Here, an investigation has been carried out in order to understand the influence of the variation of the surface roughness in the definition of the hydrological response of semiarid catchments. In the original version of TOPMODEL the convolution rout- ing procedure used takes in account the distribution of predicted inflow with distance along the channel network from the outflow, considering the distributed nature of the channel network, but does not allow for the routing on the hillslopes. This type of approach is appropriate for humid basins but not for semiarid catchments which are mainly characterised by steep and straight hillslopes. In previous studies, same au- thors proposed a modified version of TOPMODEL in which the convolution routing procedure has been extended to the hillslopes by specifying the routing velocity for each pixel of the watershed. These velocities have been linked to the watershed land use because the different surface roughness whose coefficients has been derived on the basis of Engman's table. In this new study, roughness coefficients distribution are expressed as function of a unique value treated as a calibration parameter. The original and modified versions of TOPMODEL have been applied for the simulation of flood events in a Sicilian catchment.