Recent research has uncovered that the spring runoff heading to reservoirs in the Western United States is often several years old, challenging traditional perceptions of snowmelt water availability. Paul D. Brooks, a professor of geology and geophysics at the U.W., directed the work. Using tritium isotope analysis, researchers were able to identify the specific age of water samples collected from various mountain ranges. The results show that much of our mountain snowfall undergoes a multi-year subterranean journey before eventually emerging as streamflow. If validated, this finding might have profound implications on long-term water management strategies across the West.
The residual research team collected runoff samples from 42 field sites across Idaho, Wyoming, Utah, Colorado, California, and New Mexico. Together, these places span all five of the country’s largest river basins. Brooks is currently conducting sampling in 2022, with plans to visit each site twice—once during midwinter and again during the peak spring runoff. The study’s results highlight a substantial discrepancy between the amount of water stored underground and the assumptions held by many Western water managers.
Tritium Analysis and Findings
The study employed tritium isotope analysis, allowing direct calculation of the half-life of tritium at 12.3 years. This innovative approach led to a much more representative assessment of water age. This new technique provided the first ability for researchers to follow water’s path from snowpack to streamflow with greater detail and precision than ever before. Brooks stated, “On average, it takes over five years for a snowflake that falls in the mountains to exit as streamflow.” This discovery further emphasizes the need to better understand the role of groundwater storage in developing successful water management practices.
The implications of these results are profound. The research demonstrated that there is an order of magnitude more water stored underground than many water management models account for. “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,” Brooks explained. Now as it turns out, that’s not true. Fortunately, most of that water doesn’t run off. The vast majority of it soaks into the ground. It hangs around there for three to 15 years before being absorbed by plants or seeping into streams.
Impact on Water Management
The results contradict widely held assumptions about water availability across the Western US. Thriving communities and agricultural output feed millions in part thanks to the meltwater that pours off our snow-capped Rockies every spring. Snowpack monitoring sites have been the main method federal and state water managers have used for decades to forecast water availability. Yet, Brooks and his modeling team discovered that these models are not well equipped to accurately represent changes that are happening in the subsurface water storage.
Sara Warix, a colleague of Brooks, noted, “We know if our streams are being supported by water that’s 5 to 15 years old, there’s got to be a lag between input storage and response. Even though our models have been good in the past, there are changes throughout the subsurface that are reflected in streams.” This underscores the need for improved modeling approaches that include longer groundwater residence times.
Utah’s state-run tracking system is one of the best in the nation, boasting a 120-year history of continuous streamflow monitoring. This dense historical data is a game-changer for predictive models. It does this by including new science on groundwater storage and age. As communities and agricultural sectors continue to expand in this region, understanding the dynamics of groundwater can help optimize water use and sustainability.
Regional Implications
With a large geographical scope that included unique landscapes throughout each state, the study offered valuable insight into water dynamics across a wide range of conditions. “The sampling sites are locations where there was a fair amount of existing research, a geographical distribution from the front range of Colorado to the eastern slopes of the Sierra,” Brooks remarked. This sweeping, comprehensive approach serves to support the implications found in the study, while shining a light on their importance and application in varied regional settings.
This is only set to become more acute with the combined effects of climate change and population pressures. This cutting-edge research will help inform critical tradeoff decisions facing policymakers and water managers. By recognizing that existing models may underestimate groundwater resources, stakeholders can better prepare for future challenges related to water scarcity.