PALM SPRINGS, Calif. — Humans are dramatically altering water supply in many places worldwide, say NASA scientists who have been tracking regional changes via satellite.
The researchers analyzed 14 years of data from NASA’s twin Gravity Recovery and Climate Experiment satellites, which the space agency has dubbed GRACE. They studied areas with large increases or decreases in freshwater — including water stored in aquifers, ice, lakes, rivers, snow and soil — to determine the most likely causes of these changes.
Changes in two-thirds of the 34 hot spots from California to China may be linked to climate change or human activities, such as excessive groundwater pumping for farming, according to their new study.
“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 who is the senior water scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. “The study shows that humans have really drastically altered the global water landscape in a very profound way.”
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In 14 areas — more than 40% of the hot spots — the scientists associated the water shifts partially or largely with human activity. That included groundwater depletion and drought in Southern California, the southern Great Plains from Kansas to the Texas Panhandle, the northern Middle East, northern Africa, southern Russia, Ukraine and Kazakhstan.
In eight of the regions, the trends reflected possible or probable effects of climate change, researchers said:
• Loss of ice sheets in Greenland and Antarctica
• Precipitation increases in northern Eurasia and North America
• Retreat of Alaska’s glaciers
• Melting ice fields in Patagonia at the southern tip of South America
They ascribed changes in 12 regions to natural variability, including a progression from dry to wet in the Northern Great Plains, a drought in eastern Brazil and wetter periods in the Amazon and tropical West Africa.
Many areas where researchers saw direct human effects are farming regions that have relied heavily on groundwater pumping, including northern India, the North China Plain and parts of Saudi Arabia.
Other water diversions have led to declines in sea levels for the Caspian Sea. Construction of the world's largest hydroelectric plant, Three Gorges Dam on the Yangtze River in China, and other reservoirs in China also have affected the environment.
“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 satellite data to examine regional water changes or have used hydrological models to estimate global trends.
The researchers said their study, published Wednesday in the journal Nature, is the first to combine direct satellite measurements and other data sets 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 in Laxenburg, Austria, who wasn’t involved in the study, praised the research for the detailed view it provides of the likely drivers of water trends.
“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 declining freshwater 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.
“Those are critical food-producing regions that are depending on a resource that’s dwindling," Rodell said. "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."
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.
In Greenland, where ice is melting rapidly as the planet warms, 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 a 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.
Pressures from agriculture 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 twin GRACE satellites were launched in 2002 as a joint mission involving 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. Then both spacecraft re-entered 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, is scheduled to launch Tuesday into orbit from California.
“That map, it quantifies the pace of change. We can see and quantify the pace of change. It’s happening rapidly,” said Famiglietti, recently named director of the University of Saskatchewan’s Global Institute for Water Security. “So, there’s no excuses when it comes to water resources planning.”
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