Introduction to the Symposium “Progress in Radiation and Energy Balance Measurement Systems”

O 2004, an all-day symposium entitled “Progress in Radiation and Energy Balance Measurement Systems” was convened at the ASA–CSSA–SSSA annual meetings in Seattle, WA. The symposium was sponsored by Division A-3 (Agroclimatology and Agronomic Modeling) with cosponsorship from Division C-2 (Crop Physiology and Metabolism), Division S-1 (Soil Physics), Campbell Scientific, and Decagon Devices. Dr. Carl Bernacchi presided over the symposium, which included 13 invited oral presentations on a far-ranging suite of topics spanning spatial scales from millimeters to kilometers. From these presentations, the following seven papers have been prepared for publication as a symposium in Agronomy Journal. A sincere debt of gratitude is owed to Dr. Robert Lascano, who served as the interim Technical Editor for the manuscripts, and to a team of anonymous reviewers for their efforts in preparing these papers for publication. Interest in the measurement of radiation and energy balance components at soil and plant canopy surfaces has seen a resurgence in recent years. This interest is partly due to increasing public awareness of potential global climate change impacts and related research initiatives (Ramanathan et al., 1989; Kiehl and Trenberth, 1997; Linderholm, 2006). Marked advances in sensitivity, signal processing capabilities, and affordability of surface flux instrumentation have also led to increased numbers and distribution of measurement systems. This has not only enabled integration of surface flux information into formal and informal networks (e.g., FLUXNET, AmeriFlux, EUROFLUX, etc.) but also exposed important issues regarding sensor calibration and standardization of measurement and data analysis protocols. Much effort has been devoted to the eddy covariance technique, which has become, arguably, the de facto standard for quantifying evapotranspiration and many scalar fluxes at surface stations (Leuning and Moncrieff, 1990; Baldocchi, 2003). Nonetheless, both theoretical (density effects, frequency response, advection, heat storage) and practical (uneven terrain, limited fetch) problems remain with this technique. Hatfield et al. (p. 285, this issue) illustrate some of the capabilities and limitations of the eddy covariance technique for determining energy and CO2 fluxes and their variation over space and time. Particularly vexing is the persistent inability to achieve energy balance closure when eddy covariance sensors are coupled with standard net radiation and soil heat flux sensors (Wilson et al., 2002; Richardson et al., 2006). Sophisticated correction techniques have been developed to deal with closure problems, but it has become clear that significant errors are also associated with the measurement of net radiation and soil heat flux and these cannot be ignored (Halldin and Lindroth, 1992; Mayocchi and Bristow, 1995; Sauer et al., 2003). Central issues for accurate measurement of net radiation and soil heat flux are the lack of standardized sensor calibration procedures (Fritschen and Fritschen, p. 297, this issue) and spatial heterogeneity of these fluxes at a scale much smaller than the eddy covariance footprint. The standard heat flux plate technique has also come under scrutiny as Sauer et al. (p. 304, this issue) address heat flow distortion and thermal contact resistance errors of flux plates and Ochsner et al. (p. 311, this issue) compared approaches to the calculation of heat storage in the soil above the flux plates. Remote sensing technologies are increasingly being used to bridge the gap that invariably exists between the local scale (at which most agricultural meteorological measurements are made) and the much more heterogeneous, field or regional scales (toward which most applications are aimed). As an example, new applications for older technologies such as the scanning Raman Lidar (Light Detection And Ranging) system described by Cooper et al. (p. 272, this issue) and Eichinger et al. (p. 255, this issue) provide a means to characterize 3-dimensional plumes of water vapor and some scalars (particulates and ammonia) above fields and farms where only point measurements were possible before. Likewise, Anderson et al. (p. 240, this issue) used thermal and visible/NIR imagery from different satellites along with a land–atmosphere–transfer modeling approach to upscale discrete tower and aircraft transect data to regional scales. Innovative application of these types of techniques will provide insight into complex source/sink relationships and optimal placement for point sensor systems in heterogeneous landscapes. This symposium was designed to not only catalog advancements in sensor technology and data interpretation but to also address areas for concern and challenges that still lie ahead. The presenters did an excellent job of encouraging the audience to re-evaluate current measureT.J. Sauer , USDA-ARS, National Soil Tilth Lab., 2150 Pammel Drive, Ames, IA 50011; and P.J. Pinter, Jr., USDA-ARS, U.S. Arid Lands Agricultural Res. Ctr., 21881 North Cardon Lane, Maricopa, AZ 85239. Received 3 Nov. 2006. *Corresponding author (sauer@nstl.gov).