The variability in Hortonian surface runoff discharge and volume produced by stationary rainstorms on watersheds with spatially distributed soil saturated hydraulic conductivity is examined using a two-dimensional runoff model and a Monte Carlo methodology. Results indicate that rainfall duration tr, rainfall intensity i, representative time to equilibrium tre, mean saturated hydraulic conductivity Km, and coefficient of variation Cv play major roles in the variability of surface runoff. Similarity in surface runoff generated on heterogeneous soils is governed by the following dimensionless parameters: T*=tr/tre, K*=Km/i, and Cv. The variability in both discharge and runoff volume for randomly distributed systems increases with K* and Cv, compared to the runoff generated from uniformly distributed systems. Runoff variability decreases when T* increases unless the mean value of hydraulic conductivity approaches the rainfall intensity (K*→1). In highly pervious watersheds the steady state discharge depends on the spatial distribution of hydraulic conductivity. Lumped values of saturated hydraulic conductivity are found to typically underestimate the peak discharge and runoff volume. ¿ American Geophysical Union 1995