Scientists have long modeled how shifts in climate will affect precipitation and other water cycles aboveground, which will lead to changes in groundwater conditions. Now, researchers are beginning to find that the feedback goes both ways. New research pinpoints the zone most impacted by such feedbacks in modeling work published in Nature Geoscience (DOI 10.1038/ngeo315) this month.
Water is a major conveyor of heat on land and in the air, and water movements—in the form of soil moisture, river flow, plant transpiration, or evaporation, among others—all affect groundwater. Previous approaches to modeling climate shifts and water did not include this two-way conversation.
To rectify this situation, Reed Maxwell of the Colorado School of Mines and Stefan Kollet of the University of Bonn (Germany) modeled precipitation and evapotranspiration as indicators of changes in latent heat flux, or energy at the earth’s surface that can warm the lower atmosphere. The researchers then compared possible climate scenarios in a watershed in Oklahoma by evaluating different characteristics, including water-table depths, atmospheric temperatures, and precipitation.
They found a “critical zone” at depths of 2–5 meters in which groundwater feedbacks influenced energy fluxes at the surface. At deeper groundwater levels the communication between surface and subsurface is disconnected, and land–air surface interactions are dominated by rainfall and plant cover. In water tables at shallower depths, plants’ roots get plenty of water for transpiration, so energy fluxes don’t change as much.
“This is the beginning of an integrated approach, including groundwater in [modeling] the hydrologic cycle” with regard to climate, says Maxwell. Pinning down changes in regional precipitation remains difficult because of uncertainty in climate change models. And mapping groundwater reservoirs to watersheds above with different boundaries may confound the modeling of these interactions. However, Maxwell says, the new modeling results reinforce energy feedbacks in these systems, particularly where temperatures are on the rise under drought conditions.
On the basis of the new work, Patrick Reed of Pennsylvania State University says, “You could safely say [that] if subsurface water is heavily impacted”—whether by agricultural pumping, sea level rise, or massive parking lots that prevent groundwater recharge—“you literally could be affecting the weather.” —NAOMI LUBICK

                                    Reed Maxwell/ Google Earth
A stream basin in Oklahoma is a starting point for mapping groundwater and climate interactions.
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