A worker walks on the parched ground of the Kouris dam in Alassa, Cyprus. Cyprus authorities are resorting to tapping the island's aquifer to make up a 565 million cubic foot shortfall in dam reserves to meet the needs of the divided island's southern, government-controlled areas. (Andreas Lazarou/Associated Press)

WHEN RIVERS, streams and reservoirs are low, as they are in California, people start digging holes. Large, unseen pools of water are trapped in the spongy rock and soil of the world’s aquifers, sometimes fairly close to the surface, sometimes deep underground. Tapped groundwater can save communities from economic or humanitarian catastrophe. But in too many places, humans are depleting this crucial reserve, just as climate change begins taking its toll.

A NASA study released this week provides authoritative estimates of the state of 37 major aquifers around the globe. Using satellites that measured minor variations in the earth’s gravitational pull between 2003 and 2013, researchers found that 21 of these aquifers are being run down.

A handful of massive aquifers in Africa, the Middle East and Central Asia were worst off, seeing little to no refilling to offset withdrawals over the decade of study. The Arabian Aquifer System and the Indus Basin were particularly troubled. But Western countries haven’t done a great job of aquifer management, either. Miners in Australia’s Canning Basin and farmers in California’s Central Valley are tapping groundwater resources at unsustainable rates. Groundwater already accounts for more than half of California’s water use, a proportion that may well continue to rise.

In wealthy countries with well-developed governing institutions, there’s simply no excuse for failing to take care of this vital resource. The best way to promote rational consumption is to end government subsidies, explicit or implicit, for overusing water, particularly in dry areas, then to establish functioning, transparent and efficient markets for surfacewater and groundwater alike. Yet places such as California still operate on a system of confusing historical water rights and claims that aren’t conducive to conservation or to getting water where it’s most needed. The state is finally starting to regulate groundwater, but its leaders should aim for fundamental reform.

The challenge is harder in poorer parts of the world, where historical and cultural claims to groundwater are also often strong but economies are less diverse and institutions are weaker. In the long term, the solution is similar to that in developed nations: Remove irrational incentives to overuse or inefficiently divert water, then build transparent markets that force people to account for the full costs of the resources they use. In the short term, transition costs might be high and political forces incapable of making such a leap.

According to J.S. Famiglietti, the author of the NASA study, the first thing to do is establish the size of groundwater reserves, in a process akin to estimating subterranean oil reserves, so that people know how much is left. From there should come effective monitoring of groundwater use, pinpointing who’s taking how much and for what purpose. This information will make it easier to identify opportunities to use water more efficiently, particularly in agriculture, without suddenly pushing large numbers of people out of agricultural work. Along the way, economic growth will diversify employment and reduce opposition to broader reform, a 2003 United Nations report notes, and institutions might strengthen enough to effect broader reform.

Many people didn’t need satellites to know that aquifers are in trouble. They are drilling deeper and deeper for groundwater. The poor can’t afford to construct ever-deeper wells, so they will be out of the game in stressed areas. Governments cannot ignore this issue for long.