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

Abstract

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 observatoriesPublished by Copernicus Publications on behalf of the European Geosciences Union. 842 S. L. Brantley et al.: Designing a network of critical zone 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.

OZCAR: The French Network of Critical Zone Observatories

“…Socio‐ecosystems are typically the setting of the Long Term Socio‐Ecological Research (LTSER) observatories (Haase et al, 2018). This organization in clusters is also close to the “hub‐and‐spoke” topology proposed by Brantley et al (2017) in the United States. A hub is a highly instrumented CZO (essentially a river catchment) in which the broader common metrics of measurements have been defined and which is connected to “satellite” sites focused on a particular compartment of the CZ and in which fewer parameters are monitored.…”

“…So far there is no “official” definition for how a CZO should be designed. Multidisciplinary and systemic approaches (“the CZ as an entity”, Brantley et al, 2017) seem to be common denominators of all the so‐called CZOs. In the United States, CZOs were first established in 2007 (Anderson et al, 2008; White et al, 2015) and presently feature nine instrumented sites, generally river catchments or a whole landscape of limited size (Brantley et al, 2017).…”

Hillslope Hydrology in Global Change Research and Earth System Modeling

“…Field instruments are traditionally aligned with, and hillslope‐ and catchment‐scale models are built to capture, such fundamental gradients across the landscape (e.g., Angermann et al, ; Band, , ; Band et al, 1993, ; Bergstrom et al, ; Duffy, ; Dunne & Black, ; Ebel et al, ; Hwang et al, ; McDonnell, ; McGuire & McDonnell, ; Montgomery et al, ; Nippgen et al, ; Staudinger et al, ; Wilson & Dietrich, ). Recently, Critical Zone (CZ) science—the study of the structure, function, and evolution of the soil and underlying regolith (the weathered mantle overlying fresh bedrock) that support terrestrial life (e.g., Brantley et al, ; Brantley, McDowell, et al, )—has catalyzed an integrative, system‐level approach toward understanding the coevolution of the landscape and terrestrial life, bringing together diverse perspectives from the broader Earth Surface Processes community.…”

“…Over the past six decades or so, hillslope and catchment research has produced prodigious field observations, theories, and models, at individual research sites and across organized research networks including the long‐term ecological research network and the US Department of Agriculture Agricultural Research Service and Forest Service experimental watersheds (e.g., Stringer et al, ). The nascent international network of Critical Zone Observatories (CZOs) is laying the foundation for cross‐site syntheses (e.g., Baatz et al, ; Brantley, McDowell, et al, ; Paola et al, ), making possible a global assessment of the most salient structures and functions of hillslope processes that may matter to large‐scale fluxes in ESMs. Thus, catchment and CZ scientists, as a community, can offer deep insights into the lay of the land, and we hope to tap into their collective wisdom so that we can identify the most critical processes to implement in ESMs.…”

Ideas and perspectives: Proposed best practices for collaboration at cross-disciplinary observatories

“…Earth's CZ is the thin near-surface zone spanning from bedrock to the atmospheric boundary layer (National Research Council, 2001;Brantley et al, 2007). Since the mid-2000s, scientists have been viewing this zone through a new interdisciplinary lens that brings together biology, soil science, geology, hydrology, and meteorology to make co-located measurements of chemical and biological transport and transformation that describe past landscape evolution and improve projections of future conditions (Brantley et al, 2017;Sullivan et al, 2019). This interdisciplinary approach has precipitated important insights that link hydrology, weathering rates, soil characteristics, nutrient availability, microbial processes, land-air fluxes, and plant dynamics (e.g., Hahm et al, 2014;Richter and Billings, 2015).…”