In 2008, drought hung over South America. Rivers shrank, pastures yellowed, and farmers stared at the sky waiting for rain that never came. Meanwhile, 450 kilometers above, two satellites flying 220 kilometers apart continued their invisible work: they were weighing the continent.
These satellites, named GRACE, measured Earth's gravity so precisely that they could detect water shifting from one place to another. When they passed over a region that had lost moisture, the gravitational pull weakened slightly, and the satellites drifted a bit farther apart. From these barely perceptible variations in distance, scientists calculated how much water had vanished from the La Plata basin.
Until now, such losses were estimated using models that account for rainfall and evaporation but do not see groundwater. The GLDAS model showed a loss of only 2.2 centimeters per year, while the satellites recorded up to 3.5 centimeters. The one-third difference arose because the model did not sense the depletion of underground reserves. When scientists added data from 27 wells, the model estimates nearly matched reality.
The connection to ocean currents was direct: dry years coincided with La Niña, when the Pacific Ocean cools and rains in South America diminish. Wet years, conversely, came with El Niño. This means the drought was not a random event but part of a larger natural cycle.
Today, this method is already in use: it can monitor droughts in other major basins and check how accurate climate models are. The scales that once sensed only mountains and oceans now weigh the water slipping away beneath our feet.
The deficit of 248 gigatons of water is equivalent to a layer 12 centimeters thick spread over an area comparable to the combined territory of Argentina and Uruguay.
The idea of weighing water from space echoes Arthur C. Clarke's novel 'The Fountains of Paradise,' where orbital engineers use gravitational anomalies to build a space elevator.