is tricky. Only about 1% of Earth’s freshwater is above ground, making it easy to measure. The rest is underground, where the depth and porosity of the ground vary widely.
Reed Maxwell, a hydrologist at Princeton University, compares this to money management. He sees rainfall and surface water as a checking account for immediate needs, while groundwater is like a savings account. However, he notes, “We don’t know how much is being added to our savings or how much is actually there.”
Recently, Maxwell and his team produced a high-resolution groundwater map of the contiguous United States. They estimate there’s around 306,500 cubic kilometers of groundwater there. That’s a staggering figure—13 times the water volume of all the Great Lakes combined! This new estimate is significantly refined compared to earlier ones, which ranged from about 159,000 to 570,000 cubic kilometers. The map even accounts for depths reaching 392 meters.
Grant Ferguson, a hydrogeologist from the University of Saskatchewan, points out that this new mapping effort is refreshing. He says it employs better resolution than what has been used in the past.
Historically, estimates came mainly from well data. Laura Condon, a hydrologist at the University of Arizona, highlights this limitation. “We only have measurements from specific wells, which can miss vast areas,” she explains. Some places, especially those with less groundwater, have fewer wells, leading to gaps in data.
While researchers previously relied on well data and large-scale satellite observations—for example, NASA’s GRACE mission—their resolution is often much coarser, around 300 kilometers, compared to the new map’s 30 meters. A lower resolution can mean underestimating available groundwater by as much as 18%!
The new map also provides insights into groundwater sources. Interestingly, it reveals that about 40% of the land in the contiguous U.S. features a water table less than 10 meters deep. Condon explains, “This range allows for interactions between groundwater and plants, underscoring how interconnected these systems are.”
The research combined nearly a million groundwater measurements taken between 1895 and 2023, satellite data, and various environmental maps to develop a machine learning model. This model analyzes vast amounts of data much more effectively than previous methods. Instead of just looking at raw figures, it understands the complexities of groundwater usage and depletion.
Maxwell emphasizes that this machine learning approach introduces a beneficial form of bias. Traditional methods didn’t account for how humans extract groundwater. However, this new method integrates this data, giving a clearer picture of water availability.
Maxwell and his team hope this map assists water management and agriculture, helping farmers make informed irrigation choices. Condon adds that she wants to raise awareness about groundwater. “It’s everywhere,” she states. “Wherever you dig down, there’s water at some depth.”
As concerns about water scarcity grow, understanding how much groundwater we have is crucial. This map is not just numbers; it’s a tool for better resource management and ecological awareness.
This article was first published in EOS Magazine.
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