A comparative study of mass and energy exchange over a closed C3 (wheat) and an open C4 (corn) canopy: I. The partitioning of available energy into latent and sensible heat exchange

Eddy correlation measurements of mass and energy were made simultaneously over a closed wheat and an open corn crop. This combination of crops, differing in canopy coverage and stomatal physiology, facilitated an investigation on how surface and environmental variables control the partitioning of net radiation into sensible, latent and soil heat exchange. Contrasts in canopy closure had a minor affect on the flux densities of net radiation (Rn) measured over open corn and closed wheat canopies. On the other hand, differences in canopy closure perturbed the partitioning of net radiation into sensible, latent and soil heat exchange. On average, measurements of latent heat flux densities (LE) over the sparse corn canopy were 59% less than LE measured over the closed wheat crop. Compensation for this bias caused greater flux densities of sensible (H) and soil (G) heat to be generated from the sparse corn canopy. Sensible heat flux densities were, on average, 22% greater over the open corn canopy. During the day, soil heat flux densities were, on average, 64% greater under the corn canopy than under the wheat. Available energy (A) governed daytime latent heat flux densities over the closed wheat canopy. Consequently, LE was proportional to equilibrium evaporation rates (LE~q). When the corn canopy was dry, latent heat flux densities were less coupled to available energy (A). This response occurred because exposed soil and the corn's C4 physiology increased the corn's canopy resistance to water vapor transfer. When the sparse corn canopy was wet, LE approached values experienced by the closed wheat canopy. A rule of thumb, explaining the surface control of latent heat exchange, was extracted from these data. The ratio between LE and LE~q diminished linearly with logarithmic increases of surface resistance. Different processes controlled canopy evaporation at night. Nocturnal evaporation flux densities measured over the closed wheat crop were independent of available energy, and instead were a function of the atmosphere's vapor pressure deficit. Evaporation flux densities measured over the sparse crop were weakly dependent on available energy.