TOPMODEL (Beven and Kirkby, 1979; Beven et al., 1995) was one of the first attempts to model distributed hydrological responses based on variable contributing area concepts. It makes use of an index of hydrological similarity based on an analysis of the topographic data. The index approach was later generalised to take account of spatial variability of soil transmissivities, but no similar spatial analysis of the variability in the rate of the decrease of the transmissivity with depth has yet been examined. This paper shows how the TOPMODEL theory can be extended to handle this spatial variability, using a 2D distribution function of a new soil depth-topographic index of hydrological similarity. A first sensitivity analysis of the effect of variable soil depths on the model predictions for the Maurets catchment, France, is presented. Predicted discharges and calibrated parameter values are not sensitive to the patterns of effective soil depth investigated. Distributed predictions may be more sensitive but raise questions of how to obtain the parameter data required.