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Jul 29, 2015

Groundwater monitoring via GRACE satellites

NASA satellites monitoring changes in groundwater levels show imperiled aquifers around the globe. Some hope that a recent study, the most comprehensive assessment of trends in groundwater storage to date, will serve as a wake-up call for better management of groundwater resources. However, more data alone cannot solve the problem. Governments need to acknowledge the interconnectedness of surface and groundwater in order to accurately account for and apportion this valuable resource.
Credit: NASA

Case Study: Groundwater monitoring via Gravity Recovery and Climate Experiment (GRACE)

Scope: Global

Launched: March 17, 2002, groundwater study in Water Resources Research June 2015

Intended Audience: Water resource managers, government policymakers, corporate risk managers, academic researchers, general public             

Potential Application: Sheds light on trends in groundwater storage, an opaque and poorly understood topic. Specifically, the satellites monitor the change in volume of underground aquifers, and can highlight aquifers under the greatest stress from human actions as well as other stressors. This can lead to more informed and responsible management of groundwater resources.  

Developers: UC Irvine, NASA Jet Propulsion Laboratory, National Taiwan University, UN Santa Barbara, National Center for Atmospheric Research, NASA Goddard Space Flight Center

Link to study: http://onlinelibrary.wiley.com/doi/10.1002/2015WR017349/full

Groundwater has always been thought of and managed differently than surface waters. While we can see rivers and lakes rise after storms and recede during droughts, the behavior of groundwater is mysterious. We still struggle to define the extent of groundwater aquifers, the rates at which they recharge, and understand the impacts of pumping. A variety of management regimes are in place to manage surface water withdrawals and discharges in order to ensure a continued, usable, and safe supply. Yet, due to our limited understanding, groundwater management is cursory at best.

From an insurance policy to a critical supply

Amidst an historic drought, California is supplementing its dwindling surface water supply through withdrawals from aquifers underlying its Central Valley. Approximately 60 percent of the water Californians use now comes from underground sources. After the completion of the Central Valley and State Water projects in the mid-20th century, which transfer massive amounts of surface water to more arid sections of the state, these aquifers were used sparingly. Instead of first options, groundwater was held as a contingency plan for dryer years.

Now, with many of the state’s largest reservoirs below historical capacity, and snowpack levels in the Sierra Nevada at record lows, surface water allotments are being cut back and groundwater supplies are becoming increasingly important. Around the globe, over two billion people now rely on groundwater for their primary water supply. But without a greater understanding of groundwater formations and a more accurate accounting of stocks and flows, we risk tapping these resources in an unsustainable manner, putting livelihoods and ecosystems at risk.

How can satellites shed light on groundwater?

NASA’s Gravity Recovery and Climate Experiment (GRACE) began in 2002 as a means of observing minute changes in Earth’s gravitational field. The Earth’s pull of gravity is fairly uniform, but there are small variations caused by large masses of water (glaciers, aquifers, oceans, etc.) or relatively dense rock formations. The two GRACE satellites orbit the earth in tandem, observing these variations in both space and time by tracking slight changes in their distance to one another. Since rock formations are relatively stable through short timescales, much of the temporal change in gravitational pulls are likely due to changes in mass of a water body. This data can be cross-referenced with the locations of known aquifers to estimate changes in groundwater storage.  

Recently, GRACE data was used to quantify storage changes in some of the world’s largest aquifers. Like in California, increasing dependence on groundwater has led to rapid rates of pumping, and groundwater levels are plummeting. Of the 37 aquifers studied, 21 have decreased in storage over the last decade. Distributed around the globe, these aquifers face pressure from expanding agriculture operations, urbanization, and mining operations that require large water inputs. California’s aquifers, despite experiencing some recharge from the surface, have lost about 7 trillion gallons of water since 2011.

Both red and orange aquifers are in decline. Although orange aquifers do experience some replenishment, it is not enough to keep pace with withdrawals. Source: Richey et al. 2015

Limits of GRACE

These massive losses in storage seem catastrophic, but without estimates of the total water stored within a given aquifer, it’s still an incomplete picture. Unfortunately, GRACE can only measure changes in storage, and existing estimates of total storage in major aquifers are flawed. According to an accompanying study, these 40 year old estimates overstate groundwater stores by between 10 and 1,000 times, presenting a rosier picture than the actual conditions underground.

In addition, the coarse spatial resolution of GRACE data means that only large reservoirs can be monitored, and variation within complex systems are missed. For example, GRACE found that the Ogallala aquifer, stretching from Texas to South Dakota, actually had a net increase in storage, which is due entirely to gains in its northern half. Meanwhile, groundwater levels in the southern half have dropped dramatically, and many wells are projected to go dry within a few decades.       

Better data is needed, but current water accounting is flawed

Compounding data issues is the occasional willful disregard for the intertwining nature of surface and groundwater resources. While some so called “fossil aquifers” recharge at rates so slow that they are effectively isolated from the broader hydrological system, most aquifers are intimately connected with surface waters. For example, discharge from nearby aquifers sustain river flows between rain events. Conversely, large withdrawals made from aquifers can result in rivers drying up, as the system tries to regain its steady state. While this relationship is now widely understood, regulations in place often treat groundwater and surface water supplies as separate entities, which leads to double counting.

Last year, California passed a law to regulate groundwater withdrawals for the first time. However, the law prohibits regulators from addressing the double counting issue until 2025, giving water districts more time to adjust to the fundamental shift in water accounting. This provision was the result of fierce lobbying from agriculture and industry groups opposed to more immediate change.

GRACE provides the most accurate and timely information to date on groundwater storage trends. Combined with a greater focus on determining total storage levels, we can begin to determine whether our groundwater withdrawals are sustainable. The millions of individuals operating private wells for homes, businesses, and agriculture could even be tapped for a public groundwater data gathering program, providing tons of useful data. Yet, without an acknowledgement of the interconnectedness of our water resources and honest accounting, this data will be of limited use. The good news is that unlike with drought, this is strictly a manmade problem. We choose how to regulate, manage, and count our water resources, and we can do so in a more intelligent manner.