How can we track the presence of a water resource that we can’t see? Groundwater—water that is contained within fractured rocks or soil pores beneath the ground surface—is by its nature invisible. Yet it comprises over 26 percent of U.S. water withdrawals. That percentage is dramatically higher in arid regions like California, where groundwater is an important source for irrigated agriculture. In addition, groundwater from shallow private wells is a relatively small but important source of drinking water for nearly 43 million people who are not connected to a public drinking water distribution system.

Understanding the amount of available groundwater—and how quickly that groundwater is being depleted—is thus vital to the sustainable management of water resources. Yet, many well owners rebuff efforts to meter groundwater usage. Historically, groundwater levels have been estimated using data from monitoring wells (also known as piezometers) at specific points throughout the aquifer. These data can then be used to create groundwater contour maps that show the direction of flow in the aquifer. Geologic data about bedrock depth and aquifer material and in situ pumping tests allow hydrologists to use groundwater contour maps to determine aquifer storage. However, these estimates will always be uncertain, due to the limited spatial resolution of available data. (See videos on groundwater via "Interlude: County of Maui v. Hawaii Wildlife Fund")

Satellite-assisted groundwater monitoring techniques have improved this spatial resolution, revolutionizing the way that data are collected, and have improved hydrologists’ ability to estimate the storage, replenishment, and depletion of major aquifers. The original GRACE (Gravity Recovery and Climate Experiment) satellites launched in 2002 were used for fifteen years to collect information about the mass of water and ice on and beneath the earth’s surface. The twinned GRACE satellites used a microwave-based tracking system to detect minute changes (less than the fraction of a human hair over 137 miles) in the distance between the two satellites resulting from changes in the pull of earth’s gravity. These changes in gravitational pull allowed GRACE to record changes in the earth’s mass due to, for example, groundwater depletion.

The GRACE Follow-on (GRACE-FO) mission launched in 2017 to replace the decommissioned satellites is being used to determine weekly drought indicators reported by the National Drought Mitigation Center.

Multimedia Video: GRACE and GRACE-FO Track California's Land Water Changes, Jet Propulsion Laboratory, California Institute of Technology and NASA.