Measurement of Fluxes Over Land: Capabilities, Origins, and Remaining Challenges
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Measurement of Fluxes Over Land: Capabilities, Origins, and Remaining Challenges Bruce B. Hicks1
· Dennis D. Baldocchi2
Received: 11 February 2020 / Accepted: 12 May 2020 © Springer Nature B.V. 2020
Abstract In the first half of the twentieth century the measurement of rates of exchange of momentum, heat, and especially water between the terrestrial surface and the atmosphere was an elusive challenge. A wide variety of methods evolved, ranging from reliance on drag plates, evaporation pans, and weighed lysimeters to a variety of methods based on measurement of vertical gradients in the air. None of these provided great confidence in the results. The promise of direct measurement by eddy covariance was held at bay by the lack of suitable fast-response instrumentation and the analytical requirement to average the products of pairs of these fast-response signals. Developments starting in the 1950s have now largely solved the problems that previously limited eddy covariance. Here, the history of flux measurement is summarized, and the path to now-standard capabilities is explored. The currently active major international programs making use of the modern instrumentation are addressed (with the focus on CO2 and CH4 , as befitting global concerns regarding climate change), with particular attention to the emerging need to revisit the fetch and footprint constraints that confronted early workers. Finally, some areas of continuing uncertainty are mentioned, with an implicit request for increased experimental attention. Keywords Carbon dioxide · Eddy covariance · Energy budget · Gradient methods · Methane
1 Introduction For the last fifty years, Boundary-Layer Meteorology has been a favoured outlet for reports on developments relating to understanding of air–surface interaction and the characteristics of the surface boundary layer. These reports have covered the spectrum from instrumentation to theory. Here, it is the role of Boundary-Layer Meteorology in the evolution of methods for conducting experiments that will be addressed. The focus will be on the development of methods to measure the fluxes of importance, initially the sensible and latent heat fluxes
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Bruce B. Hicks [email protected]
1
Metcorps, P.O. Box 1510, Norris, TN 37828, USA
2
Department of Environmental Science, Policy and Management, University of California, Berkeley, CA 94720-3110, USA
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B. B. Hicks, D. D. Baldocchi
contributing to the surface heat budget (H and L w E) and the momentum flux (τ ). As time progressed, the application of the flux-measuring technologies expanded to matters of air quality and climate relevance, and in all of these Boundary-Layer Meteorology has played a pivotal part. Here, the development of flux-measuring methods will be reviewed, with the central role of Boundary-Layer Meteorology in mind. The discussion will be arranged to provide evidence of current research trends and anticipated future needs, recommendations for which the present text is concluded. The developments occurring during the period of B
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