Forests survive droughts through hidden sources of rock moisture

Groundwater is an essential component of the ecosystems that surround us, as well as a major contributor to our own water supply. However, when it comes to studying hydrological processes, the transitional zone beneath soils and above groundwater is commonly overlooked. The underground layer of bedrock that often exists under soil in mountainous environments has now been found to hold significant amounts of water. Researchers believe that it plays a role as an important reservoir for trees, particularly in times of drought.

A recent study from The University of Texas at Austin and University of California, Berkeley, found that the water contained within the fractures and pores of the rock may play a vital role in the water cycle, at both a local and global level.

“There are significant hydrologic dynamics in weathered bedrock environments, but they are not traditionally investigated because they are hard to access,” says Daniella Rempe, an assistant professor in the Department of Geological Sciences at the UT Austin Jackson School of Geosciences and lead author of the paper. “The study was designed to investigate this region directly.”

Through analysis of a field site in Northern California, the researchers found that up to 27 percent of annual rainfall was stored as “rock moisture,” the water within the pores and cracks of the bedrock. This rock moisture varies depending on the region and topography, but the study’s authors believe that it could explain how trees in the areas studied experienced very little stress from the significant 2010-2015 drought, which killed over 100 million trees in California.

“How trees can survive extended periods of severe drought has been a mystery,” explains Richard Yuretich, director of the National Science Foundation’s Critical Zone Observatories program, which funded the study. “This study has revealed a significant reservoir of trapped water that has gone unnoticed in the past. Research of this kind can help greatly in managing natural resources during times of environmental stress.”

Using a neutron probe to measure levels of hydrogen — and thus water levels — in nine wells drilled into bedrock along a steep forested hillslope, the researchers found that this rock layer built up a supply of 4 to 21 inches of rock moisture during the winter wet season. Even in a significant drought year, the maximum amount of rock moisture in each well stayed about the same. This finding shows that it doesn’t matter whether there is a lot of rain or a drought during the winter dry season, as the total rainfall amount does not influence rock moisture levels.

Their results also showed that average rock moisture at all of the wells exceeded the average soil moisture measurements for all locations studied. “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,” says Rempe. “This is the first time this has been demonstrated in a multi-year field study.”

These findings hold implications beyond water availability in the locations studied. The authors believe that the potential for rock moisture to travel back to the atmosphere via evaporation from tree leaves, or flow down into groundwater means that it could have a significant impact on the environment and climate.

“At present, most, if not all, of global climate and hydrological models neglect moisture stored in rocks,” says Zong-Liang Yang, a professor in the Jackson School’s Department of Geological Sciences, who was not involved in the research. “This study fills a critical gap in our understanding.”

Image Credit: University of Texas at Austin Jackson School of Geosciences.

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By Connor Ertz, Earth.com Staff Writer