The current popularity of the rainfall-runoff model Topmodel is a direct result of the widespread availability of catchment GIS systems and particularly of digital terrain maps. Water flows downhill, therefore topography must be hydrologically significant, therefore how can the digital terrain data be used in hydrological modelling to improve the realism of the predictions? There are not many choices available: Topmodel is one of them but the concepts on which it is based will not be applicable everywhere (see some of the papers in this volume). Indeed, one of the most important aspects of the use of Topmodel is the possibility of mapping the simulations back into space so that the distributed predictions can be compared with field observations. The experiences reported here in applications from around the world represent an excellent summary of the success and limitations of the concepts in a wide variety of environments. Topmodel was never intended to be a fixed model structure but rather a set of concepts that could be modified if required. As such it is important that the experiences of other uses can be gathered together and passed on.
