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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.

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Category: Science