Thanks to record heat and wildfires, issues of drought in the United States are finally starting to get some needed attention. But images of bleached bones and dry, cracked creek beds don’t convey how deep our water depletion runs.

A new assessment makes the worldwide reduction in groundwater crystal clear. The analysis, published in Nature this month, examined groundwater stores and use in relation to rates of renewal and found the “groundwater footprint” (defined by the authors as “the area required to sustain groundwater use and groundwater-dependent ecosystem services”) was about 3.5 times larger than the actual area of the world’s aquifers.

“That said,” the authors write in their abstract, “80 percent of aquifers have a groundwater footprint that is less than their area, meaning that the net global value is driven by a few heavily overexploited aquifers.”

Among those threatened aquifers is the Ogallala, which spreads about 174,000 square miles from Texas to the Dakotas. The aquifer is the source for 30 percent of the country’s irrigation groundwater and also provides drinking water for 82 percent of the more than 2.3 million people that live in its boundaries, according to a U.S. Geological Survey fact sheet.

According to the recent assessment, the High Plains aquifer system (of which the Ogallala is a part) has a groundwater footprint of 5-10 times its actual size. Even in moister times, too much water is pumped from the aquifer—and these aren’t moist times. The lack of surface water is driving farmers to rely even more on the aquifer for irrigation, compounding the problem.

“The problem with drawing too much water from an aquifer, which has been stored in these geologic formations for thousands of years, is that it can’t easily be restored once pumped dry,” writes Scientific American editor David Biello. “That’s the crisis facing farmers who rely on the Ogallala Aquifer, which once contained enough water to cover the entire continental U.S. roughly half-a-meter deep. Once pumped dry, the Ogallala would take at least 6,000 years to refill.”

While the new science of groundwater footprint calculations may bring troubles brewing deep underground into the light, whether it will be compelling enough to change entrenched thinking about what we can't see with our own eyes is another question. A recent California Energy Commission legal analysis of barriers to water sector climate adaptation makes it clear just how far we have to go.

In California, groundwater use continues to depend on the legal categories of "surface water," "percolating groundwater," and "subterranean streams." The Commission report sparely explains the problem with this scheme by quoting Berkeley Law Professor Joseph Sax. “To put the matter as simply as possible, the above categories do not accord with scientific understanding of the occurrence and distribution of water on and in the earth."