Twin satellites track water’s rise and fall

A pair of satellites launched in 2002 has detected small, localized changes in Earth’s gravitational field caused by seasonal variations in rainfall and soil moisture.

Earth’s gravity can vary from place to place as large masses of air and water—such as weather systems and ocean tides—move across the planet’s surface. Some of the largest gravitational variations result from water flow across land, says Byron D. Tapley of the University of Texas at Austin. The Gravity Recovery and Climate Experiment (GRACE) satellites, launched in March 2002, are designed to measure such gravity variations on a monthly basis (SN: 1/4/03, p. 6: Mapping with GRACE).

Analyses of GRACE data gathered between April 2002 and December 2003 suggest that scientists can track shifts in Earth’s gravity caused by a change in mass equal to that of as little as 6 centimeters of water covering a 400-kilometer-by-400-km region.

The data suggest that between April 2003 and October 2003, the average amount of water present in some Amazonian locales dropped about 65 cm. If such a change were to occur throughout the 5-million-square-km basin, it would represent an outpouring of water that could easily exceed that of the Mississippi River, says Tapley.

He and his colleagues, who report their findings in the July 23 Science, also used GRACE’s gravity measurements to estimate year-to-year fluctuations in water levels around the world. For example, the scientists estimate that the area north of Africa’s tropical rainforest was substantially drier in April 2003 than it was a year earlier. Data from the GRACE satellites will enable scientists to discern the regional hydrological effects of climate phenomena such as El Niño and La Niña, Tapley asserts.