In the present study the water movement in a bottom ash landfill from a municipal solid waste incinerator (MSWI) was investigated. The pore regime of such landfills consists of macropores (with diameter > 50 microm), which make up about two-thirds of the total porosity and micropores. The program MACRO, which describes flow through porous media and takes both macro- and micropore flows into account, was applied. The model was calibrated with a time series from the landfill Riet, near Winterthur in Switzerland. In the present study the model was recalibrated at a time series for 1 year. With this scenario the influence of an expected reduction or increase of the porosity on leachate behaviour of such landfills over a long time (> 100 years) was studied ('long-term behaviour'). It has been shown that reliable information about water percolation can only be provided by obtaining more information about the hydraulic structure of such landfills. In particular, the number of macropores and the porosity exert great influence on the water movement.