BOX 2.5

Critical Zone Observatories and Critical Zone Exploration Network

The Critical Zone Observatories (CZOs) are natural laboratories built around watersheds or groups of watersheds that are investigated by interdisciplinary teams of National Science Foundation (NSF)-funded ecologists, geochemists, geologists, geomorphologists, hydrologists, and soil scientists using field, laboratory, and modeling approaches. Based on the initiatives put forward in a document “Frontiers in Exploration of the Critical Zone” (Brantley et al., 2006) as well as discussions with the Consortium of Universities for Advancement of Hydrologic Science, Inc. (CUAHSI),a the first three CZOs were funded from the Division of Earth Sciences (EAR) within the NSF Geosciences Directorate. These three CZOs received five-year grants beginning in 2007 to investigate chemical, physical, and biological processes within the natural laboratories. The funding is being used to emplace instrumentation, collect data, and use models to answer forefront questions at the crossroads of hydrology, geomorphology, soil science, geophysics, ecology, and geochemistry. Implicit to the goals of the CZO program is recognition of the contribution that Earth scientists make toward understanding the Critical Zone.

The three CZOs represent sites that vary in some attributes; for example, two sites lie on granitic material (one in the Sierra foothills of California and one in Colorado) and one on shale (in Pennsylvania). Furthermore, each of the three CZOs is designed around a different set of questions: the Southern Sierra CZO is studying how a change from snow- to rain-dominated climate is impacting hydrology and biogeochemistry in the CZO; the Boulder Creek CZO is studying the influence of differing erosion histories on Critical Zone architecture and function; the Susquehanna-Shale Hills CZO is measuring water, energy, and solute budgets from the water table through the atmospheric boundary layer with particular focus on rates of regolith formation. Three new CZOs have been funded in 2009.

Although each CZO is different, integrative modeling efforts are going on at both the intra-site and the cross-site levels. For example, at the Sierra CZO, models are addressing how the snow-rain transition affects the Critical Zone across an elevation gradient. Similarly, researchers at all three CZOs are using cosmogenic nuclides and models to quantify the rates and mechanisms of the bedrock-to-regolith transformation. As part of this latter initiative, data from the CZOs are being compared to data from seed sites within the Critical Zone Exploration Network (CZEN) as well as to the six satellite sites associated with the Pennsylvania CZO to investigate how variations in temperature and precipitation affect regolith formation

Aspects of this challenge have been addressed—though as yet incompletely. The Critical Zone Observatories (CZOs) will add considerably to this knowledge (Box 2.5).

Water chemistry can be monitored intensively, but the chemical and material properties of the soil and bedrock that influence this chemistry remain technically challenging to document, especially at larger spatial scales. Models for evolution of soil chemistry have been developed but are limited by our lack of knowledge with respect to (1) the kinetics of chemical reactions in nature; (2) fundamental thermodynamic equilibrium constants;



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