We investigated the impact of the 2011 severe drought on summer evapotranspiration (ET) and energy partitioning of a managed Old World bluestem grassland in the Southern Great Plains and compared the results with those from 2010, a hydrologically wet year. Measurements of CO2, latent heat (LE), and sensible heat (H) fluxes were made using an eddy covariance flux tower. In 2010, the total precipitation received during the summer months (June–August) was 82% above the 9-year average precipitation, and in 2011, it was only 8% of the 9-year average precipitation. The amount of net radiation (Rn) available at the surface was higher in the wet year (54.3% of the incident radiation) compared with the drought year (41.1% of the incident radiation). In 2010, LE consumed the majority of available energy resulting in a high evaporative fraction (EF). In 2011, less energy was converted to LE as soil moisture remained low, which also resulted in reduced plant growth. Thus, the energy partitioning in 2011 was dominated by H. Significant differences in bulk canopy conductance (Gs) were also observed between the two years. In both years, Gs was linearly correlated with soil volumetric water content in the top 4 cm of the soil profile. The gross primary production (GPP) was linearly correlated to ET. The water use efficiency was 2.63 g C kg−1 of water in 2010 and 0.69 g C kg−1 of water in 2011. Results from this study showed that, in semi-arid grasslands, the ET and energy balance are strongly affected by water availability. Copyright © 2014 John Wiley & Sons, Ltd.