SALT LAKE CITY — Plants suck up more water and more of that water is lost to evaporation in a warming climate, with a new study suggesting those dynamics may impact how reservoir storage is built and managed one day in the West.
The study published in Environmental Research Letters Tuesday featured a team of hydrologists from two states, including University of Utah's Paul Brooks from the Department of Geology and Geophysics.
Researchers looked at two Colorado stream watersheds on either side of the Continental Divide and how runoff and streamflow are impacted by precipitation that falls as rain and also how the "energy budget" of the system drives the extent of groundwater recharge and runoff.
"Our results suggest that land-energy changes have a larger effect on total water available for use in the mountain hydrologic system, both in surface and groundwater, than a phase change from snow to rain," the study concluded. "This reduction in usable water is mostly driven by an increase in summer evapotranspiration due to warming."
What those conclusions mean is that warmer temperatures have more long-term debilitating effects on water supply by prompting an earlier runoff and leaving less water in the system overall by not giving groundwater time to recharge.
"Changes in energy, which result in changes in evapotranspiration, outweighed the changes in the form of precipitation," said Reed Maxwell of the Colorado School of Mines.
While the stream system becomes "flashier" when precipitation falls as rain instead of snow, there was less impact to streamflow than by upping the temperature by 4 degrees Celsius, researchers found.
In a rain versus snow scenario, researchers found that streamflow decreased 11 percent in the watershed east of the Continental Divide and by 18 percent on the western side. Under warmer temperatures, the reductions were 19 percent in the east and 23 percent on the western side.
"We know two things are possible: that we could have warmer storms with more rain, or just on average, we become warmer," Brooks said. "They both have problems — both changes give us less water."
The study, which applied 11 simulations of various temperature alterations to determine watershed response, could be a guidepost of sorts as water managers strive to be the most effective caretakers possible in the 21st century.
"The switch from snow to rain is tough because snowpack is such a good storage mechanism for us," Brooks said. "If we did not have the snowpack, we would have to build much larger reservoirs and change how we store water. Neither one is a good scenario."
The big U.S. Bureau of Reclamation dam building projects across the West is what drove the settlement of population centers 100 years ago, Brooks added, and now managing and adapting that infrastructure in this century will be key.
"All our big dams, small dams, pipelines and canals are built on assumptions that we will have a predictable release of snowmelt," he added.
The research, funded with a grant from the National Science Foundation, involved three universities in Colorado and Utah.
"It was a really great team and a fun project, an important one," Brooks said. "We both share the Colorado River, we share these mountains and water is gold."
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