Using measurements from Earth-observing satellites, NASA scientists have tracked changes in water supplies worldwide and they’ve found that in many places humans are dramatically altering the global water map.
The team of researchers analyzed 14 years of data from NASA’s twin GRACE satellites and studied regions that have seen large increases or decreases in the total amount of freshwater, including water in lakes and rivers and water stored in underground aquifers, soil, snow and ice.
The scientists examined precipitation trends and other data to determine the most likely causes of these huge losses and gains of water around the world. Their findings in a new study reveal that of the 34 “hotspots” of water change in places from California to China, the trends in about two-thirds of those areas may be linked to climate change or human activities, such as excessive groundwater pumping in farming regions.
“The human fingerprint is all over changing freshwater availability. We see it in large-scale overuse of groundwater. We see it as a driver of climate change,” said Jay Famiglietti, a co-author of the research and senior water scientist at NASA’s Jet Propulsion Laboratory. “The study shows that humans have really drastically altered the global water landscape in a very profound way.”
In eight of the 34 regions, the researchers said the trends reflect “possible” or “probable” impacts of climate change, including losses of ice sheets in Greenland and Antarctica, precipitation increases in the high latitudes of Eurasia and North America, the retreat of Alaska’s glaciers and melting ice fields in Patagonia.
They ascribed changes in 12 regions to natural variability, including a progression from a dry period to a wet period in the northern Great Plains, a drought in eastern Brazil and wetter periods in the Amazon and tropical West Africa.
In 14 of the areas — more than 40 percent of the hotspots — the scientists associated the water shifts partially or largely with human activity. That included groundwater depletion combined with drought in Southern California and the southern High Plains from Kansas to the Texas Panhandle, as well as in the northern Middle East, northern Africa, southern Russia, Ukraine and Kazakhstan.
Many of the areas where the researchers saw direct human impacts are farming regions that have relied heavily on groundwater pumping, including northern India, the North China Plain and parts of Saudi Arabia. The scientists also identified other human-driven impacts, including water diversions that have led to declines in the Caspian Sea and the construction of Three Gorges Dam and other reservoirs in China.
“This is the first time that the global trend map from GRACE has been thoroughly analyzed in this way,” said Matthew Rodell, the lead author and chief of the Hydrological Sciences Laboratory at NASA’s Goddard Space Flight Center in Greenbelt, Md. “We get this global picture of how water storage is changing and what the various causes are.”
Previous studies have used data from the satellites to examine regional water changes, or have used hydrological models to estimate global trends.
The researchers said their study, which was published Wednesday in the journal Nature, is the first to combine direct satellites measurements and other datasets to assess shifts in freshwater everywhere on the planet and analyze the causes. The research enabled them to produce what they describe as the first satellite-based map of changes in the availability of freshwater worldwide.
Yoshihide Wada, a water scientist at the International Institute for Applied Systems Analysis who wasn’t involved in the study, praised the research for the detailed view it provides of the likely drivers of water trends. He said attributing changes to specific causes is generally a difficult task and will require further research, but he said the findings are important.
“Water scarcity is getting much severer and we need to consider better water management practices in many intensively irrigated regions,” Wada said. “In these regions, human impacts are expected to put much bigger pressure on freshwater resources than climate change.”
The scientists estimated the water losses and gains in gigatons per year. Each gigaton of water is one billion tons, enough to fill 400,000 Olympic swimming pools.
For a sense of scale, the largest reservoir in the United States, Lake Mead, holds about 32 gigatons when it’s full. And during the 14 years of satellite measurements, nearly all the regions lost or gained at least that much. Eleven of the regions lost or gained 10 times that or more.
“The numbers are huge. It’s pretty staggering,” Rodell said. “A large portion of them, either direct or indirect human impacts were factors, if not outright the major cause.”
In Greenland, where ice is rapidly melting as the planet warms, the researchers estimated water losses at a rate of 279 gigatons per year — an amount equivalent to eight Lake Meads at full capacity, flowing into the oceans and contributing to sea-level rise. In Antarctica, they estimated losses of 128 gigatons of ice annually.
The results show that Southern California, including the farmlands of the San Joaquin Valley, lost more than 4 gigatons of water per year between 2002 and 2016 — a period in which growers relied heavily on groundwater pumping during the most severe drought in the state’s modern history.
