Bruce Hicksa,b, Joel Oettingc, Neal Eashc, Taqi Razac
a Metcorps, P.O. Box 1510, Norris, TN 37828
b NOAA/ARL/ATDD, Oak Ridge, TN 37830
c University of Tennessee, Biosystems Engineering & Soil Science, Knoxville, TN 37996
The addition of infrared surface temperature measurements to the familiar suite of observations associated with the Bowen Ratio method for quantifying heat fluxes (sensible and latent) from a crop canopy permits a more detailed examination of the energy imbalance than otherwise possible. The Augmented Bowen Ratio Analysis (ABRA) methodology is a statistical approach that allows the energy balance term Imb in the surface heat balance relationship
Rn = H + LwE + G + Imb (1)
to be examined as a function of time of day.
The ABRA method centers around consideration of three variables:
X1 = Rn – G (the energy driving biological as well as physical processes at the surface)
X2 = Δη (the difference in enthalpy between the two heights of measurement — z1 and z2)
X3 = dT0/dt (the rate of change of infrared surface temperature)
Uncertainties in the quantification of the ground heat flux (G) doubtlessly contribute to the uncertainties classically associated with BREB results — G cannot be easily measured, but can be closely estimated using a variety of established techniques. In the present series of analyses, the measurement techniques described by Sauer and Horton ( 2005) have been used. The enthalpy difference is conveniently computed as
Δη = ΔTa˖(1 + 1/β) (2)
where β is the familiar Bowen ratio and ΔTa is the potential temperature difference between the two heights of BREB atmospheric temperature and humidity measurement. It represents the total rate of loss of heat by covariance.
Initial application of the ABRA methodology (over a crop of maize in Zimbabwe) led to the conclusion that the magnitude of the morning imbalance exceeded that explicable in terms of heat storage by the canopy biomass and the air contained within it. Subsequent studies (in Ohio) confirmed the Zimbabwe results and indicated that photosynthesis was a contributor to Imb, in line with expectations. Subsequent field experimentation (at sites located in eastern Tennessee, near Loudon) have provided considerable supporting evidence
The addition of PAR sensors to the field programs has enabled an extension of the ABRA multiple regression approach to refine examination of the energy imbalance following sunrise. Measurements of carbon dioxide concentrations at the two heights already used for temperature and humidity gradient measurement (in the present cases, 1.5 m apart vertically and elevated as the crop grew to maintain a lower level of measurement about 0.5 m above the crown) reveal that CO2 night-time pooling is a major feature of the diurnal cycle, the source of the CO2 being biological activity subsurface. Immediately following sunrise, when PAR passes about 40 μmol m-2 s-1, photosynthesis commences, much sooner than when (Rn – G) increases to a level permitting turbulent (convective) exchange. This behavior is clearly evident in the results of a refined ABRA examination of the data record, this time considering the four variables
Y1 = PAR (the intensity of available photosynthetically active radiation)
Y2 = Δ[CO2] (the CO2 concentration difference between heights z1 and z2)
Y3 = d[CO2]/dt (the rate of change of CO2 concentrations)
Y4 = Δη (as before, the enthalpy difference between z1 and z2)
Clearly, Y2 and Y3 are not independent variables but constitute two different indicators of the way in which CO2 concentrations respond to changes in radiation intensity, on the one hand, and to turbulent exchange, on the other.
Results indicate that when the crop is most rapidly growing, the appetite for CO2 is such that during the morning (when the energy imbalance term Imb is maximized) photosynthetic demands override the consequences of turbulent exchange within the layer of the atmosphere containing the crop and extending up to about 5 m. During the remainder of the day, changes in radiation and turbulent exchange appear to have similar impact on CO2 concentrations and, apparently, on the magnitude of CO2 fluxes. At night, of course, turbulence is controlling.
The material discussed here is too extensive to permit presentation is satisfying detail in the time allotted. Hence, it is planned to summarize the methodologies and to present a sample of results, with the hope that others will be sufficiently intrigued to examine these matters further and to adapt the methodologies to issues of their own interest.
Sauer, T.J. and Horton, R., 2005. Soil heat flux. Micrometeorol. in Agricult. Systems, 47:131-154.
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