Where does water go when it rains? How does soil form? How did forests and other ecosystems develop? And how can understanding those processes inform human behaviors in the face of changes in land use and climate? Those are among the questions that scientists are asking about the critical zone—defined loosely as the region at Earth’s surface that extends from the treetops down to bedrock.

Although critical-zone research is tuned to local environments, scientists also look for broader implications. At the Boulder Creek Critical Zone Observatory in Colorado, part of the US network of nine CZOs, the main focus is on how erosion and weathering shape topography. At the Calhoun CZO in the former Cotton Belt of South Carolina, sediment formation and land erosion and recovery are the thrusts. “If you follow gradients, monitor fluxes, intensively observe over time, you have a chance of understanding process,” says William Dietrich, an Earth and planetary scientist at the University of California, Berkeley, and the principal investigator of the Eel River CZO in Northern California. “And that process, not the properties of a specific place, is what you can generalize.”

Jon Chorover is a soil chemist at the University of Arizona and head of the CZO that has sites in the Santa Catalina Mountains in Arizona and the Jemez River basin in New Mexico. Like other scientists who study the critical zone, he is trying to understand how climate interacts with rock to create a zone that sustains and supports life. “We want to know how the structure developed over millennia and understand the dynamics in a real-time way,” he says.

The roots of a ponderosa pine pry open fractures in bedrock in the Betasso catchment in Colorado. Measurements of contact forces between the rocks and the roots show enigmatic daily cycles. And the cracks speed up rock weathering by allowing faster penetration of water and organic matter.

The roots of a ponderosa pine pry open fractures in bedrock in the Betasso catchment in Colorado. Measurements of contact forces between the rocks and the roots show enigmatic daily cycles. And the cracks speed up rock weathering by allowing faster penetration of water and organic matter.

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In the US and elsewhere, critical-zone researchers and the organizations that fund them also aim to influence policymaking. Says Jérôme Gaillardet, who coordinates France’s network of roughly 50 CZOs, “Our responsibility as scientists is to come up with unified answers. When the scientific community is divided, policymakers have too many answers. Together, we will have a stronger voice.”

Germany has set up CZOs in areas that are sensitive to climate change, with the intent of providing long-term data to the science community and informing policy. An Indo-French collaboration in the Kabini River basin in southern India looks at how to balance changes in land fertility with economic, energy, and water considerations. In China, CZOs are being created to tackle problems related to farming and other activities that affect soil and water resources. And in its 2012 competition, NSF, which funds the US CZOs, required that proposals include a socially relevant component that connects critical-zone research to human health, safety, or water or to other environmental factors on which societies depend.

Research on the critical zone—a term that first gained prominence in the 2001 National Research Council report Basic Research Opportunities in Earth Science—is growing worldwide with an enthusiasm akin to nanotechnology two decades ago. The field brings together scientists from many disciplines, including geophysics, geochemistry, hydrology, and microbiology. It encompasses monitoring, measuring, and computer modeling.

NSF began its CZO program a decade ago and has invested about $80 million in it. Despite positive signals from NSF officials, as Physics Today went to press no funding mechanism was yet in place to continue the program.

The US CZOs dot the country. In addition to the four already mentioned, others are located in the southern Sierra Nevada in California, the Reynolds Creek watershed in southwest Idaho, Susquehanna Shale Hills in Pennsylvania, and the Luquillo Mountains in Puerto Rico. The Intensively Managed Landscapes CZO spreads over three sites in Illinois, Iowa, and Minnesota. The nine CZOs range in size from a few tens to many thousands of square kilometers.

The critical zone extends from treetops down to bedrock. Scientists from many disciplines are teaming up to understand how its components interact and to predict how they react to changes in land use and other anthropogenic activities. (Based on an image by Ralph Kindlimann. Courtesy of Jon Chorover and colleagues, Catalina-Jemez Critical Zone Observatory.)

The critical zone extends from treetops down to bedrock. Scientists from many disciplines are teaming up to understand how its components interact and to predict how they react to changes in land use and other anthropogenic activities. (Based on an image by Ralph Kindlimann. Courtesy of Jon Chorover and colleagues, Catalina-Jemez Critical Zone Observatory.)

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Water runs as a common research component through all the sites. So do carbon and nutrient fluxes, rock weathering, geologic processes, plant and microbial life, and the impacts of agriculture and other human activities. Crosscutting research goals include measuring the properties and structure of the critical zone and understanding its evolution and function. Researchers probe the regolith, which is the weathered mantle above the bedrock, and balances of mass and energy—water and transport of nutrients, metals, and chemicals in the case of mass, and exchanges of heat and electromagnetic radiation for energy. “We are looking intensively at all aspects of the Earth surface system, and trying to nail down what is going on,” says Susan Brantley, director of Penn State’s Earth and Environmental Systems Institute and head of the Shale Hills CZO.

