The recent widespread availability of digital terrain data has resulted in increasing use, in a variety of hydrological models, of different degrees of complexity. Previous experience suggests that neither model results nor effective parameter values are independent of the resolution of the digital terrain data used. Calibration of parameter values can compensate for lack of resolution in the digital terrain data. This paper will show, for one particular model, TOPMODEL, how an analytical link can be established between the grid size of a raster digital terrain model and the effective saturated hydraulic conductivity value used in the model. The work generalizes the results of the recent study by Franchini et al. (1996) and allows the change in effective conductivity to be estimated on the basis of keeping a realistic simulation of saturated contributing areas as the DTM grid size changes.