Bringing GRACE Down to Earth.

Introduction NASA’s Gravity Recovery and Climate Experiment (GRACE), which is a joint mission of the United States and Germany, uses a pair of coupled satellites to measure spatial and temporal changes in the Earth’s gravity field. From these data, estimates of changes (time-variable anomalies) in mass are derived. In turn, the mass changes are attributed primarily to changes in water content (Tapley et al. 2004; Tiwari et al. 2009; Rodell et al. 2009; Famiglietti and Rodell 2013). Changes in water mass can arise from several hydrologic components, including soil moisture, surface water, snow and ice, and groundwater storage. Surface water and snow water equivalent contributions to total mass changes are estimated using auxiliary datasets and models and subtracted from GRACE-derived estimates of change in water mass. The residual is attributed to subsurface water storage changes. Assuming that soil moisture changes (trends) are negligible or can be estimated allows volumetric groundwater storage depletion to be identified (noting that fluid density provides a straightforward linkage between water mass and water volume). GRACE has provided useful information about global groundwater depletion. In particular, the GRACE results have been highly effective in getting large numbers of people to start thinking about groundwater and the sustainability of its use. GRACE’s color-coded maps show at a glance where groundwater is being rapidly depleted around the globe. At the same time, oversimplifications in the interpretations of GRACE results have promulgated some misperceptions about groundwater resources. We address these misperceptions and discuss how, with appropriate caveats on their limitations, GRACE results can be placed in a stronger hydrogeologic context.