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The savannah of central Brazil occupies 208 million ha, of which about 54 million ha have been converted to cultivated pasture. The aim of this study was to provide the dynamics of evapotranspiration (ET) in a Brachiaria brizantha cultivated pasture in the Brazilian Savannah region at the beginning of the rainy season and determine biological and environmental factors that influence ET by using the decoupling approach. A meteorological station and an eddy covariance system were placed at the studied site. The above-ground biomass production of the B. brizantha pasture responded rapidly to the onset of the rainy season. Leaf area index (LAI) increased from 0·4 to 1·1 between 15 November and 3 December, with a corresponding increase in live biomass. Average ET during the study period was 2·6 ± 0·9 mm. Daily ET was significantly correlated with maximum vapour pressure deficit and precipitation. The average of daytime decoupling factor obtained over 2 d without rain was 0·50 ± 0·06. The B. brizantha pasture deployed in this region showed an intermediate coupling which takes ET to be influenced by both atmospheric conditions and by the available radiation at the canopy.