Groundwater mining — pumping aquifers faster than they can be replenished — can have nasty consequences. Mining the Ogallala Aquifer (also called the High Plains aquifer), for example, has infamously run the White River dry where it once gushed over Texas’ Silver Falls (above in 1891 and today, right).
Most of the groundwater sprayed on thirsty croplands across America makes its way into streams and rivers. Even though much of the water seeps into the soil first, the vast majority never makes its way back into the aquifer. Instead, it heads toward the sea, where it eventually contributes a surprising share of global sea level rise, reports Leonard Konikow, a hydrogeologist with the U.S. Geological Survey.
In a rigorous new analysis of global groundwater depletion published earlier this month, Konikow estimated that global aquifers lost 4,500 cubic kilometers between 1900 and 2008 — enough to raise global sea level by about 12.6 millimeters. That’s a little more than 6 percent of the total sea level rise that took place over that time.
The actual impact on global sea level would have been much greater had it not been for dams, which stall groundwater’s trip to the sea. In a second paper out last week, Konikow and a group of colleagues showed that between 1972 and 2008, dams retained more water in their reservoirs than people pumped out of aquifers. Specifically, groundwater depletion contributed an average of 0.3 millimeters per year to sea level rise, whereas surface water retention decreased sea level rise by 0.4 millimeters per year.
Dams and pumps may be locked in a zero sum game for now, but the balance of power is shifting. Konikow points out a startling acceleration in groundwater depletion since 1950: one quarter of the depletion from 1900 to 2008 occurred in the final eight years. Indeed, the volume of groundwater lost between 2000 and 2008 was equal to 13 percent of global sea level rise. That means groundwater loss is accelerating right at a time when dams may be unable to keep pace; the reservoirs of old dams continue to fill with sediment, and new dam construction is expected to slow.
We need not fear groundwater will flood Florida anytime soon. Only the mega-melting of the planet’s ice sheets and glaciers are up to that task.
Still, clarifying how much groundwater depletion does (or doesn’t) contribute to sea level rise is critically important for climate scientists. They must understand all pluses and minuses in the global sea level equation, because it is against those accounts that they check the global energy budgets that are so critical to predicting future climate change.
For the rest of us, the most pressing concern about the future groundwater mining is pragmatic: will we have enough fresh water? Konikow’s analysis reveals that the acceleration of groundwater loss in the U.S. is due increasing extraction from the High Plains (Ogallala) and other Gulf Coast aquifers, despite recent initiatives to change farming practices and conserve water.
The Ogallala Aquifer took 30,000 years to fill the first time, so it won’t replenish itself any time soon. We’ll have to make due.
Silver Falls along the White River in Texas in 1891 and today, when agricultural pumping and the resulting depletion of the Ogallala Aquifer have dried the once year-round spring. Courtesy Wikimedia Commons
An aquifer is a layer of saturated, porous rock trapped between dry or less permeable ones. Courtesy Wikimedia Commons
Cumulative net groundwater depletion in major U.S. aquifers, from 1900 to 2008. Leonard F. Konikow