In the vast and arid landscapes of the Western United States, water reigns as one of the most critical and scarce resources. Communities and agriculture in this region heavily depend on the seasonal pulse of snowmelt trickling down from towering mountain ranges every spring. For decades, hydrologists and water managers have operated under the assumption that the majority of this snowmelt swiftly transitions into surface runoff, rapidly feeding rivers and streams that supply reservoirs and irrigation networks. However, groundbreaking research conducted by the University of Utah’s hydrology team challenges this long-held paradigm, revealing a far more intricate and nuanced journey that mountain snow undergoes before replenishing the waterways below.
Traditionally, hydrologic models estimating streamflow volumes have presumed that only a minimal portion of snowmelt infiltrates shallow soils, with the greater bulk transforming immediately into fast-moving surface flow. Yet, the recent study led by Professor Paul Brooks and his colleagues paints an entirely different picture. Their research demonstrates that most of the water contributing to spring runoff is, in fact, several years old. This indicates that the majority of mountain precipitation embarks on a prolonged and largely hidden subterranean voyage, residing as groundwater beneath the surface for periods ranging from three to 15 years before eventually resurfacing in streams and rivers.
This revelation holds profound implications for water resource management, suggesting that the volume of groundwater storage in mountain catchments is an order of magnitude greater than previously incorporated in existing predictive models. Brooks emphasizes, “It takes over five years on average for a snowflake that falls in the mountains to exit as streamflow. This fundamentally alters our understanding of mountain hydrology and challenges the efficacy of current models that underestimate subterranean water storage.” The findings underscore that groundwater storage acts as a buffering reservoir, releasing water slowly rather than immediately, which influences the timing and reliability of streamflow used for environmental and human needs.
To unravel this hidden timeline of water flow, the research team sampled runoff from 42 sites across major river basins in the western U.S., including locations in Utah, Colorado, Idaho, Wyoming, California, and New Mexico. Employing tritium isotope analysis, a precise method that assesses the radioactive decay of tritium—a hydrogen isotope with a well-characterized half-life—the investigators could accurately gauge the "age" of water samples. Tritium, produced naturally in the upper atmosphere and introduced into the environment through mid-20th century nuclear weapons testing, serves as a reliable chronometer that deciphers when precipitation fell from the sky, up to approximately 100 years prior.
According to co-author and geology professor Kip Solomon, tritium analysis provides a crucial window into subsurface hydrological processes. The concentration of tritium relative to other hydrogen isotopes in water reflects its residence time underground. This isotope-based technique permits researchers to move beyond surface measurements and infer complex, longer-term interactions within groundwater systems, something traditional monitoring methods fail to capture. The ability to date water samples with such sophistication offers a transformative tool to untangle the intricate processes governing water storage and release in montane environments.
The study reveals distinct variations in the age of runoff depending on regional geology and hydrologic conditions. In basins underlain by highly porous, permeable substrates, groundwater residence times tend to be longer, leading to older water being released during snowmelt runoff. In sharp contrast, areas characterized by impermeable bedrock or glaciated canyons, such as Utah’s Little Cottonwood Canyon, feature more rapid surface runoff with much younger water ages due to limited subsurface storage. These findings highlight the heterogeneity of Chinese megafauna across the diverse mountain terrains of the Western U.S. and the consequent variability of water flow paths and timing.
Beyond pushing the boundaries of hydrologic science, the implications for water management are urgent and consequential. Current forecasting models, heavily reliant on snowpack measurements alone, overlook the critical subsurface component that significantly modulates streamflow. As Paul Brooks points out, “Our decades-old reliance on snowpack data risks providing an incomplete and potentially misleading portrait of water availability, particularly as climatic variability and human impacts alter hydrologic cycles.” Indeed, the 2022 water year starkly demonstrated this disconnect. Despite snowpack levels hovering near average, groundwater reservoirs were depleted to record lows, culminating in historically low spring streamflow. Such disparities expose vulnerabilities in resource planning and the necessity for updated, integrated models encompassing groundwater dynamics.
