Current Micrometeorological Flux Methodologies with Applications in Agriculture

History has shown that many of the technological advances in micrometeorological measurement methods and techniques were facilitated by agronomic research into understanding plant–water relationships and photosynthesis (Deacon & Swinbank, 1958; Lemon, 1960; Suomi & Tanner, 1960; Tanner, 1960). Even today, many research programs are focusing on a better understanding of the water balance on regional and continental scales with a goal of providing useful probabilistic forecasts that will lead to more effective planning of the use of water resources in areas where water is limited or stricken by drought (Lawford, 1999). The need to understand and predict components of the water and carbon cycle on seasonal and annual time scales has pushed the micrometeorological technology to work not only for short duration intensive experiments, but for much longer periods (Baldocchi et al., 2001; Running et al., 1999; Grelle & Lindroth, 1996).\nMany agricultural research activities include short-term intensive experiments in which the emission or deposition of some chemical species is to be quantified. Along with the more common species like water vapor and CO2, fluxes of other chemical species have become important in agriculture. They include N compounds such as NH3, HNO3, NO, other greenhouse gases such as CH4 and N2O, pollutants such as O3 and SO2, and many pesticides (Majewski et al., 1989). Measured fluxes also are being integrated during the year to compile annual budgets of water and C. Potential biases in the measured fluxes that could lead to potentially large errors in these annual budgets has spurred several efforts to re-examine the methodologies of current flux measurement techniques (Massman & Lee, 2002), and how the data should be processed to insure the highest data integrity (Foken & Wichura, 1995). Although key findings and recommendations from these articles will be discussed here, reading these articles is encouraged.\nHere, we will described the various micrometeorological methodologies that are currently used to determine the vertical turbulent fluxes of water, CO2, and other scalar entities on both short and long time scales, incorporating the latest strategies on flux measurement techniques. There are many well-written historical articles and book chapters on micrometeorological methodologies (Webb, 1965; Kanemasu et al., 1979; Hicks, 1984). The reader is encouraged to read these references as these provide basic information on the fundamental processes of air-surface exchange, which is necessary to appreciate and understand the application of the various measurement methodologies.\nFor many agricultural applications, micrometeorological methods are preferred since they are generally non-intrusive, can be applied on a semicontinuous basis, and provide information about vertical fluxes that are aerially averaged on scales ranging from tens of meters to several kilometers, depending on the roughness of the surface, the height of the instrumentation, and the stability of the atmosphere surface layer. Interpretations of the data obtained from these methods are also constrained by the assumptions of stationarity in the scalar concentration field, horizontal homogeneity of the surface conditions, and relatively flat terrain. These constraints insure that the vertical turbulent flux is due to the scalar source–sink at the surface and not by advective horizontal mean or turbulent fluxes.\nThe most commonly used micrometeorological methods can be separated into four categories. They are: (i) eddy covariance, (ii) flux-gradient, (iii) accumulation, and (iv) mass balance. Each of these techniques are suited for applications that depend on the scalar of interest and surface type, over which the flux is to be measured. For example, the eddy covariance method can only be used for trace species for which fast response instrumentation is available. Likewise, flux gradient or accumulation methods are often used in systems for which the atmospheric concentrations can be determined with a high degree of accuracy using either slow response sensors or accumulation devices such as flasks, canisters, filter packs, or annular denuders.\nThere are other methodologies that have been used to measure fluxes including the variance method (Padro, 1993) and the inertial dissipation technique (Fairall & Larsen, 1986). These methods are usually applied in very special circumstances and cannot be necessarily generalized for all trace gases and environmental conditions. In this chapter, we will discuss the basics and strengths of the four methodologies above. Our intent is not to elaborate in detail on the theoretical derivations and complete historical background of these methods, but to discuss and describe how these can be used to measure fluxes on time scales that are pertinent to research in agriculture. There are many excellent reviews, both current and historical, that provide excellent background material that need not be repeated here.