Recalcitrant Problems in Environmental Instrumentation

mental research (e.g., human impacts on water quality, efficiency of agricultural water use, and trace gas imFrontiers in any science are generally defined by measurement pacts on global climate change) similarly contain stublimitations, and that is especially true in environmental biophysics. born instrumentation problems. I have made no attempt Among the more persistent issues are surface–atmosphere exchange, to cover all of them, instead choosing a few that are soil water and solute fluxes, plant water status, and plant/soil nutrient status. Measurement of surface–atmosphere exchange is particularly notable either for their importance or their longevity. critical to global climate change research. Despite advances in instruCoincidentally, most of the issues chosen are among the mentation, accuracy of flux measurements, particularly eddy covarimany problem areas to which Gaylon Campbell has ance, remains unacceptable, partly because the underlying assumpmade seminal contributions. In the arena of surface– tions of stationarity and surface homogeneity are so restrictive. Even atmosphere exchange, he was an early principal in the when these assumptions are valid, the method appears to systematidevelopment of sonic anemometry and open-path hycally underestimate for reasons that are not yet well understood. grometry (Campbell and Unsworth, 1979; Campbell and Similarly, soil water and solute fluxes cannot yet be measured accuTanner, 1985). He and his coworkers have also made rately and routinely, hampering water quality research. Recent adnumerous contributions to the subject of soil water and vances in tension lysimetry offer hope for improvement, but most solute fluxes (e.g., Riha and Campbell, 1985; Campbell, field experiments still rely on modeling of water and solute flow, supported by indirect measurements of ancillary variables, e.g., soil 1985, 1988; Campbell and Anderson, 1998) and to the water content, soil water potential, and solute concentration, at dismeasurement of plant water status (Campbell et al., crete points in time and space. A third area of ongoing concern is 1973, 1979; Rawlins and Campbell, 1986). These efforts that of plant water status. The major uncertainty here concerns which have considerably advanced our understanding, but rouproperty should be measured. Nearly all of the effort over the past 30 tine, accurate determination of these and other environyr has been directed at measuring water potential, but water potential mental variables continues to provide an energizing measurements are equilibrium measurements, and plants operate in challenge for those of us with a passion for measurement dynamic environments. Furthermore, many physiological processes and instrumentation. appear to be more related to relative water content than to water potential. Finally, more accurate and more timely (e.g., in situ) measurements of plant/soil nutrient status are sorely needed to take advanSURFACE–ATMOSPHERE EXCHANGE tage of the promise of precision agriculture. This is the central issue of micrometeorology. Originally, water vapor transport was the primary interest, and its measurement remains important for a variety of A in the understanding of our environment purposes, but the transport of other scalars has assumed cannot proceed too far ahead of our ability to increasing attention in recent years. Particular emphasis measure. Even the rare individual with the Einsteinian has been placed on the exchange of gases such as CO2, intuition to see an explanation without the benefit of methane, and nitrous oxide that absorb appreciable indata must await the development of instruments and frared radiation, since human-induced increases in their their confirming numbers before the concept is generatmospheric concentrations may alter the earth’s energy ally accepted, which can take years or decades. For the balance. Indeed, there are now flux measurement netrest of us, conceptual understanding and measurement works throughout the world devoted to this effort (Balcapability are more tightly coupled. For example, our docchi et al., 1996). Considering the attention and understanding of the full environmental impact of chlomoney that has been devoted to measuring atmospheric rofluorocarbons (CFCs) developed only after the adtransport, it is surprising that it remains poorly resolved. vent of high-accuracy instruments for measuring UV Comparisons with energy balance measurements show radiation and stratospheric ozone concentrations. The mean errors in water vapor transport measurement that underlying concepts regarding CFC reactions with often exceed 20%. For other gases, there is generally ozone were already known, as were the UV-absorbing no check of accuracy available, but presumably their properties of ozone, but the extent of depletion and behavior is no better. its unusual spatial pattern would likely not have been However, the barriers to more accurate measurement guessed, and a global CFC ban would certainly not have of surface exchange processes may be more conceptual occurred, without direct field measurements. Most if than instrumental. The fundamental validity of eddy not all of the issues currently in the forefront of environcovariance has always been unquestioned, in part because of its powerful theoretical simplicity, but perhaps USDA-ARS Soil and Water Manage. Unit, Dep. of Soil, Water, and more because there were no fast-response sensors capaClim., Univ. of Minnesota, St. Paul, MN 55108. Received 20 Nov. ble of rigorously testing and applying it. Now that there 2002. *Corresponding author ([email protected]). are infrared gas analyzers for water vapor and CO2, and tunable diode laser spectroscopy systems for a range of Published in Agron. J. 95:1404–1407 (2003).  American Society of Agronomy 677 S. Segoe Rd., Madison, WI 53711 USA Abbreviations: CFC, chlorofluorocarbon.