Changes in Earth's Gravity Reveal Changes in Groundwater Storage

Did you know that changes in the amount of water in aquifers cause small changes in Earth’s gravitational field? When the amount of groundwater in an aquifer changes, either by recharge or by discharge to surface water or wells, the gravitational acceleration at the land surface also changes. In this photo, USGS Research Hydrologist Jeff Kennedy is making measurements of very small changes in the earth’s gravity field to help us understand changes in groundwater storage. This very small change is detectable with highly precise instruments. The measurements allow scientists to map changes in groundwater storage.

Gravity changes can also be measured by satellites, which provide an average value of water-storage changes over several thousand square kilometers. While these data sources are complementary, land-based measurements provide finer spatial resolution at the local or regional scales that resource managers typically need to inform groundwater management and planning decisions.

The yellow gravity meter in the photo is an absolute-gravity meter, which provides a direct measurement of the absolute force of gravity at a location. Absolute-gravity meters measure the acceleration of a mass falling in a vacuum, using a laser interferometer to measure distance and an atomic oscillator to measure time.

This photo was taken near the All American Canal in Southeast California Desert west of Yuma, Arizona.