September 10, 2009 | In 2008, NCAR scientist Sean Swenson used data from twin satellites named GRACE (Gravity Recovery and Climate Experiment) to measure the mass of water in the region around east Africa's Lake Victoria, revealing that human management—not just drought—was responsible for declining water levels.
Swenson is drawing from GRACE again, this time to assess northern India's groundwater. The stretch of the subcontinent that runs from eastern Pakistan across northern India into Bangladesh is likely the world's most intensively irrigated region. It's also home to 600 million people. In a study published in Geophysical Research Letters in August, Swenson and his collaborators found that, between 2002 and 2008, the region depleted groundwater at a rate of around 13 cubic miles (54 cubic kilometers) per year, enough to produce a drop of about 4 inches (10 centimeters) per year in the water table when averaged over the entire region.
The research team's estimate of yearly groundwater loss is roughly equivalent to the loss from melting Alaskan glaciers during the 2002-2008 period. It represents the total volume of water leaving the area through evapotranspiration and runoff in excess of precipitation.
"This is probably the largest rate of groundwater loss in any comparable-sized region on Earth," Swenson and his co-authors write. "If this trend is sustained, it will lead to a major water crisis in the region when this nonrenewable resource is exhausted."
GRACE works by measuring subtle variations in Earth's gravity caused by water moving around and under Earth's surface. The two satellites follow each other through space about 85 miles (220 kilometers) apart, making passes over the same spots. Scientists can extrapolate changes in gravity from the rates of acceleration between the satellites, combining this data with hydrological models to give a holistic picture of groundwater.