Hidden 'rock moisture' possible key to forest response to drought

The study, published today in the journal Proceedings of the National Academy of Sciences (PNAS), looked at the water stored inside the layer of weathered bedrock that lies under soils in mountain forest ecosystems.

This transitional zone beneath soils and above groundwater is often overlooked when it comes to studying hydrologic processes, but researchers found that the water contained in the fractures and pores of the rock could play an important role in the water cycle at local and global levels.

"There are significant hydrologic dynamics in weathered bedrock environments, but traditionally they are not investigated because they are hard to access," said lead PNAS author Daniella Rempe, a geoscientist at UT Austin. "Our study was designed to investigate this region."

Researchers found that water in bedrock can sustain trees through droughts even after the soil has become parched.

At a field site in Northern California's Mendocino County, scientists found that up to 27 percent of annual rainfall was stored as "rock moisture," the water clinging to cracks and pores in the bedrock.

The impact of rock moisture varies, the researchers said, depending on region and topography. But it likely explains why trees in the study area showed little effect from the severe 2010-2015 drought, which killed more than 100 million trees throughout California.

"How trees can survive extended periods of severe drought has been a mystery," said Richard Yuretich, director of NSF's Critical Zone Observatory (CZO) program. The research was conducted at the NSF Eel River Critical Zone Observatory, one of nine NSF CZO research sites across the country.

"This study reveals a significant reservoir of trapped water that had gone unnoticed in the past," says Yuretich. "Research of this kind can help greatly in managing natural resources during times of environmental stress."

To conduct the study, scientists monitored rock moisture from 2013 to 2016 at nine wells drilled into weathered bedrock along a steep, forested hillside. They used a neutron probe, a precision tool that measures the amount of water in a sample area by detecting hydrogen.

They found that the weathered rock layer built up a supply of 4 to 21 inches of rock moisture during the winter wet season, depending on the well.

The maximum amount of rock moisture in each well stayed about the same throughout the study period, which included a significant drought year. The finding indicates that the total rainfall amount does not influence the rock moisture levels.

"It doesn't matter how much it rains in the winter; rock moisture builds up to the same maximum value," Rempe said. "That leads to the same amount of water being available every summer for use by trees."

Researchers also found that the average rock moisture at all wells exceeded the average soil moisture measurements at all locations.

"Soils are important, but when it comes to determining if a place is going to experience water stress, it could be the underlying rock that matters most," Rempe said. "This is the first time this has been demonstrated in a multi-year field study."

The potential for rock moisture to travel back to the atmosphere by evaporation from tree leaves or to trickle down into groundwater indicates that it could affect the environment and climate on a larger scale.

The study provides a glimpse into rock moisture at a small, intensive research site, according to paper co-author William Dietrich of the University of California, Berkeley. He said the data collected during the study should be a starting point for more research. "The future paths are many. Now we have just one well-studied site."

The research was also supported by the Keck Foundation and the University of California Reserve System.