The sites host instruments and their supporting structures, not office buildings. To characterize topography and vegetation, the US CZOs have used airborne lidar surveys. Researchers use sensors and take samples: An assortment of field instruments keep tabs on the temperature, humidity, chemical composition, and other parameters related to the atmosphere, plants, water, soil, and rocks. Typical depths of investigation extend to a few tens of meters.

The NSF funding, roughly $1 million a year apiece for most of the US CZOs, pays postdocs, students, and technicians, as well as some faculty summer salaries. The funding is also used for data storage and distribution, instruments, maintenance, and travel. The Shale Hills CZO, says Brantley, is located in a sustainable forest, for which it is charged an annual maintenance fee. In addition, the site’s eddy flux tower has been hit by lightning many times, and equipment has had to be replaced due to vandalism. “It’s amazing how much maintenance is needed,” she says.

The US CZOs each have their own principal investigator, but they are intended as user facilities and are open to all researchers. For the most part, however, visiting researchers must secure their own funding. Data collected at the sites are freely available. However, since the CZOs grew up individually, they don’t employ a shared data format.

The first CZOs in Europe were launched in 2009. They were five-year experiments with a more applied bent than their US counterparts. The US focus was initially on undisturbed systems, explains Nikolaos Nikolaidis, who heads Greece’s Koiliaris CZO on Crete, but in Europe the soils are degraded. “It’s more important to understand how soil ecosystems function, and how they provide services—biomass production, biodiversity, clean water, carbon and nutrient sequestration—and sustain humanity.”

Germany’s handful of CZOs were started in 2010 with the aim of collecting data over at least 20 years. They monitor groundwater levels; soil moisture, nutrients, carbon, and emissions; and river-water composition. “We want to establish a long-term database to see how the critical zone reacts to climate change and land use. How can the critical zone sustain food, feed, and energy?” says Harry Vereecken, head of Germany’s Jülich Research Center and a prime mover in forming a network of Europe’s CZOs.

Many of France’s CZOs, located in France, Nepal, South America, and former French colonies in Africa, predate the concept of the critical zone. A site may have originally focused on hydrology, soil, or ecology, for example, but has broadened its purview. “We put money on the table to form a network of our existing observatories,” says Gaillardet. The network encompasses glaciers, drainage basins, and swamps, which, as “sentinels of global change,” are good places to study the effects of global warming and land use, he says. “The challenge is how to integrate [data], to pass from individual observatories to the larger-scale questions they can work on together.”

In addition, sites in Australia, Denmark, Italy, Turkey, and elsewhere are being expanded to fit into the critical-zone paradigm. And across Europe, the CZOs aim to tighten links with each other and with socioecological research sites. The growing numbers of CZOs around the globe collaborate and share methods and data.

Last August, scientists submitted a proposal to the European Commission for their CZOs, in concert with ecological and social research networks, to join the road map of the European Strategy Forum on Research Infrastructures. A successful bid would put them on the path to sustained, long-term funding. A decision is expected late this year. In encouraging stronger interdisciplinary interactions, says Vereecken, “we expect people to think holistically. It forces them to think in terms of the whole system.”

Eddy flux towers—so named because they measure vertical turbulent fluxes—are installed at many critical-zone observatories to monitor the exchange of water and carbon dioxide between the ecosystem and atmosphere. This one is in the foothills of California’s Sierra Nevada and is part of the Southern Sierra Critical Zone Observatory.

Eddy flux towers—so named because they measure vertical turbulent fluxes—are installed at many critical-zone observatories to monitor the exchange of water and carbon dioxide between the ecosystem and atmosphere. This one is in the foothills of California’s Sierra Nevada and is part of the Southern Sierra Critical Zone Observatory.

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For the past couple of years, the UK has worked with China on five CZOs. The projects focus on the impacts of land-use change—rapid development of cities, changes in agriculture, shifts in dietary demand, and so on. Andrew Binley, a hydrogeophysicist at Lancaster University, and colleagues collect radar and resistivity measurements to improve predictions of, for example, how nitrates from fertilizers move to deep groundwater. They also look at variations in moisture content across a hill slope and ask such questions as, “If I grow peanuts next to citrus trees, do I get a better crop of peanuts?” The research, he stresses, is not about how the critical zone behaves under natural conditions. China has “more pressing problems, namely, keeping the supply of food and water for future populations.”

