Comparison of Factors Affecting Water Transfer in a Cultivated Paleotropical Grass (Brachiaria decumbens Stapf) Field and a Neotropical Savanna during the Dry Season of the Orinoco Lowlands

J. J. San José Centro de Ecología, Instituto Venezolano de Investigaciones Científicas, Caracas, Venezuela

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N. Nikonova Centro de Ecología, Instituto Venezolano de Investigaciones Científicas, Caracas, Venezuela

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R. Bracho Centro de Ecología, Instituto Venezolano de Investigaciones Científicas, Caracas, Venezuela

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Abstract

Partitioning of available energy into sensible (H) and latent (LE) heat exchange was simultaneously measured in a closed Brachiaria field and an open neotropical savanna during the dry season of the Orinoco lowlands. These communities, differing in canopy coverage and aerodynamic forcing conditions, had major effects on the radiation and energy budgets. Thus, in the Brachiaria canopy, seasonally averaged daily LE was 55% (6.635 ± 0.92 MJ m−2 day−1) of the seasonally averaged daily available energy. In the savanna, it was 30% (2.841 ± 0.65 MJ m−2 day−1). The high degree of coupling between the Brachiaria and savanna surfaces to the prevailing weather of the dry season, as expressed by the seasonally averaged daily omega factor (Ω = 0.056 ± 0.005 and 0.042 ± 0.003, respectively), indicates that LE depended on the atmospheric driving potential and the canopy conductance (gc) to vapor diffusion. Consequently, daily LE was proportional to the imposed evapotranspiration rate. The hourly LE measured/LE equilibrium ratio for the Brachiaria canopy was logarithmically related to the canopy resistance (rc); in the savanna, the ratio was exponentially related to rc with a threshold value ranging from 60 to 90 s m−1. This contrasting behavior was unrelated to plant water status as maximum hourly leaf water potential in both canopies was, on average, similar (−0.7 to −0.8 MPa). The difference between the two canopies in the control exerted by the canopy conductance on water losses was associated with the size of the evaporating surface and the proportional contribution of the foliage and the dry soil resistance.

Corresponding author address: Dr. J. J. San José, Centro de Ecologia, Inst. Venezolano de Investigaciones Cientificas, Apartado Postal 21827, Caracas 1020A, Venezuela.

jsanjose@oikos.ivic.ve

Abstract

Partitioning of available energy into sensible (H) and latent (LE) heat exchange was simultaneously measured in a closed Brachiaria field and an open neotropical savanna during the dry season of the Orinoco lowlands. These communities, differing in canopy coverage and aerodynamic forcing conditions, had major effects on the radiation and energy budgets. Thus, in the Brachiaria canopy, seasonally averaged daily LE was 55% (6.635 ± 0.92 MJ m−2 day−1) of the seasonally averaged daily available energy. In the savanna, it was 30% (2.841 ± 0.65 MJ m−2 day−1). The high degree of coupling between the Brachiaria and savanna surfaces to the prevailing weather of the dry season, as expressed by the seasonally averaged daily omega factor (Ω = 0.056 ± 0.005 and 0.042 ± 0.003, respectively), indicates that LE depended on the atmospheric driving potential and the canopy conductance (gc) to vapor diffusion. Consequently, daily LE was proportional to the imposed evapotranspiration rate. The hourly LE measured/LE equilibrium ratio for the Brachiaria canopy was logarithmically related to the canopy resistance (rc); in the savanna, the ratio was exponentially related to rc with a threshold value ranging from 60 to 90 s m−1. This contrasting behavior was unrelated to plant water status as maximum hourly leaf water potential in both canopies was, on average, similar (−0.7 to −0.8 MPa). The difference between the two canopies in the control exerted by the canopy conductance on water losses was associated with the size of the evaporating surface and the proportional contribution of the foliage and the dry soil resistance.

Corresponding author address: Dr. J. J. San José, Centro de Ecologia, Inst. Venezolano de Investigaciones Cientificas, Apartado Postal 21827, Caracas 1020A, Venezuela.

jsanjose@oikos.ivic.ve

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