The contribution of the upwind source area to the surface fluxes determined from lower boundary layer profiles was studied. Use was made of the boundary layer data obtained from a 213-m meteorological tower and from a tethered sonde, and of surface fluxes from five stations deployed during the 3-week Tsukuba 92 intensive field experiment whose 16 by 16 km experimental area can be characterized by its generally flat topography and by its complex surface land usages. The profile-derived regional sensible heat fluxes were compared with the area-averaged fluxes from the flux data of the surface stations, evaluated for the upwind areas of different sizes from approximately 3¿10-3 km2 to 70 km2. The best agreement was found for the comparison with the results for the upwind area of around 17 km2. This roughly corresponds to the height to fetch ratio of 200 when one considers the mean surface layer height of 40 m.¿ 1997 American Geophysical Union