China is also starting CZOs on its own. The focus depends on the agency in charge, says Weiya Ge of the China Geological Survey’s hydrology and environmental geology department in Beijing. Some emphasize basic science questions related to biochemistry, geochemistry, and the carbon cycle, while others characterize critical-zone structure and function with an eye to advising the government on ecological and environmental problems.

As a graduate student doing fieldwork at Eel River, Daniella Rempe worked out a method with her colleagues to sample water and gases from weathered bedrock. They collect samples every two weeks from discrete depth intervals in diagonally drilled holes. “We capture both freely draining water and water that is more tightly held, to identify how fractures influence where water is stored and what its chemical composition is,” says Rempe, who is now an assistant professor at the University of Texas at Austin. “At our site, the rock moisture is 30% of the water budget,” says Eel River leader Dietrich. “But this rock moisture is not in any climate model.”

Other CZO findings are that critical-zone thickness controls water availability and that different types of trees get their water from different sources. For example, Douglas firs use rock moisture, while hardwoods such as oaks and madrones appear to get their water from shallower sources. Those differences have consequences: In Northern California, fire suppression has led to firs encroaching on hardwoods, and as firs take over they may sap water that would normally feed salmon-bearing streams. Those streams are also threatened by intense diversion of water by marijuana growers. Such changes have implications for drought survival and biodiversity, Dietrich says. “We feel a responsibility to sort out where the different tree types get water and how changes in the forest can change water use.”

Geophysicist Steven Holbrook of Virginia Tech has conducted research at several US CZOs. He and his team measure electrical resistivity and seismic velocity in the regolith. A student swings a sledgehammer at a metal plate on the ground. The more porous the subsurface, the slower the sound travels to a nearby array of seismographs. High velocities indicate bedrock and give the regolith thickness. The team found two thickness patterns and tied them to tectonic stress levels. Where compressive stress is high, the bedrock is shallow beneath stream valleys and deeper beneath ridges, so that the base of the regolith mirrors surface topography. Where stress is low, bedrock lies at a constant depth, so that the regolith forms an even crust beneath topography. “It was a jaw-dropping Eureka moment to see that critical-zone structure might be related to large-scale tectonic stress,” Holbrook says.

Pamela Sullivan is an assistant professor in geography and atmospheric science at the University of Kansas. While a postdoc at Shale Hills, she wanted to address the questions, “As we change the surface, how does it alter the subsurface, and what does it mean for nutrient availability and soil formation in the future?” As a proxy for climate change, she used variations in solar radiation. She compared rock weathering in two hill slopes that naturally soak up different amounts of sunlight and found that an increase of 0.8 °C in soil temperature enhances the chemical breakdown rate of silicate minerals by up to 13%. Given that plant nutrients such as potassium are stored in silicate minerals, “a warming climate may mean a faster loss of important nutrients,” she says.

In the Ploemeur fractured rock observatory in Brittany, France, Tanguy Le Borgne and colleagues work on the deep critical zone, to depths of 200 m. They inject fluorescent dyes, salt, heat, and dissolved gases as tracers into boreholes to quantify mixing rates and residence times in the subsurface. They also track subsurface temperatures using optical fibers: A laser is shined into the fiber, and because Raman scattering is temperature-dependent, the output signals can be fit to models. Whereas previously they may have had 10 sensors and gathered samples once a month, now the data are nearly continuous. Temperature, Le Borgne explains, gives information about the distribution of water flow and chemical transport.

Among their findings is the existence of deep hot spots of microorganism activity. “Fractures can create unexpected mixing between nutrients provided by recently infiltrated water and dissolved minerals from old groundwater,” says Le Borgne. Before the creation of the critical-zone observatory, he says, “I studied flow in fractures. Someone else studied biogeochemistry and microbes. Now we are working together.” The missions of many CZOs are completely determined by their local features, which are highly heterogeneous, he says. “The challenge is to go beyond this local variability to make observations that are relevant globally.”

Dust is important everywhere, and everywhere has a dust story, says Brantley. In Pennsylvania, dust is enhanced in manganese that appears to originate from the Industrial Revolution. Dust at the Luquillo CZO is deposited from the Sahara. The Sierras have dust from the Gobi Desert and from California’s Central Valley. Dust enriches soils with nutrients, feeds forests, and affects clouds and the penetration to Earth of solar radiation.

