Groundwater Research at Goshen College

A pair of satellites measures very small changes in earth's gravitational pull across the globe. Changes in local gravity are due primarily to changes in groundwater. Using satellite data rather than test wells gives groundwater data in a resolution of 1 degree latitude by 1 degree longitude across the whole world, even in otherwise inaccessible areas

How It Works

The Gravity Recovery and Climate Experiment (GRACE) mission was launched in 2002 as a collaboration between NASA and DLR (the German space agency). The two satellites, nicknamed Tom and Jerry, coast around the earth, separated by 220 km (137 miles) and 500 km (311 miles) above sea level. Higher gravity ahead - due to different densities on earth's surface - causes the front satellite to speed up. Microwave transmissions between satellites are used to detect tiny changes (less than 1 μ/s, or 3.6 mm/hr ) in relative velocity.

Changes in local gravity are due almost exclusively to changes in total water storage (TWS). This includes lakes and rivers (surface water), soil moisture, underground aquifers, and snow/glaciers. NASA, the DLR, and the University of Texas Center for Space Research (CSR) process the raw gravity data about once a month, and make the TWS data publically availible.