acceptance by the community of researchers addressing matters of air-surface exchange.
Historic methodologies relying on measurement of the available heat energy and its partitioning
according to measured temperature and humidity gradients (as in the well-known Bowen ratio
energy balance technique — BREB) are no longer favored, largely because of the repeated
demonstration that the surface energy budget is not always balanced (i.e. that Rn – G = H +
LwE is not always satisfied). There are many reasons for the imbalance, principally the omission
of heat storage terms, the disregard of photosynthesis and the assumption that G can be well
measured using ground heat flux plates inserted into the soil. All three of these matters have
been subject to considerable investigation that has failed to provide answers sufficient to revive
the BREB approach, given that EC yields direct measurement of the fluxes of prime interest
without making the energy balance assumption. The fluxes of interest are not only those of heat
and moisture but also the trace chemicals and particles of air pollution and climate concern,
most notably including carbon dioxide and nitrous oxide.
However, EC requires either sites that satisfy specific micrometeorological specifications
or extensive supporting experimentation to derive accurate representations of the fluxes
affecting the surface from covariances measured at the usual heights (many meters above the
surface). To a large extent, BREB avoids this complexity by permitting gradient measurements
to be made very close to the surface, the corresponding assumption being only that the
turbulent diffusivities of heat and moisture are the same. The considerations involved have been
well demonstrated in recent examinations of flux “footprints.”
The matter came to a heat in planning for experiments intended to investigate the
atmospheric CO 2 sensitivity to maize cropping in Africa, where subsistence farming is common
and where plot sizes are declining as generations progress. Use of EC methods was rejected
because of plot size limitations (of scale O(100m)) and characteristic terrain complexity.
Instead, BREB methods were used, but with the addition of downward-looking infrared
thermometry. The measurements of surface temperature permitted and augmented Bowen
ratio approach (ABRA) to be used to provise average diurnal cycles of the desired eddy fluxes
without the assumption of surface heat energy balance. Instead, ABRA quantified the heat
stored in the canopy biomass, its air mass, and in the soil above the depth of G measurement.
It also provided determinations of the extent of heat energy imbalance after heat storage is
accounted for.
It is not the present intent to promote ABRA as an optimal improvement on BREB but
rather to provide it as an example of how it is possible to adjust experimentation methods to
address specific goals. It is proposed that considerable thought is needed before decisions are
made about experimental protocols and that standardization of them will necessarily impose a
need for site-specific modification unless micrometeorological site requirements are satisfied.
If the intent is to study the processes involved in air-surface exchange, then a principal
requirement is to make measurements of the contributing processes as close to the interface as
is possible. Otherwise, the experimental program risks evolving into an extended study of how
to interpret the measurements that are made. Results obtained in studies of maize crops in
typical farming land in Tennessee will be used to illustrate the difficulties associated with BREB,
ABRA and EC methodologies. Examinations of the processes contributing to the correction of
EC measurements are continuing.
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