The object of this paper is to study how heterogeneities in basin erosivity and smoothing processes like diffusive erosion lead to different patterns of runoff production, different width functions and hence basin response (IUH). In other words, we want to study how, all else being equal, heterogeneous basins and diffusion dominated basins behave when compared to homogeneous and fluvially eroded basins. We use a physically based basin evolution model (Moglen and Bras, 1995) as to generate our test beds. We can vary heterogeneity and the strength of diffusive erosion in the model to generate a series of river basins whose response is then studied.