Weighing Brazil’s drought from space

The drought that has been afflicting southeastern Brazil for three years has become the country’s worst since the 1920s. Water is rationed in São Paolo and other cities. Crop yields are plummeting. If the drought does not break, energy rationing could follow, as Brazil’s hydropower stations struggle to operate. To gain a continent-scale overview of the disaster, Augusto Getirana of NASA’s Goddard Space Flight Center turned to data from the Gravity Recovery and Climate Experiment (GRACE). The two GRACE spacecraft circle Earth in a low-altitude polar orbit, one trailing the other by 220 km. Whenever the lead craft flies over, say, a mountain, it feels a slightly increased gravitational tug, which temporarily pulls it farther away from the trailing craft. Measured interferometrically, such fluctuations in separation—positive and negative—are translated into a time-dependent map of Earth’s gravity. On seasonal time scales, the fluctuations arise largely from changes in the disposition of the planet’s liquid and frozen water. When Getirana looked at GRACE maps of Brazil, he could see the seasonal changes in the total amount of water above and below ground. As the figure shows, by 2014 a severe drought had stricken the southeast. Because droughts arise from planet-scale shifts in climate, Getirana’s study suggests that gravitational data could help tie those shifts to local shifts in ground and surface water. (A. C. V. Getirana, J. Hydrometeor., in press. )