An international experiment to investigate the total surface energy balance took place in July and August 2000 the San Joaquin Valley, California. The primary objective of this experiment was to determine why micrometeorological measurements of individual terms of this basic budget equation (net radiation balanced by sensible and latent heat flux, soil heat flux and storage) often cannot achieve closure. A flood-irrigated cotton field was selected to have the physical complexity of a forest canopy but with simpler logistics so that multiple locations could be sampled. The typical meteorological conditions were cloud free skies with northerly winds and evaporation rates of up to 0.6 mm h-1 (400 Wm-2 ). The experimental design featured an array of ten towers with along wind tower spacing of 200 m and a typically fetch of more than 400 m. Furthermore, most commercially-available turbulence instruments were deployed for a comparison phase of experiment. A closure of the energy balance of about 20 W m-2 was found for daily averaged data while for 30-minute averages residual values up to 100-200 W m-2 were found. In this paper the factors affecting the residual of the energy balance closure will be discussed, including: instrumental corrections, heat storage in the canopy and soil, and mesoscale meteorological phenomena.