Introduction to the Critical Zone

Document Type

Contribution to Book

Publication Date


Publication Title

Developments in Earth Surface Processes



First Page



Critical Zone, Critical Zone Observatories, Regolith, Soil, Water

Last Page



The National Research Council (NRC, 2001, p. 2) defined the Critical Zone as ". the heterogeneous, near surface environment in which complex interactions involving rock, soil, water, air and living organisms regulate the natural habitat and determine availability of life sustaining resources." From this original definition, many, now loosely worded, definitions have been crafted to define the limits of this zone as ranging from the top of the canopy layer down to the bottom of the aquifer, so that the Critical Zone includes all the upper zone of Earth sensu lato. The term Critical Zone, referring to this near-surface and surface zone, was first introduced by Gail Ashley in 1988 to recognize that soil connects the vegetation canopy to the soil; the soil connects to the weathered materials and the weathered materials connect to bedrock, and bedrock provides the connection to the aquifer. In recognition of the importance of this cause-and-effect relationship between previously unconnected spheres, the US National Science Foundation (NSF) established 10 Critical Zone Observatories (CZOs) supported by a national office. This was followed by the establishment of Soil Transformations in European Catchments (SoilTrEC) by the consortium of European Union members. Today there are 64 CZOs spread across the planet but within a narrow range of biophysical environments. The raison d'être of the Critical Zone network is driven by basic principles of science: all the research at each location is focused on asking fundamental integrated biophysical questions and collecting and building long-term data banks from a well-studied environment. In this respect, one of the most important applied aspects of the global CZO network will be the development of reliable data that will lead to enlightened policy and management of the geoscience base of Earth. The name "Critical Zone" has become a fashionable term, but very little, truly integrated work occurs at the CZOs, which we argue will only be possible if it recognized that the real thread that connects all the components of the Critical Zone is water. That said, we think the Critical Zone concept is still a step in the right direction to serve as a unifying principle for the geosciences and an opportunity for truly integrative research of the biophysical environment and the role of humans within that environment. © 2015 Elsevier B.V.