Tucked in the foothills of the San Gabriel Mountains in Pasadena, California, NASA’s Jet Propulsion Laboratory (JPL) looks and feels like a high-security college campus for engineering nerds. From the streetlight banners celebrating the latest satellite launch to the bungee rocket toys in the gift shop, the facility—which is formally a part of the California Institute of Technology—leaves no doubt about its mission: “It is really an aerospace company,” is how Jay Famiglietti, NASA’s senior water scientist who runs a project called the Western States Water Mission, puts it. The vast majority of JPL’s thousands of employees are engineers and technicians who build robots and rockets to ship into space. Scientists like Famiglietti are there to make sure that all that hardware serves some sort of greater purpose benefitting humanity. Right now, for his research team, that purpose is mapping the dwindling water supply of the drought-gripped western United States.

Famiglietti is a hydrologist—a specialist in the world’s water systems. Before he arrived at JPL, he was accustomed to looking at rivers, oceans, and aquifers from ground level. Now, as a NASA researcher, he looks at them from space. His project seeks to create a computer model that will track and predict the movement of every concentration of water in the western part of the country: how much snow is in the mountains, how much moisture is in the soil, how much groundwater the aquifers contain, how much water is in the rivers and how quickly it’s flowing, where floods will occur, and what's the water volume of the reservoirs. Once his team shows it can be done regionally, they will expand the project to a global scale.

The ongoing drought presents an obvious case for such an undertaking, as do the outdated methods we use to measure water concentration. Nearly two-thirds of California’s water supply comes from snowmelt out of the Sierra Nevada Mountains, but estimates of the volume of snowpack are rough at best. As they have been doing for over a century, state surveyors make their estimates by planting measuring poles in a handful of locations in the mountains. They complement that method with a technology they have been using since the 1980s: scattering the landscape with scales that weigh the snow that falls on them.

To measure the volume of the aquifers, which are being depleted in California at a catastrophic rate, researchers can’t do much better than making educated guesses based on how far wells have to be drilled in order to hit water tables. Even reservoirs defy accurate measurement, due to our limited understanding of the ways that sediment settles and how that changes the shape of lake floors. “Hydrology is nothing but a data gap,” Cédric David, a young, scruffy French hydrologist on Famiglietti’s team, who specializes in river systems, told me. “Pretty much, we don’t know anything.”

NASA took a big step toward filling that data gap in 2002 with the launch of the Gravity Recovery and Climate Experiment—GRACE—by engineers at JPL and the University of Texas. GRACE is composed of two satellites, each about the size of a Volkswagen Bug. The pair orbits around the world, with a distance of 250 kilometers between them. As one approaches a mountain range, or any part of the world where the crust of the earth is thicker or denser than elsewhere, the force of gravity acting upon that satellite increases as a function of the greater planetary mass beneath it. The result is a slight acceleration. Where there is a basin, the pull is slightly less; the satellite drags.