After a wet winter that refilled reservoirs, California Gov. Jerry Brown declared the drought over in 2017. But the scientists said it’s “doubtful that aquifer storage will recover completely without large usage reductions,” in part because when aquifers are depleted the water-storing spaces between rocks, clay and sand can compact, permanently reducing how much water they hold.
Groundwater levels have been declining for decades beneath the farmlands of the San Joaquin Valley, and in 2014 California passed legislation that calls for combating overdraft and adopting local plans for sustainable groundwater management.
Pressures from agriculture have also depleted groundwater in Saudi Arabia, which lost 6.1 gigatons of water per year during a period in which irrigated farmland expanded in the desert.
The two satellites were launched in 2002 to begin the GRACE mission, which stands for Gravity Recovery and Climate Experiment and has involved both NASA and Germany’s space agency. The satellites monitored changes in Earth’s gravity field, acting as a “scale in the sky” and measuring shifts in the total amounts of water, both above and below ground.
The satellites generated data until last year, when the mission ended. Then both spacecraft reentered the atmosphere and burned up over the oceans, one in December and the other in March.
The next generation of twin satellites, called GRACE Follow-On, are scheduled to launch into orbit from California on Tuesday.
Some of the trends that appear in the satellite data have long been studied and attributed to specific causes, such as the heavy groundwater pumping from the Ogallala Aquifer in Kansas and neighboring states for farms that produce corn and other crops.
To assess the likely causes of the trends worldwide, the research team needed to use other data sources beyond the satellite measurements.
“We examined information on precipitation, agriculture and groundwater pumping, and from these trends we can find a possible explanation for the trends estimated from GRACE,” said Hiroko Beaudoing, a co-author with the Goddard Space Flight Center and the University of Maryland.
In some areas, the researchers found a complex mix of causes driving water depletion. In northwestern China, for example, glaciers are melting, population is growing and more water is being used for agriculture, while an infrastructure project is transferring water from one river to another river, Rodell said.
The declining water trends showed up clearly in many of the world’s major food-producing regions, from California and the American Southwest to India, the North China Plain, parts of the Middle East and southern Russia.
“It’s scary. Those are critical food-producing regions that are depending on a resource that’s dwindling. And either they’re going to have to be more efficient in their water usage, or eventually that food will have to be grown elsewhere,” Rodell said. “One of the important things about this map is it provides information for policymakers and decisionmakers to think about longer-term strategies for how we’re going to make sure that the world continues to have enough water to grow food for a growing population.”
In the study, the scientists wrote that groundwater is often unseen and difficult to monitor and measure, but that about half of the world’s people get their water from aquifers, many of which are receding as water is drawn out to irrigate farmland.
Famiglietti said even after years of studying the satellite data, he’s struck by how much human civilization is affecting the world’s water supplies.
“When I sit back and look at it, I still am surprised by the human fingerprint — how strong it is, how we have really drastically altered the freshwater landscape,” said Famiglietti, who was recently named director of the University of Saskatchewan’s Global Institute for Water Security.
“That map, it quantifies the pace of change. We can see and quantify the pace of change. It’s happening rapidly,” Famiglietti said. “So, there’s no excuses when it comes to water resources planning.”
Regions that he sees as especially worrisome include not only food-producing regions in the United States and elsewhere but also the Middle East, where there’s a history of violent conflict “in which water is frequently involved, either directly or indirectly.”
“Human security in South Asia is really at risk, in my opinion, due to decreasing water availability, due to disappearing glaciers, groundwater depletion, and changing extremes,” Famiglietti said. “The disappearance of Himalayan glaciers could result in millions of climate refugees in the coming decades.”
Those water risks will become critical, Famiglietti said, and demand urgent attention internationally.
The scientists said they found a trend of wet areas getting wetter, especially in the high latitudes and the tropics, and many dry mid-latitude areas getting drier.
“While the pattern of wet-getting-wetter, dry-getting-drier is predicted by the Intergovermental Panel on Climate Change models for the end of the 21st century, we can’t yet attribute the emergence of a similar pattern in the GRACE data to climate change,” Famiglietti said. “But it’s consistent with what the climate models project.”