Co-author Sara Warix stresses that this newfound understanding mandates a paradigm shift in hydrologic prediction and management frameworks. There is a critical lag between snowfall input, groundwater storage, and eventual streamflow output, meaning that climatic and land-use changes will manifest in water systems on multiyear timescales. “To make informed decisions about water allocations and ecosystem health for the coming decades, it is essential to incorporate this lagged groundwater component,” Warix asserts. Without revising models to incorporate long-term groundwater storage and release, forecasts will likely fall short of accuracy, compromising sustainability and crisis preparedness in the water-stressed West.
The meticulous sampling campaign conducted by Brooks and his team involved collection during two key periods in 2022: winter base flow conditions, representing groundwater-fed stream contributions, and the active spring runoff season fueled by melting snow. This dual approach enabled a comprehensive evaluation of water sources underpinning streamflow throughout the transition from frozen mountain accumulation to riverine discharge. By spanning a broad geographic range of 42 sites, the study effectively captures diverse hydrogeological settings, thereby strengthening the generalizability of its conclusions.
Institutional support and long-term monitoring programs were pivotal for this endeavor. Many sampling sites benefited from ongoing research infrastructure maintained by the U.S. Geological Survey, the National Science Foundation, and the Department of Energy. Notably, Utah’s exceptional continuous streamflow data record, extending over 120 years, provides an invaluable historical context to corroborate and interpret results. Such rich datasets allow hydrologists to discern historic patterns and cycles that would otherwise remain elusive, enhancing confidence in the study’s identification of underlying hydrological mechanisms.
This pioneering research, published in Nature Communications Earth & Environment, emerges at a critical juncture as Western water systems confront intensifying pressures from climate change, population growth, and competing demands. The realization that groundwater storage in mountainous regions dominates snowmelt runoff—and thus streamflow efficiency—upends conventional wisdom and compels a reevaluation of water resource strategies. Incorporating the role of groundwater as a vast, slow-moving reservoir not only refines scientific understanding but also equips policymakers and stakeholders with a more robust framework to anticipate future water availability.
Far from a mere academic curiosity, the study’s insights have tangible, far-reaching consequences. Effective water supply forecasting and ecosystem management hinge upon recognizing that the hydrologic cycle’s pace in this region is modulated by subterranean storage processes with inherent time lags. Strategies that fail to account for these delayed responses risk misallocating water, underestimating drought severity, and overlooking opportunities to enhance resilience. As the West increasingly grapples with the challenges of drought and climate variability, embracing this groundwater-centric perspective is essential for securing the region’s hydrologic and ecological future.
In conclusion, the research led by University of Utah hydrologists marks a transformative advancement in understanding Western mountain hydrology. It illuminates the prolonged subterranean passage of snowmelt water, challenges prevailing assumptions about rapid runoff dominance, and underscores groundwater’s pivotal role in controlling the timing and volume of streamflow. By leveraging innovative tritium isotope analysis and extensive field sampling, this study provides critical knowledge necessary to refine water management models amid a rapidly changing climate. As the Western United States seeks to adapt to evolving water realities, embracing the hidden longevity of groundwater storage will be paramount in crafting sustainable, forward-looking solutions.
Subject of Research: Not applicable
Article Title: Groundwater dominates snowmelt runoff and controls streamflow efficiency in the western United States
News Publication Date: May 3, 2025
Web References:
References:
Brooks, P., Warix, S., Solomon, K., et al. (2025). Groundwater dominates snowmelt runoff and controls streamflow efficiency in the western United States. Nature Communications Earth & Environment. DOI: 10.1038/s43247-025-02303-3
Image Credits: Brian Maffly, University of Utah
Keywords: groundwater storage, snowmelt runoff, streamflow efficiency, tritium isotope analysis, mountain hydrology, Western United States, water resource management, climate change, hydrologic models, subsurface water, snowpack, hydrology
