Groundwater hides in many ways. Groundwater is hidden from view on the surface while it also hides from general knowledge that it’s Earth’s largest source of freshwater beside what is locked in glaciers and ice sheets. Another hidden aspect is that groundwater overpumping not only depletes aquifers, but also the overlaying rivers and surface waters, a recent study published in Nature ascertains.
Over time, societies have come to rely more heavily on aquifers as water sources, yet the rate of pumping has become unsustainable as groundwater cannot be recharged as fast it’s extracted. As National Geographic reports, the result is a “slow desiccation” of river ecosystems that number in the thousands globally. As the authors note, between 15 and 21 percent of watersheds that have significant groundwater extraction currently fall below a critical ecological threshold, with the number estimated to increase to between 40 and 79 percent by 2050.
According to the study’s lead author, hydrologist Inge de Graaf,
“We can really consider this ecological effect like a ticking time bomb. If we pump the groundwater now, we don’t see the impacts until like 10 years further or even longer. So what we do right now will impact our environment for many years to come.”
The research team found that major ecological problems occurred when groundwater extraction rates caused water levels to drop under 90 percent of average flow during the dry season, a period when groundwater contributes more to stream or river flow. If that 90 percent and below threshold is passed for more than three months per year in at least two consecutive years, freshwater ecosystems and the species that depend on them are threatened, according Brian Richter, a scientist at Sustainable Waters.
“There can be just a very small depletion of water at those sensitive times, but ecologically it’s meaningful,” says Richter.