Annual snowpack in the Western United States gets a lot of coverage each year in part due to the perception that the that the amount of snow on the mountains directly determines how much freshwater will flow into streams, rivers, lakes, and reservoirs.
When precipitation falls on mountains, the water will either collect on the surface as snow (during the winter), immediately runoff, or percolate down into the ground where it gets stored as groundwater. Mountain water is continuously moving both above and below ground.
Measuring the source of water runoff in western U.S. mountains
Led by University of Utah, a team of researchers is countering that common assumption that the spring runoff is mostly made up of that year’s snowmelt. To see how old the water is during spring and summer runoff, researchers used a technique called tritium dating. Tritium dating measures the age of water based on the presence of a radioactive form of hydrogen that decays over time.
42 headwater catchment locations in five watersheds in the interior Western United States were analyzed. All of these catchment areas were chosen because of long-term water measurements already in place. Where possible, samples were collected in the winter when streamflow is low and mostly comes from groundwater, and again in the spring and early summer during snowmelt, when streamflow is high.
Water sampled during the winter, when runoff is seasonally low, had a higher average age. The mean age of winter baseflow (groundwater) for these sites was 10.4 years (with a standard deviation of 4.5 years). During the spring, snowmelt introduces a much younger source of water to runoff. What surprised the researchers was that the mean age of water during peak runoff was 5.7 years (with a standard deviation of 4.3 years). This means that long-time groundwater was still a significant contributor of water to spring runoff. Researchers used a simple, two-component linear mixing model to estimate that 58% of spring runoff was old water (defined as water that been stored as groundwater for at least one year prior to the current season).
The amount of old water in snowmelt runoff varied depending on the type of bedrock beneath each watershed. Watersheds with hard, low-permeability rock (like granite or shale) had younger water and less groundwater storage. Watersheds with more porous rock (like sandstone) had older water and stored much more groundwater.
What does this mean for water modeling?
The results from this study challenge assumptions used by many hydrology models that assume most spring runoff originates from that year’s snow and rainfall. In contrast, the water dating from this study suggests that water seepage from groundwater can continue to contribute to both streamflow and plant water use for years. Based on the results of this study, the authors recommend collecting tritium samples twice a year to help water managers more accurately forecast streamflows and track groundwater recovery after droughts.
The study:
Brooks, P. D., Solomon, D. K., Kampf, S., Warix, S., Bern, C., Barnard, D., … & Wolf, M. (2025). Groundwater dominates snowmelt runoff and controls streamflow efficiency in the western United States. Communications Earth & Environment, 6(1), 341. DOI: /10.1038/s43247-025-02303-3