In many water-stressed regions of the world, the signs are obvious. Just look at the dramatically shrunken Colorado River, or the parched riverbed of the Danube. Far less conspicuous is the global water crisis unfolding beneath our feet, though it is severe.

That’s according to a recent study published in the journal Proceedings of the National Academy of Sciences. The researchers analyzed NASA satellite data to map global trends in freshwater storage at high-resolution, revealing regions where human activities such as agricultural and urban water use are putting dire strain on groundwater supplies.

“The regions of greatest concern are those already recognized as water-stressed, where we now observe losses even greater than previously estimated,” Mary Michael O’Neill, an assistant research scientist at the University of Maryland and NASA’s Goddard Space Flight Center, told Earth.com.

While it’s natural for Earth’s freshwater stores to fluctuate, human activity has become a dominant driver of groundwater depletion. In many parts of the world, extracting groundwater for irrigation, industry, and municipal uses has become a fundamental component of the water cycle, and climate change is only adding to this strain.

“Estimates of global total freshwaters withdrawals range from 3,800 to 5,000 gigatons per year, and unsustainable practices have resulted in pervasive water scarcity,” O’Neill and her colleagues write in their report. “Identifying and accurately quantifying anthropogenic, long-term changes in freshwater resources is critical for implementing strategies to ensure water and food security.”

To that end, scientists have used data from NASA’s Gravity Recovery and Climate Experiment (GRACE) and its successor (GRACE-FO) in hundreds of studies to track changes in global freshwater supplies.

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