Monitoring Ground Water Variability Using the GRACE Satellite Gravity Mission: An Application to Ground Water Withdrawal in Northern India

    NASA in collaboration with the German Space Agency launched the GRACE satellite mission in 2002.   The mission  is presently projected to last through 2013.  GRACE provides highly accurate solutions for the Earth's global gravity field every month.  Because gravity signals from water storage variability are particularly prominent in the data, GRACE can be used to monitor changes in  total stored water volume at scales of a few hundred km and larger.   Total water volume includes contributions from ground water, soil moisture, and surface water.  If soil moisture and surface water contributions are subtracted using either observations or model output, the residuals represent ground water variability.
      In this talk we illustrate these capabilities of GRACE by showing water storage estimates from around the globe.  We will focus, especially, on northern India and its surroundings. GRACE measurements reveal a steady, large-scale mass loss in this region that we attribute to excessive extraction of ground water. We use output from hydrological models to remove the contributions from natural water storage variability and thus to isolate the anthropogenic component. Our results suggest that this region lost groundwater at a rate of 54 km³/yr between April, 2002 (the start of the GRACE mission) and August, 2008.  This is probably the largest rate of groundwater loss in any comparable-sized region on Earth.  Its likely contribution to sea level rise is  roughly equivalent to that from melting Alaskan glaciers.