Predicting Water Age to Improve Estimates of Water Availability Within Watersheds

The Science

Key knowledge gaps exist around the concept of water transit time—how long water stays in different parts of a watershed system—starting from the time it falls on the land as precipitation until it exits as stream or river discharge. To overcome current limitations, a multi-institutional team of researchers developed a hydrologic framework called Sequential Precipitation Input Tagging (SPIT). The SPIT framework allows for continuous tracking of precipitation through its journey in both surface and subsurface environments and subsequently down streams and into rivers. Its straightforward implementation simplifies the process of estimating water transit times, requiring only basic precipitation data and commonly measured water isotopes. This new tracking method offers a clearer understanding of water movement, helping to improve predictions of water availability in the future. Moreover, the framework can be easily applied to other locations to inform water management across the United States.

The Impact

Watershed processes play a crucial role in regulating the timing and amount of surface water and groundwater that electricity-producing systems, such as hydropower dams and nuclear power plants, depend on for their ongoing operation. Supporting reliable energy production while also anticipating challenges requires a thorough understanding of water movement across scales, from hillslopes to whole watersheds. The new SPIT method developed here is poised for broad use to increase the fundamental understanding of how water moves through watersheds and, in turn, enhance the ability of models to make reliable predictions of water delivery to downstream energy infrastructure. This knowledge provides the foundation for understanding and anticipating current water availability, as well as how it will change over time.

Summary

Understanding how long water remains underground before reaching streams is essential for evaluating water storage and movement. Researchers developed a new approach called SPIT to simulate belowground movement of water by assigning monthly tags to rainfall in a water-tracking model over a seven-year period. Using this method, they tracked how tagged precipitation moved through soil and groundwater and into rivers at six research sites across the continental United States. The model estimated two key indicators of water age: mean transit time (MTT), which describes the average time water takes to travel through a watershed, and fraction of young water, which is the proportion of stream water less than 90 days old.

The multidisciplinary team of researchers found that a significant amount of tagged rainfall was needed—typically more than three years’ worth—before it could substantially contribute to streamflow. The MTT ranged from about 190 to 850 days when tagged water made up at least 75 percent of the streamflow. The researchers also estimated the chemical signatures of stream water by using oxygen and hydrogen isotopes, achieving good agreement with observed data. They found that the MTT and fraction of young water varied in response to changes in rainfall, discharge, and groundwater levels, revealing power-law relationships that reflect consistent patterns of water movement. The SPIT framework allows for reproducible estimation of water transit times and tracer dynamics in models that simulate tagged precipitation. By enhancing predictive capabilities, the SPIT framework can also anticipate water availability challenges, thereby providing managers with critical information to better minimize power disruptions. This is vital as nearly all U.S. domestic energy production occurs in land-based ecosystems, and much of that energy production capacity relies on water delivered via watershed processes.

The initial draft of the text above was created using ChatGPT (version 5.5 or lower, OpenAI). The language and content were subsequently edited by the author for grammar, clarity, and accuracy, and the final document was reviewed by the author.

Contacts

James Stegen, Pacific Northwest National Laboratory

Zach Butler, Oregon State University

Funding

This research was supported by the Department of Energy (DOE), Office of Science, Biological and Environmental Research (BER) program, as part of BER’s Environmental System Science program. This contribution originates from the River Corridor Scientific Focus Area at Pacific Northwest National Laboratory (PNNL), facilitated by a distinguished graduate research fellowship between PNNL and Oregon State University. PNNL is operated for DOE by Battelle Memorial Institute.