An observation experiment about water vapour transportation over desert is carried out at desert station in the HEIFE on July 1991 to find the specific humidity profiles often appear minimum in the surface layer.The level of the minimun value is at the height of zero water vapour flux.The water vapour is transported downwards and the profile of specific humidity appears inverse humidity character above the zero flux height,but the vapour is transported upwards and the profile is evaporation character under the zero flux height.The water vapour flux is unconstant in the surface laved at a fixed level,can not represent the evaporativity and the latent heat flux on ground surface.