Water’s Age and What It Can Tell Us

By Elaina Hancock

When it rains, what happens to the water once it enters the soil? Does the new precipitation mix with all of the water that was already there? In their recent paper in Water Resources Research, Department of Natural Resources and the Environment Ph.D. student Joshua Snarski and assistant professor James Knighton show the answer is more complicated than previously assumed, but knowing the age of water gives a more accurate picture.

Hydrologists use models to simulate what is happening in natural systems. Since hydrologic processes are complex, researchers need to make assumptions about some aspects, such as how water mixes within the soil profile. Though previous hydrologic research is focused on the amount and timing of precipitation, Snarski says shifting the focus to the age of water within the soil profile can reveal more about what is happening beneath the surface.

For this project, the researchers determined the age of water in the soil by looking at the stable water isotope compositions of soil water samples through time. Stable water isotopes are naturally abundant in the environment and do not interact with other elements within the system, Snarski explains, which makes them powerful tracers. Each rainstorm releases water with a unique isotope signature, allowing each precipitation event to be tracked.

“Precipitation acts as water inputs to the soil and assigning these ‘new’ water inputs with an age of zero days allows us to ‘start the clock’ on the soil water aging,” says Snarski. “We collect precipitation and soil water samples to create a record of the volume and isotope signature of both the new water entering the soil and the existing water within the soil.”

The researchers use these two water records to estimate the age of water in the soil profile over time. If you imagine a drop of rain traveling down through layers of soil, knowing the age of that water indicates its pace of movement, and that can provide insight into how water is stored within and released from soils. This information is especially crucial in agricultural settings, as farmers need to decide when and how much fertilizer to apply to fields to support crop growth.