Critical Zone Research and Observatories: Current Status and Future Perspectives

Abstract

The Critical Zone (CZ) is the thin layer of the Earth's terrestrial surface and near-surface environment that ranges from the top of the vegetation canopy to the bottom of the weathering zone and plays fundamental roles in sustaining life and humanity. The past few years have seen a number of Critical Zone Observatories (CZOs) being developed following the first CZOs established in the United States in 2007. This update summarizes major research findings in CZ science achieved in the past 5 yr or so (2011)(2012)(2013)(2014)(2015)(2016), especially those obtained from recognized CZOs. A conceptual framework of "deep" science-deep time, deep depth, and deep coupling-is used to synthesize recent CZ studies across a broad range of spatial and temporal scales. This "deep" science concept emphasizes the integration of Earth surface processes that underlies the contributions of CZ science to terrestrial environmental research. We identify some main knowledge gaps and major opportunities to advance the frontiers of CZ science. We advocate that the CZ scientific community work toward a global network of CZOs to link sites, people, ideas, data, models, and tools. We hope that this update can stimulate continuous scientific advancement and practical applications of CZ science worldwide.Abbreviations: CZ, Critical Zone; CZO, Critical Zone Observatory; DOC, dissolved organic carbon; DOM, dissolved organic matter; EEMT, effective energy and mass transfer; ET, evapotranspiration; GPR, ground-penetrating radar; SOC, soil organic carbon; TDR, time-domain reflectometry; WTT, water transmit times.The Earth's Critical Zone (CZ) is defined as the thin layer of the Earth's surface and near-surface terrestrial environment from the top of the vegetation canopy (or atmosphere-vegetation interface) to the bottom of the weathering zone (or freshwater-bedrock interface) (National Research Council, 2001). This zone encompasses the near-surface biosphere, the entire pedosphere, the surface and near-surface portion of the hydrosphere and the atmosphere, and the shallow lithosphere (Lin, 2010). This concept of the CZ provides a unifying framework for integrating belowground-aboveground, abiotic-biotic, and time-space in mass and energy flows to holistically understand complex terrestrial ecosystems and offers a fertile ground for interdisciplinary research (Anderson et al., 2007;Lin et al., 2011). Thus, the integrated study of the CZ has been recognized as one of the most compelling research fields in Earth and environmental sciences in the 21st century (National Research Council, 2001.Environmental processes within the CZ, such as mass and energy exchange, soil formation, streamflow generation, and landscape evolution are crucial to sustaining biodiversity and humanity Field et al., 2015). The CZ supplies nearly every life-sustaining resource on which life originates, evolves, and thrives (National Research Council, 2001;Lin, 2014). This zone provides diverse services to human society and determines human livelihood (Lin, 2014;Field et al., 2...

The global distribution of Earth's critical zone and its controlling factors

“…In addition to the first definition of the critical zone “the outermost layers of the continental crust from the canopy top to the base of the groundwater zone” [ NRC , ], there are also several other definitions. For example, Guo and Lin [] stated that the critical zone is the thin layer of the Earth's terrestrial surface and near surface environment that ranges from the top of the vegetation canopy to the bottom of the weathering zone; Fan [] refers to the critical zone as the upper terrestrial environment from canopy top to the base of active groundwater. Although there are some discrepancies among them, all definitions emphasize the location of the CZ in the “near surface” and the important roles of the critical zone in “sustaining terrestrial lives.” Gleeson et al [] emphasized that storage of fresh groundwater (<50 years old) is a critical component of hydrologic and climatic processes; it is the most recently recharged and also the most vulnerable to global change.…”

“…LiDAR has also been used to detect wet channel distribution [ Hooshyar et al , ] and vegetation structure [ Dubayah et al , ; Saito et al , ; Simard et al , ; Tao et al , ]. However, the CZ extends to depth [ Guo and Lin , ; Richter and Mobley , ], and subsurface characterization is challenging. Therefore, new techniques are urgently needed.…”

Effects of differential hillslope‐scale water retention characteristics on rainfall–runoff response at the Landscape Evolution Observatory

“…LEO is unique in its field due to its fully controlled environment, state‐of‐the‐art measuring equipment, and hillslope‐size scale. Other projects with similar research goals include the Critical Zone Observatory network in the USA (Anderson, Bales, & Duffy, ; Guo & Lin, ), the artificial catchment “Chicken Creek” located in Germany (Gerwin, Raab, Biemelt, Bens, & Hüttl, ; Hofer, Lehmann, Biemelt, Stähli, & Krafczyk, ), and the TERENO program (Bogena et al, ; Zacharias et al, ), also located in Germany. Although these projects are similar in the sense that they also attempt to improve understanding of coupled processes in catchments, they take place at a different spatial scale.…”

The integrated water balance and soil data set of the Rollesbroich hydrological observatory

“…However, the highly heterogeneous pattern of soil water content leading to complex and scale-dependent patterns of water, energy, and matter fluxes makes it challenging to predict terrestrial system responses for both scientists and policymakers (Jaeger and Seneviratne, 2011;Seneviratne et al, 2010). Therefore, integrated observations of soil water content and the exchange of water and heat between the soil, vegetation, and atmosphere are critical to improving our understanding of the terrestrial system response to changes in climatic conditions and land management (Dirnbock et al, 2003;Foley et al, 1998;Hinzman et al, 2005;Refsgaard, 1997;Seneviratne et al, 2010;Guo and Lin, 2016) and serve as key data in validating remote sensing data products (e.g., Rötzer et al, 2014;Cosh et al, 2016).…”