Articles | Volume 5, issue 4
Earth Surf. Dynam., 5, 841–860, 2017
Earth Surf. Dynam., 5, 841–860, 2017

Research article 18 Dec 2017

Research article | 18 Dec 2017

Designing a network of critical zone observatories to explore the living skin of the terrestrial Earth

Susan L. Brantley1, William H. McDowell2, William E. Dietrich3, Timothy S. White1, Praveen Kumar4, Suzanne P. Anderson5, Jon Chorover6, Kathleen Ann Lohse7, Roger C. Bales8, Daniel D. Richter9, Gordon Grant10, and Jérôme Gaillardet11 Susan L. Brantley et al.
  • 1Earth and Environmental Systems Institute, Department of Geosciences, The Pennsylvania State University, Univ. Park, PA 16802, USA
  • 2Department of Natural Resources and the Environment, University of New Hampshire, Durham, NH, 03824, USA
  • 3Department of Earth and Planetary Science, University of California, Berkeley, CA 94720, USA
  • 4Department of Civil and Environmental Engineering, and Department of Atmospheric Science, University of Illinois, Urbana, Illinois 61801, USA
  • 5Department of Geography, Institute of Arctic and Alpine Research (INSTAAR), University of Colorado, CO 80309-0450, USA
  • 6Department of Soil, Water and Environmental Science, University of Arizona, Tucson, AZ 85750, USA
  • 7Department of Biological Sciences, Idaho State University, Pocatello, ID 83209, USA
  • 8Sierra Nevada Research Institute and School of Engineering, University of California, Merced 94530, USA
  • 9Nicholas School of the Environment, Duke University, Durham, North Carolina 27708, USA
  • 10Pacific Northwest Research Station, USDA Forest Service, Corvallis, OR 97331, USA
  • 11Institut de Physique du Globe de Paris, Sorbonne Paris Cité, CNRS, Paris, France

Abstract. The critical zone (CZ), the dynamic living skin of the Earth, extends from the top of the vegetative canopy through the soil and down to fresh bedrock and the bottom of the groundwater. All humans live in and depend on the CZ. This zone has three co-evolving surfaces: the top of the vegetative canopy, the ground surface, and a deep subsurface below which Earth's materials are unweathered. The network of nine CZ observatories supported by the US National Science Foundation has made advances in three broad areas of CZ research relating to the co-evolving surfaces. First, monitoring has revealed how natural and anthropogenic inputs at the vegetation canopy and ground surface cause subsurface responses in water, regolith structure, minerals, and biotic activity to considerable depths. This response, in turn, impacts aboveground biota and climate. Second, drilling and geophysical imaging now reveal how the deep subsurface of the CZ varies across landscapes, which in turn influences aboveground ecosystems. Third, several new mechanistic models now provide quantitative predictions of the spatial structure of the subsurface of the CZ.
Many countries fund critical zone observatories (CZOs) to measure the fluxes of solutes, water, energy, gases, and sediments in the CZ and some relate these observations to the histories of those fluxes recorded in landforms, biota, soils, sediments, and rocks. Each US observatory has succeeded in (i) synthesizing research across disciplines into convergent approaches; (ii) providing long-term measurements to compare across sites; (iii) testing and developing models; (iv) collecting and measuring baseline data for comparison to catastrophic events; (v) stimulating new process-based hypotheses; (vi) catalyzing development of new techniques and instrumentation; (vii) informing the public about the CZ; (viii) mentoring students and teaching about emerging multidisciplinary CZ science; and (ix) discovering new insights about the CZ. Many of these activities can only be accomplished with observatories. Here we review the CZO enterprise in the United States and identify how such observatories could operate in the future as a network designed to generate critical scientific insights. Specifically, we recognize the need for the network to study network-level questions, expand the environments under investigation, accommodate both hypothesis testing and monitoring, and involve more stakeholders. We propose a driving question for future CZ science and a hubs-and-campaigns model to address that question and target the CZ as one unit. Only with such integrative efforts will we learn to steward the life-sustaining critical zone now and into the future.

Short summary
The layer known as the critical zone extends from the tree tops to the groundwater. This zone varies globally as a function of land use, climate, and geology. Energy and materials input from the land surface downward impact the subsurface landscape of water, gas, weathered material, and biota – at the same time that differences at depth also impact the superficial landscape. Scientists are designing observatories to understand the critical zone and how it will evolve in the future.