Groundwater dominates snowmelt runoff and controls streamflow efficiency in the western United States

Authors: BROOKS, PAUL - University Of Utah; SOLOMON, D - University Of Utah; KAMPF, STEPHANIE - Colorado State University; WARIX, SARA - University Of Utah; BERN, CARLETON - Us Geological Survey (USGS); Barnard, David; BARNARD, HOLLY - University Of Colorado; CARLING, GREGORY - Brigham Young University; CARROLL, ROSEMARY - Desert Research Institute; CHOROVER, JON - University Of Arizona; HARPOLD, ADRIAN - University Of Nevada; LOHSE, KATHLEEN - Idaho State University; MEZA, FABIOLA - University Of Arizona; MCINTOSH, JENNIFER - University Of Arizona; NEILSON, BETHANY - Utah State University; SEARS, MEGAN - Colorad0 State University; WOLF, MARGARET - University Of Idaho

Submitted to: Communications Earth & Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/16/2025
Publication Date: 5/3/2025
Citation: Brooks, P.D., Solomon, D.K., Kampf, S., Warix, S., Bern, C., Barnard, D.M., Barnard, H.R., Carling, G.T., Carroll, R.W., Chorover, J., Harpold, A., Lohse, K., Meza, F., McIntosh, J., Neilson, B., Sears, M., Wolf, M. 2025. Groundwater dominates snowmelt runoff and controls streamflow efficiency in the western United States. Communications Earth & Environment. 6. Article e341. https://doi.org/10.1038/s43247-025-02303-3.
DOI: https://doi.org/10.1038/s43247-025-02303-3

Interpretive Summary: Climate change is making it harder to predict how much water will flow into rivers from melting snow. This is because temperatures are rising, snowfall is becoming more unpredictable, snow is melting earlier, and people are using more water. These changes, along with more wildfires, insect outbreaks, and droughts, are making it difficult for scientists and water managers to accurately predict how much water will flow from snow-covered mountains. A big reason for this is that we've been assuming that snowmelt water flows quickly into rivers. But this study shows that most of the water flowing from melting snow is actually old groundwater. This means that the streamflow behavior is influenced by past climate conditions and snowfall from previous years, not just the current weather. To improve our predictions, we need to include the impact of this old groundwater in studies. This will help us manage our water resources more effectively.

Technical Abstract: Warming temperatures, increasing variability in the amount and form of precipitation, earlier snowmelt, and growing demands are stressing snowmelt-derived water supplies. These direct effects of climate change, combined with increasing levels of disturbance from fire, insects, and drought have reduced the ability of operational models to accurately predict snowmelt-driven streamflow. A potential contributor to the decline in predictability is the assumption that melt water is routed quickly to streams through surface runoff. Here we use tritium age dating to show that streamflow during snowmelt instead is dominated by preexisting groundwater and that runoff efficiency is negatively related to the age of these groundwater stores. The average age of snowmelt runoff (5.5 +/- 4.5 years) from 42 catchments across the western U.S. was closer to the average age of groundwater (10.4 +/- 4.5 years) than to recent precipitation. The large variability in ages was related to geology with water from hard rock catchments being younger and more evaporated than water from clastic catchments. These results demonstrate that snowmelt runoff is the result of multiple years of climate interacting with catchment geology to control water balance. Including a groundwater mediated memory of past climate in operational models holds great promise improving streamflow prediction and efficient management of limited water resources.