DRI’s Rosemary Carroll, a research professor of hydrology, has co-authored a study revealing that spring runoff in the Western U.S. mountains is older than previously thought. Using advanced dating techniques, researchers found that mountain snowpacks initially replenish groundwater reserves before filling springs several years later. This suggests water takes over five years from snowflake to streamflow, impacting water management and understanding of water availability.
The study adds to Carroll’s extensive research on groundwater in the Western U.S., where population growth and volatile weather affect water availability. It follows another recent study by Carroll examining how groundwater depth influences mountain streamflow response during droughts.
“Groundwater matters in mountain systems and should not be ignored,” said Carroll. She emphasized the need to understand groundwater for planning during prolonged droughts, noting that deeper groundwater systems support streamflow longer but delay recovery compared to shallow basins.
Paul Brooks, a geology and geophysics professor at the University of Utah, led the research indicating most spring runoff is several years old due to its journey as groundwater before reaching streams. “On average, it takes over five years for a snowflake that falls in the mountains to exit as streamflow,” Brooks stated.
Researchers used tritium isotope analysis on samples from 42 sites to determine water age since it fell as snow. Published in Nature Communications Earth & Environment, the findings were co-authored by Utah geology professors Sara Warix and Kip Solomon with other scientists across the West.
Warix highlighted that understanding streamflow age is crucial for predicting hydrological responses to climate change and land use changes. She noted past models are becoming less reliable as inputs change.
Brooks conducted sampling at sites across Idaho, Wyoming, Utah, Colorado, California, and New Mexico during his 2022 sabbatical. The sites span major river basins with long-term research catchments funded by federal agencies.
Solomon explained that much of Earth's fresh water is underground but remains difficult to quantify. Tritium dating offers clues about precipitation timing up to a century ago.
Federal and state managers have traditionally relied on snowpack monitoring for annual water forecasts; however, researchers argue this data alone is insufficient given large subsurface stores' variability affecting snowfall-streamflow relationships.
Brooks pointed out declining model accuracy driven by unquantified subsurface stores' variability as seen in 2022 when average snowpacks coincided with record low groundwater storage causing reduced spring streamflow.