Federal scientists probing the Colorado River, the primary water source for the West, have found that, on average, 56 percent of the flow starts underground and seeps to the surface from below.
This U.S. Geological Survey conclusion, published last week, is expected to help city water supply managers who increasingly must anticipate future volumes as rising temperatures favor a shift from snow to rain.
A river basin fed mostly from underground could be less sensitive, at least in the short term, if the snowpack disappears.
But itap too early to know the full implications because scientists don’t know how long water remains underground before seeping into the river, said Matthew Miller, a Utah-based USGS hydrologist and lead author of a study published in the science journal Water Resources Research.
“We estimate most of the groundwater is discharged into Colorado River Basin streams at higher elevations,” Miller said.
All water originates as precipitation. Altered precipitation, due to climate change, eventually affects surface water and groundwater.
The question is how water volumes in the river may change and how long water has been underground before it reaches the river, said Miller, who is embarking on further study.
“The timing of change may differ depending on how much of your water originates as groundwater or as snow,” he said.
“How old is the groundwater coming into the river system? If it is young, you might expect the flow would change more rapidly in response to a change in precipitation,” Miller said.
One thing is certain: Water mangers must recognize that groundwater and surface water is connected, Miller said.
More than 50 million people across the western United States . The population in arid western states is expected to increase by another 23 million before 2030. Westerners rely on the rivers for irrigating crops to produce food, for domestic water in homes, to generate electricity, for industrial processes in mining, and to nourish livestock and wildlife including fish.
Federal researchers looked at water data collected at 146 sites in the Upper Colorado River Basin in Colorado, Utah, New Mexico and Arizona. They analyzed the data and developed a model to show how water in the river entered the river.
Secure supplies may require knowing the age of the water and how it reaches the river, Miller said.
“If the groundwater is very old water, then you can say we have a cushion,” he said. “But we don’t know yet how old the groundwater is.”
