Degree to which aquifers important for farming are under stress. Gleeson, T. et al.
Groundwater use is unsustainable in many of the world's major agricultural zones.
Amanda Mascarelli 08 August 2012
Almost one-quarter of the world’s population lives in regions where groundwater is being used up faster than it can be replenished, concludes a comprehensive global analysis of groundwater depletion, published this week in Nature1.
Across the world, human civilizations depend largely on tapping vast reservoirs of water that have been stored for up to thousands of years in sand, clay and rock deep underground. These massive aquifers — which in some cases stretch across multiple states and country borders — provide water for drinking and crop irrigation, as well as to support ecosystems such as forests and fisheries.
Yet in most of the world’s major agricultural regions, including the Central Valley in California, the Nile delta region of Egypt, and the Upper Ganges in India and Pakistan, demand exceeds these reservoirs' capacity for renewal.
“This overuse can lead to decreased groundwater availability for both drinking water and growing food,” says Tom Gleeson, a hydrogeologist at McGill University in Montreal, Quebec, and lead author of the study. Eventually, he adds, it “can lead to dried up streams and ecological impacts”.
Gleeson and his colleagues combined a global hydrological model and a data set of groundwater use to estimate how much groundwater is being extracted by countries around the world. They also estimated each aquifer's rate of ‘recharge’ — the speed at which groundwater is being replenished. Using this approach, the researchers were able to determine the groundwater ‘footprint’ for nearly 800 aquifers worldwide (see map above).
In calculating how much stress each source of groundwater is under, Gleeson and colleagues also looked in detail at the water flows needed to sustain the health of ecosystems such as grasses, trees and streams.
“To my knowledge, this is the first water-stress index that actually accounts for preserving the health of the environment,” says Jay Famiglietti, a hydrologist at the University of California, Irvine, who was not involved in the study. “That’s a critical step.”
Overexploitation
The authors found that 20% of the world’s aquifers are being overexploited, some massively so. For example, the groundwater footprint for the Upper Ganges aquifer is more than 50 times the size of its aquifer, “so the rate of extraction is quite unsustainable there”, says Gleeson.
Yet Famiglietti notes that the study, which focuses on quantifying the rate of groundwater tapping versus recharging, underscores the lack of data we have on the amount of water currently in the world's aquifers. “The only way to answer the sustainability question is to answer how much water we actually have,” he says.
He predicts that a comprehensive picture would reveal that many more of the world’s aquifers are being tapped unsustainably. As certain regions face more frequent droughts and population growth, full characterization of aquifers worldwide, although expensive, will be necessary, adds Famiglietti.
But Gleeson adds that there is at least one significant source of hope. As much as 99% of the fresh, unfrozen water on the planet is groundwater. “It’s this huge reservoir that we have the potential to manage sustainably,” he says. “If we choose to.”
Both the US and parts of Asia, including India and Pakistan, are extremely overusing groundwater. Indian and China are most impacted by this lack of sustainability.
Wednesday, August 8th 2012, 05:26 PM
Heavily-populated regions of Asia, the arid Middle East and parts of the US corn belt are dangerously over-exploiting their underground water supplies, according to a study published on Wednesday in the journal Nature.
"The countries that are overusing groundwater most significantly are the United States, India, China, Pakistan, Iran, Saudi Arabia and Mexico, and the highest number of people that are impacted by this live in India and China," Canadian hydrologist Tom Gleeson told AFP.
"Over a quarter of the world's population live in these regions where groundwater is being overused," he said in a phone interview.
Many places are rapidly pumping out "fossil" water, or water that was laid down sometimes thousands of years ago and cannot be replaced on a human timescale.
Seeking a yardstick of sustainability, the study creates a measure called the groundwater footprint.
It calculates the area of land sustained by extracted water and compares this to the size of the aquifer beneath.
The global groundwater footprint is a whopping 3.5 times the size of the world's aquifers, the study found.
However, this stress is accounted for by a small number of countries.
For instance, in the South Caspian region of northern Iran, the footprint is 98 times the size of the aquifer; in the Upper Ganges in India and Pakistan, it is 54; while in the US High Plains, the figure is nine.
"Humans are over-exploiting groundwater in many large aquifers that are crucial to agriculture, especially in North America and Asia," said Gleeson.
"Irrigation for agriculture is largely causing the problem but it is already impacting in some regions the ability to use groundwater for irrigation, so it is almost like a self-reinforcing problem."
The study aims at adding a new analytical tool to help policymakers cope with the world's intensifying water problems.
In March, the UN warned in its Fourth World Water Report that water problems in many parts of the world were chronic, and without a crackdown on wastage would worsen as demand for food rises and climate change intensifies.
By 2050, agricultural use of water will rise by nearly 20 percent, on the basis of current farming methods, to meet food demands from a population set to rise from seven billion today more than nine billion.
Gleeson, a specialist at McGill University in Montreal, Canada, used a computer model in collaboration with scientists at Utrecht University in the Netherlands and crunched national statistics on water use. The next step will be to use satellite data, which should be a more reliable source, he said.
Water from surface sources -- rivers and lakes -- is well documented, but use of aquifers is poorly understood.
According to the UN report, extraction from aquifers has tripled in the past 50 years and now accounts for nearly half of all drinking water today. But how this use breaks down in finer detail, notably its impact on the watersheds that feed rivers, is less well known.
News 
Market Data 
Discover 
AI
