Growing communities and extensive agriculture throughout the Western United States rely on meltwater that spills out of snow-capped mountains every spring. The models for predicting the amount of this streamflow available each year have long assumed that a small fraction of snowmelt each year enters shallow soil, with the remainder rapidly exiting in rivers and creeks.

New research from University of Utah hydrologists, however, suggests that streamflow generation is much more complicated. Most spring runoff heading to reservoirs is actually several years old, indicating that most mountain snowfall has a years-long invisible journey as groundwater before it leaves the mountains.

The findings also indicate there is an order of magnitude more water stored underground than most Western water managers account for, said research leader Paul Brooks, a professor of geology and geophysics.

“On average, it takes over five years for a snowflake that falls in the mountains to exit as streamflow,” Brooks said. “Most of our models, whether for predicting streamflow or predicting how much water trees will have in dry years, are based on the idea that there’s very little water stored in the mountains. Now we know that that’s not the case. Most of the water goes into the ground and it sits there for somewhere between three and 15 years before it’s either used by plants or it goes into the streams.”

The team collected runoff samples at 42 sites and used tritium isotope analysis to determine the age of the water, that is how much time elapsed since it fell from the sky as snow.

Published this week in the journal Nature Communications Earth & Environment, the findings were co-authored by U geology professors Sara Warix and Kip Solomon in collaboration with research scientists around the West.

Read the full story by Brian Maffly in @TheU