In over half of the world’s largest aquifers, water is being drawn faster than it is being replenished, while some of those wellsprings may be much smaller than previously assumed, according to two studies released on Tuesday.

As climate change and population growth increasingly stress the world’s water supplies, understanding how much groundwater exists and at what rates it can be sustainably drawn is critical, said scientists behind the studies.

“Until improved storage estimates exist to determine a system’s full capacity to buffer against renewable ground water stress, continued pressure on aquifer systems could lead to irreversible depletion that seriously threaten the sustainability of groundwater dependent regions,” said a University of California at Irvine report titled “Uncertainty in Global Groundwater Storage.”

Although surface water is the world’s main freshwater supply, it has become less reliable and predictable. Groundwater is becoming an increasingly important source, with some 2 billion people depending on it, said researchers behind a second study, also by U.C. Irvine.

“Understanding the amount of groundwater used versus the volume available is crucial,” said the second report, titled “Quantifying Renewable Groundwater Stress.”

It is critical to understand how human and natural dynamics are impacting the world's available water resources in order to determine levels of sustainable use, the second study said.

The first report found that there is great uncertainty in the total groundwater storage and estimates of depletion, which is becoming increasingly relevant in regions prone to drought.

From satellite observations using NASA’s Gravity Recovery and Climate Experiment (GRACE) mission, researchers found that some aquifers were much smaller than previously thought.

Groundwater aquifers take thousands of years to be replenished, so unsustainable use could make access to freshwater impossible in regions that do not have correct data on supply versus use, the first report said.

The second report found that the use of 21 of the world’s 37 largest aquifers has become unsustainable, meaning more water is being drawn than replenished. Among the aquifers with the least sustainable use were two in India, one in China, and California’s Central Valley aquifer, the report said.

Researchers characterized the Central Valley aquifer as “highly stressed” because due it use for irrigation and the drought that has gripped the state.

“The current depletion rate shows that the aquifer is unable to balance the combined impact of groundwater use and drought,” the report said.

Such information is crucial for leaders to govern groundwater resources sustainably and to ensure active management of those aquifers, scientists said. Lacking such information, but responding to stress put on groundwater from the drought, California lawmakers passed the first regulations to govern the resource in 2014. Unfortunately, the report added, the laws may not go into effect until 2040.

Three aquifers in the middle of the United States were being drawn from in a sustainable manner, the second report said. But it warned that the Atlantic and Gulf Coastal Plains aquifer under Florida and the Gulf Coast was being depleted.

The author of the two studies, Jay Famiglietti, a senior water scientist at NASA’s Jet Propulsion Laboratory and the principle investigator for the studies, said the water table is dropping throughout the world, according to the Washington Post.

“We need to get our heads together on how we manage groundwater — because we’re running out of it,” Famiglietti told the Post.