High in the Rocky Mountains, at the headwaters of the Colorado River, Shemin Ge is studying water that flows at times when it shouldn’t. More specifically, Ge, a geological science professor at the University of Colorado, is looking at subsurface water flows that are trickling into streams during late fall and winter — times when the ground should be frozen and water flowing beneath it nonexistent.
As surface temperatures warm due to climate change, Ge’s models show winter subsurface flows increasing, likely becoming bigger contributors to surface streamflow in the short term. In the long term though, the trend raises questions about the sustainability of those high-altitude groundwater sources if they aren’t being adequately replenished.
“So we could see short term water abundance but a long-term water shortage,” said Ge, who presented her research to a room full of hydrogeology students, professors and other scientists on Thursday.
Ge’s research also matters for states like Arizona that are highly dependent on what happens at the Colorado River’s headwaters, said Abe Springer a hydrogeologist at Northern Arizona University.
Increasing subsurface contributions to Colorado River tributaries during the winter months could impact the long term base flows of the mainstem in ways that river managers haven’t anticipated, he said.
“It may not do much at all to peak flow, but of that low flow it’s an important component of it,” Springer said.
Ge said she zeroed in on the subsurface flow phenomenon by observing stream flows during the year’s coldest months when inflow from rain or melting snow is negligible. Somehow though, Ge and her research team observed the volume of water increasing in the streams as they ran downhill, suggesting they were being fed by some other source of water.
It turns out it was subsurface groundwater seeping into the water channels.
It’s not just the headwaters regions of the Rockies where this pattern is becoming apparent. In Canada, there are more than 20 locations where streamflow is increasing in the fall and winter and a similar phenomenon is occurring among the six largest Eurasian rivers that flow into the Arctic Ocean, Ge said.
Since logging those initial observations in the mountains above Boulder, Colo., Ge has been working on modeling how winter subsurface flows in these headwaters regions will continue to change as the planet warms.
Ge’s models use the Intergovernmental Panel on Climate Change’s assumption that global temperatures will rise 3 degrees celsius by 2100. Then, considering those surface temperature trends, she made projections 40 years into the future for what will happen below the surface in high altitude, high latitude regions that serve as the headwaters for the world’s great rivers.
The models found that the layer of ground above the permafrost layer, the one that freezes and thaws seasonally, will grow threefold over the next four decades. That’s important because subsurface water more easily discharges to surface streams and rivers through that more permeable layer of ground. In addition, advective heat transport, or heat that is transported by water, will increase fivefold over those 40 years, leading to even greater redistribution of thermal energy within the system.
Those factors add up to the conclusion that by the middle of the century, groundwater contribution to stream baseflow will have tripled, Ge said.
The phenomenon is worth paying attention to because decades from now it could lead to groundwater shortages as more of that subsurface flow percolates to the surface, Ge said.
“Increasing streamflow is a trend I would argue may not be able to sustain itself hydrologically unless there is an increase in water recharging the system,” she said.
As the subsurface flows seep downslope into streams or lakes the highland areas also could get drier as lowlands get more saturated, Ge said.
Additionally, she pointed out that increased groundwater flow in subsurface regions promotes nutrient transport and mobilizes chemical constituents in permafrost, which will require further study.
“There are a lot of things happening below the subsurface that are not very well understood,” she said.