by Bob Heath

Most people in the world, including everyone in the city of Kent, depend on groundwater as their source for drinking water–and for good reasons: ground water is 100 times more abundant than surfacewater, its abundance is less variable than surface water, and ground water in general does not have many of the problems that constantly plague surface water. For example, surface water is susceptible to pollution from airborne contaminants (such as mercury from coal-fired power plants) and contaminants in stormwater runoff (such as nitrogen and phosphorus from agricultural activities and the associated toxins from hazardous algal blooms). Surface water also is more susceptible to the vicissitudes of weather patterns such as the three-year drought in Georgia that nearly drained Atlanta’s major drinking water reservoir.

Groundwater aquifers have one major threat: drying up–either from water being withdrawn faster than the aquifer can be replenished by natural process (typically because of population increases that lead to increased demand for water) or from an aquifer replenishment rate that is too slow (typically because too much pervious surface has been lost to roadways and urban landscapes). Another potential major threat to groundwater sources is changes in weather patterns as a result of global climate change. The danger here is a slower groundwater recharge rate. Because most people in the world depend on groundwater as their sole source of water for drinking and for commercial activities, if aquifers shrink to the point of being unable to support the people who depends on them, major populations shifts may occur. If you think that immigration is a problem as people seek a better life, think about what will happen as people immigrate to stay alive. This potential exodus is why the U.S. military views loss of water resilience as a major threat to international security.

Never has it been more important to monitor the size of global groundwater aquifers as global populations increase and aquifers potentially decrease. The problem is that groundwater is out of sight and not as easy to monitor as surface water is. But help is on the horizon and passing overhead every 90 minutes. GRACE (Gravity Recovery and Climate Experiment) is not just another satellite; rather, it’s two satellites in exactly the same orbit but spaced about 137 miles apart at an altitude of 300 miles. A balance between the momentum of the satellites and gravitational pull exerted on them by the Earth keeps the two satellites from colliding with each other. For example, as one satellite passes over a region of higher gravity, such as a mountain range, it is pulled into a lower orbit, increasing the distance between the two satellites, which is measured precisely by microwaves that are constantly beamed from each satellite to the other. Satellite GPS instruments record the exact coordinates of the satellites. In this way a “gravity map” of the entire Earth is determined.

GRACE satellite observations from 2002 to 2017 have shown that while the mass of mountains does not change over time, the mass of water does change. The simple explanation is that water moves, but mountains do not. In other words, water moves and therefore its mass at any given point is a sum of the parts that can move (water) and the parts that do not move (the mountains). As a region on Earth become wetter, the mass of that region increases; conversely, as that region becomes drier, the mass of the region decreases. After comparing the gravity maps from several passes of the GRACE satellites over several years, NASA scientists were able to determine which regions are becoming wetter and which regions are getting drier. The original GRACE satellites stopped working in 2017; however, a set of replacements–the GRACE-Follow On satellites, or GRACE-FO, for
short–was launched in May 2018.

Analysis of the gravity maps during the 15-year life of the original GRACE satellites led NASA scientists to detect trends in water-distribution changes. The scientists summarized their findings by saying, “The wet places are getting wetter and the dry places are getting drier.” The wet places are the high latitudes (i.e., polar regions) and the tropics (i.e., regions near the equator). The temperate regions (the middle latitudes, which span between the tropics and the polar regions) are getting drier. The scientists note in their report that “within the dry areas we see multiple spots resulting from groundwater depletion.”

But why? These changes were examined for causes ranging from changes in rainfall patterns to cyclical changes in weather patterns to changes in human activities and demography. The scientists found that

one of the consistent causes of groundwater depletion is agricultural activity and that the effect is visible in such diverse places as Saudi Arabia, western Australia and the Central Valley of California. In California, for example, farmers in the Central Valley, an area often referred to as the “fruit basket of the United States,” were forced to rely more and more on groundwater during a three-year snowfall drought in the northern part of the state diminished the supply of surface water typically used to irrigate crops. Whether these trends are the result of climate change or the result of cyclical weather patterns remains unknown. The data-collection period was not long enough to answer that question. Thanks to GRACE-FO, that question will be addressed. Stay tuned.

Want more information? Go to https://gracefo.jpl.nasa.gov/resources/38/grace-fo-fact-sheet/ for a fact sheet and go to https://gracefo.jpl.nasa.gov/resources/73/for-15-years-grace-tracked-freshwater-movements-around-the-world/ for a NASA video.