Nikolaidis and colleagues have run a multiyear experiment with tomato crops at their CZO on Crete. By adding carbon to enhance soil fertility, he says, they see a 60% increase in tomato yield compared with typical commercial yields.

At some CZOs, natural disasters have become natural experiments. According to Chorover of the Catalina-Jemez CZO, almost all the long-term erosion at the observatory takes place in the years immediately following a wildfire. That understanding, he says, was derived from a combination of before-and-after lidar landscape images and measurements of beryllium-10. Concentrations of the isotope, which forms when cosmic rays bombard rock minerals, provide a measure of erosion rate. And after the devastating 2013 Colorado Front Range flood, Boulder Creek researchers led by Suzanne Anderson studied landslides and debris flow. “We found that in the course of a few minutes, the hill slopes lost sediment produced by 300–400 years of weathering,” she says.

At the Luquillo site in Puerto Rico, researchers are reorganizing their priorities in the aftermath of Hurricane Maria, which made landfall on the island on 20 September 2017. They are monitoring greenhouse gases in the air and solutes and organic matter in streams and soil to see the effects of the hurricane. The breakdown by microbes of the pileup of organic matter—leaves, branches, dead tree roots—could accelerate the production of carbon dioxide, methane, and nitrous oxide, explains William McDowell, principal investigator at the Luquillo CZO and director of the New Hampshire Water Resources Research Center at the University of New Hampshire.

Flash floods, like this one at the Koiliaris Critical Zone Observatory on Crete, are an important part of the surface-water budget in the fractured karst limestone bedrock of the Mediterranean region. Although this tributary floods only three or four times a year, flash floods collectively carry up to 70% of its suspended sediments and many pollutants to sea.

Flash floods, like this one at the Koiliaris Critical Zone Observatory on Crete, are an important part of the surface-water budget in the fractured karst limestone bedrock of the Mediterranean region. Although this tributary floods only three or four times a year, flash floods collectively carry up to 70% of its suspended sediments and many pollutants to sea.

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They also plan to compare lidar images from before and after the hurricane for insights into erosion and the resilience of vegetation. “With all the trees knocked down, we want to quantify how much biomass is no longer in the canopy,” says McDowell. As the landscape recovers, they will look to see what controls sediment production. “That is part of what drives the availability of drinking water for this island,” he says.

The Southern Sierra CZO is located in an area that just came off a four-year drought that killed millions of trees. “Why did we have 100% conifer mortality in some places?” asks Roger Bales, a hydrologist from the University of California, Merced, who heads the CZO. Droughts and fires in California are par for the course, but the state’s forests would have survived better had they had fewer trees, he says. “We measure snowpack and subsurface water, the flux of snowmelt, the water going in and out of the soil and regolith, and evapotranspiration. We are looking at the spatial balance of water in the regolith to develop predictions, and we are scaling to the entire Sierra Nevada.”

Forest and water management “are pressing questions for California,” says Bales. “I feel good that we are getting our science results into policy and that California wants to listen.”

Critical-zone science has come into its own, says Gordon Grant, a hydrologist and geomorphologist with the US Forest Service in Oregon and chair of the CZO scientific steering committee. “It lifts the hood on a previously cryptic environment, revealing things that seem paradoxical,” he says. “For example, water flows differently than we previously thought.”

So far the US has been a leader in promoting the critical-zone concept, Grant notes, but it’s uncertain whether that will continue. Many environments—such as karst, permafrost, and cities—are not well represented by the US CZOs, yet the future funding of even the existing observatories is unclear and NSF’s silence has researchers nervous.

Funding runs out this fall, which makes planning and keeping employees on-site nerve wracking, says Boulder Creek’s Anderson. “It’s a lot to ask people to carry on with their jobs.” And some researchers can’t help wondering whether the Trump administration’s denial of climate change could trickle down to them as a loss in funding. But Richard Yuretich, program manager for integrated activities in NSF’s division of Earth sciences, says that’s not a factor.

The agency hasn’t yet put out a new call for proposals because of uncertainties in its overall budget, Yuretich says. But the CZO program “came out very positive” in an extensive evaluation last year. “We have a plan to continue critical-zone research and observatories,” he says. “We want to make some changes in how the program is run, but there is nothing official yet.” Among other considerations, he says, the agency is discussing ways of broadening participation in the CZOs. “They have spawned an enormous cadre of mostly young scientists who are thinking about the Earth’s surface in a different way. As these newer researchers move on to [other institutions], they don’t have a CZO. We are looking at ways to encourage their participation without replicating CZOs.”

Yuretich is hopeful that a call for proposals will come out by summer.