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  • Author or Editor: H. W. Loescher x
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T. W. Ocheltree and H. W. Loescher

Abstract

The AmeriFlux network continues to improve the understanding of carbon, water, and energy fluxes across temporal and spatial scales. The network includes ∼120 research sites that contribute to the understanding of processes within and among ecosystems. To improve the networks ability and confidence to synthesize data across multiple sites, the AmeriFlux quality assurance and quality control laboratory was established to reduce the within- and among-site uncertainties. This paper outlines the design of the portable eddy covariance system (PECS) and subsequent data processing procedures used for site comparisons. Because the PECS makes precision measurements of atmospheric CO2, the authors also present the results of uncertainty analyses in determining the polynomials for an infrared gas analyzer, estimating the CO2 in secondary standards, and estimating ambient CO2 in field measurements. Under field conditions, drift in the measurement of CO2 increased the uncertainty in flux measurements across 7 days by 5% and was not dependent on the magnitude or direction of the flux. The maximum relative flux measurement error for unstable conditions was 10.03 μmol CO2 m−2 s−1.

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H. W. Loescher, C. V. Hanson, and T. W. Ocheltree

Abstract

Numerous agencies, programs, and national networks are focused on improving understanding of water and energy fluxes across temporal and spatial scales and on enhancing confidence to synthesize data across multiple sites. Enhancing the accuracy and precision in the surface energy balance and the latent energy (λE) flux lies, in part, with being able to uniformly calibrate water vapor measurements at and among sites to traceable standards. This paper examines (i) the traceable physical controls on field applications of chilled-mirror hygrometers and (ii) an automated means to accurately and precisely calibrate infrared gas analyzers for water vapor concentrations and eddy covariance (λE) data. The environmental physics and gas handling were examined in a theoretical and applied manner that found that chilled-mirror technologies can be a robust measure of dewpoint temperatures and ambient water vapor only if the unit conversions are accounted for between inlet and body temperatures. Psychrometers were also examined and a functional relationship (exponential) was developed for the psychrometric constant against the wet-bulb temperature depression (T dbT wb), across a wider range of temperature depressions than previously reported. These empirical estimates of the psychrometer constant for small temperature depressions are much lower than other reported values—that is, ∼0.000 52 K−1 for a wet-bulb temperature depression (T dbT wb) of 4.3 K.

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H. W. Loescher, G. Starr, T. A. Martin, M. Binford, and H. L. Gholz

Abstract

The daytime net ecosystem exchange of CO2 (NEE) was measured in an even-aged slash pine plantation in northern Florida from 1999 to 2001 using the eddy covariance technique. In August 2000, two clear-cuts were formed approximately 1 km west of the study site. A statistical approach was used to determine whether the clear-cuts induced changes in CO2 concentration, wind direction, horizontal and vertical wind speeds, and temperature, as measured by instruments above the plantation canopy and, in turn, whether any such changes affected daytime NEE. The NEE estimates were first transformed so that mean responses to incident radiation and vapor pressure deficit were removed using empirically derived functions for each 30-min period. Prior to the clear-cuts, there were significant interactive effects of CO2 concentration and some wind statistics on NEE at the tower when wind was flowing from the direction of the future clear-cuts. Even in this relatively homogenous forest, with flat topography, the CO2 source strength differed with wind direction prior to the clear-cuts. After the clear-cuts, additional two- and three-way interactive effects became significant during flows from the direction of the clear-cuts. There was also a 16.6% reduction in the integrated measure of daytime NEE over 487 days after the clear-cuts. The results herein suggest that the development of local circulations over the clear-cuts contributed to low-frequency effects on the NEE.

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H. W. Loescher, J. M. Jacobs, O. Wendroth, D. A. Robinson, G. S. Poulos, K. McGuire, P. Reed, B. P. Mohanty, J. B. Shanley, and W. Krajewski

The Consortium of Universities for the Advancement of Hydrologic Sciences, Inc., established the Hydrologic Measurement Facility to transform watershed-scale hydrologic research by facilitating access to advanced instrumentation and expertise that would not otherwise be available to individual investigators. We outline a committee-based process that determined which suites of instrumentation best fit the needs of the hydrological science community and a proposed mechanism for the governance and distribution of these sensors. Here, we also focus on how these proposed suites of instrumentation can be used to address key scientific challenges, including scaling water cycle science in time and space, broadening the scope of individual subdisciplines of water cycle science, and developing mechanistic linkages among these subdisciplines and spatiotemporal scales.

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