Paper Cautions Against Oversimplified Interpretation Of GRACE Groundwater Data

NASA’s Gravity Recovery and Climate Experiment (GRACE) has provided useful information about global groundwater depletion, but insufficient attention has been placed on its limitations, concludes a paper published in Groundwater, a journal of the National Ground Water Association.

Authored by Dr. William M. Alley, NGWA’s director of science and technology, and Dr. Leonard F. Konikow, U.S. Geological Survey scientist emeritus, the article is available in the Wiley Online Library database.

“The article, ‘Bringing GRACE Down to Earth,’ is a must-read for anyone who wishes to use the data from this joint U.S.-Germany space mission in the context of groundwater hydrology," said Groundwater Editor-in-Chief Hendrik M. Haitjema, Ph.D.

GRACE uses a pair of coupled satellites to measure spatial and temporal changes in the Earth’s gravity field. From these data, estimates of changes in mass are derived and then, factoring in other available data, estimates are made of groundwater depletion.

“The GRACE results have been highly effective in getting large numbers of people to start thinking about groundwater and the sustainability of its use,” the paper states. “At the same time, oversimplifications in the interpretations of GRACE results have promulgated some misperceptions about groundwater resources.”

Following are some of the misperceptions and limitations of GRACE groundwater data raised in the paper:

• GRACE provides a one-dimensional indicator of the status of a large three-dimensional groundwater body
• Many key issues associated with groundwater pumping, such as streamflow depletion and land subsidence, are not addressed by GRACE data
• Analysis of GRACE data cannot assess or apportion the contributory factors causing groundwater depletion; it cannot distinguish climatic from pumping effects
• The total volume of groundwater is much less important than many other attributes of a groundwater system and is meaningless for many (most) aquifer systems
• Different indicators from GRACE results apply to renewable vs. nonrenewable aquifers.

“GRACE is among the latest ‘tools in the toolbox’ available to hydrogeologists to evaluate groundwater resources. Planned improvement in the resolution of satellite-based gravity measurement technology will certainly increase the value and utility of this tool,” the paper states. “But all hydrologic ‘tools’ have their strengths and limitations. GRACE, a powerful tool in some respects, is no exception.”

One example is GRACE’s limitations as a practical tool for water managers. GRACE estimates change in water storage over a very large footprint — a resolution on the order of 200,000 km2. GRACE provides a “big picture” view that might be appropriate input for setting broad regional or national policies, the paper states, but “the low spatial resolution of GRACE limits its ability to provide groundwater depletion data at a scale appropriate for water managers to use effectively. As such, claims that GRACE is a water management tool should be made with great modesty.”

The paper will appear in the November-December 2015 print edition of Groundwater.

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