Honing in on Groundwater

The first steps in the analysis of GRACE data provide an estimate of total water storage change that includes all groundwater, soil moisture, snow, ice, and surface waters. When combined with additional information, the GRACE technique may be taken a step further and used to focus solely on groundwater changes. This raises the very exciting possibility of tracking changes in aquifer water storage from space. Rodell and Famiglietti demonstrated this potential by examining data from the High Plains Aquifer located in the midwestern United States.

“GRACE will provide estimates of changes in total terrestrial water storage over large areas [greater than 200,000 square kilometers] on a monthly basis,” Rodell states. “We hydrologists need to find a way to [separate] those total changes into changes in soil moisture, snow, groundwater, etc.”

The hydrologists want to take GRACE’s measure of continental water storage and break it down into its component parts so they can determine how much groundwater is stored in aquifers. In their study, Rodell and Famiglietti found that for the High Plains region, the variability of snow and surface water made insignificant contributions to total water storage variability when compared to the contributions made by groundwater and soil moisture. This means that soil moisture is the sole component that has to be removed from the gravity data to get at elusive groundwater changes. It turns out that the U.S. High Plains Aquifer has been well sampled and studied, and so scientists have long-term, detailed records on soil moisture change in the region. So, by subtracting the soil moisture contribution, the remaining change in GRACE’s measure of total water storage is likely due to changes in groundwater.

Rodell and Famiglietti’s study shows that the GRACE technique will allow for estimates of annual groundwater change over the High Plains that are within about 8.7 millimeters of their actual value. This level of accuracy may only represent a modest improvement for a well-sampled aquifer like the High Plains Aquifer, but there are many places in the world, such as northern Africa, India, and the Arabian peninsula, where an estimate of the water levels within a centimeter or less of the actual value would be extremely valuable. GRACE may help to reveal groundwater depletion in areas of the world where such measurements are not systematically recorded or where they are not disclosed for political reasons. Even over more extensively sampled areas, the prospect of a technique that is less labor intensive and does not require an extensive network of wells makes GRACE an attractive option.

Overall, the GRACE technique offers an objective, unbiased method for monitoring water storage changes on a global scale. According to GRACE Principal Investigator Byron Tapley, of the University of Texas, “GRACE is our first real chance to demonstrate the measurement of gravity as a new remote-sensing tool that will play a major role in our understanding of the Earth and its climate.”

 Planning for the Future
 Challenges and Limitations to Using the GRACE Technique