A glimpse into the sustainability of global groundwater extraction for rivers, streams and lakes in the next few decades has revealed a worrying picture.
The hydrological model, published in the journal Nature, shows that in nearly 20% of regions that pump groundwater, rivers are already flowing too low to maintain healthy aquatic ecosystems.
By 2050, more than half those localities will have surpassed their environmental flow limits, or streamflow critical point.
The most striking finding, says lead author Inge de Graaf from the University of Freiburg in Germany, is that just one small drop in groundwater level will create these critically low river flows.
“This shows that riverine water freshwater ecosystems are extremely sensitive to water decline,” she adds.
And in contrast to surface-water use, which has immediate impacts on streamflow, it can take decades for groundwater extraction to show a noticeable reduction of groundwater influx.
“Groundwater pumping can thus be considered a ticking time bomb whose ecological effects become visible only years later,” says de Graaf.
As the world’s largest source of fresh water, groundwater is vital for irrigation and global food security, particularly during dry spells. But it’s under increasing threat from human activities and climate change.
Already, groundwater extraction exceeds recharge rates from rain and rivers, particularly in areas that are intensively irrigated, depleting this precious resource with “potentially devastating effects on aquatic ecosystems,” the authors write.
Flow-on effects include land subsidence, unstable infrastructure and heightened flood risks in coastal areas, and depleting surface water resources of groundwater discharge that is vital for sustaining lakes, rivers, streams and wetlands, and related ecosystems.
Depletion ‘hotspots’, regions that already reached their limit before 2010 due to low discharge and high dependence on groundwater, include the High Plains aquifer in Kansas, US, part of California’s Central Valley aquifer, areas of Mexico, and the Upper Ganges and Indus basins in India.
But environmental flow limits have been reached outside these areas as well, including north-east USA and parts of Argentina.
The researchers calculate that before 2050, more regions will reach their threshold due to drier climates and increased reliance on irrigation, such as southern and central Europe and parts of Canada, Asia, Australia and Africa.
They add that their estimates are likely optimistic, as they don’t factor in increased demand for groundwater resulting from population growth or newly emerging economies.
And while physical and economic limits will develop afterward, the authors caution that healthy stream flows are vital for ecosystems. As such, they write that environmental groundwater needs should be factored into global water resource assessments.
The study is the first model to simulate the impact of groundwater abstractions on rivers worldwide.
“We point out that only small changes in groundwater levels are needed to convert a stream well supported by groundwater to a stream not supported by groundwater,” says de Graaf.