Comparative energy exchange from cowpeas (Vigna unguiculata (L.) Walp cvs. TC-9-6 and M-28-6-6) with differences in canopy architectures and growth durations at the Orinoco llanos

This study analyzes the eddy correlation measurements of mass and energy taken simultaneously over two contrasting cowpeas (Vigna unguiculata (L.) Walp cvs. TC-9-6 and M-28-6-6). Here, flux densities were measured over an early-maturing and postrate-canopy (TC-9-6) and a medium-maturity and erect-canopy cowpea (M-28-6-6). The results indicates that at maximum crop growth stage, leaf area index was similar in the two cultivars (i.e. 2.13 and 2.15 m2 m-2 for TC-9-6 and M-28-6-6, respectively), and also that different architectures of the canopy had a minor effect on the flux densities of net radiation as well as the partitioning of available energy into sensible and latent heat. In Tejero, the phenological averaged daytime latent heat flux (LE) to available energy ratio throughout the vegetative and reproductive stages was 0.73 and 0.82, respectively. Over the erect M-28-6-6 canopy, this ratio was 0.75 and 0.86, respectively. As the senescence stage proceeded, the ratio in the early-maturing TC-9-6 (i.e. 0.72) was significantly lower than that in the medium-maturing M-28-6-6 canopy (i.e. 0.81). As consequence of this difference in leaf delayed senescence as well as seasonal variations in daily Bowen ratio, the accumulated seasonal LE over the TC-9-6 (14.8 mm) was 8% lower than that measured over M-28-6-6 (161 mm). Results indicate that the phenological trend of the daily LE was controlled by the leaf area index (LAI) development. When LAI reached its maximum value at the flowering and pod-filling stages, LE was controlled mainly by the available energy and temperature. Consequently, LE was related to equilibrium LE, and the Penman evapotranspiration model for equilibrium was most appropriate for modeling LE. During this stage, the LE/LEL ratio increased linearly with increases of surface conductance. By contrast, during the vegetative and senescence stages, the partial exposed soil surface by the sparsely canopies and the low surface conductance were able to decrease LE. Although the reported results are relevant only to similar agricultural sites in the Orinoco lowlands, the processes and controls described here are representative of cowpeas growing during savanna wet seasons, which are subject to short rainless periods.