The Critical Zone is defined as the permeable layer from the top of the vegetation canopy to the bottom of freely circulating groundwater where rock, soil, water, air and life meet. The structure and function of the Critical Zone (CZ) has evolved in response to climatic and tectonic perturbations throughout Earth history. CZ science is motivated by an overall lack of understanding of the coupled physical, chemical, and biological processes in the zone at differing spatial and temporal scales. Degradation and impacts on the CZ by human activities cause further complications, and challenge the scientific community’s ability to predict the response of CZ attributes and processes to projected climate and land-use changes.
The Critical Zone Observatories (CZOs), supported by the U.S. National Science Foundation’s Geosciences Directorate, aim to provide infrastructure, data and models to gain understanding of the evolution and function of the CZ from grain-to-watershed scales. Nine observatories span a range of climatic, ecologic, geologic, and physiographic environments from California to Puerto Rico, working on site-specific hypotheses and network-scale goals. Each observatory strives to apply common infrastructure, protocols and measurements that help quantify the composition and fluxes of energy, water, solutes, sediments, energy, and mass across boundaries of the CZ system through both space and time. This type of approach enables researchers to access and integrate data in a way that allows for the isolation of environmental variables and comparison of processes and responses across environmental gradients.
Cross-CZO science emerges from a set of common CZ science questions and hypotheses focused on CZ structure and evolution, event-based and continuous fluxes across CZ interfaces, and changes in storage of major CZ reservoirs at the catchment scale. The current capacity for each CZO to quantify these processes is based on the number of observatories posing the same question AND which observatories have the infrastructure in place to address that question. Each observatory’s status with respect to a cross-site science question and common measurement infrastructure to address that question has been weighed against one another and used to determine that the CZO network is currently best set to address the following questions: (i) how does CZ evolution depend on lithology? (ii) how does vegetation influence critical zone processes? A series of eight workshops have been convened during summer and fall 2015 to develop and refine cross-site science questions and the common measurements required to address those questions. The results of these workshops may be cast within the five core research areas of the Long-Term Ecological Research Network (LTER) to form the basis of ongoing collaborations between the CZOs and LTER network.