Spring Outflows are older than you think: Research reveals years of long underground journeys

Growing communities and large-scale agriculture in the western United States rely on meltwater spilling from snowy mountains every spring. The model for predicting the amount of stream flow available each year has long assumed that a small number of snowmelts enter shallow soil each year, with the rest coming out rapidly in rivers and streams.

However, a new study by a hydrologist at the University of Utah suggests that river generation is much more complicated. Most spring runoffs heading to the reservoir are actually a few years old, and most mountain snowfalls indicate that there are years of invisible journeys as groundwater before leaving the mountain.

Findings also show that more water is stored underground than most Western water managers explain, said Paul Brooks, a research leader who is a professor of geology and geophysics.

“On average, it takes more than five years for snowflakes to fall into the mountains to emerge as river flow,” Brooks said. “Most of our models are based on the idea that mountains have very little water, whether it is to predict river flow or how much water trees will have in dry years, and we see that this is not the case now.

The team collected effluent samples at 42 sites and determined the age of water using Tritium isotopic analysis. It’s how long it’s been since it fell from the sky like snow.

The findings, published in the Journal Communications Earth & Environment, were co-authored by U Geology Professors Sara Warix and Kip Solomon, in collaboration with research scientists in the West.

Determining the age of mountain flow is a prerequisite for predicting how mountain hydrology responds to changes in climate and land use, the researchers said.

“We know if our stream is supported by water ages 5-15. There is a need for a delay between input storage and response. So, even if the model has been good in the past, it’s good enough to make decisions about water use.

“The past mechanisms, past processes are not the same for 20 or 50 years, so if you want to make a good decision to move forward, you need to incorporate that groundwater storage component.”

Brooks conducted sampling in 2022 and visited 42 sites twice on Sabbatical. One visit was once in the middle of winter and captured the “base flow” of the stream, presumably fully fed by groundwater and again fed during the spring runoff.

“The sampling site is where there was a significant amount of existing research, a geographical distribution from the Colorado frontline to the eastern slopes of the Sierra,” Brooks said. The site was located in Idaho, Wyoming, Utah, Colorado, California and New Mexico, and represented five major river basins.

Utah’s tracking status is particularly robust, providing continuous streamflow data 120 years ago. This is an unparalleled data set that allowed hydrologists to record historical cycles of climate and river flow, and would have missed it otherwise, Brooks said.

According to Solomon, most of the Earth’s fresh and usable water is underground, but how much puzzle remains. Dating water provides clues and to determine the age of water, Solomon turns into tritium, a radioactive isotope of hydrogen with a half-life of 12.3 years.

Tritium is naturally produced in the upper atmosphere, a by-product of nuclear reactors, and was once produced during weapon testing during the Cold War. By determining the number of tritium atoms in a water sample compared to other hydrogen atoms, scientists can calculate when water falls from the sky as precipitation, but dates back to the first century.

The average age of runoff sampled in the study varies between catchments depending on geology. The more porous the ground, the older the water becomes, as the underground can hold more water. In contrast, glacial canyons with transparent capacity and shallow bedrock, such as Utah’s Little Cotton Wood Canyon, provide much less underground storage and younger oceans, according to research.

For decades, federal and state water managers have relied on a network of snow monitoring sites to provide data to guide forecasts of water availability for next year. Researchers say that such snow pack data does not provide a complete situation.

“In most parts of the West, particularly in the inner west where this study is based, our model is losing its skill,” Brooks said.

The growing cutting between snowfall, snow pack volumes, and river flows is driven by variation in these large, previously quantitative groundwater reservoirs. As an example, Brooks highlighted the year of 2022 in the Wed. This caused snow to fall in many western provinces, near or below average. However, low groundwater storage was recorded that year, far below average spring river flow.