A new U.S. Geological Survey finds groundwater levels in the Upper Colorado River Basin may hold steady over the rest of this century despite a warming climate.
The revelation comes just months after another study by the same agency found that groundwater accounts for 56 percent of streamflow in the Upper Colorado basin.
The studies offer some hope of groundwater helping mitigate other water-related impacts of a changing climate in coming decades. Perhaps most importantly, groundwater could help maintain later-season streamflows at a time when snowpack runoff is expected to occur earlier in the year, resulting in additional strains on water supplies and reservoir storage during the summer months.
The climate-change study is published in the journal Geophysical Research Letters. It finds that increased precipitation predicted by climate-change models should be enough to offset the impacts of warmer temperatures on groundwater levels in the basin.
Fred Tillman, a USGS scientist and the report’s lead author, said researchers considered how groundwater outputs and inputs would be affected by climate change. Earlier snowmelt as temperatures warm will mean native vegetation greens up and begins using water through a process called transpiration earlier each year. This and more evaporation of water from soils and water bodies will increase water loss from the basin.
“We knew that we had more going out from the higher temperatures. What we found was, well, we actually had more precipitation coming in too, according to these (climate-change) models,” Tillman said.
Researchers view the base flow of streams as a proxy for the groundwater discharge into them, apart from surface flows from snowmelt and rain.
Eric Kuhn, general manager of the Colorado River District in Glenwood Springs, said one thing water managers have been concerned about is that future late-summer base streamflows will be extremely low as the climate changes. He said he hasn’t yet read the new study, but it may be that groundwater levels will hold up better than expected due to higher precipitation.
“Perhaps the late-season, low-flow period will not be as bad as we once thought it would be,” he said.
“With a little more groundwater it offers a little more help” late in the season, Kuhn said.
However, Kuhn also said climate models generally suggest that precipitation will increase in the basin in more northern and eastern regions, while dropping off to the south and west.
Tillman noted the same thing, and said a follow-up study by the Geological Survey will break down its groundwater projections by sub-regions, and will likely show that some sub-regions to the south will show losses in groundwater, while some in the north will gain.
The study also doesn’t try to factor in how changes in future land use and human activities could affect groundwater levels, something Tillman said would require accounting for a whole other set of projections. Kuhn said increased transpiration will occur not just for native vegetation but things such as lawns, parks and crops.
The river district has been involved with another study on how vulnerable Lake Powell may be to drought in coming decades. Upper Colorado River Basin states rely on water stored in the reservoir to meet water delivery obligations to downstream states under an interstate compact, so they can avoid a so-called compact call that would affect water users in Colorado. Powell also is a sizable source of hydroelectric power in the region.
Kuhn said that while average precipitation may go up in the region, climate models also suggest a warming atmosphere would lead to more severe dry and wet periods. The preliminary findings of the study the river district is leading are that another drought like the one that occurred in the early 2000s could empty Powell if it’s half-full, as is now the case.
As water officials try to figure out how to address future supplies, Geological Survey researchers believe the findings of their studies point to the importance of thinking about surface water and groundwater in the Upper Colorado basin as a single resource, and managing water accordingly. Tillman said hydrologists know that generally speaking, groundwater and surface water are all the same water, originating from precipitation, but it’s good to get that message out.
He said researchers on the two Geological Survey studies weren’t working together, but their results dovetail in terms of helping people understand the importance of groundwater in the basin and then looking at how that groundwater might change in the future.
The earlier study evaluated water chemistry and streamflow data at 146 sites to help separate out what role groundwater plays in streamflows in the basin. It found that a greater percentage of streamflow is from groundwater lower in the basin, with a greater percentage of snowmelt and precipitation directly contributing to streamflows in the high-elevation headwaters.
It also estimates that 82 percent of groundwater that discharges to streams in the basin is lost to factors such as evaporation, plant transpiration and water diversions, and it points to the threat that groundwater pumping poses to groundwater levels.
Kuhn said that in Colorado, almost all groundwater in the Colorado River basin is considered tributary groundwater that originates as snow and ice and rain and makes its way out of the ground and into streams within months or years. Such groundwater is different from deep groundwater supplies in places like eastern Colorado, and Kuhn said that from a management perspective, tributary groundwater already is subject to surface-water-type appropriation laws in the state.
“That groundwater, basically it is Colorado River water,” he said.