TThis paper is essentially devoted to the study of the interfacial coefficient problem and to a characterization of the falling rate period.By means of a 2D numerical simulation, it is shown that the falling rate period is characterized by strong humidity heterogeneities at the interface. These heterogeneities affect significantly the interfacial mass transfer coefficient to be used in the traditional ID approach of drying. In this paper. the heterogeneities are due to the coupling between external and internal transfer in a boundary layer flow leading edge region.0ther phenomena that may also cause the occurence of humidity heterogeneities at the interface are discussed.