An international experiment to investigate the total surface energy balance took place in July and August 2000 near Fresno, California, on a flood irrigated cotton field in the San Joaquin Valley. The primary objective of this experiment was to determine why micrometeorological measurements of individual terms of this basic budget equation (sensible and latent heat flux, net radiation, soil heat flux and storage) often cannot achieve closure. 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 was an array of ten towers with along wind tower spacing of 200 m and a typical fetch of more than 400 m. Furthermore, all currently used turbulence instruments were compared during a special comparison experiment at the beginning. 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 the paper the instrumental corrections, heat storage effects, meso-meteorological and other reasons for the residuum of the energy balance closure will be discussed.