A warmer world could for billions be drier. The resultant deeper wells spell trouble for those reliant on groundwater.
By Tim Radford
As many as one fifth of the world’s wells could be about to run dry, as levels of the subterranean water table continue to fall. And if they do, the resultant deeper wells spell trouble for billions of people who will face diminishing supplies of clean water, and water for their crops.
Most of the world’s freshwater is truly out of sight: 96% of all available water is held in aquifers, rock and sediment layers just below, and sometimes well below, the Earth’s surface. It sustains almost half of global agriculture. The world’s drylands are also home to more than a third of all humanity.
All this is at risk because in many places water tables are falling. According to a new study in the journal Science, if groundwater levels decline a few metres more, then the wells will run dry. Somewhere between 6% and 20% of the world’s wells are no more than five metres deeper than the water table.
And water levels almost certainly will decline. Researchers have for years been warning about global demand for groundwater. In urban areas the demand has been so great that many cities are literally going downhill: throughout the 20th century Tokyo sank by four metres, Shanghai in China and New Orleans in the US by two to three metres.
“Wells are already running dry because of groundwater level declines”
Climate change − which promises to distort global rainfall patterns still further − is steadily scorching the world’s already parched regions and as a consequence groundwater is being extracted at an accelerated rate.
And that means more water stress for millions. All the evidence is that, as greenhouse gas emissions rise as a consequence of profligate fossil fuel use, things could get a lot worse.
Californian scientists report that they compiled 39 million records of groundwater well locations, along with their depths, the reasons they were sunk, and the dates they were dug, in 40 countries that collectively make up 40% of all the lands free of ice. This landscape accounts for probably half of all groundwater extraction.
To test their simulations of overall groundwater availability, they compiled and analysed 100 million measurements made in a million wells monitored individually, and they found that in half of these there were seasonal fluctuations of around a metre or more.
Newer means deeper
They checked the big picture of water table decline against 15 years of data from the US space agency Nasa’s Gravity Recovery and Climate Experiment satellites.
They also looked at the age data of their sample, to find that in many areas, the newer the well, the more likely it was to be deeper than an old well. That alone was evidence of gradually falling water tables.
“From India to the United States, wells are already running dry because of groundwater level declines,” the authors write. “In California’s Central Valley and several other agricultural hubs around the globe, typical agricultural wells are deeper than domestic wells; as a result, domestic wells are running dry…”
Where wells are already running dry, that decline will continue, and even expand into areas that have not yet seen any depletion. And, they warn, it may not help to simply sink even deeper wells: the costs would become prohibitive and the water quality at greater depth might anyway be not good enough.