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Quality Assurance wizards Andres Schmidt and James Kathilankal with
a portable eddy covariance system.

Quality Assurance and Control

Goals

    Build cohesive network of research sites to quantify and understand carbon sources and sinks and the response of terrestrial ecosystems to climate and disturbance.

Objectives

    The major objective of this project is to contribute to the AmeriFlux network by continuing to build consistency in AmeriFlux measurements by addressing objectives stated in the AmeriFlux strategic plan and self evaluation, the North American Carbon Program, and the US Carbon Cycle Science Program. The project directly contributes to NACP and CCSP goals to establish an integrated, near-real time network of observations to inform climate change science.

 

AmeriFlux is a network of 92 long-term research sites across North America measuring the exchange of water vapor, sensible heat, and carbon fluxes between the atmosphere and biosphere using the eddy covariance technique. As with every precision measurement endeavor, there are sources of systematic and random error resulting from variances in hardware, software, and the instrument installation. To reduce and quantify these uncertainties, the AmeriFlux quality assurance and quality control (QA/QC) laboratory was created to enhance data quality and ensure consistency in eddy covariance measurements within and among sites.

The primary activities of the AmeriFlux QA/QC laboratory involve the use of a portable eddy covariance system (PECS). This system includes all necessary hardware and software to make eddy covariance measurements as well as other meteorological data. During the active growing season the PECS is transported to selected AmeriFlux sites, set up next to the existing EC systems, and 5 daysí worth of data are collected and compared. The goal of site comparisons is to identify systematic errors due to instrument bias, malfunctioning hardware, outdated calibrations, and software. We identify errors due solely to different software routines by asking each site to process an independent raw data file developed by the Euroflux and AmeriFlux networks. Concerns regarding the installation strategy of permanent instruments may be discussed during site comparisons, but the ultimate decision on instrument configuration and site representativeness is left to the principal investigators.

The specific project objectives were i) conduct 10-20 robust site comparisons each year, ii) separate uncertainties associated with site instrumentation and process software through the use of "Gold Files", iii) continually enhance and enforce the flux measurement guidelines, iv) develop and disseminate the current suite of roving standards to assist sites in maintaining internal QA/QC protocols, v) report findings to the AmeriFlux Science Chair and Steering Committee, vi) lead and develop synthesis activities across the network, vii) serve as a resource for measurement strategies among the network and other ad hoc, coordinated QA/QC activities, and viii) handle logistics of the annual AmeriFlux Meeting.

In addition, we continued to address four specific recommendations made in the AmeriFlux internal evaluation report 2005 (and supported by the AmeriFlux steering committee and the TCCRP final report, Running et al. 2005): i) Provided additional roving temperature standards to each research group for calibrating their systems (High Priority); ii) provided a photosynthetic photon flux density (PPFD) sensor to each research group as a standard that is kept in the site lab, maintained annually by the AmeriFlux QA/QC lab, and maintain all calibration records in a our repository (High Priority); iii) provide all research groups not currently using the World Meteorological Organization-NOAA Global Monitoring division (WMO-GMD, formally CMDL) standards with (2) Ďarchivalí CO2 standards approximately 10 ppm apart in concentration within the range of ambient concentrations, and with an ensemble uncertainty of +/- 0.12 ppm, (High Priority), and iv) explored an appropriate means to automate H2O calibrations for all IRGAs (High Priority).

Our primary activity was comparison of data from the AmeriFlux portable EC system (PECS) and individual AmeriFlux/NACP sites to achieve the QA/QC goals. We prioritized selection of sites we would visit each year by i) existing sites that had not been evaluated for more than three years; ii) sites located in the mid-continental US in support of the NACP mid-continent intensive; iii) recently established sites that were initiated as part of the NACP, iv) sites where more than three years have passed since the last comparison. We made the NACP regional projects a high priority to ensure coherency of flux site data quality within each region and reduce uncertainty in the synthesis of flux and meteorological data within each region.

Our instruments were calibrated in our QA lab between the site visits. We derived our own IRGA polynomials rather than use those provided by Li-Cor, which helped us to achieve higher accuracy and precision (see Ocheltree 2007 for a detailed description). To ensure rapid turnaround of each comparison report, we informed PIs that we will only visit sites that agree a priori to provide data within two weeks of the site visit. We also encouraged the PIs to provide processed data while the site visit was underway to allow us to perform preliminary analyses. This improved the quality and amount of data we had available for analysis after the site visit.

We communicate directly with individual site PIs through the process of evaluating their data quality, and iterate until we feel they address issues we found with their measurements or data processing. General communication with AmeriFlux investigators is conducted through the AmeriFlux list server, conferences and development of web-based materials for the AmeriFlux web site at Oak Ridge National Lab.

 

Terrestrial Ecosystems Research & Regional Analysis - Pacific Northwest
Oregon State University, Corvallis, OR 97331
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