Comparison of water transfer as a component of the energy balance in a cultivated grass ( Brachiaria decumbens Stapf.) field and a savanna during the wet season of the Orinoco Llanos

Abstract

Flux densities of sensible ( H) and latent heat (LE) were comparatively measured over a closed canopy of a cultivated paleotropical grass ( Brachiaria decumbens) field and an open canopy of a native neotropical savanna with upright-tiller grasses. The measurements were conducted during non-rainy days of the wet season (May–November). Results indicated that these two canopies, differing in closure and forcing conditions, were able to affect the energy budget. Consequently, seasonally averaged daily net radiation ( R n) was, on average, 1.1 fold higher over the Brachiaria canopy. However, contrast in canopy closure had a major effect on the partitioning of seasonally averaged daily R n into LE, H and soil heat flux ( G) as seasonally averaged daily LE over the Brachiaria canopy dissipated 85% of the available energy (7.5±0.6 MJ m −2 day −1). Over the savanna, the value was 64% (4.9±0.3 MJ m −2 day −1). Seasonally averaged daily measurements of H and G in the closed canopy were 53 and 66% lower than those in the open canopy. In the Brachiaria canopy, physical factors related to the hourly available energy were mainly controlling the energy partitioning and hourly LE was proportionally related to the equilibrium evaporation rate (LE eq). In the savanna canopy, the canopy cover and relatively higher soil exposure to solar radiation ( R s) were mainly controlling the stand evapotranspiration as the differential effect of R s and the air water saturation deficit on the vegetation and soil evaporating surfaces. The evapotranspirative adaptation of Brachiaria in relation to the dynamic between water-transpirating and water absorbing surfaces as well as surface conductance sensitivity to soil transfer and yielding capacity are probably responsible for the widespread nature of this forage in the Orinoco Llanos. However, the difference in production between the Brachiaria and savanna stands is economically determined under the limiting environmental conditions of the Orinoco Llanos, where yield increases, which may be gained throughout the agricultural inputs must justify the required addition of resources.