Mean diel cycle of latent (E), sensible (H), net longwave (Lw_net), net shortwave (Sw), and net surface heat flux balance (S) were estimated from hourly meteorological and subsurface water temperature time series acquired for ~1 month during mid-austral autumn by a buoy system in a large tropical reservoir in Brazil. E and H were in phase and had their maximum (E = 163 Wm⁻²; H = 39 Wm⁻²) at early morning and minimum (E = 112 Wm⁻²; H = 6 Wm⁻²) midafter-noon, resulting in Bowen ratios of 0.24 and 0.06, respectively. Heat loss by evaporation therefore dominates over sensible heat used to warm surface atmosphere. Atmospheric instability was present almost all the time, increasing latent and sensible heat flux exchange coefficients by ~50% over their neutral values (from 1.4 × 10⁻³ to 2.2 × 10⁻³). Mean Lw_net varied from 76 at late afternoon to 89 Wm⁻² at early morning, indicating its importance in the overall surface heat flux balance. All 3 fluxes (E, H, and Lw_net) were positive (reservoir losing energy) throughout the day. The integrated daily average net energy budget S (net short wave radiation minus E + H + Lw_net) was ~ −60 Wm⁻²; nighttime energy loss exceeded daytime gain, with consequent cooling of the reservoir. A mean temperature drop of about −0.1 °C d−1 was obtained by fitting a linear trend line to observed daily mean surface temperatures. In a qualitative way, diel time variations of surface water temperature were consistent to the time variability of S, indicating the dominant role of the surface heat budget in modulating surface layer temperatures of the reservoir.