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 silver gravity meter in the photo is a relative-gravity meter, which measures differences in the force exerted on a mechanical spring as the meter is moved from place to place. These meters are highly portable but require extreme care during field surveys to minimize random drift and offsets in readings.
The photo was taken along the Colorado River south of Yuma, Arizona.