Deforestation and climate change have the capacity to alter rainfall regimes, water availability, and surface-atmosphere flux of water and energy of tropical forests. This may be especially true in ecotonal, semi-deciduous tropical forests of the southern Amazon Basin, which have experienced some of the most rapid recent climate change and deforestation and are expected to decline by 70-80% by 2050 under the "business-as-usual" land management scenario. To reduce uncertainty regarding current and future energy and water flux, measurements of latent and sensible heat flux were combined with measurements of sap flux density and maximum leaf conductance to characterize the seasonal controls on energy flux density over the 2005-06 annual cycle in a tropical semi-deciduous forest of the rainforest-savanna ecotone in northern Mato Grosso, Brazil. Average diel patterns and daily rates of energy and sap flux density were remarkably stable despite large seasonal variations in rainfall and soil moisture; however, variations in stomatal and canopy conductance were large and positively correlated with seasonal variations in rainfall and surface soil moisture. These data suggest that semi-deciduous trees have access to deep water reserves that support high rates evapotranspiration and sap flow during the dry season, but because high vapor pressure deficit and evapotranspiration cause a decline in leaf water potential there is a concomitant decline in stomatal and canopy conductance.