Using Gravity To Redefine Hydrology

Water planners and managers do their best to plan for water usage patterns but there are often many assumptions that have to factor into those plans. Specifically, how will private wells be used, and how will farmers use natural water sources such as streams and rivers adjacent to their land? How robust is the aquifer? What is the recharge rate of that aquifer relative to rainfall patterns? A poor assumption relating to any of these questions can unhinge a well-crafted plan. Now, planners may have a new tool for evaluating water sustainability – satellite imagery.

Scientists at the University of California have a project called GRACE which stands for “Gravity Recovery and Climate Experiment” and the project appear poised to redefine the science of hydrology. The project team looks for small variations in the Earth’s gravity to identify trouble spots where people could be making unsustainable demands on groundwater. Jay S. Famiglietti, director of the University of California’s Center for Hydrologic Modeling, says that GRACE can detect changes in ice, snow, surface water and soil moisture.

In an article in the journal Nature, the GRACE team identified and evaluated a region in northwest India. Indirect evidence had been mounting that groundwater was being consumed faster than it was being replenished but no direct evidence was available. The team used changes in observations from the NASA Gravity Recovery and Climate Experiment to show that groundwater was being depleted at a mean rate of 4.0cm annually over the Indian states of Rajasthan, Punjab and Haryana, including Delhi.

The Indian government suspected that this was happening, but there had been no regional assessment of the rate of groundwater depletion. The GRACE team used terrestrial water storage-change observations and simulated soil-water variations and what they found was startling. During the period from August 2002 to October 2008, the groundwater depletion was equivalent to a net loss of 109 km3 of water, which is double the capacity of India’s largest surface-water reservoir. The detected depletion was especially significant given that the annual rainfall was close to normal throughout the period which removes abnormal weather as a possible culprit.

After using the GRACE data to look at the complete picture, the team ruled out significant changes in soil moisture, water volume in fresh water bodies, glaciers and biomass that could have otherwise accounted for the water use. Based on these facts, the team concluded that human consumption was the only factor that could account for the drop in groundwater levels. Although the observation period was relatively brief, the available data indicates that an unsustainable rate of consumption is present and in the absence of measures taken to address the issue, problems could manifest including a reduction of agricultural output.

GRACE has also been used to show similar patterns of consumption in the western United States, African nations and elsewhere. In contrast to the reaction in India, where the data was viewed as a confirmation and reinforcement of what was already suspected, findings in other regions have been viewed with some suspicion. California water managers, for example, were skeptical of data that showed from 2003 to 2010, aquifers under the state’s Central Valley were drawn down by 25 million acre-feet. Additionally, Greg Zlotnick, a board member of the Association of California Water Agencies, said that the managers feared that the data could be used to demand a reallocation of the region’s freshwater resources.

As with many new technologies, there will be a learning curve with this emerging science and healthy skepticism many lead to improvements in the science or greater understanding of its limitations. Such skepticism, however, should not be used as a singular reason to dismiss findings. Rather, efforts should be made to directly confirm or challenge the GRACE findings. We should acknowledge that the entire water community is searching for the same set of answers and new methods such as the GRACE project may offer valuable tools in the quest to obtain those answers.