The Water Information System, WIS, has been developed at the Institute of Hydrology, Wallingford, as a four-dimensional database and GIS. TOPMODEL is a set of concepts for distributed rainfall-runoff modelling, that makes use of detailed DTM data but is computationally efficient. Incorporation of TOPMODEL into WIS raises a number of problems, the most critical of which is the generalization of the model structure to allow for varying topographic and hydrological conditions with different levels of data availability. One approach to this problem is the use of "on-line; sensitivity analysis, parameter calibration and estimation of predictive uncertainty for different possible model structures, with the graphical presentation of the results within WIS for direct visual evaluation by the user. -from Authors