The Secret Life of a Raindrop

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According to a widely held belief, you can’t squeeze water from a rock. But researchers from UC Berkeley who are trying to better understand where water is stored in nature are challenging that old adage.

After nearly ten years of studying a steep, 20-square-mile area near the South Fork Eel River in coastal Mendocino County, the scientists have shown that for trees and other plants, deep and highly fractured rock formations beneath the Earth’s surface are a much larger water reservoir than was previously known.

The work to understand the role that “rock water” plays in the hydrologic cycle began in 2006 when researchers from UC Berkeley embarked on a multi-year study sponsored by the Keck Foundation called the Hydrowatch project. It was designed to precisely monitor and measure the pathways of water in Mendocino County’s Angelo Coast Range Reserve as it cycles from the groundwater table to the tops of trees and into the atmosphere.

“We were really interested in learning the fate of precipitation in the land surface,” explains Todd Dawson, professor of Integrative Biology at UC Berkeley. “So really trying to figure out when precipitation arrives at the site, where does it get into the rock, where does it get into the stream, how does is recharge the ground water, how much of it is used by the vegetation, and ultimately, how much of it ends up in the streams and going back out to the Pacific Ocean.”

Learning more about where water consumed by forests, or flowing through streams actually comes from is important, the scientists say, in better understanding the impact of climate change.

In 2013, the project expanded to become part of a landmark study sponsored by the National Science Foundation called the Critical Zone Observatories Program. Today, the site is called the Eel River CZO and it’s part of a national network of ten similar watershed observation sites across the United States - each with unique climate, geology and vegetation.

The term “critical zone” is relatively new and is being used by scientists to define the zone that is tectonically, geologically and biologically active across the Earth’s surface. It represents a groundbreaking new approach to studying the hydrologic cycle.

“The critical zone really tries to capture this idea of the zone between bedrock beneath our feet, and the top of the vegetation where the trees are interacting with the atmosphere,” explains Dawson. “So it’s everything in between. It’s rock, it’s soil, it’s the vegetation, and it’s the atmosphere that’s coupled to that vegetation. That’s the critical zone. It’s where life meets rock.”

Scientists across a broad range of earth, life and computer sciences – from microbiologists to geologists to electrical engineers - are now working together to conduct research and share data within the most comprehensive hydrologic science network in the world.

“We’ve rarely studied all those things together at one site,” says William Dietrich, professor of Earth and Planetary Science at UC Berkeley and lead Investigator at the Eel River CZO. “Geologists rarely work with microbiologists, and now all of us are working together at the same site to merge our information to see how each of the pieces work interdependently and impact the other pieces.”

To gather information, researchers at the ten national sites scale trees and towers hundreds of feet tall and drill deep into bedrock to place sensors that collect climate information. Their instruments transmit real-time measurements of things like air temperature, rock moisture, soil, air and water content and stream flow.

Some of the sites have so many instruments that the vegetation and landscapes look almost bionic. One tree in UC Merced’s Southern Sierra CZO on the North Fork Kings River in Fresno County has been dubbed the “critical zone tree” because it’s adorned with nearly 200 sensors that measure things like humidity, temperature, and water movement through the tree via sap flux.

As a plant physiologist, Dawson’s part in the project is to provide information on the role that plants and trees are playing in how water moves through the Eel River watershed.

“Seventy-five to eighty percent of the water on this planet is recycled through agriculture, through forests, through the plants,” he explains. “You take those plants away, you remove that straw in the Earth, that conduit for water to move out of the soil and back into the atmosphere, and that eventually can lead to deserts expanding. It changes the climate. We know for example when trees were cut down in the Amazon, there was less precipitation.”

One of the team’s main discoveries was that large amounts of water in the Eel River watershed is stored in the massive network of fractures in the rock that can be tens to hundreds of feet thick. This “rock water reservoir” is hidden deep inside the Earth, away from the influence of evaporation. It sits beneath the soil and above the saturated layer commonly called ground water and occupies the deepest part of what hydrologists call the “unsaturated zone”. Many trees reach their deep roots into this matrix of water-filled rock fissures and use the water stored there when other water sources dry out or become unavailable. Different types of rock store water in different ways.

“We are thinking of them as different types of sponges in the subsurface in the way they take up and retain moisture and give back that moisture to the vegetation that is rooted into them,” says Dietrich.

In additional to discovering the amount of water stored in underground rock fractures, Dawson and his team have learned that different types of trees actually use the “rock water” in very different ways depending on climate conditions. For example, the rock matrix inside slopes of hills is a key water resource for the largest trees in the watershed, like Douglas firs.

Hardwood trees like tanoak, madrone and live oaks rely largely on precipitation. But when drier times come, they shift to using the more stable groundwater below the surface and then may draw on some "rock water" in later summer and fall.

Conifers play a larger role in moving water out of the subsurface areas in winter and early spring, Dawson says. And hardwoods are playing a larger role in summer and fall.

“As climate and the forest change over time,” he adds, “this will lead to changes in how water enters and leaves these ecosystems because of what the vegetation on the land surface is composed of.”

The work being done within the National Critical Zone Observatories Program is timely because of a growing sense of urgency within the scientific community that as climate is changing and lands are changing because of human use of the land surface, we’re permanently disturbing the way the Earth works.

“If we don’t put a singular focus on understanding the critical zone,” explains Dawson, “as we march into the future and climate continues to change we’re not going to know how to mitigate for the kinds of impacts that humans and climate are actually having on resource balance on planet Earth.”