Groundwater is often contained between different confining layers, like a layer cake of aquifers under the ground. It’s a huge resource—there’s more freshwater underground than there is on the surface—but it is a finite resource: When it’s gone, it’s gone. Drawing groundwater out more quickly than it can be recharged, either naturally or artificially, creates a situation of overdraft.

“It’s harder to see the impact of global warming on groundwater than on surface water, but we’re getting fewer slow, steady rains that enable water to percolate into the ground and refill aquifers. The storms we get more often now produce huge amounts of rain all at once that the ground can’t absorb quickly. It runs off downstream and is less likely to refill aquifers. Frequent drought increases reliance on groundwater, and rising temperatures affect how much rain falls as snow—a bank account for water when it melts slowly and filters into the ground. Warmer temperatures also mean trees need to consume more water. As surface water declines, people indirectly use more groundwater, so we’re starting to drain aquifers along with surface water. And when we use too much water, we can actually diminish aquifer storage capacity.

“Groundwater regulation is important, as is enforcing that regulation. But allocation is a state purview. At the national level, our ability to understand the nation’s water resources has diminished over time, in part because the federal investment in monitoring has gone down. An important first step is for the federal government to provide strong leadership in understanding the state of the nation's groundwater."