Evapotranspiration (ET), ecosystem water use efficiency (EWUE), and energy partitioning in switchgrass (Panicum virgatum L.) ecosystems are crucial to understand its water and energy balances since switchgrass acreage is expanding for cellulosic biofuels. We measured CO2, H2O, and energy fluxes over a switchgrass field in Chickasha, OK, USA, using eddy covariance method. The objective of this study was to quantify seasonal variations in ET, EWUE, and seasonal distributions of energy partitioning in response to controlling factors. Seasonal (May to mid-November) cumulative ET (450 mm) was similar to cumulative rainfall (432 mm). During June to September, ET was 1·92 times of rainfall, indicating that the crop experienced severe drought during the mid-growing season. ET showed clear seasonality with 3–4 mm day−1 during the active growing season (late May and June) to low rates of about 0·5 mm day−1 during the late growing season (November). The ET rate decreased during dry periods. On seasonal scale, more energy was partitioned to sensible heat flux (H) than latent heat (LE) because of drought. Estimation of EWUE by dividing time integrals of gross ecosystem photosynthesis (GEP) by respective H2O fluxes (ET) at monthly time scale provided EWUE from 10·46 (August) to 14·08 g CO2 mm−1 ET (October) with a seasonal average of 12·01 g CO2 mm−1 ET. Seasonal patterns in EWUE were observed with smaller values during drought because of more rapid reduction in carbon assimilation (GEP) than ET. These findings confirm the major role of precipitation in determining water and energy balances in switchgrass. Copyright © 2012 John Wiley & Sons, Ltd.